docs: Correct wording in LangRef relating to available_externally linkage.

[oota-llvm.git] / docs / LangRef.rst

==============================
LLVM Language Reference Manual
==============================

.. contents::
   :local:
   :depth: 4

Abstract
========

This document is a reference manual for the LLVM assembly language. LLVM
is a Static Single Assignment (SSA) based representation that provides
type safety, low-level operations, flexibility, and the capability of
representing 'all' high-level languages cleanly. It is the common code
representation used throughout all phases of the LLVM compilation
strategy.

Introduction
============

The LLVM code representation is designed to be used in three different
forms: as an in-memory compiler IR, as an on-disk bitcode representation
(suitable for fast loading by a Just-In-Time compiler), and as a human
readable assembly language representation. This allows LLVM to provide a
powerful intermediate representation for efficient compiler
transformations and analysis, while providing a natural means to debug
and visualize the transformations. The three different forms of LLVM are
all equivalent. This document describes the human readable
representation and notation.

The LLVM representation aims to be light-weight and low-level while
being expressive, typed, and extensible at the same time. It aims to be
a "universal IR" of sorts, by being at a low enough level that
high-level ideas may be cleanly mapped to it (similar to how
microprocessors are "universal IR's", allowing many source languages to
be mapped to them). By providing type information, LLVM can be used as
the target of optimizations: for example, through pointer analysis, it
can be proven that a C automatic variable is never accessed outside of
the current function, allowing it to be promoted to a simple SSA value
instead of a memory location.

.. _wellformed:

Well-Formedness
---------------

It is important to note that this document describes 'well formed' LLVM
assembly language. There is a difference between what the parser accepts
and what is considered 'well formed'. For example, the following
instruction is syntactically okay, but not well formed:

.. code-block:: llvm

    %x = add i32 1, %x

because the definition of ``%x`` does not dominate all of its uses. The
LLVM infrastructure provides a verification pass that may be used to
verify that an LLVM module is well formed. This pass is automatically
run by the parser after parsing input assembly and by the optimizer
before it outputs bitcode. The violations pointed out by the verifier
pass indicate bugs in transformation passes or input to the parser.

.. _identifiers:

Identifiers
===========

LLVM identifiers come in two basic types: global and local. Global
identifiers (functions, global variables) begin with the ``'@'``
character. Local identifiers (register names, types) begin with the
``'%'`` character. Additionally, there are three different formats for
identifiers, for different purposes:

#. Named values are represented as a string of characters with their
   prefix. For example, ``%foo``, ``@DivisionByZero``,
   ``%a.really.long.identifier``. The actual regular expression used is
   '``[%@][-a-zA-Z$._][-a-zA-Z$._0-9]*``'. Identifiers that require other
   characters in their names can be surrounded with quotes. Special
   characters may be escaped using ``"\xx"`` where ``xx`` is the ASCII
   code for the character in hexadecimal. In this way, any character can
   be used in a name value, even quotes themselves. The ``"\01"`` prefix
   can be used on global variables to suppress mangling.
#. Unnamed values are represented as an unsigned numeric value with
   their prefix. For example, ``%12``, ``@2``, ``%44``.
#. Constants, which are described in the section Constants_ below.

LLVM requires that values start with a prefix for two reasons: Compilers
don't need to worry about name clashes with reserved words, and the set
of reserved words may be expanded in the future without penalty.
Additionally, unnamed identifiers allow a compiler to quickly come up
with a temporary variable without having to avoid symbol table
conflicts.

Reserved words in LLVM are very similar to reserved words in other
languages. There are keywords for different opcodes ('``add``',
'``bitcast``', '``ret``', etc...), for primitive type names ('``void``',
'``i32``', etc...), and others. These reserved words cannot conflict
with variable names, because none of them start with a prefix character
(``'%'`` or ``'@'``).

Here is an example of LLVM code to multiply the integer variable
'``%X``' by 8:

The easy way:

.. code-block:: llvm

    %result = mul i32 %X, 8

After strength reduction:

.. code-block:: llvm

    %result = shl i32 %X, 3

And the hard way:

.. code-block:: llvm

    %0 = add i32 %X, %X           ; yields i32:%0
    %1 = add i32 %0, %0           ; yields i32:%1
    %result = add i32 %1, %1

This last way of multiplying ``%X`` by 8 illustrates several important
lexical features of LLVM:

#. Comments are delimited with a '``;``' and go until the end of line.
#. Unnamed temporaries are created when the result of a computation is
   not assigned to a named value.
#. Unnamed temporaries are numbered sequentially (using a per-function
   incrementing counter, starting with 0). Note that basic blocks and unnamed
   function parameters are included in this numbering. For example, if the
   entry basic block is not given a label name and all function parameters are
   named, then it will get number 0.

It also shows a convention that we follow in this document. When
demonstrating instructions, we will follow an instruction with a comment
that defines the type and name of value produced.

High Level Structure
====================

Module Structure
----------------

LLVM programs are composed of ``Module``'s, each of which is a
translation unit of the input programs. Each module consists of
functions, global variables, and symbol table entries. Modules may be
combined together with the LLVM linker, which merges function (and
global variable) definitions, resolves forward declarations, and merges
symbol table entries. Here is an example of the "hello world" module:

.. code-block:: llvm

    ; Declare the string constant as a global constant.
    @.str = private unnamed_addr constant [13 x i8] c"hello world\0A\00"

    ; External declaration of the puts function
    declare i32 @puts(i8* nocapture) nounwind

    ; Definition of main function
    define i32 @main() {   ; i32()*
      ; Convert [13 x i8]* to i8  *...
      %cast210 = getelementptr [13 x i8], [13 x i8]* @.str, i64 0, i64 0

      ; Call puts function to write out the string to stdout.
      call i32 @puts(i8* %cast210)
      ret i32 0
    }

    ; Named metadata
    !0 = !{i32 42, null, !"string"}
    !foo = !{!0}

This example is made up of a :ref:`global variable <globalvars>` named
"``.str``", an external declaration of the "``puts``" function, a
:ref:`function definition <functionstructure>` for "``main``" and
:ref:`named metadata <namedmetadatastructure>` "``foo``".

In general, a module is made up of a list of global values (where both
functions and global variables are global values). Global values are
represented by a pointer to a memory location (in this case, a pointer
to an array of char, and a pointer to a function), and have one of the
following :ref:`linkage types <linkage>`.

.. _linkage:

Linkage Types
-------------

All Global Variables and Functions have one of the following types of
linkage:

``private``
    Global values with "``private``" linkage are only directly
    accessible by objects in the current module. In particular, linking
    code into a module with an private global value may cause the
    private to be renamed as necessary to avoid collisions. Because the
    symbol is private to the module, all references can be updated. This
    doesn't show up in any symbol table in the object file.
``internal``
    Similar to private, but the value shows as a local symbol
    (``STB_LOCAL`` in the case of ELF) in the object file. This
    corresponds to the notion of the '``static``' keyword in C.
``available_externally``
    Globals with "``available_externally``" linkage are never emitted into
    the object file corresponding to the LLVM module. From the linker's
    perspective, an ``available_externally`` global is equivalent to
    an external declaration. They exist to allow inlining and other
    optimizations to take place given knowledge of the definition of the
    global, which is known to be somewhere outside the module. Globals
    with ``available_externally`` linkage are allowed to be discarded at
    will, and allow inlining and other optimizations. This linkage type is
    only allowed on definitions, not declarations.
``linkonce``
    Globals with "``linkonce``" linkage are merged with other globals of
    the same name when linkage occurs. This can be used to implement
    some forms of inline functions, templates, or other code which must
    be generated in each translation unit that uses it, but where the
    body may be overridden with a more definitive definition later.
    Unreferenced ``linkonce`` globals are allowed to be discarded. Note
    that ``linkonce`` linkage does not actually allow the optimizer to
    inline the body of this function into callers because it doesn't
    know if this definition of the function is the definitive definition
    within the program or whether it will be overridden by a stronger
    definition. To enable inlining and other optimizations, use
    "``linkonce_odr``" linkage.
``weak``
    "``weak``" linkage has the same merging semantics as ``linkonce``
    linkage, except that unreferenced globals with ``weak`` linkage may
    not be discarded. This is used for globals that are declared "weak"
    in C source code.
``common``
    "``common``" linkage is most similar to "``weak``" linkage, but they
    are used for tentative definitions in C, such as "``int X;``" at
    global scope. Symbols with "``common``" linkage are merged in the
    same way as ``weak symbols``, and they may not be deleted if
    unreferenced. ``common`` symbols may not have an explicit section,
    must have a zero initializer, and may not be marked
    ':ref:`constant <globalvars>`'. Functions and aliases may not have
    common linkage.

.. _linkage_appending:

``appending``
    "``appending``" linkage may only be applied to global variables of
    pointer to array type. When two global variables with appending
    linkage are linked together, the two global arrays are appended
    together. This is the LLVM, typesafe, equivalent of having the
    system linker append together "sections" with identical names when
    .o files are linked.
``extern_weak``
    The semantics of this linkage follow the ELF object file model: the
    symbol is weak until linked, if not linked, the symbol becomes null
    instead of being an undefined reference.
``linkonce_odr``, ``weak_odr``
    Some languages allow differing globals to be merged, such as two
    functions with different semantics. Other languages, such as
    ``C++``, ensure that only equivalent globals are ever merged (the
    "one definition rule" --- "ODR"). Such languages can use the
    ``linkonce_odr`` and ``weak_odr`` linkage types to indicate that the
    global will only be merged with equivalent globals. These linkage
    types are otherwise the same as their non-``odr`` versions.
``external``
    If none of the above identifiers are used, the global is externally
    visible, meaning that it participates in linkage and can be used to
    resolve external symbol references.

It is illegal for a function *declaration* to have any linkage type
other than ``external`` or ``extern_weak``.

.. _callingconv:

Calling Conventions
-------------------

LLVM :ref:`functions <functionstructure>`, :ref:`calls <i_call>` and
:ref:`invokes <i_invoke>` can all have an optional calling convention
specified for the call. The calling convention of any pair of dynamic
caller/callee must match, or the behavior of the program is undefined.
The following calling conventions are supported by LLVM, and more may be
added in the future:

"``ccc``" - The C calling convention
    This calling convention (the default if no other calling convention
    is specified) matches the target C calling conventions. This calling
    convention supports varargs function calls and tolerates some
    mismatch in the declared prototype and implemented declaration of
    the function (as does normal C).
"``fastcc``" - The fast calling convention
    This calling convention attempts to make calls as fast as possible
    (e.g. by passing things in registers). This calling convention
    allows the target to use whatever tricks it wants to produce fast
    code for the target, without having to conform to an externally
    specified ABI (Application Binary Interface). `Tail calls can only
    be optimized when this, the GHC or the HiPE convention is
    used. <CodeGenerator.html#id80>`_ This calling convention does not
    support varargs and requires the prototype of all callees to exactly
    match the prototype of the function definition.
"``coldcc``" - The cold calling convention
    This calling convention attempts to make code in the caller as
    efficient as possible under the assumption that the call is not
    commonly executed. As such, these calls often preserve all registers
    so that the call does not break any live ranges in the caller side.
    This calling convention does not support varargs and requires the
    prototype of all callees to exactly match the prototype of the
    function definition. Furthermore the inliner doesn't consider such function
    calls for inlining.
"``cc 10``" - GHC convention
    This calling convention has been implemented specifically for use by
    the `Glasgow Haskell Compiler (GHC) <http://www.haskell.org/ghc>`_.
    It passes everything in registers, going to extremes to achieve this
    by disabling callee save registers. This calling convention should
    not be used lightly but only for specific situations such as an
    alternative to the *register pinning* performance technique often
    used when implementing functional programming languages. At the
    moment only X86 supports this convention and it has the following
    limitations:

    -  On *X86-32* only supports up to 4 bit type parameters. No
       floating point types are supported.
    -  On *X86-64* only supports up to 10 bit type parameters and 6
       floating point parameters.

    This calling convention supports `tail call
    optimization <CodeGenerator.html#id80>`_ but requires both the
    caller and callee are using it.
"``cc 11``" - The HiPE calling convention
    This calling convention has been implemented specifically for use by
    the `High-Performance Erlang
    (HiPE) <http://www.it.uu.se/research/group/hipe/>`_ compiler, *the*
    native code compiler of the `Ericsson's Open Source Erlang/OTP
    system <http://www.erlang.org/download.shtml>`_. It uses more
    registers for argument passing than the ordinary C calling
    convention and defines no callee-saved registers. The calling
    convention properly supports `tail call
    optimization <CodeGenerator.html#id80>`_ but requires that both the
    caller and the callee use it. It uses a *register pinning*
    mechanism, similar to GHC's convention, for keeping frequently
    accessed runtime components pinned to specific hardware registers.
    At the moment only X86 supports this convention (both 32 and 64
    bit).
"``webkit_jscc``" - WebKit's JavaScript calling convention
    This calling convention has been implemented for `WebKit FTL JIT
    <https://trac.webkit.org/wiki/FTLJIT>`_. It passes arguments on the
    stack right to left (as cdecl does), and returns a value in the
    platform's customary return register.
"``anyregcc``" - Dynamic calling convention for code patching
    This is a special convention that supports patching an arbitrary code
    sequence in place of a call site. This convention forces the call
    arguments into registers but allows them to be dynamically
    allocated. This can currently only be used with calls to
    llvm.experimental.patchpoint because only this intrinsic records
    the location of its arguments in a side table. See :doc:`StackMaps`.
"``preserve_mostcc``" - The `PreserveMost` calling convention
    This calling convention attempts to make the code in the caller as
    unintrusive as possible. This convention behaves identically to the `C`
    calling convention on how arguments and return values are passed, but it
    uses a different set of caller/callee-saved registers. This alleviates the
    burden of saving and recovering a large register set before and after the
    call in the caller. If the arguments are passed in callee-saved registers,
    then they will be preserved by the callee across the call. This doesn't
    apply for values returned in callee-saved registers.

    - On X86-64 the callee preserves all general purpose registers, except for
      R11. R11 can be used as a scratch register. Floating-point registers
      (XMMs/YMMs) are not preserved and need to be saved by the caller.

    The idea behind this convention is to support calls to runtime functions
    that have a hot path and a cold path. The hot path is usually a small piece
    of code that doesn't use many registers. The cold path might need to call out to
    another function and therefore only needs to preserve the caller-saved
    registers, which haven't already been saved by the caller. The
    `PreserveMost` calling convention is very similar to the `cold` calling
    convention in terms of caller/callee-saved registers, but they are used for
    different types of function calls. `coldcc` is for function calls that are
    rarely executed, whereas `preserve_mostcc` function calls are intended to be
    on the hot path and definitely executed a lot. Furthermore `preserve_mostcc`
    doesn't prevent the inliner from inlining the function call.

    This calling convention will be used by a future version of the ObjectiveC
    runtime and should therefore still be considered experimental at this time.
    Although this convention was created to optimize certain runtime calls to
    the ObjectiveC runtime, it is not limited to this runtime and might be used
    by other runtimes in the future too. The current implementation only
    supports X86-64, but the intention is to support more architectures in the
    future.
"``preserve_allcc``" - The `PreserveAll` calling convention
    This calling convention attempts to make the code in the caller even less
    intrusive than the `PreserveMost` calling convention. This calling
    convention also behaves identical to the `C` calling convention on how
    arguments and return values are passed, but it uses a different set of
    caller/callee-saved registers. This removes the burden of saving and
    recovering a large register set before and after the call in the caller. If
    the arguments are passed in callee-saved registers, then they will be
    preserved by the callee across the call. This doesn't apply for values
    returned in callee-saved registers.

    - On X86-64 the callee preserves all general purpose registers, except for
      R11. R11 can be used as a scratch register. Furthermore it also preserves
      all floating-point registers (XMMs/YMMs).

    The idea behind this convention is to support calls to runtime functions
    that don't need to call out to any other functions.

    This calling convention, like the `PreserveMost` calling convention, will be
    used by a future version of the ObjectiveC runtime and should be considered
    experimental at this time.
"``cxx_fast_tlscc``" - The `CXX_FAST_TLS` calling convention for access functions
    Clang generates an access function to access C++-style TLS. The access
    function generally has an entry block, an exit block and an initialization
    block that is run at the first time. The entry and exit blocks can access
    a few TLS IR variables, each access will be lowered to a platform-specific
    sequence.

    This calling convention aims to minimize overhead in the caller by
    preserving as many registers as possible (all the registers that are
    perserved on the fast path, composed of the entry and exit blocks).

    This calling convention behaves identical to the `C` calling convention on
    how arguments and return values are passed, but it uses a different set of
    caller/callee-saved registers.

    Given that each platform has its own lowering sequence, hence its own set
    of preserved registers, we can't use the existing `PreserveMost`.

    - On X86-64 the callee preserves all general purpose registers, except for
      RDI and RAX.
"``cc <n>``" - Numbered convention
    Any calling convention may be specified by number, allowing
    target-specific calling conventions to be used. Target specific
    calling conventions start at 64.

More calling conventions can be added/defined on an as-needed basis, to
support Pascal conventions or any other well-known target-independent
convention.

.. _visibilitystyles:

Visibility Styles
-----------------

All Global Variables and Functions have one of the following visibility
styles:

"``default``" - Default style
    On targets that use the ELF object file format, default visibility
    means that the declaration is visible to other modules and, in
    shared libraries, means that the declared entity may be overridden.
    On Darwin, default visibility means that the declaration is visible
    to other modules. Default visibility corresponds to "external
    linkage" in the language.
"``hidden``" - Hidden style
    Two declarations of an object with hidden visibility refer to the
    same object if they are in the same shared object. Usually, hidden
    visibility indicates that the symbol will not be placed into the
    dynamic symbol table, so no other module (executable or shared
    library) can reference it directly.
"``protected``" - Protected style
    On ELF, protected visibility indicates that the symbol will be
    placed in the dynamic symbol table, but that references within the
    defining module will bind to the local symbol. That is, the symbol
    cannot be overridden by another module.

A symbol with ``internal`` or ``private`` linkage must have ``default``
visibility.

.. _dllstorageclass:

DLL Storage Classes
-------------------

All Global Variables, Functions and Aliases can have one of the following
DLL storage class:

``dllimport``
    "``dllimport``" causes the compiler to reference a function or variable via
    a global pointer to a pointer that is set up by the DLL exporting the
    symbol. On Microsoft Windows targets, the pointer name is formed by
    combining ``__imp_`` and the function or variable name.
``dllexport``
    "``dllexport``" causes the compiler to provide a global pointer to a pointer
    in a DLL, so that it can be referenced with the ``dllimport`` attribute. On
    Microsoft Windows targets, the pointer name is formed by combining
    ``__imp_`` and the function or variable name. Since this storage class
    exists for defining a dll interface, the compiler, assembler and linker know
    it is externally referenced and must refrain from deleting the symbol.

.. _tls_model:

Thread Local Storage Models
---------------------------

A variable may be defined as ``thread_local``, which means that it will
not be shared by threads (each thread will have a separated copy of the
variable). Not all targets support thread-local variables. Optionally, a
TLS model may be specified:

``localdynamic``
    For variables that are only used within the current shared library.
``initialexec``
    For variables in modules that will not be loaded dynamically.
``localexec``
    For variables defined in the executable and only used within it.

If no explicit model is given, the "general dynamic" model is used.

The models correspond to the ELF TLS models; see `ELF Handling For
Thread-Local Storage <http://people.redhat.com/drepper/tls.pdf>`_ for
more information on under which circumstances the different models may
be used. The target may choose a different TLS model if the specified
model is not supported, or if a better choice of model can be made.

A model can also be specified in an alias, but then it only governs how
the alias is accessed. It will not have any effect in the aliasee.

For platforms without linker support of ELF TLS model, the -femulated-tls
flag can be used to generate GCC compatible emulated TLS code.

.. _namedtypes:

Structure Types
---------------

LLVM IR allows you to specify both "identified" and "literal" :ref:`structure
types <t_struct>`. Literal types are uniqued structurally, but identified types
are never uniqued. An :ref:`opaque structural type <t_opaque>` can also be used
to forward declare a type that is not yet available.

An example of an identified structure specification is:

.. code-block:: llvm

    %mytype = type { %mytype*, i32 }

Prior to the LLVM 3.0 release, identified types were structurally uniqued. Only
literal types are uniqued in recent versions of LLVM.

.. _globalvars:

Global Variables
----------------

Global variables define regions of memory allocated at compilation time
instead of run-time.

Global variable definitions must be initialized.

Global variables in other translation units can also be declared, in which
case they don't have an initializer.

Either global variable definitions or declarations may have an explicit section
to be placed in and may have an optional explicit alignment specified.

A variable may be defined as a global ``constant``, which indicates that
the contents of the variable will **never** be modified (enabling better
optimization, allowing the global data to be placed in the read-only
section of an executable, etc). Note that variables that need runtime
initialization cannot be marked ``constant`` as there is a store to the
variable.

LLVM explicitly allows *declarations* of global variables to be marked
constant, even if the final definition of the global is not. This
capability can be used to enable slightly better optimization of the
program, but requires the language definition to guarantee that
optimizations based on the 'constantness' are valid for the translation
units that do not include the definition.

As SSA values, global variables define pointer values that are in scope
(i.e. they dominate) all basic blocks in the program. Global variables
always define a pointer to their "content" type because they describe a
region of memory, and all memory objects in LLVM are accessed through
pointers.

Global variables can be marked with ``unnamed_addr`` which indicates
that the address is not significant, only the content. Constants marked
like this can be merged with other constants if they have the same
initializer. Note that a constant with significant address *can* be
merged with a ``unnamed_addr`` constant, the result being a constant
whose address is significant.

A global variable may be declared to reside in a target-specific
numbered address space. For targets that support them, address spaces
may affect how optimizations are performed and/or what target
instructions are used to access the variable. The default address space
is zero. The address space qualifier must precede any other attributes.

LLVM allows an explicit section to be specified for globals. If the
target supports it, it will emit globals to the section specified.
Additionally, the global can placed in a comdat if the target has the necessary
support.

By default, global initializers are optimized by assuming that global
variables defined within the module are not modified from their
initial values before the start of the global initializer. This is
true even for variables potentially accessible from outside the
module, including those with external linkage or appearing in
``@llvm.used`` or dllexported variables. This assumption may be suppressed
by marking the variable with ``externally_initialized``.

An explicit alignment may be specified for a global, which must be a
power of 2. If not present, or if the alignment is set to zero, the
alignment of the global is set by the target to whatever it feels
convenient. If an explicit alignment is specified, the global is forced
to have exactly that alignment. Targets and optimizers are not allowed
to over-align the global if the global has an assigned section. In this
case, the extra alignment could be observable: for example, code could
assume that the globals are densely packed in their section and try to
iterate over them as an array, alignment padding would break this
iteration. The maximum alignment is ``1 << 29``.

Globals can also have a :ref:`DLL storage class <dllstorageclass>`.

Variables and aliases can have a
:ref:`Thread Local Storage Model <tls_model>`.

Syntax::

    [@<GlobalVarName> =] [Linkage] [Visibility] [DLLStorageClass] [ThreadLocal]
                         [unnamed_addr] [AddrSpace] [ExternallyInitialized]
                         <global | constant> <Type> [<InitializerConstant>]
                         [, section "name"] [, comdat [($name)]]
                         [, align <Alignment>]

For example, the following defines a global in a numbered address space
with an initializer, section, and alignment:

.. code-block:: llvm

    @G = addrspace(5) constant float 1.0, section "foo", align 4

The following example just declares a global variable

.. code-block:: llvm

   @G = external global i32

The following example defines a thread-local global with the
``initialexec`` TLS model:

.. code-block:: llvm

    @G = thread_local(initialexec) global i32 0, align 4

.. _functionstructure:

Functions
---------

LLVM function definitions consist of the "``define``" keyword, an
optional :ref:`linkage type <linkage>`, an optional :ref:`visibility
style <visibility>`, an optional :ref:`DLL storage class <dllstorageclass>`,
an optional :ref:`calling convention <callingconv>`,
an optional ``unnamed_addr`` attribute, a return type, an optional
:ref:`parameter attribute <paramattrs>` for the return type, a function
name, a (possibly empty) argument list (each with optional :ref:`parameter
attributes <paramattrs>`), optional :ref:`function attributes <fnattrs>`,
an optional section, an optional alignment,
an optional :ref:`comdat <langref_comdats>`,
an optional :ref:`garbage collector name <gc>`, an optional :ref:`prefix <prefixdata>`,
an optional :ref:`prologue <prologuedata>`,
an optional :ref:`personality <personalityfn>`,
an optional list of attached :ref:`metadata <metadata>`,
an opening curly brace, a list of basic blocks, and a closing curly brace.

LLVM function declarations consist of the "``declare``" keyword, an
optional :ref:`linkage type <linkage>`, an optional :ref:`visibility
style <visibility>`, an optional :ref:`DLL storage class <dllstorageclass>`,
an optional :ref:`calling convention <callingconv>`,
an optional ``unnamed_addr`` attribute, a return type, an optional
:ref:`parameter attribute <paramattrs>` for the return type, a function
name, a possibly empty list of arguments, an optional alignment, an optional
:ref:`garbage collector name <gc>`, an optional :ref:`prefix <prefixdata>`,
and an optional :ref:`prologue <prologuedata>`.

A function definition contains a list of basic blocks, forming the CFG (Control
Flow Graph) for the function. Each basic block may optionally start with a label
(giving the basic block a symbol table entry), contains a list of instructions,
and ends with a :ref:`terminator <terminators>` instruction (such as a branch or
function return). If an explicit label is not provided, a block is assigned an
implicit numbered label, using the next value from the same counter as used for
unnamed temporaries (:ref:`see above<identifiers>`). For example, if a function
entry block does not have an explicit label, it will be assigned label "%0",
then the first unnamed temporary in that block will be "%1", etc.

The first basic block in a function is special in two ways: it is
immediately executed on entrance to the function, and it is not allowed
to have predecessor basic blocks (i.e. there can not be any branches to
the entry block of a function). Because the block can have no
predecessors, it also cannot have any :ref:`PHI nodes <i_phi>`.

LLVM allows an explicit section to be specified for functions. If the
target supports it, it will emit functions to the section specified.
Additionally, the function can be placed in a COMDAT.

An explicit alignment may be specified for a function. If not present,
or if the alignment is set to zero, the alignment of the function is set
by the target to whatever it feels convenient. If an explicit alignment
is specified, the function is forced to have at least that much
alignment. All alignments must be a power of 2.

If the ``unnamed_addr`` attribute is given, the address is known to not
be significant and two identical functions can be merged.

Syntax::

    define [linkage] [visibility] [DLLStorageClass]
           [cconv] [ret attrs]
           <ResultType> @<FunctionName> ([argument list])
           [unnamed_addr] [fn Attrs] [section "name"] [comdat [($name)]]
           [align N] [gc] [prefix Constant] [prologue Constant]
           [personality Constant] (!name !N)* { ... }

The argument list is a comma separated sequence of arguments where each
argument is of the following form:

Syntax::

   <type> [parameter Attrs] [name]


.. _langref_aliases:

Aliases
-------

Aliases, unlike function or variables, don't create any new data. They
are just a new symbol and metadata for an existing position.

Aliases have a name and an aliasee that is either a global value or a
constant expression.

Aliases may have an optional :ref:`linkage type <linkage>`, an optional
:ref:`visibility style <visibility>`, an optional :ref:`DLL storage class
<dllstorageclass>` and an optional :ref:`tls model <tls_model>`.

Syntax::

    @<Name> = [Linkage] [Visibility] [DLLStorageClass] [ThreadLocal] [unnamed_addr] alias <AliaseeTy>, <AliaseeTy>* @<Aliasee>

The linkage must be one of ``private``, ``internal``, ``linkonce``, ``weak``,
``linkonce_odr``, ``weak_odr``, ``external``. Note that some system linkers
might not correctly handle dropping a weak symbol that is aliased.

Aliases that are not ``unnamed_addr`` are guaranteed to have the same address as
the aliasee expression. ``unnamed_addr`` ones are only guaranteed to point
to the same content.

Since aliases are only a second name, some restrictions apply, of which
some can only be checked when producing an object file:

* The expression defining the aliasee must be computable at assembly
  time. Since it is just a name, no relocations can be used.

* No alias in the expression can be weak as the possibility of the
  intermediate alias being overridden cannot be represented in an
  object file.

* No global value in the expression can be a declaration, since that
  would require a relocation, which is not possible.

.. _langref_comdats:

Comdats
-------

Comdat IR provides access to COFF and ELF object file COMDAT functionality.

Comdats have a name which represents the COMDAT key. All global objects that
specify this key will only end up in the final object file if the linker chooses
that key over some other key. Aliases are placed in the same COMDAT that their
aliasee computes to, if any.

Comdats have a selection kind to provide input on how the linker should
choose between keys in two different object files.

Syntax::

    $<Name> = comdat SelectionKind

The selection kind must be one of the following:

``any``
    The linker may choose any COMDAT key, the choice is arbitrary.
``exactmatch``
    The linker may choose any COMDAT key but the sections must contain the
    same data.
``largest``
    The linker will choose the section containing the largest COMDAT key.
``noduplicates``
    The linker requires that only section with this COMDAT key exist.
``samesize``
    The linker may choose any COMDAT key but the sections must contain the
    same amount of data.

Note that the Mach-O platform doesn't support COMDATs and ELF only supports
``any`` as a selection kind.

Here is an example of a COMDAT group where a function will only be selected if
the COMDAT key's section is the largest:

.. code-block:: llvm

   $foo = comdat largest
   @foo = global i32 2, comdat($foo)

   define void @bar() comdat($foo) {
     ret void
   }

As a syntactic sugar the ``$name`` can be omitted if the name is the same as
the global name:

.. code-block:: llvm

  $foo = comdat any
  @foo = global i32 2, comdat


In a COFF object file, this will create a COMDAT section with selection kind
``IMAGE_COMDAT_SELECT_LARGEST`` containing the contents of the ``@foo`` symbol
and another COMDAT section with selection kind
``IMAGE_COMDAT_SELECT_ASSOCIATIVE`` which is associated with the first COMDAT
section and contains the contents of the ``@bar`` symbol.

There are some restrictions on the properties of the global object.
It, or an alias to it, must have the same name as the COMDAT group when
targeting COFF.
The contents and size of this object may be used during link-time to determine
which COMDAT groups get selected depending on the selection kind.
Because the name of the object must match the name of the COMDAT group, the
linkage of the global object must not be local; local symbols can get renamed
if a collision occurs in the symbol table.

The combined use of COMDATS and section attributes may yield surprising results.
For example:

.. code-block:: llvm

   $foo = comdat any
   $bar = comdat any
   @g1 = global i32 42, section "sec", comdat($foo)
   @g2 = global i32 42, section "sec", comdat($bar)

From the object file perspective, this requires the creation of two sections
with the same name. This is necessary because both globals belong to different
COMDAT groups and COMDATs, at the object file level, are represented by
sections.

Note that certain IR constructs like global variables and functions may
create COMDATs in the object file in addition to any which are specified using
COMDAT IR. This arises when the code generator is configured to emit globals
in individual sections (e.g. when `-data-sections` or `-function-sections`
is supplied to `llc`).

.. _namedmetadatastructure:

Named Metadata
--------------

Named metadata is a collection of metadata. :ref:`Metadata
nodes <metadata>` (but not metadata strings) are the only valid
operands for a named metadata.

#. Named metadata are represented as a string of characters with the
   metadata prefix. The rules for metadata names are the same as for
   identifiers, but quoted names are not allowed. ``"\xx"`` type escapes
   are still valid, which allows any character to be part of a name.

Syntax::

    ; Some unnamed metadata nodes, which are referenced by the named metadata.
    !0 = !{!"zero"}
    !1 = !{!"one"}
    !2 = !{!"two"}
    ; A named metadata.
    !name = !{!0, !1, !2}

.. _paramattrs:

Parameter Attributes
--------------------

The return type and each parameter of a function type may have a set of
*parameter attributes* associated with them. Parameter attributes are
used to communicate additional information about the result or
parameters of a function. Parameter attributes are considered to be part
of the function, not of the function type, so functions with different
parameter attributes can have the same function type.

Parameter attributes are simple keywords that follow the type specified.
If multiple parameter attributes are needed, they are space separated.
For example:

.. code-block:: llvm

    declare i32 @printf(i8* noalias nocapture, ...)
    declare i32 @atoi(i8 zeroext)
    declare signext i8 @returns_signed_char()

Note that any attributes for the function result (``nounwind``,
``readonly``) come immediately after the argument list.

Currently, only the following parameter attributes are defined:

``zeroext``
    This indicates to the code generator that the parameter or return
    value should be zero-extended to the extent required by the target's
    ABI (which is usually 32-bits, but is 8-bits for a i1 on x86-64) by
    the caller (for a parameter) or the callee (for a return value).
``signext``
    This indicates to the code generator that the parameter or return
    value should be sign-extended to the extent required by the target's
    ABI (which is usually 32-bits) by the caller (for a parameter) or
    the callee (for a return value).
``inreg``
    This indicates that this parameter or return value should be treated
    in a special target-dependent fashion while emitting code for
    a function call or return (usually, by putting it in a register as
    opposed to memory, though some targets use it to distinguish between
    two different kinds of registers). Use of this attribute is
    target-specific.
``byval``
    This indicates that the pointer parameter should really be passed by
    value to the function. The attribute implies that a hidden copy of
    the pointee is made between the caller and the callee, so the callee
    is unable to modify the value in the caller. This attribute is only
    valid on LLVM pointer arguments. It is generally used to pass
    structs and arrays by value, but is also valid on pointers to
    scalars. The copy is considered to belong to the caller not the
    callee (for example, ``readonly`` functions should not write to
    ``byval`` parameters). This is not a valid attribute for return
    values.

    The byval attribute also supports specifying an alignment with the
    align attribute. It indicates the alignment of the stack slot to
    form and the known alignment of the pointer specified to the call
    site. If the alignment is not specified, then the code generator
    makes a target-specific assumption.

.. _attr_inalloca:

``inalloca``

    The ``inalloca`` argument attribute allows the caller to take the
    address of outgoing stack arguments. An ``inalloca`` argument must
    be a pointer to stack memory produced by an ``alloca`` instruction.
    The alloca, or argument allocation, must also be tagged with the
    inalloca keyword. Only the last argument may have the ``inalloca``
    attribute, and that argument is guaranteed to be passed in memory.

    An argument allocation may be used by a call at most once because
    the call may deallocate it. The ``inalloca`` attribute cannot be
    used in conjunction with other attributes that affect argument
    storage, like ``inreg``, ``nest``, ``sret``, or ``byval``. The
    ``inalloca`` attribute also disables LLVM's implicit lowering of
    large aggregate return values, which means that frontend authors
    must lower them with ``sret`` pointers.

    When the call site is reached, the argument allocation must have
    been the most recent stack allocation that is still live, or the
    results are undefined. It is possible to allocate additional stack
    space after an argument allocation and before its call site, but it
    must be cleared off with :ref:`llvm.stackrestore
    <int_stackrestore>`.

    See :doc:`InAlloca` for more information on how to use this
    attribute.

``sret``
    This indicates that the pointer parameter specifies the address of a
    structure that is the return value of the function in the source
    program. This pointer must be guaranteed by the caller to be valid:
    loads and stores to the structure may be assumed by the callee
    not to trap and to be properly aligned. This may only be applied to
    the first parameter. This is not a valid attribute for return
    values.

``align <n>``
    This indicates that the pointer value may be assumed by the optimizer to
    have the specified alignment.

    Note that this attribute has additional semantics when combined with the
    ``byval`` attribute.

.. _noalias:

``noalias``
    This indicates that objects accessed via pointer values
    :ref:`based <pointeraliasing>` on the argument or return value are not also
    accessed, during the execution of the function, via pointer values not
    *based* on the argument or return value. The attribute on a return value
    also has additional semantics described below. The caller shares the
    responsibility with the callee for ensuring that these requirements are met.
    For further details, please see the discussion of the NoAlias response in
    :ref:`alias analysis <Must, May, or No>`.

    Note that this definition of ``noalias`` is intentionally similar
    to the definition of ``restrict`` in C99 for function arguments.

    For function return values, C99's ``restrict`` is not meaningful,
    while LLVM's ``noalias`` is. Furthermore, the semantics of the ``noalias``
    attribute on return values are stronger than the semantics of the attribute
    when used on function arguments. On function return values, the ``noalias``
    attribute indicates that the function acts like a system memory allocation
    function, returning a pointer to allocated storage disjoint from the
    storage for any other object accessible to the caller.

``nocapture``
    This indicates that the callee does not make any copies of the
    pointer that outlive the callee itself. This is not a valid
    attribute for return values.

.. _nest:

``nest``
    This indicates that the pointer parameter can be excised using the
    :ref:`trampoline intrinsics <int_trampoline>`. This is not a valid
    attribute for return values and can only be applied to one parameter.

``returned``
    This indicates that the function always returns the argument as its return
    value. This is an optimization hint to the code generator when generating
    the caller, allowing tail call optimization and omission of register saves
    and restores in some cases; it is not checked or enforced when generating
    the callee. The parameter and the function return type must be valid
    operands for the :ref:`bitcast instruction <i_bitcast>`. This is not a
    valid attribute for return values and can only be applied to one parameter.

``nonnull``
    This indicates that the parameter or return pointer is not null. This
    attribute may only be applied to pointer typed parameters. This is not
    checked or enforced by LLVM, the caller must ensure that the pointer
    passed in is non-null, or the callee must ensure that the returned pointer
    is non-null.

``dereferenceable(<n>)``
    This indicates that the parameter or return pointer is dereferenceable. This
    attribute may only be applied to pointer typed parameters. A pointer that
    is dereferenceable can be loaded from speculatively without a risk of
    trapping. The number of bytes known to be dereferenceable must be provided
    in parentheses. It is legal for the number of bytes to be less than the
    size of the pointee type. The ``nonnull`` attribute does not imply
    dereferenceability (consider a pointer to one element past the end of an
    array), however ``dereferenceable(<n>)`` does imply ``nonnull`` in
    ``addrspace(0)`` (which is the default address space).

``dereferenceable_or_null(<n>)``
    This indicates that the parameter or return value isn't both
    non-null and non-dereferenceable (up to ``<n>`` bytes) at the same
    time. All non-null pointers tagged with
    ``dereferenceable_or_null(<n>)`` are ``dereferenceable(<n>)``.
    For address space 0 ``dereferenceable_or_null(<n>)`` implies that
    a pointer is exactly one of ``dereferenceable(<n>)`` or ``null``,
    and in other address spaces ``dereferenceable_or_null(<n>)``
    implies that a pointer is at least one of ``dereferenceable(<n>)``
    or ``null`` (i.e. it may be both ``null`` and
    ``dereferenceable(<n>)``). This attribute may only be applied to
    pointer typed parameters.

.. _gc:

Garbage Collector Strategy Names
--------------------------------

Each function may specify a garbage collector strategy name, which is simply a
string:

.. code-block:: llvm

    define void @f() gc "name" { ... }

The supported values of *name* includes those :ref:`built in to LLVM
<builtin-gc-strategies>` and any provided by loaded plugins. Specifying a GC
strategy will cause the compiler to alter its output in order to support the
named garbage collection algorithm. Note that LLVM itself does not contain a
garbage collector, this functionality is restricted to generating machine code
which can interoperate with a collector provided externally.

.. _prefixdata:

Prefix Data
-----------

Prefix data is data associated with a function which the code
generator will emit immediately before the function's entrypoint.
The purpose of this feature is to allow frontends to associate
language-specific runtime metadata with specific functions and make it
available through the function pointer while still allowing the
function pointer to be called.

To access the data for a given function, a program may bitcast the
function pointer to a pointer to the constant's type and dereference
index -1. This implies that the IR symbol points just past the end of
the prefix data. For instance, take the example of a function annotated
with a single ``i32``,

.. code-block:: llvm

    define void @f() prefix i32 123 { ... }

The prefix data can be referenced as,

.. code-block:: llvm

    %0 = bitcast void* () @f to i32*
    %a = getelementptr inbounds i32, i32* %0, i32 -1
    %b = load i32, i32* %a

Prefix data is laid out as if it were an initializer for a global variable
of the prefix data's type. The function will be placed such that the
beginning of the prefix data is aligned. This means that if the size
of the prefix data is not a multiple of the alignment size, the
function's entrypoint will not be aligned. If alignment of the
function's entrypoint is desired, padding must be added to the prefix
data.

A function may have prefix data but no body. This has similar semantics
to the ``available_externally`` linkage in that the data may be used by the
optimizers but will not be emitted in the object file.

.. _prologuedata:

Prologue Data
-------------

The ``prologue`` attribute allows arbitrary code (encoded as bytes) to
be inserted prior to the function body. This can be used for enabling
function hot-patching and instrumentation.

To maintain the semantics of ordinary function calls, the prologue data must
have a particular format. Specifically, it must begin with a sequence of
bytes which decode to a sequence of machine instructions, valid for the
module's target, which transfer control to the point immediately succeeding
the prologue data, without performing any other visible action. This allows
the inliner and other passes to reason about the semantics of the function
definition without needing to reason about the prologue data. Obviously this
makes the format of the prologue data highly target dependent.

A trivial example of valid prologue data for the x86 architecture is ``i8 144``,
which encodes the ``nop`` instruction:

.. code-block:: llvm

    define void @f() prologue i8 144 { ... }

Generally prologue data can be formed by encoding a relative branch instruction
which skips the metadata, as in this example of valid prologue data for the
x86_64 architecture, where the first two bytes encode ``jmp .+10``:

.. code-block:: llvm

    %0 = type <{ i8, i8, i8* }>

    define void @f() prologue %0 <{ i8 235, i8 8, i8* @md}> { ... }

A function may have prologue data but no body. This has similar semantics
to the ``available_externally`` linkage in that the data may be used by the
optimizers but will not be emitted in the object file.

.. _personalityfn:

Personality Function
--------------------

The ``personality`` attribute permits functions to specify what function
to use for exception handling.

.. _attrgrp:

Attribute Groups
----------------

Attribute groups are groups of attributes that are referenced by objects within
the IR. They are important for keeping ``.ll`` files readable, because a lot of
functions will use the same set of attributes. In the degenerative case of a
``.ll`` file that corresponds to a single ``.c`` file, the single attribute
group will capture the important command line flags used to build that file.

An attribute group is a module-level object. To use an attribute group, an
object references the attribute group's ID (e.g. ``#37``). An object may refer
to more than one attribute group. In that situation, the attributes from the
different groups are merged.

Here is an example of attribute groups for a function that should always be
inlined, has a stack alignment of 4, and which shouldn't use SSE instructions:

.. code-block:: llvm

   ; Target-independent attributes:
   attributes #0 = { alwaysinline alignstack=4 }

   ; Target-dependent attributes:
   attributes #1 = { "no-sse" }

   ; Function @f has attributes: alwaysinline, alignstack=4, and "no-sse".
   define void @f() #0 #1 { ... }

.. _fnattrs:

Function Attributes
-------------------

Function attributes are set to communicate additional information about
a function. Function attributes are considered to be part of the
function, not of the function type, so functions with different function
attributes can have the same function type.

Function attributes are simple keywords that follow the type specified.
If multiple attributes are needed, they are space separated. For
example:

.. code-block:: llvm

    define void @f() noinline { ... }
    define void @f() alwaysinline { ... }
    define void @f() alwaysinline optsize { ... }
    define void @f() optsize { ... }

``alignstack(<n>)``
    This attribute indicates that, when emitting the prologue and
    epilogue, the backend should forcibly align the stack pointer.
    Specify the desired alignment, which must be a power of two, in
    parentheses.
``alwaysinline``
    This attribute indicates that the inliner should attempt to inline
    this function into callers whenever possible, ignoring any active
    inlining size threshold for this caller.
``builtin``
    This indicates that the callee function at a call site should be
    recognized as a built-in function, even though the function's declaration
    uses the ``nobuiltin`` attribute. This is only valid at call sites for
    direct calls to functions that are declared with the ``nobuiltin``
    attribute.
``cold``
    This attribute indicates that this function is rarely called. When
    computing edge weights, basic blocks post-dominated by a cold
    function call are also considered to be cold; and, thus, given low
    weight.
``convergent``
    This attribute indicates that the callee is dependent on a convergent
    thread execution pattern under certain parallel execution models.
    Transformations that are execution model agnostic may not make the execution
    of a convergent operation control dependent on any additional values.
``inlinehint``
    This attribute indicates that the source code contained a hint that
    inlining this function is desirable (such as the "inline" keyword in
    C/C++). It is just a hint; it imposes no requirements on the
    inliner.
``jumptable``
    This attribute indicates that the function should be added to a
    jump-instruction table at code-generation time, and that all address-taken
    references to this function should be replaced with a reference to the
    appropriate jump-instruction-table function pointer. Note that this creates
    a new pointer for the original function, which means that code that depends
    on function-pointer identity can break. So, any function annotated with
    ``jumptable`` must also be ``unnamed_addr``.
``minsize``
    This attribute suggests that optimization passes and code generator
    passes make choices that keep the code size of this function as small
    as possible and perform optimizations that may sacrifice runtime
    performance in order to minimize the size of the generated code.
``naked``
    This attribute disables prologue / epilogue emission for the
    function. This can have very system-specific consequences.
``nobuiltin``
    This indicates that the callee function at a call site is not recognized as
    a built-in function. LLVM will retain the original call and not replace it
    with equivalent code based on the semantics of the built-in function, unless
    the call site uses the ``builtin`` attribute. This is valid at call sites
    and on function declarations and definitions.
``noduplicate``
    This attribute indicates that calls to the function cannot be
    duplicated. A call to a ``noduplicate`` function may be moved
    within its parent function, but may not be duplicated within
    its parent function.

    A function containing a ``noduplicate`` call may still
    be an inlining candidate, provided that the call is not
    duplicated by inlining. That implies that the function has
    internal linkage and only has one call site, so the original
    call is dead after inlining.
``noimplicitfloat``
    This attributes disables implicit floating point instructions.
``noinline``
    This attribute indicates that the inliner should never inline this
    function in any situation. This attribute may not be used together
    with the ``alwaysinline`` attribute.
``nonlazybind``
    This attribute suppresses lazy symbol binding for the function. This
    may make calls to the function faster, at the cost of extra program
    startup time if the function is not called during program startup.
``noredzone``
    This attribute indicates that the code generator should not use a
    red zone, even if the target-specific ABI normally permits it.
``noreturn``
    This function attribute indicates that the function never returns
    normally. This produces undefined behavior at runtime if the
    function ever does dynamically return.
``norecurse``
    This function attribute indicates that the function does not call itself
    either directly or indirectly down any possible call path. This produces
    undefined behavior at runtime if the function ever does recurse.
``nounwind``
    This function attribute indicates that the function never raises an
    exception. If the function does raise an exception, its runtime
    behavior is undefined. However, functions marked nounwind may still
    trap or generate asynchronous exceptions. Exception handling schemes
    that are recognized by LLVM to handle asynchronous exceptions, such
    as SEH, will still provide their implementation defined semantics.
``optnone``
    This function attribute indicates that most optimization passes will skip
    this function, with the exception of interprocedural optimization passes.
    Code generation defaults to the "fast" instruction selector.
    This attribute cannot be used together with the ``alwaysinline``
    attribute; this attribute is also incompatible
    with the ``minsize`` attribute and the ``optsize`` attribute.

    This attribute requires the ``noinline`` attribute to be specified on
    the function as well, so the function is never inlined into any caller.
    Only functions with the ``alwaysinline`` attribute are valid
    candidates for inlining into the body of this function.
``optsize``
    This attribute suggests that optimization passes and code generator
    passes make choices that keep the code size of this function low,
    and otherwise do optimizations specifically to reduce code size as
    long as they do not significantly impact runtime performance.
``readnone``
    On a function, this attribute indicates that the function computes its
    result (or decides to unwind an exception) based strictly on its arguments,
    without dereferencing any pointer arguments or otherwise accessing
    any mutable state (e.g. memory, control registers, etc) visible to
    caller functions. It does not write through any pointer arguments
    (including ``byval`` arguments) and never changes any state visible
    to callers. This means that it cannot unwind exceptions by calling
    the ``C++`` exception throwing methods.

    On an argument, this attribute indicates that the function does not
    dereference that pointer argument, even though it may read or write the
    memory that the pointer points to if accessed through other pointers.
``readonly``
    On a function, this attribute indicates that the function does not write
    through any pointer arguments (including ``byval`` arguments) or otherwise
    modify any state (e.g. memory, control registers, etc) visible to
    caller functions. It may dereference pointer arguments and read
    state that may be set in the caller. A readonly function always
    returns the same value (or unwinds an exception identically) when
    called with the same set of arguments and global state. It cannot
    unwind an exception by calling the ``C++`` exception throwing
    methods.

    On an argument, this attribute indicates that the function does not write
    through this pointer argument, even though it may write to the memory that
    the pointer points to.
``argmemonly``
    This attribute indicates that the only memory accesses inside function are
    loads and stores from objects pointed to by its pointer-typed arguments,
    with arbitrary offsets. Or in other words, all memory operations in the
    function can refer to memory only using pointers based on its function
    arguments.
    Note that ``argmemonly`` can be used together with ``readonly`` attribute
    in order to specify that function reads only from its arguments.
``returns_twice``
    This attribute indicates that this function can return twice. The C
    ``setjmp`` is an example of such a function. The compiler disables
    some optimizations (like tail calls) in the caller of these
    functions.
``safestack``
    This attribute indicates that
    `SafeStack <http://clang.llvm.org/docs/SafeStack.html>`_
    protection is enabled for this function.

    If a function that has a ``safestack`` attribute is inlined into a
    function that doesn't have a ``safestack`` attribute or which has an
    ``ssp``, ``sspstrong`` or ``sspreq`` attribute, then the resulting
    function will have a ``safestack`` attribute.
``sanitize_address``
    This attribute indicates that AddressSanitizer checks
    (dynamic address safety analysis) are enabled for this function.
``sanitize_memory``
    This attribute indicates that MemorySanitizer checks (dynamic detection
    of accesses to uninitialized memory) are enabled for this function.
``sanitize_thread``
    This attribute indicates that ThreadSanitizer checks
    (dynamic thread safety analysis) are enabled for this function.
``ssp``
    This attribute indicates that the function should emit a stack
    smashing protector. It is in the form of a "canary" --- a random value
    placed on the stack before the local variables that's checked upon
    return from the function to see if it has been overwritten. A
    heuristic is used to determine if a function needs stack protectors
    or not. The heuristic used will enable protectors for functions with:

    - Character arrays larger than ``ssp-buffer-size`` (default 8).
    - Aggregates containing character arrays larger than ``ssp-buffer-size``.
    - Calls to alloca() with variable sizes or constant sizes greater than
      ``ssp-buffer-size``.

    Variables that are identified as requiring a protector will be arranged
    on the stack such that they are adjacent to the stack protector guard.

    If a function that has an ``ssp`` attribute is inlined into a
    function that doesn't have an ``ssp`` attribute, then the resulting
    function will have an ``ssp`` attribute.
``sspreq``
    This attribute indicates that the function should *always* emit a
    stack smashing protector. This overrides the ``ssp`` function
    attribute.

    Variables that are identified as requiring a protector will be arranged
    on the stack such that they are adjacent to the stack protector guard.
    The specific layout rules are:

    #. Large arrays and structures containing large arrays
       (``>= ssp-buffer-size``) are closest to the stack protector.
    #. Small arrays and structures containing small arrays
       (``< ssp-buffer-size``) are 2nd closest to the protector.
    #. Variables that have had their address taken are 3rd closest to the
       protector.

    If a function that has an ``sspreq`` attribute is inlined into a
    function that doesn't have an ``sspreq`` attribute or which has an
    ``ssp`` or ``sspstrong`` attribute, then the resulting function will have
    an ``sspreq`` attribute.
``sspstrong``
    This attribute indicates that the function should emit a stack smashing
    protector. This attribute causes a strong heuristic to be used when
    determining if a function needs stack protectors. The strong heuristic
    will enable protectors for functions with:

    - Arrays of any size and type
    - Aggregates containing an array of any size and type.
    - Calls to alloca().
    - Local variables that have had their address taken.

    Variables that are identified as requiring a protector will be arranged
    on the stack such that they are adjacent to the stack protector guard.
    The specific layout rules are:

    #. Large arrays and structures containing large arrays
       (``>= ssp-buffer-size``) are closest to the stack protector.
    #. Small arrays and structures containing small arrays
       (``< ssp-buffer-size``) are 2nd closest to the protector.
    #. Variables that have had their address taken are 3rd closest to the
       protector.

    This overrides the ``ssp`` function attribute.

    If a function that has an ``sspstrong`` attribute is inlined into a
    function that doesn't have an ``sspstrong`` attribute, then the
    resulting function will have an ``sspstrong`` attribute.
``"thunk"``
    This attribute indicates that the function will delegate to some other
    function with a tail call. The prototype of a thunk should not be used for
    optimization purposes. The caller is expected to cast the thunk prototype to
    match the thunk target prototype.
``uwtable``
    This attribute indicates that the ABI being targeted requires that
    an unwind table entry be produced for this function even if we can
    show that no exceptions passes by it. This is normally the case for
    the ELF x86-64 abi, but it can be disabled for some compilation
    units.


.. _opbundles:

Operand Bundles
---------------

Note: operand bundles are a work in progress, and they should be
considered experimental at this time.

Operand bundles are tagged sets of SSA values that can be associated
with certain LLVM instructions (currently only ``call`` s and
``invoke`` s).  In a way they are like metadata, but dropping them is
incorrect and will change program semantics.

Syntax::

    operand bundle set ::= '[' operand bundle (, operand bundle )* ']'
    operand bundle ::= tag '(' [ bundle operand ] (, bundle operand )* ')'
    bundle operand ::= SSA value
    tag ::= string constant

Operand bundles are **not** part of a function's signature, and a
given function may be called from multiple places with different kinds
of operand bundles.  This reflects the fact that the operand bundles
are conceptually a part of the ``call`` (or ``invoke``), not the
callee being dispatched to.

Operand bundles are a generic mechanism intended to support
runtime-introspection-like functionality for managed languages.  While
the exact semantics of an operand bundle depend on the bundle tag,
there are certain limitations to how much the presence of an operand
bundle can influence the semantics of a program.  These restrictions
are described as the semantics of an "unknown" operand bundle.  As
long as the behavior of an operand bundle is describable within these
restrictions, LLVM does not need to have special knowledge of the
operand bundle to not miscompile programs containing it.

- The bundle operands for an unknown operand bundle escape in unknown
  ways before control is transferred to the callee or invokee.
- Calls and invokes with operand bundles have unknown read / write
  effect on the heap on entry and exit (even if the call target is
  ``readnone`` or ``readonly``), unless they're overriden with
  callsite specific attributes.
- An operand bundle at a call site cannot change the implementation
  of the called function.  Inter-procedural optimizations work as
  usual as long as they take into account the first two properties.

More specific types of operand bundles are described below.

Deoptimization Operand Bundles
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Deoptimization operand bundles are characterized by the ``"deopt"``
operand bundle tag.  These operand bundles represent an alternate
"safe" continuation for the call site they're attached to, and can be
used by a suitable runtime to deoptimize the compiled frame at the
specified call site.  There can be at most one ``"deopt"`` operand
bundle attached to a call site.  Exact details of deoptimization is
out of scope for the language reference, but it usually involves
rewriting a compiled frame into a set of interpreted frames.

From the compiler's perspective, deoptimization operand bundles make
the call sites they're attached to at least ``readonly``.  They read
through all of their pointer typed operands (even if they're not
otherwise escaped) and the entire visible heap.  Deoptimization
operand bundles do not capture their operands except during
deoptimization, in which case control will not be returned to the
compiled frame.

The inliner knows how to inline through calls that have deoptimization
operand bundles.  Just like inlining through a normal call site
involves composing the normal and exceptional continuations, inlining
through a call site with a deoptimization operand bundle needs to
appropriately compose the "safe" deoptimization continuation.  The
inliner does this by prepending the parent's deoptimization
continuation to every deoptimization continuation in the inlined body.
E.g. inlining ``@f`` into ``@g`` in the following example

.. code-block:: llvm

    define void @f() {
      call void @x()  ;; no deopt state
      call void @y() [ "deopt"(i32 10) ]
      call void @y() [ "deopt"(i32 10), "unknown"(i8* null) ]
      ret void
    }

    define void @g() {
      call void @f() [ "deopt"(i32 20) ]
      ret void
    }

will result in

.. code-block:: llvm

    define void @g() {
      call void @x()  ;; still no deopt state
      call void @y() [ "deopt"(i32 20, i32 10) ]
      call void @y() [ "deopt"(i32 20, i32 10), "unknown"(i8* null) ]
      ret void
    }

It is the frontend's responsibility to structure or encode the
deoptimization state in a way that syntactically prepending the
caller's deoptimization state to the callee's deoptimization state is
semantically equivalent to composing the caller's deoptimization
continuation after the callee's deoptimization continuation.

.. _moduleasm:

Module-Level Inline Assembly
----------------------------

Modules may contain "module-level inline asm" blocks, which corresponds
to the GCC "file scope inline asm" blocks. These blocks are internally
concatenated by LLVM and treated as a single unit, but may be separated
in the ``.ll`` file if desired. The syntax is very simple:

.. code-block:: llvm

    module asm "inline asm code goes here"
    module asm "more can go here"

The strings can contain any character by escaping non-printable
characters. The escape sequence used is simply "\\xx" where "xx" is the
two digit hex code for the number.

Note that the assembly string *must* be parseable by LLVM's integrated assembler
(unless it is disabled), even when emitting a ``.s`` file.

.. _langref_datalayout:

Data Layout
-----------

A module may specify a target specific data layout string that specifies
how data is to be laid out in memory. The syntax for the data layout is
simply:

.. code-block:: llvm

    target datalayout = "layout specification"

The *layout specification* consists of a list of specifications
separated by the minus sign character ('-'). Each specification starts
with a letter and may include other information after the letter to
define some aspect of the data layout. The specifications accepted are
as follows:

``E``
    Specifies that the target lays out data in big-endian form. That is,
    the bits with the most significance have the lowest address
    location.
``e``
    Specifies that the target lays out data in little-endian form. That
    is, the bits with the least significance have the lowest address
    location.
``S<size>``
    Specifies the natural alignment of the stack in bits. Alignment
    promotion of stack variables is limited to the natural stack
    alignment to avoid dynamic stack realignment. The stack alignment
    must be a multiple of 8-bits. If omitted, the natural stack
    alignment defaults to "unspecified", which does not prevent any
    alignment promotions.
``p[n]:<size>:<abi>:<pref>``
    This specifies the *size* of a pointer and its ``<abi>`` and
    ``<pref>``\erred alignments for address space ``n``. All sizes are in
    bits. The address space, ``n``, is optional, and if not specified,
    denotes the default address space 0. The value of ``n`` must be
    in the range [1,2^23).
``i<size>:<abi>:<pref>``
    This specifies the alignment for an integer type of a given bit
    ``<size>``. The value of ``<size>`` must be in the range [1,2^23).
``v<size>:<abi>:<pref>``
    This specifies the alignment for a vector type of a given bit
    ``<size>``.
``f<size>:<abi>:<pref>``
    This specifies the alignment for a floating point type of a given bit
    ``<size>``. Only values of ``<size>`` that are supported by the target
    will work. 32 (float) and 64 (double) are supported on all targets; 80
    or 128 (different flavors of long double) are also supported on some
    targets.
``a:<abi>:<pref>``
    This specifies the alignment for an object of aggregate type.
``m:<mangling>``
    If present, specifies that llvm names are mangled in the output. The
    options are

    * ``e``: ELF mangling: Private symbols get a ``.L`` prefix.
    * ``m``: Mips mangling: Private symbols get a ``$`` prefix.
    * ``o``: Mach-O mangling: Private symbols get ``L`` prefix. Other
      symbols get a ``_`` prefix.
    * ``w``: Windows COFF prefix:  Similar to Mach-O, but stdcall and fastcall
      functions also get a suffix based on the frame size.
    * ``x``: Windows x86 COFF prefix:  Similar to Windows COFF, but use a ``_``
      prefix for ``__cdecl`` functions.
``n<size1>:<size2>:<size3>...``
    This specifies a set of native integer widths for the target CPU in
    bits. For example, it might contain ``n32`` for 32-bit PowerPC,
    ``n32:64`` for PowerPC 64, or ``n8:16:32:64`` for X86-64. Elements of
    this set are considered to support most general arithmetic operations
    efficiently.

On every specification that takes a ``<abi>:<pref>``, specifying the
``<pref>`` alignment is optional. If omitted, the preceding ``:``
should be omitted too and ``<pref>`` will be equal to ``<abi>``.

When constructing the data layout for a given target, LLVM starts with a
default set of specifications which are then (possibly) overridden by
the specifications in the ``datalayout`` keyword. The default
specifications are given in this list:

-  ``E`` - big endian
-  ``p:64:64:64`` - 64-bit pointers with 64-bit alignment.
-  ``p[n]:64:64:64`` - Other address spaces are assumed to be the
   same as the default address space.
-  ``S0`` - natural stack alignment is unspecified
-  ``i1:8:8`` - i1 is 8-bit (byte) aligned
-  ``i8:8:8`` - i8 is 8-bit (byte) aligned
-  ``i16:16:16`` - i16 is 16-bit aligned
-  ``i32:32:32`` - i32 is 32-bit aligned
-  ``i64:32:64`` - i64 has ABI alignment of 32-bits but preferred
   alignment of 64-bits
-  ``f16:16:16`` - half is 16-bit aligned
-  ``f32:32:32`` - float is 32-bit aligned
-  ``f64:64:64`` - double is 64-bit aligned
-  ``f128:128:128`` - quad is 128-bit aligned
-  ``v64:64:64`` - 64-bit vector is 64-bit aligned
-  ``v128:128:128`` - 128-bit vector is 128-bit aligned
-  ``a:0:64`` - aggregates are 64-bit aligned

When LLVM is determining the alignment for a given type, it uses the
following rules:

#. If the type sought is an exact match for one of the specifications,
   that specification is used.
#. If no match is found, and the type sought is an integer type, then
   the smallest integer type that is larger than the bitwidth of the
   sought type is used. If none of the specifications are larger than
   the bitwidth then the largest integer type is used. For example,
   given the default specifications above, the i7 type will use the
   alignment of i8 (next largest) while both i65 and i256 will use the
   alignment of i64 (largest specified).
#. If no match is found, and the type sought is a vector type, then the
   largest vector type that is smaller than the sought vector type will
   be used as a fall back. This happens because <128 x double> can be
   implemented in terms of 64 <2 x double>, for example.

The function of the data layout string may not be what you expect.
Notably, this is not a specification from the frontend of what alignment
the code generator should use.

Instead, if specified, the target data layout is required to match what
the ultimate *code generator* expects. This string is used by the
mid-level optimizers to improve code, and this only works if it matches
what the ultimate code generator uses. There is no way to generate IR
that does not embed this target-specific detail into the IR. If you
don't specify the string, the default specifications will be used to
generate a Data Layout and the optimization phases will operate
accordingly and introduce target specificity into the IR with respect to
these default specifications.

.. _langref_triple:

Target Triple
-------------

A module may specify a target triple string that describes the target
host. The syntax for the target triple is simply:

.. code-block:: llvm

    target triple = "x86_64-apple-macosx10.7.0"

The *target triple* string consists of a series of identifiers delimited
by the minus sign character ('-'). The canonical forms are:

::

    ARCHITECTURE-VENDOR-OPERATING_SYSTEM
    ARCHITECTURE-VENDOR-OPERATING_SYSTEM-ENVIRONMENT

This information is passed along to the backend so that it generates
code for the proper architecture. It's possible to override this on the
command line with the ``-mtriple`` command line option.

.. _pointeraliasing:

Pointer Aliasing Rules
----------------------

Any memory access must be done through a pointer value associated with
an address range of the memory access, otherwise the behavior is
undefined. Pointer values are associated with address ranges according
to the following rules:

-  A pointer value is associated with the addresses associated with any
   value it is *based* on.
-  An address of a global variable is associated with the address range
   of the variable's storage.
-  The result value of an allocation instruction is associated with the
   address range of the allocated storage.
-  A null pointer in the default address-space is associated with no
   address.
-  An integer constant other than zero or a pointer value returned from
   a function not defined within LLVM may be associated with address
   ranges allocated through mechanisms other than those provided by
   LLVM. Such ranges shall not overlap with any ranges of addresses
   allocated by mechanisms provided by LLVM.

A pointer value is *based* on another pointer value according to the
following rules:

-  A pointer value formed from a ``getelementptr`` operation is *based*
   on the first value operand of the ``getelementptr``.
-  The result value of a ``bitcast`` is *based* on the operand of the
   ``bitcast``.
-  A pointer value formed by an ``inttoptr`` is *based* on all pointer
   values that contribute (directly or indirectly) to the computation of
   the pointer's value.
-  The "*based* on" relationship is transitive.

Note that this definition of *"based"* is intentionally similar to the
definition of *"based"* in C99, though it is slightly weaker.

LLVM IR does not associate types with memory. The result type of a
``load`` merely indicates the size and alignment of the memory from
which to load, as well as the interpretation of the value. The first
operand type of a ``store`` similarly only indicates the size and
alignment of the store.

Consequently, type-based alias analysis, aka TBAA, aka
``-fstrict-aliasing``, is not applicable to general unadorned LLVM IR.
:ref:`Metadata <metadata>` may be used to encode additional information
which specialized optimization passes may use to implement type-based
alias analysis.

.. _volatile:

Volatile Memory Accesses
------------------------

Certain memory accesses, such as :ref:`load <i_load>`'s,
:ref:`store <i_store>`'s, and :ref:`llvm.memcpy <int_memcpy>`'s may be
marked ``volatile``. The optimizers must not change the number of
volatile operations or change their order of execution relative to other
volatile operations. The optimizers *may* change the order of volatile
operations relative to non-volatile operations. This is not Java's
"volatile" and has no cross-thread synchronization behavior.

IR-level volatile loads and stores cannot safely be optimized into
llvm.memcpy or llvm.memmove intrinsics even when those intrinsics are
flagged volatile. Likewise, the backend should never split or merge
target-legal volatile load/store instructions.

.. admonition:: Rationale

 Platforms may rely on volatile loads and stores of natively supported
 data width to be executed as single instruction. For example, in C
 this holds for an l-value of volatile primitive type with native
 hardware support, but not necessarily for aggregate types. The
 frontend upholds these expectations, which are intentionally
 unspecified in the IR. The rules above ensure that IR transformations
 do not violate the frontend's contract with the language.

.. _memmodel:

Memory Model for Concurrent Operations
--------------------------------------

The LLVM IR does not define any way to start parallel threads of
execution or to register signal handlers. Nonetheless, there are
platform-specific ways to create them, and we define LLVM IR's behavior
in their presence. This model is inspired by the C++0x memory model.

For a more informal introduction to this model, see the :doc:`Atomics`.

We define a *happens-before* partial order as the least partial order
that

-  Is a superset of single-thread program order, and
-  When a *synchronizes-with* ``b``, includes an edge from ``a`` to
   ``b``. *Synchronizes-with* pairs are introduced by platform-specific
   techniques, like pthread locks, thread creation, thread joining,
   etc., and by atomic instructions. (See also :ref:`Atomic Memory Ordering
   Constraints <ordering>`).

Note that program order does not introduce *happens-before* edges
between a thread and signals executing inside that thread.

Every (defined) read operation (load instructions, memcpy, atomic
loads/read-modify-writes, etc.) R reads a series of bytes written by
(defined) write operations (store instructions, atomic
stores/read-modify-writes, memcpy, etc.). For the purposes of this
section, initialized globals are considered to have a write of the
initializer which is atomic and happens before any other read or write
of the memory in question. For each byte of a read R, R\ :sub:`byte`
may see any write to the same byte, except:

-  If write\ :sub:`1`  happens before write\ :sub:`2`, and
   write\ :sub:`2` happens before R\ :sub:`byte`, then
   R\ :sub:`byte` does not see write\ :sub:`1`.
-  If R\ :sub:`byte` happens before write\ :sub:`3`, then
   R\ :sub:`byte` does not see write\ :sub:`3`.

Given that definition, R\ :sub:`byte` is defined as follows:

-  If R is volatile, the result is target-dependent. (Volatile is
   supposed to give guarantees which can support ``sig_atomic_t`` in
   C/C++, and may be used for accesses to addresses that do not behave
   like normal memory. It does not generally provide cross-thread
   synchronization.)
-  Otherwise, if there is no write to the same byte that happens before
   R\ :sub:`byte`, R\ :sub:`byte` returns ``undef`` for that byte.
-  Otherwise, if R\ :sub:`byte` may see exactly one write,
   R\ :sub:`byte` returns the value written by that write.
-  Otherwise, if R is atomic, and all the writes R\ :sub:`byte` may
   see are atomic, it chooses one of the values written. See the :ref:`Atomic
   Memory Ordering Constraints <ordering>` section for additional
   constraints on how the choice is made.
-  Otherwise R\ :sub:`byte` returns ``undef``.

R returns the value composed of the series of bytes it read. This
implies that some bytes within the value may be ``undef`` **without**
the entire value being ``undef``. Note that this only defines the
semantics of the operation; it doesn't mean that targets will emit more
than one instruction to read the series of bytes.

Note that in cases where none of the atomic intrinsics are used, this
model places only one restriction on IR transformations on top of what
is required for single-threaded execution: introducing a store to a byte
which might not otherwise be stored is not allowed in general.
(Specifically, in the case where another thread might write to and read
from an address, introducing a store can change a load that may see
exactly one write into a load that may see multiple writes.)

.. _ordering:

Atomic Memory Ordering Constraints
----------------------------------

Atomic instructions (:ref:`cmpxchg <i_cmpxchg>`,
:ref:`atomicrmw <i_atomicrmw>`, :ref:`fence <i_fence>`,
:ref:`atomic load <i_load>`, and :ref:`atomic store <i_store>`) take
ordering parameters that determine which other atomic instructions on
the same address they *synchronize with*. These semantics are borrowed
from Java and C++0x, but are somewhat more colloquial. If these
descriptions aren't precise enough, check those specs (see spec
references in the :doc:`atomics guide <Atomics>`).
:ref:`fence <i_fence>` instructions treat these orderings somewhat
differently since they don't take an address. See that instruction's
documentation for details.

For a simpler introduction to the ordering constraints, see the
:doc:`Atomics`.

``unordered``
    The set of values that can be read is governed by the happens-before
    partial order. A value cannot be read unless some operation wrote
    it. This is intended to provide a guarantee strong enough to model
    Java's non-volatile shared variables. This ordering cannot be
    specified for read-modify-write operations; it is not strong enough
    to make them atomic in any interesting way.
``monotonic``
    In addition to the guarantees of ``unordered``, there is a single
    total order for modifications by ``monotonic`` operations on each
    address. All modification orders must be compatible with the
    happens-before order. There is no guarantee that the modification
    orders can be combined to a global total order for the whole program
    (and this often will not be possible). The read in an atomic
    read-modify-write operation (:ref:`cmpxchg <i_cmpxchg>` and
    :ref:`atomicrmw <i_atomicrmw>`) reads the value in the modification
    order immediately before the value it writes. If one atomic read
    happens before another atomic read of the same address, the later
    read must see the same value or a later value in the address's
    modification order. This disallows reordering of ``monotonic`` (or
    stronger) operations on the same address. If an address is written
    ``monotonic``-ally by one thread, and other threads ``monotonic``-ally
    read that address repeatedly, the other threads must eventually see
    the write. This corresponds to the C++0x/C1x
    ``memory_order_relaxed``.
``acquire``
    In addition to the guarantees of ``monotonic``, a
    *synchronizes-with* edge may be formed with a ``release`` operation.
    This is intended to model C++'s ``memory_order_acquire``.
``release``
    In addition to the guarantees of ``monotonic``, if this operation
    writes a value which is subsequently read by an ``acquire``
    operation, it *synchronizes-with* that operation. (This isn't a
    complete description; see the C++0x definition of a release
    sequence.) This corresponds to the C++0x/C1x
    ``memory_order_release``.
``acq_rel`` (acquire+release)
    Acts as both an ``acquire`` and ``release`` operation on its
    address. This corresponds to the C++0x/C1x ``memory_order_acq_rel``.
``seq_cst`` (sequentially consistent)
    In addition to the guarantees of ``acq_rel`` (``acquire`` for an
    operation that only reads, ``release`` for an operation that only
    writes), there is a global total order on all
    sequentially-consistent operations on all addresses, which is
    consistent with the *happens-before* partial order and with the
    modification orders of all the affected addresses. Each
    sequentially-consistent read sees the last preceding write to the
    same address in this global order. This corresponds to the C++0x/C1x
    ``memory_order_seq_cst`` and Java volatile.

.. _singlethread:

If an atomic operation is marked ``singlethread``, it only *synchronizes
with* or participates in modification and seq\_cst total orderings with
other operations running in the same thread (for example, in signal
handlers).

.. _fastmath:

Fast-Math Flags
---------------

LLVM IR floating-point binary ops (:ref:`fadd <i_fadd>`,
:ref:`fsub <i_fsub>`, :ref:`fmul <i_fmul>`, :ref:`fdiv <i_fdiv>`,
:ref:`frem <i_frem>`, :ref:`fcmp <i_fcmp>`) have the following flags that can
be set to enable otherwise unsafe floating point operations

``nnan``
   No NaNs - Allow optimizations to assume the arguments and result are not
   NaN. Such optimizations are required to retain defined behavior over
   NaNs, but the value of the result is undefined.

``ninf``
   No Infs - Allow optimizations to assume the arguments and result are not
   +/-Inf. Such optimizations are required to retain defined behavior over
   +/-Inf, but the value of the result is undefined.

``nsz``
   No Signed Zeros - Allow optimizations to treat the sign of a zero
   argument or result as insignificant.

``arcp``
   Allow Reciprocal - Allow optimizations to use the reciprocal of an
   argument rather than perform division.

``fast``
   Fast - Allow algebraically equivalent transformations that may
   dramatically change results in floating point (e.g. reassociate). This
   flag implies all the others.

.. _uselistorder:

Use-list Order Directives
-------------------------

Use-list directives encode the in-memory order of each use-list, allowing the
order to be recreated. ``<order-indexes>`` is a comma-separated list of
indexes that are assigned to the referenced value's uses. The referenced
value's use-list is immediately sorted by these indexes.

Use-list directives may appear at function scope or global scope. They are not
instructions, and have no effect on the semantics of the IR. When they're at
function scope, they must appear after the terminator of the final basic block.

If basic blocks have their address taken via ``blockaddress()`` expressions,
``uselistorder_bb`` can be used to reorder their use-lists from outside their
function's scope.

:Syntax:

::

    uselistorder <ty> <value>, { <order-indexes> }
    uselistorder_bb @function, %block { <order-indexes> }

:Examples:

::

    define void @foo(i32 %arg1, i32 %arg2) {
    entry:
      ; ... instructions ...
    bb:
      ; ... instructions ...

      ; At function scope.
      uselistorder i32 %arg1, { 1, 0, 2 }
      uselistorder label %bb, { 1, 0 }
    }

    ; At global scope.
    uselistorder i32* @global, { 1, 2, 0 }
    uselistorder i32 7, { 1, 0 }
    uselistorder i32 (i32) @bar, { 1, 0 }
    uselistorder_bb @foo, %bb, { 5, 1, 3, 2, 0, 4 }

.. _typesystem:

Type System
===========

The LLVM type system is one of the most important features of the
intermediate representation. Being typed enables a number of
optimizations to be performed on the intermediate representation
directly, without having to do extra analyses on the side before the
transformation. A strong type system makes it easier to read the
generated code and enables novel analyses and transformations that are
not feasible to perform on normal three address code representations.

.. _t_void:

Void Type
---------

:Overview:


The void type does not represent any value and has no size.

:Syntax:


::

      void


.. _t_function:

Function Type
-------------

:Overview:


The function type can be thought of as a function signature. It consists of a
return type and a list of formal parameter types. The return type of a function
type is a void type or first class type --- except for :ref:`label <t_label>`
and :ref:`metadata <t_metadata>` types.

:Syntax:

::

      <returntype> (<parameter list>)

...where '``<parameter list>``' is a comma-separated list of type
specifiers. Optionally, the parameter list may include a type ``...``, which
indicates that the function takes a variable number of arguments. Variable
argument functions can access their arguments with the :ref:`variable argument
handling intrinsic <int_varargs>` functions. '``<returntype>``' is any type
except :ref:`label <t_label>` and :ref:`metadata <t_metadata>`.

:Examples:

+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``i32 (i32)``                   | function taking an ``i32``, returning an ``i32``                                                                                                                    |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``float (i16, i32 *) *``        | :ref:`Pointer <t_pointer>` to a function that takes an ``i16`` and a :ref:`pointer <t_pointer>` to ``i32``, returning ``float``.                                    |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``i32 (i8*, ...)``              | A vararg function that takes at least one :ref:`pointer <t_pointer>` to ``i8`` (char in C), which returns an integer. This is the signature for ``printf`` in LLVM. |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``{i32, i32} (i32)``            | A function taking an ``i32``, returning a :ref:`structure <t_struct>` containing two ``i32`` values                                                                 |
+---------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------+

.. _t_firstclass:

First Class Types
-----------------

The :ref:`first class <t_firstclass>` types are perhaps the most important.
Values of these types are the only ones which can be produced by
instructions.

.. _t_single_value:

Single Value Types
^^^^^^^^^^^^^^^^^^

These are the types that are valid in registers from CodeGen's perspective.

.. _t_integer:

Integer Type
""""""""""""

:Overview:

The integer type is a very simple type that simply specifies an
arbitrary bit width for the integer type desired. Any bit width from 1
bit to 2\ :sup:`23`\ -1 (about 8 million) can be specified.

:Syntax:

::

      iN

The number of bits the integer will occupy is specified by the ``N``
value.

Examples:
*********

+----------------+------------------------------------------------+
| ``i1``         | a single-bit integer.                          |
+----------------+------------------------------------------------+
| ``i32``        | a 32-bit integer.                              |
+----------------+------------------------------------------------+
| ``i1942652``   | a really big integer of over 1 million bits.   |
+----------------+------------------------------------------------+

.. _t_floating:

Floating Point Types
""""""""""""""""""""

.. list-table::
   :header-rows: 1

   * - Type
     - Description

   * - ``half``
     - 16-bit floating point value

   * - ``float``
     - 32-bit floating point value

   * - ``double``
     - 64-bit floating point value

   * - ``fp128``
     - 128-bit floating point value (112-bit mantissa)

   * - ``x86_fp80``
     -  80-bit floating point value (X87)

   * - ``ppc_fp128``
     - 128-bit floating point value (two 64-bits)

X86_mmx Type
""""""""""""

:Overview:

The x86_mmx type represents a value held in an MMX register on an x86
machine. The operations allowed on it are quite limited: parameters and
return values, load and store, and bitcast. User-specified MMX
instructions are represented as intrinsic or asm calls with arguments
and/or results of this type. There are no arrays, vectors or constants
of this type.

:Syntax:

::

      x86_mmx


.. _t_pointer:

Pointer Type
""""""""""""

:Overview:

The pointer type is used to specify memory locations. Pointers are
commonly used to reference objects in memory.

Pointer types may have an optional address space attribute defining the
numbered address space where the pointed-to object resides. The default
address space is number zero. The semantics of non-zero address spaces
are target-specific.

Note that LLVM does not permit pointers to void (``void*``) nor does it
permit pointers to labels (``label*``). Use ``i8*`` instead.

:Syntax:

::

      <type> *

:Examples:

+-------------------------+--------------------------------------------------------------------------------------------------------------+
| ``[4 x i32]*``          | A :ref:`pointer <t_pointer>` to :ref:`array <t_array>` of four ``i32`` values.                               |
+-------------------------+--------------------------------------------------------------------------------------------------------------+
| ``i32 (i32*) *``        | A :ref:`pointer <t_pointer>` to a :ref:`function <t_function>` that takes an ``i32*``, returning an ``i32``. |
+-------------------------+--------------------------------------------------------------------------------------------------------------+
| ``i32 addrspace(5)*``   | A :ref:`pointer <t_pointer>` to an ``i32`` value that resides in address space #5.                           |
+-------------------------+--------------------------------------------------------------------------------------------------------------+

.. _t_vector:

Vector Type
"""""""""""

:Overview:

A vector type is a simple derived type that represents a vector of
elements. Vector types are used when multiple primitive data are
operated in parallel using a single instruction (SIMD). A vector type
requires a size (number of elements) and an underlying primitive data
type. Vector types are considered :ref:`first class <t_firstclass>`.

:Syntax:

::

      < <# elements> x <elementtype> >

The number of elements is a constant integer value larger than 0;
elementtype may be any integer, floating point or pointer type. Vectors
of size zero are not allowed.

:Examples:

+-------------------+--------------------------------------------------+
| ``<4 x i32>``     | Vector of 4 32-bit integer values.               |
+-------------------+--------------------------------------------------+
| ``<8 x float>``   | Vector of 8 32-bit floating-point values.        |
+-------------------+--------------------------------------------------+
| ``<2 x i64>``     | Vector of 2 64-bit integer values.               |
+-------------------+--------------------------------------------------+
| ``<4 x i64*>``    | Vector of 4 pointers to 64-bit integer values.   |
+-------------------+--------------------------------------------------+

.. _t_label:

Label Type
^^^^^^^^^^

:Overview:

The label type represents code labels.

:Syntax:

::

      label

.. _t_token:

Token Type
^^^^^^^^^^

:Overview:

The token type is used when a value is associated with an instruction
but all uses of the value must not attempt to introspect or obscure it.
As such, it is not appropriate to have a :ref:`phi <i_phi>` or
:ref:`select <i_select>` of type token.

:Syntax:

::

      token



.. _t_metadata:

Metadata Type
^^^^^^^^^^^^^

:Overview:

The metadata type represents embedded metadata. No derived types may be
created from metadata except for :ref:`function <t_function>` arguments.

:Syntax:

::

      metadata

.. _t_aggregate:

Aggregate Types
^^^^^^^^^^^^^^^

Aggregate Types are a subset of derived types that can contain multiple
member types. :ref:`Arrays <t_array>` and :ref:`structs <t_struct>` are
aggregate types. :ref:`Vectors <t_vector>` are not considered to be
aggregate types.

.. _t_array:

Array Type
""""""""""

:Overview:

The array type is a very simple derived type that arranges elements
sequentially in memory. The array type requires a size (number of
elements) and an underlying data type.

:Syntax:

::

      [<# elements> x <elementtype>]

The number of elements is a constant integer value; ``elementtype`` may
be any type with a size.

:Examples:

+------------------+--------------------------------------+
| ``[40 x i32]``   | Array of 40 32-bit integer values.   |
+------------------+--------------------------------------+
| ``[41 x i32]``   | Array of 41 32-bit integer values.   |
+------------------+--------------------------------------+
| ``[4 x i8]``     | Array of 4 8-bit integer values.     |
+------------------+--------------------------------------+

Here are some examples of multidimensional arrays:

+-----------------------------+----------------------------------------------------------+
| ``[3 x [4 x i32]]``         | 3x4 array of 32-bit integer values.                      |
+-----------------------------+----------------------------------------------------------+
| ``[12 x [10 x float]]``     | 12x10 array of single precision floating point values.   |
+-----------------------------+----------------------------------------------------------+
| ``[2 x [3 x [4 x i16]]]``   | 2x3x4 array of 16-bit integer values.                    |
+-----------------------------+----------------------------------------------------------+

There is no restriction on indexing beyond the end of the array implied
by a static type (though there are restrictions on indexing beyond the
bounds of an allocated object in some cases). This means that
single-dimension 'variable sized array' addressing can be implemented in
LLVM with a zero length array type. An implementation of 'pascal style
arrays' in LLVM could use the type "``{ i32, [0 x float]}``", for
example.

.. _t_struct:

Structure Type
""""""""""""""

:Overview:

The structure type is used to represent a collection of data members
together in memory. The elements of a structure may be any type that has
a size.

Structures in memory are accessed using '``load``' and '``store``' by
getting a pointer to a field with the '``getelementptr``' instruction.
Structures in registers are accessed using the '``extractvalue``' and
'``insertvalue``' instructions.

Structures may optionally be "packed" structures, which indicate that
the alignment of the struct is one byte, and that there is no padding
between the elements. In non-packed structs, padding between field types
is inserted as defined by the DataLayout string in the module, which is
required to match what the underlying code generator expects.

Structures can either be "literal" or "identified". A literal structure
is defined inline with other types (e.g. ``{i32, i32}*``) whereas
identified types are always defined at the top level with a name.
Literal types are uniqued by their contents and can never be recursive
or opaque since there is no way to write one. Identified types can be
recursive, can be opaqued, and are never uniqued.

:Syntax:

::

      %T1 = type { <type list> }     ; Identified normal struct type
      %T2 = type <{ <type list> }>   ; Identified packed struct type

:Examples:

+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``{ i32, i32, i32 }``        | A triple of three ``i32`` values                                                                                                                                                      |
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``{ float, i32 (i32) * }``   | A pair, where the first element is a ``float`` and the second element is a :ref:`pointer <t_pointer>` to a :ref:`function <t_function>` that takes an ``i32``, returning an ``i32``.  |
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| ``<{ i8, i32 }>``            | A packed struct known to be 5 bytes in size.                                                                                                                                          |
+------------------------------+---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+

.. _t_opaque:

Opaque Structure Types
""""""""""""""""""""""

:Overview:

Opaque structure types are used to represent named structure types that
do not have a body specified. This corresponds (for example) to the C
notion of a forward declared structure.

:Syntax:

::

      %X = type opaque
      %52 = type opaque

:Examples:

+--------------+-------------------+
| ``opaque``   | An opaque type.   |
+--------------+-------------------+

.. _constants:

Constants
=========

LLVM has several different basic types of constants. This section
describes them all and their syntax.

Simple Constants
----------------

**Boolean constants**
    The two strings '``true``' and '``false``' are both valid constants
    of the ``i1`` type.
**Integer constants**
    Standard integers (such as '4') are constants of the
    :ref:`integer <t_integer>` type. Negative numbers may be used with
    integer types.
**Floating point constants**
    Floating point constants use standard decimal notation (e.g.
    123.421), exponential notation (e.g. 1.23421e+2), or a more precise
    hexadecimal notation (see below). The assembler requires the exact
    decimal value of a floating-point constant. For example, the
    assembler accepts 1.25 but rejects 1.3 because 1.3 is a repeating
    decimal in binary. Floating point constants must have a :ref:`floating
    point <t_floating>` type.
**Null pointer constants**
    The identifier '``null``' is recognized as a null pointer constant
    and must be of :ref:`pointer type <t_pointer>`.
**Token constants**
    The identifier '``none``' is recognized as an empty token constant
    and must be of :ref:`token type <t_token>`.

The one non-intuitive notation for constants is the hexadecimal form of
floating point constants. For example, the form
'``double    0x432ff973cafa8000``' is equivalent to (but harder to read
than) '``double 4.5e+15``'. The only time hexadecimal floating point
constants are required (and the only time that they are generated by the
disassembler) is when a floating point constant must be emitted but it
cannot be represented as a decimal floating point number in a reasonable
number of digits. For example, NaN's, infinities, and other special
values are represented in their IEEE hexadecimal format so that assembly
and disassembly do not cause any bits to change in the constants.

When using the hexadecimal form, constants of types half, float, and
double are represented using the 16-digit form shown above (which
matches the IEEE754 representation for double); half and float values
must, however, be exactly representable as IEEE 754 half and single
precision, respectively. Hexadecimal format is always used for long
double, and there are three forms of long double. The 80-bit format used
by x86 is represented as ``0xK`` followed by 20 hexadecimal digits. The
128-bit format used by PowerPC (two adjacent doubles) is represented by
``0xM`` followed by 32 hexadecimal digits. The IEEE 128-bit format is
represented by ``0xL`` followed by 32 hexadecimal digits. Long doubles
will only work if they match the long double format on your target.
The IEEE 16-bit format (half precision) is represented by ``0xH``
followed by 4 hexadecimal digits. All hexadecimal formats are big-endian
(sign bit at the left).

There are no constants of type x86_mmx.

.. _complexconstants:

Complex Constants
-----------------

Complex constants are a (potentially recursive) combination of simple
constants and smaller complex constants.

**Structure constants**
    Structure constants are represented with notation similar to
    structure type definitions (a comma separated list of elements,
    surrounded by braces (``{}``)). For example:
    "``{ i32 4, float 17.0, i32* @G }``", where "``@G``" is declared as
    "``@G = external global i32``". Structure constants must have
    :ref:`structure type <t_struct>`, and the number and types of elements
    must match those specified by the type.
**Array constants**
    Array constants are represented with notation similar to array type
    definitions (a comma separated list of elements, surrounded by
    square brackets (``[]``)). For example:
    "``[ i32 42, i32 11, i32 74 ]``". Array constants must have
    :ref:`array type <t_array>`, and the number and types of elements must
    match those specified by the type. As a special case, character array
    constants may also be represented as a double-quoted string using the ``c``
    prefix. For example: "``c"Hello World\0A\00"``".
**Vector constants**
    Vector constants are represented with notation similar to vector
    type definitions (a comma separated list of elements, surrounded by
    less-than/greater-than's (``<>``)). For example:
    "``< i32 42, i32 11, i32 74, i32 100 >``". Vector constants
    must have :ref:`vector type <t_vector>`, and the number and types of
    elements must match those specified by the type.
**Zero initialization**
    The string '``zeroinitializer``' can be used to zero initialize a
    value to zero of *any* type, including scalar and
    :ref:`aggregate <t_aggregate>` types. This is often used to avoid
    having to print large zero initializers (e.g. for large arrays) and
    is always exactly equivalent to using explicit zero initializers.
**Metadata node**
    A metadata node is a constant tuple without types. For example:
    "``!{!0, !{!2, !0}, !"test"}``". Metadata can reference constant values,
    for example: "``!{!0, i32 0, i8* @global, i64 (i64)* @function, !"str"}``".
    Unlike other typed constants that are meant to be interpreted as part of
    the instruction stream, metadata is a place to attach additional
    information such as debug info.

Global Variable and Function Addresses
--------------------------------------

The addresses of :ref:`global variables <globalvars>` and
:ref:`functions <functionstructure>` are always implicitly valid
(link-time) constants. These constants are explicitly referenced when
the :ref:`identifier for the global <identifiers>` is used and always have
:ref:`pointer <t_pointer>` type. For example, the following is a legal LLVM
file:

.. code-block:: llvm

    @X = global i32 17
    @Y = global i32 42
    @Z = global [2 x i32*] [ i32* @X, i32* @Y ]

.. _undefvalues:

Undefined Values
----------------

The string '``undef``' can be used anywhere a constant is expected, and
indicates that the user of the value may receive an unspecified
bit-pattern. Undefined values may be of any type (other than '``label``'
or '``void``') and be used anywhere a constant is permitted.

Undefined values are useful because they indicate to the compiler that
the program is well defined no matter what value is used. This gives the
compiler more freedom to optimize. Here are some examples of
(potentially surprising) transformations that are valid (in pseudo IR):

.. code-block:: llvm

      %A = add %X, undef
      %B = sub %X, undef
      %C = xor %X, undef
    Safe:
      %A = undef
      %B = undef
      %C = undef

This is safe because all of the output bits are affected by the undef
bits. Any output bit can have a zero or one depending on the input bits.

.. code-block:: llvm

      %A = or %X, undef
      %B = and %X, undef
    Safe:
      %A = -1
      %B = 0
    Unsafe:
      %A = undef
      %B = undef

These logical operations have bits that are not always affected by the
input. For example, if ``%X`` has a zero bit, then the output of the
'``and``' operation will always be a zero for that bit, no matter what
the corresponding bit from the '``undef``' is. As such, it is unsafe to
optimize or assume that the result of the '``and``' is '``undef``'.
However, it is safe to assume that all bits of the '``undef``' could be
0, and optimize the '``and``' to 0. Likewise, it is safe to assume that
all the bits of the '``undef``' operand to the '``or``' could be set,
allowing the '``or``' to be folded to -1.

.. code-block:: llvm

      %A = select undef, %X, %Y
      %B = select undef, 42, %Y
      %C = select %X, %Y, undef
    Safe:
      %A = %X     (or %Y)
      %B = 42     (or %Y)
      %C = %Y
    Unsafe:
      %A = undef
      %B = undef
      %C = undef

This set of examples shows that undefined '``select``' (and conditional
branch) conditions can go *either way*, but they have to come from one
of the two operands. In the ``%A`` example, if ``%X`` and ``%Y`` were
both known to have a clear low bit, then ``%A`` would have to have a
cleared low bit. However, in the ``%C`` example, the optimizer is
allowed to assume that the '``undef``' operand could be the same as
``%Y``, allowing the whole '``select``' to be eliminated.

.. code-block:: llvm

      %A = xor undef, undef

      %B = undef
      %C = xor %B, %B

      %D = undef
      %E = icmp slt %D, 4
      %F = icmp gte %D, 4

    Safe:
      %A = undef
      %B = undef
      %C = undef
      %D = undef
      %E = undef
      %F = undef

This example points out that two '``undef``' operands are not
necessarily the same. This can be surprising to people (and also matches
C semantics) where they assume that "``X^X``" is always zero, even if
``X`` is undefined. This isn't true for a number of reasons, but the
short answer is that an '``undef``' "variable" can arbitrarily change
its value over its "live range". This is true because the variable
doesn't actually *have a live range*. Instead, the value is logically
read from arbitrary registers that happen to be around when needed, so
the value is not necessarily consistent over time. In fact, ``%A`` and
``%C`` need to have the same semantics or the core LLVM "replace all
uses with" concept would not hold.

.. code-block:: llvm

      %A = fdiv undef, %X
      %B = fdiv %X, undef
    Safe:
      %A = undef
    b: unreachable

These examples show the crucial difference between an *undefined value*
and *undefined behavior*. An undefined value (like '``undef``') is
allowed to have an arbitrary bit-pattern. This means that the ``%A``
operation can be constant folded to '``undef``', because the '``undef``'
could be an SNaN, and ``fdiv`` is not (currently) defined on SNaN's.
However, in the second example, we can make a more aggressive
assumption: because the ``undef`` is allowed to be an arbitrary value,
we are allowed to assume that it could be zero. Since a divide by zero
has *undefined behavior*, we are allowed to assume that the operation
does not execute at all. This allows us to delete the divide and all
code after it. Because the undefined operation "can't happen", the
optimizer can assume that it occurs in dead code.

.. code-block:: llvm

    a:  store undef -> %X
    b:  store %X -> undef
    Safe:
    a: <deleted>
    b: unreachable

These examples reiterate the ``fdiv`` example: a store *of* an undefined
value can be assumed to not have any effect; we can assume that the
value is overwritten with bits that happen to match what was already
there. However, a store *to* an undefined location could clobber
arbitrary memory, therefore, it has undefined behavior.

.. _poisonvalues:

Poison Values
-------------

Poison values are similar to :ref:`undef values <undefvalues>`, however
they also represent the fact that an instruction or constant expression
that cannot evoke side effects has nevertheless detected a condition
that results in undefined behavior.

There is currently no way of representing a poison value in the IR; they
only exist when produced by operations such as :ref:`add <i_add>` with
the ``nsw`` flag.

Poison value behavior is defined in terms of value *dependence*:

-  Values other than :ref:`phi <i_phi>` nodes depend on their operands.
-  :ref:`Phi <i_phi>` nodes depend on the operand corresponding to
   their dynamic predecessor basic block.
-  Function arguments depend on the corresponding actual argument values
   in the dynamic callers of their functions.
-  :ref:`Call <i_call>` instructions depend on the :ref:`ret <i_ret>`
   instructions that dynamically transfer control back to them.
-  :ref:`Invoke <i_invoke>` instructions depend on the
   :ref:`ret <i_ret>`, :ref:`resume <i_resume>`, or exception-throwing
   call instructions that dynamically transfer control back to them.
-  Non-volatile loads and stores depend on the most recent stores to all
   of the referenced memory addresses, following the order in the IR
   (including loads and stores implied by intrinsics such as
   :ref:`@llvm.memcpy <int_memcpy>`.)
-  An instruction with externally visible side effects depends on the
   most recent preceding instruction with externally visible side
   effects, following the order in the IR. (This includes :ref:`volatile
   operations <volatile>`.)
-  An instruction *control-depends* on a :ref:`terminator
   instruction <terminators>` if the terminator instruction has
   multiple successors and the instruction is always executed when
   control transfers to one of the successors, and may not be executed
   when control is transferred to another.
-  Additionally, an instruction also *control-depends* on a terminator
   instruction if the set of instructions it otherwise depends on would
   be different if the terminator had transferred control to a different
   successor.
-  Dependence is transitive.

Poison values have the same behavior as :ref:`undef values <undefvalues>`,
with the additional effect that any instruction that has a *dependence*
on a poison value has undefined behavior.

Here are some examples:

.. code-block:: llvm

    entry:
      %poison = sub nuw i32 0, 1           ; Results in a poison value.
      %still_poison = and i32 %poison, 0   ; 0, but also poison.
      %poison_yet_again = getelementptr i32, i32* @h, i32 %still_poison
      store i32 0, i32* %poison_yet_again  ; memory at @h[0] is poisoned

      store i32 %poison, i32* @g           ; Poison value stored to memory.
      %poison2 = load i32, i32* @g         ; Poison value loaded back from memory.

      store volatile i32 %poison, i32* @g  ; External observation; undefined behavior.

      %narrowaddr = bitcast i32* @g to i16*
      %wideaddr = bitcast i32* @g to i64*
      %poison3 = load i16, i16* %narrowaddr ; Returns a poison value.
      %poison4 = load i64, i64* %wideaddr  ; Returns a poison value.

      %cmp = icmp slt i32 %poison, 0       ; Returns a poison value.
      br i1 %cmp, label %true, label %end  ; Branch to either destination.

    true:
      store volatile i32 0, i32* @g        ; This is control-dependent on %cmp, so
                                           ; it has undefined behavior.
      br label %end

    end:
      %p = phi i32 [ 0, %entry ], [ 1, %true ]
                                           ; Both edges into this PHI are
                                           ; control-dependent on %cmp, so this
                                           ; always results in a poison value.

      store volatile i32 0, i32* @g        ; This would depend on the store in %true
                                           ; if %cmp is true, or the store in %entry
                                           ; otherwise, so this is undefined behavior.

      br i1 %cmp, label %second_true, label %second_end
                                           ; The same branch again, but this time the
                                           ; true block doesn't have side effects.

    second_true:
      ; No side effects!
      ret void

    second_end:
      store volatile i32 0, i32* @g        ; This time, the instruction always depends
                                           ; on the store in %end. Also, it is
                                           ; control-equivalent to %end, so this is
                                           ; well-defined (ignoring earlier undefined
                                           ; behavior in this example).

.. _blockaddress:

Addresses of Basic Blocks
-------------------------

``blockaddress(@function, %block)``

The '``blockaddress``' constant computes the address of the specified
basic block in the specified function, and always has an ``i8*`` type.
Taking the address of the entry block is illegal.

This value only has defined behavior when used as an operand to the
':ref:`indirectbr <i_indirectbr>`' instruction, or for comparisons
against null. Pointer equality tests between labels addresses results in
undefined behavior --- though, again, comparison against null is ok, and
no label is equal to the null pointer. This may be passed around as an
opaque pointer sized value as long as the bits are not inspected. This
allows ``ptrtoint`` and arithmetic to be performed on these values so
long as the original value is reconstituted before the ``indirectbr``
instruction.

Finally, some targets may provide defined semantics when using the value
as the operand to an inline assembly, but that is target specific.

.. _constantexprs:

Constant Expressions
--------------------

Constant expressions are used to allow expressions involving other
constants to be used as constants. Constant expressions may be of any
:ref:`first class <t_firstclass>` type and may involve any LLVM operation
that does not have side effects (e.g. load and call are not supported).
The following is the syntax for constant expressions:

``trunc (CST to TYPE)``
    Truncate a constant to another type. The bit size of CST must be
    larger than the bit size of TYPE. Both types must be integers.
``zext (CST to TYPE)``
    Zero extend a constant to another type. The bit size of CST must be
    smaller than the bit size of TYPE. Both types must be integers.
``sext (CST to TYPE)``
    Sign extend a constant to another type. The bit size of CST must be
    smaller than the bit size of TYPE. Both types must be integers.
``fptrunc (CST to TYPE)``
    Truncate a floating point constant to another floating point type.
    The size of CST must be larger than the size of TYPE. Both types
    must be floating point.
``fpext (CST to TYPE)``
    Floating point extend a constant to another type. The size of CST
    must be smaller or equal to the size of TYPE. Both types must be
    floating point.
``fptoui (CST to TYPE)``
    Convert a floating point constant to the corresponding unsigned
    integer constant. TYPE must be a scalar or vector integer type. CST
    must be of scalar or vector floating point type. Both CST and TYPE
    must be scalars, or vectors of the same number of elements. If the
    value won't fit in the integer type, the results are undefined.
``fptosi (CST to TYPE)``
    Convert a floating point constant to the corresponding signed
    integer constant. TYPE must be a scalar or vector integer type. CST
    must be of scalar or vector floating point type. Both CST and TYPE
    must be scalars, or vectors of the same number of elements. If the
    value won't fit in the integer type, the results are undefined.
``uitofp (CST to TYPE)``
    Convert an unsigned integer constant to the corresponding floating
    point constant. TYPE must be a scalar or vector floating point type.
    CST must be of scalar or vector integer type. Both CST and TYPE must
    be scalars, or vectors of the same number of elements. If the value
    won't fit in the floating point type, the results are undefined.
``sitofp (CST to TYPE)``
    Convert a signed integer constant to the corresponding floating
    point constant. TYPE must be a scalar or vector floating point type.
    CST must be of scalar or vector integer type. Both CST and TYPE must
    be scalars, or vectors of the same number of elements. If the value
    won't fit in the floating point type, the results are undefined.
``ptrtoint (CST to TYPE)``
    Convert a pointer typed constant to the corresponding integer
    constant. ``TYPE`` must be an integer type. ``CST`` must be of
    pointer type. The ``CST`` value is zero extended, truncated, or
    unchanged to make it fit in ``TYPE``.
``inttoptr (CST to TYPE)``
    Convert an integer constant to a pointer constant. TYPE must be a
    pointer type. CST must be of integer type. The CST value is zero
    extended, truncated, or unchanged to make it fit in a pointer size.
    This one is *really* dangerous!
``bitcast (CST to TYPE)``
    Convert a constant, CST, to another TYPE. The constraints of the
    operands are the same as those for the :ref:`bitcast
    instruction <i_bitcast>`.
``addrspacecast (CST to TYPE)``
    Convert a constant pointer or constant vector of pointer, CST, to another
    TYPE in a different address space. The constraints of the operands are the
    same as those for the :ref:`addrspacecast instruction <i_addrspacecast>`.
``getelementptr (TY, CSTPTR, IDX0, IDX1, ...)``, ``getelementptr inbounds (TY, CSTPTR, IDX0, IDX1, ...)``
    Perform the :ref:`getelementptr operation <i_getelementptr>` on
    constants. As with the :ref:`getelementptr <i_getelementptr>`
    instruction, the index list may have zero or more indexes, which are
    required to make sense for the type of "pointer to TY".
``select (COND, VAL1, VAL2)``
    Perform the :ref:`select operation <i_select>` on constants.
``icmp COND (VAL1, VAL2)``
    Performs the :ref:`icmp operation <i_icmp>` on constants.
``fcmp COND (VAL1, VAL2)``
    Performs the :ref:`fcmp operation <i_fcmp>` on constants.
``extractelement (VAL, IDX)``
    Perform the :ref:`extractelement operation <i_extractelement>` on
    constants.
``insertelement (VAL, ELT, IDX)``
    Perform the :ref:`insertelement operation <i_insertelement>` on
    constants.
``shufflevector (VEC1, VEC2, IDXMASK)``
    Perform the :ref:`shufflevector operation <i_shufflevector>` on
    constants.
``extractvalue (VAL, IDX0, IDX1, ...)``
    Perform the :ref:`extractvalue operation <i_extractvalue>` on
    constants. The index list is interpreted in a similar manner as
    indices in a ':ref:`getelementptr <i_getelementptr>`' operation. At
    least one index value must be specified.
``insertvalue (VAL, ELT, IDX0, IDX1, ...)``
    Perform the :ref:`insertvalue operation <i_insertvalue>` on constants.
    The index list is interpreted in a similar manner as indices in a
    ':ref:`getelementptr <i_getelementptr>`' operation. At least one index
    value must be specified.
``OPCODE (LHS, RHS)``
    Perform the specified operation of the LHS and RHS constants. OPCODE
    may be any of the :ref:`binary <binaryops>` or :ref:`bitwise
    binary <bitwiseops>` operations. The constraints on operands are
    the same as those for the corresponding instruction (e.g. no bitwise
    operations on floating point values are allowed).

Other Values
============

.. _inlineasmexprs:

Inline Assembler Expressions
----------------------------

LLVM supports inline assembler expressions (as opposed to :ref:`Module-Level
Inline Assembly <moduleasm>`) through the use of a special value. This value
represents the inline assembler as a template string (containing the
instructions to emit), a list of operand constraints (stored as a string), a
flag that indicates whether or not the inline asm expression has side effects,
and a flag indicating whether the function containing the asm needs to align its
stack conservatively.

The template string supports argument substitution of the operands using "``$``"
followed by a number, to indicate substitution of the given register/memory
location, as specified by the constraint string. "``${NUM:MODIFIER}``" may also
be used, where ``MODIFIER`` is a target-specific annotation for how to print the
operand (See :ref:`inline-asm-modifiers`).

A literal "``$``" may be included by using "``$$``" in the template. To include
other special characters into the output, the usual "``\XX``" escapes may be
used, just as in other strings. Note that after template substitution, the
resulting assembly string is parsed by LLVM's integrated assembler unless it is
disabled -- even when emitting a ``.s`` file -- and thus must contain assembly
syntax known to LLVM.

LLVM's support for inline asm is modeled closely on the requirements of Clang's
GCC-compatible inline-asm support. Thus, the feature-set and the constraint and
modifier codes listed here are similar or identical to those in GCC's inline asm
support. However, to be clear, the syntax of the template and constraint strings
described here is *not* the same as the syntax accepted by GCC and Clang, and,
while most constraint letters are passed through as-is by Clang, some get
translated to other codes when converting from the C source to the LLVM
assembly.

An example inline assembler expression is:

.. code-block:: llvm

    i32 (i32) asm "bswap $0", "=r,r"

Inline assembler expressions may **only** be used as the callee operand
of a :ref:`call <i_call>` or an :ref:`invoke <i_invoke>` instruction.
Thus, typically we have:

.. code-block:: llvm

    %X = call i32 asm "bswap $0", "=r,r"(i32 %Y)

Inline asms with side effects not visible in the constraint list must be
marked as having side effects. This is done through the use of the
'``sideeffect``' keyword, like so:

.. code-block:: llvm

    call void asm sideeffect "eieio", ""()

In some cases inline asms will contain code that will not work unless
the stack is aligned in some way, such as calls or SSE instructions on
x86, yet will not contain code that does that alignment within the asm.
The compiler should make conservative assumptions about what the asm
might contain and should generate its usual stack alignment code in the
prologue if the '``alignstack``' keyword is present:

.. code-block:: llvm

    call void asm alignstack "eieio", ""()

Inline asms also support using non-standard assembly dialects. The
assumed dialect is ATT. When the '``inteldialect``' keyword is present,
the inline asm is using the Intel dialect. Currently, ATT and Intel are
the only supported dialects. An example is:

.. code-block:: llvm

    call void asm inteldialect "eieio", ""()

If multiple keywords appear the '``sideeffect``' keyword must come
first, the '``alignstack``' keyword second and the '``inteldialect``'
keyword last.

Inline Asm Constraint String
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The constraint list is a comma-separated string, each element containing one or
more constraint codes.

For each element in the constraint list an appropriate register or memory
operand will be chosen, and it will be made available to assembly template
string expansion as ``$0`` for the first constraint in the list, ``$1`` for the
second, etc.

There are three different types of constraints, which are distinguished by a
prefix symbol in front of the constraint code: Output, Input, and Clobber. The
constraints must always be given in that order: outputs first, then inputs, then
clobbers. They cannot be intermingled.

There are also three different categories of constraint codes:

- Register constraint. This is either a register class, or a fixed physical
  register. This kind of constraint will allocate a register, and if necessary,
  bitcast the argument or result to the appropriate type.
- Memory constraint. This kind of constraint is for use with an instruction
  taking a memory operand. Different constraints allow for different addressing
  modes used by the target.
- Immediate value constraint. This kind of constraint is for an integer or other
  immediate value which can be rendered directly into an instruction. The
  various target-specific constraints allow the selection of a value in the
  proper range for the instruction you wish to use it with.

Output constraints
""""""""""""""""""

Output constraints are specified by an "``=``" prefix (e.g. "``=r``"). This
indicates that the assembly will write to this operand, and the operand will
then be made available as a return value of the ``asm`` expression. Output
constraints do not consume an argument from the call instruction. (Except, see
below about indirect outputs).

Normally, it is expected that no output locations are written to by the assembly
expression until *all* of the inputs have been read. As such, LLVM may assign
the same register to an output and an input. If this is not safe (e.g. if the
assembly contains two instructions, where the first writes to one output, and
the second reads an input and writes to a second output), then the "``&``"
modifier must be used (e.g. "``=&r``") to specify that the output is an
"early-clobber" output. Marking an ouput as "early-clobber" ensures that LLVM
will not use the same register for any inputs (other than an input tied to this
output).

Input constraints
"""""""""""""""""

Input constraints do not have a prefix -- just the constraint codes. Each input
constraint will consume one argument from the call instruction. It is not
permitted for the asm to write to any input register or memory location (unless
that input is tied to an output). Note also that multiple inputs may all be
assigned to the same register, if LLVM can determine that they necessarily all
contain the same value.

Instead of providing a Constraint Code, input constraints may also "tie"
themselves to an output constraint, by providing an integer as the constraint
string. Tied inputs still consume an argument from the call instruction, and
take up a position in the asm template numbering as is usual -- they will simply
be constrained to always use the same register as the output they've been tied
to. For example, a constraint string of "``=r,0``" says to assign a register for
output, and use that register as an input as well (it being the 0'th
constraint).

It is permitted to tie an input to an "early-clobber" output. In that case, no
*other* input may share the same register as the input tied to the early-clobber
(even when the other input has the same value).

You may only tie an input to an output which has a register constraint, not a
memory constraint. Only a single input may be tied to an output.

There is also an "interesting" feature which deserves a bit of explanation: if a
register class constraint allocates a register which is too small for the value
type operand provided as input, the input value will be split into multiple
registers, and all of them passed to the inline asm.

However, this feature is often not as useful as you might think.

Firstly, the registers are *not* guaranteed to be consecutive. So, on those
architectures that have instructions which operate on multiple consecutive
instructions, this is not an appropriate way to support them. (e.g. the 32-bit
SparcV8 has a 64-bit load, which instruction takes a single 32-bit register. The
hardware then loads into both the named register, and the next register. This
feature of inline asm would not be useful to support that.)

A few of the targets provide a template string modifier allowing explicit access
to the second register of a two-register operand (e.g. MIPS ``L``, ``M``, and
``D``). On such an architecture, you can actually access the second allocated
register (yet, still, not any subsequent ones). But, in that case, you're still
probably better off simply splitting the value into two separate operands, for
clarity. (e.g. see the description of the ``A`` constraint on X86, which,
despite existing only for use with this feature, is not really a good idea to
use)

Indirect inputs and outputs
"""""""""""""""""""""""""""

Indirect output or input constraints can be specified by the "``*``" modifier
(which goes after the "``=``" in case of an output). This indicates that the asm
will write to or read from the contents of an *address* provided as an input
argument. (Note that in this way, indirect outputs act more like an *input* than
an output: just like an input, they consume an argument of the call expression,
rather than producing a return value. An indirect output constraint is an
"output" only in that the asm is expected to write to the contents of the input
memory location, instead of just read from it).

This is most typically used for memory constraint, e.g. "``=*m``", to pass the
address of a variable as a value.

It is also possible to use an indirect *register* constraint, but only on output
(e.g. "``=*r``"). This will cause LLVM to allocate a register for an output
value normally, and then, separately emit a store to the address provided as
input, after the provided inline asm. (It's not clear what value this
functionality provides, compared to writing the store explicitly after the asm
statement, and it can only produce worse code, since it bypasses many
optimization passes. I would recommend not using it.)


Clobber constraints
"""""""""""""""""""

A clobber constraint is indicated by a "``~``" prefix. A clobber does not
consume an input operand, nor generate an output. Clobbers cannot use any of the
general constraint code letters -- they may use only explicit register
constraints, e.g. "``~{eax}``". The one exception is that a clobber string of
"``~{memory}``" indicates that the assembly writes to arbitrary undeclared
memory locations -- not only the memory pointed to by a declared indirect
output.


Constraint Codes
""""""""""""""""
After a potential prefix comes constraint code, or codes.

A Constraint Code is either a single letter (e.g. "``r``"), a "``^``" character
followed by two letters (e.g. "``^wc``"), or "``{``" register-name "``}``"
(e.g. "``{eax}``").

The one and two letter constraint codes are typically chosen to be the same as
GCC's constraint codes.

A single constraint may include one or more than constraint code in it, leaving
it up to LLVM to choose which one to use. This is included mainly for
compatibility with the translation of GCC inline asm coming from clang.

There are two ways to specify alternatives, and either or both may be used in an
inline asm constraint list:

1) Append the codes to each other, making a constraint code set. E.g. "``im``"
   or "``{eax}m``". This means "choose any of the options in the set". The
   choice of constraint is made independently for each constraint in the
   constraint list.

2) Use "``|``" between constraint code sets, creating alternatives. Every
   constraint in the constraint list must have the same number of alternative
   sets. With this syntax, the same alternative in *all* of the items in the
   constraint list will be chosen together.

Putting those together, you might have a two operand constraint string like
``"rm|r,ri|rm"``. This indicates that if operand 0 is ``r`` or ``m``, then
operand 1 may be one of ``r`` or ``i``. If operand 0 is ``r``, then operand 1
may be one of ``r`` or ``m``. But, operand 0 and 1 cannot both be of type m.

However, the use of either of the alternatives features is *NOT* recommended, as
LLVM is not able to make an intelligent choice about which one to use. (At the
point it currently needs to choose, not enough information is available to do so
in a smart way.) Thus, it simply tries to make a choice that's most likely to
compile, not one that will be optimal performance. (e.g., given "``rm``", it'll
always choose to use memory, not registers). And, if given multiple registers,
or multiple register classes, it will simply choose the first one. (In fact, it
doesn't currently even ensure explicitly specified physical registers are
unique, so specifying multiple physical registers as alternatives, like
``{r11}{r12},{r11}{r12}``, will assign r11 to both operands, not at all what was
intended.)

Supported Constraint Code List
""""""""""""""""""""""""""""""

The constraint codes are, in general, expected to behave the same way they do in
GCC. LLVM's support is often implemented on an 'as-needed' basis, to support C
inline asm code which was supported by GCC. A mismatch in behavior between LLVM
and GCC likely indicates a bug in LLVM.

Some constraint codes are typically supported by all targets:

- ``r``: A register in the target's general purpose register class.
- ``m``: A memory address operand. It is target-specific what addressing modes
  are supported, typical examples are register, or register + register offset,
  or register + immediate offset (of some target-specific size).
- ``i``: An integer constant (of target-specific width). Allows either a simple
  immediate, or a relocatable value.
- ``n``: An integer constant -- *not* including relocatable values.
- ``s``: An integer constant, but allowing *only* relocatable values.
- ``X``: Allows an operand of any kind, no constraint whatsoever. Typically
  useful to pass a label for an asm branch or call.

  .. FIXME: but that surely isn't actually okay to jump out of an asm
     block without telling llvm about the control transfer???)

- ``{register-name}``: Requires exactly the named physical register.

Other constraints are target-specific:

AArch64:

- ``z``: An immediate integer 0. Outputs ``WZR`` or ``XZR``, as appropriate.
- ``I``: An immediate integer valid for an ``ADD`` or ``SUB`` instruction,
  i.e. 0 to 4095 with optional shift by 12.
- ``J``: An immediate integer that, when negated, is valid for an ``ADD`` or
  ``SUB`` instruction, i.e. -1 to -4095 with optional left shift by 12.
- ``K``: An immediate integer that is valid for the 'bitmask immediate 32' of a
  logical instruction like ``AND``, ``EOR``, or ``ORR`` with a 32-bit register.
- ``L``: An immediate integer that is valid for the 'bitmask immediate 64' of a
  logical instruction like ``AND``, ``EOR``, or ``ORR`` with a 64-bit register.
- ``M``: An immediate integer for use with the ``MOV`` assembly alias on a
  32-bit register. This is a superset of ``K``: in addition to the bitmask
  immediate, also allows immediate integers which can be loaded with a single
  ``MOVZ`` or ``MOVL`` instruction.
- ``N``: An immediate integer for use with the ``MOV`` assembly alias on a
  64-bit register. This is a superset of ``L``.
- ``Q``: Memory address operand must be in a single register (no
  offsets). (However, LLVM currently does this for the ``m`` constraint as
  well.)
- ``r``: A 32 or 64-bit integer register (W* or X*).
- ``w``: A 32, 64, or 128-bit floating-point/SIMD register.
- ``x``: A lower 128-bit floating-point/SIMD register (``V0`` to ``V15``).

AMDGPU:

- ``r``: A 32 or 64-bit integer register.
- ``[0-9]v``: The 32-bit VGPR register, number 0-9.
- ``[0-9]s``: The 32-bit SGPR register, number 0-9.


All ARM modes:

- ``Q``, ``Um``, ``Un``, ``Uq``, ``Us``, ``Ut``, ``Uv``, ``Uy``: Memory address
  operand. Treated the same as operand ``m``, at the moment.

ARM and ARM's Thumb2 mode:

- ``j``: An immediate integer between 0 and 65535 (valid for ``MOVW``)
- ``I``: An immediate integer valid for a data-processing instruction.
- ``J``: An immediate integer between -4095 and 4095.
- ``K``: An immediate integer whose bitwise inverse is valid for a
  data-processing instruction. (Can be used with template modifier "``B``" to
  print the inverted value).
- ``L``: An immediate integer whose negation is valid for a data-processing
  instruction. (Can be used with template modifier "``n``" to print the negated
  value).
- ``M``: A power of two or a integer between 0 and 32.
- ``N``: Invalid immediate constraint.
- ``O``: Invalid immediate constraint.
- ``r``: A general-purpose 32-bit integer register (``r0-r15``).
- ``l``: In Thumb2 mode, low 32-bit GPR registers (``r0-r7``). In ARM mode, same
  as ``r``.
- ``h``: In Thumb2 mode, a high 32-bit GPR register (``r8-r15``). In ARM mode,
  invalid.
- ``w``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s31``,
  ``d0-d31``, or ``q0-q15``.
- ``x``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s15``,
  ``d0-d7``, or ``q0-q3``.
- ``t``: A floating-point/SIMD register, only supports 32-bit values:
  ``s0-s31``.

ARM's Thumb1 mode:

- ``I``: An immediate integer between 0 and 255.
- ``J``: An immediate integer between -255 and -1.
- ``K``: An immediate integer between 0 and 255, with optional left-shift by
  some amount.
- ``L``: An immediate integer between -7 and 7.
- ``M``: An immediate integer which is a multiple of 4 between 0 and 1020.
- ``N``: An immediate integer between 0 and 31.
- ``O``: An immediate integer which is a multiple of 4 between -508 and 508.
- ``r``: A low 32-bit GPR register (``r0-r7``).
- ``l``: A low 32-bit GPR register (``r0-r7``).
- ``h``: A high GPR register (``r0-r7``).
- ``w``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s31``,
  ``d0-d31``, or ``q0-q15``.
- ``x``: A 32, 64, or 128-bit floating-point/SIMD register: ``s0-s15``,
  ``d0-d7``, or ``q0-q3``.
- ``t``: A floating-point/SIMD register, only supports 32-bit values:
  ``s0-s31``.


Hexagon:

- ``o``, ``v``: A memory address operand, treated the same as constraint ``m``,
  at the moment.
- ``r``: A 32 or 64-bit register.

MSP430:

- ``r``: An 8 or 16-bit register.

MIPS:

- ``I``: An immediate signed 16-bit integer.
- ``J``: An immediate integer zero.
- ``K``: An immediate unsigned 16-bit integer.
- ``L``: An immediate 32-bit integer, where the lower 16 bits are 0.
- ``N``: An immediate integer between -65535 and -1.
- ``O``: An immediate signed 15-bit integer.
- ``P``: An immediate integer between 1 and 65535.
- ``m``: A memory address operand. In MIPS-SE mode, allows a base address
  register plus 16-bit immediate offset. In MIPS mode, just a base register.
- ``R``: A memory address operand. In MIPS-SE mode, allows a base address
  register plus a 9-bit signed offset. In MIPS mode, the same as constraint
  ``m``.
- ``ZC``: A memory address operand, suitable for use in a ``pref``, ``ll``, or
  ``sc`` instruction on the given subtarget (details vary).
- ``r``, ``d``,  ``y``: A 32 or 64-bit GPR register.
- ``f``: A 32 or 64-bit FPU register (``F0-F31``), or a 128-bit MSA register
  (``W0-W31``). In the case of MSA registers, it is recommended to use the ``w``
  argument modifier for compatibility with GCC.
- ``c``: A 32-bit or 64-bit GPR register suitable for indirect jump (always
  ``25``).
- ``l``: The ``lo`` register, 32 or 64-bit.
- ``x``: Invalid.

NVPTX:

- ``b``: A 1-bit integer register.
- ``c`` or ``h``: A 16-bit integer register.
- ``r``: A 32-bit integer register.
- ``l`` or ``N``: A 64-bit integer register.
- ``f``: A 32-bit float register.
- ``d``: A 64-bit float register.


PowerPC:

- ``I``: An immediate signed 16-bit integer.
- ``J``: An immediate unsigned 16-bit integer, shifted left 16 bits.
- ``K``: An immediate unsigned 16-bit integer.
- ``L``: An immediate signed 16-bit integer, shifted left 16 bits.
- ``M``: An immediate integer greater than 31.
- ``N``: An immediate integer that is an exact power of 2.
- ``O``: The immediate integer constant 0.
- ``P``: An immediate integer constant whose negation is a signed 16-bit
  constant.
- ``es``, ``o``, ``Q``, ``Z``, ``Zy``: A memory address operand, currently
  treated the same as ``m``.
- ``r``: A 32 or 64-bit integer register.
- ``b``: A 32 or 64-bit integer register, excluding ``R0`` (that is:
  ``R1-R31``).
- ``f``: A 32 or 64-bit float register (``F0-F31``), or when QPX is enabled, a
  128 or 256-bit QPX register (``Q0-Q31``; aliases the ``F`` registers).
- ``v``: For ``4 x f32`` or ``4 x f64`` types, when QPX is enabled, a
  128 or 256-bit QPX register (``Q0-Q31``), otherwise a 128-bit
  altivec vector register (``V0-V31``).

  .. FIXME: is this a bug that v accepts QPX registers? I think this
     is supposed to only use the altivec vector registers?

- ``y``: Condition register (``CR0-CR7``).
- ``wc``: An individual CR bit in a CR register.
- ``wa``, ``wd``, ``wf``: Any 128-bit VSX vector register, from the full VSX
  register set (overlapping both the floating-point and vector register files).
- ``ws``: A 32 or 64-bit floating point register, from the full VSX register
  set.

Sparc:

- ``I``: An immediate 13-bit signed integer.
- ``r``: A 32-bit integer register.

SystemZ:

- ``I``: An immediate unsigned 8-bit integer.
- ``J``: An immediate unsigned 12-bit integer.
- ``K``: An immediate signed 16-bit integer.
- ``L``: An immediate signed 20-bit integer.
- ``M``: An immediate integer 0x7fffffff.
- ``Q``, ``R``, ``S``, ``T``: A memory address operand, treated the same as
  ``m``, at the moment.
- ``r`` or ``d``: A 32, 64, or 128-bit integer register.
- ``a``: A 32, 64, or 128-bit integer address register (excludes R0, which in an
  address context evaluates as zero).
- ``h``: A 32-bit value in the high part of a 64bit data register
  (LLVM-specific)
- ``f``: A 32, 64, or 128-bit floating point register.

X86:

- ``I``: An immediate integer between 0 and 31.
- ``J``: An immediate integer between 0 and 64.
- ``K``: An immediate signed 8-bit integer.
- ``L``: An immediate integer, 0xff or 0xffff or (in 64-bit mode only)
  0xffffffff.
- ``M``: An immediate integer between 0 and 3.
- ``N``: An immediate unsigned 8-bit integer.
- ``O``: An immediate integer between 0 and 127.
- ``e``: An immediate 32-bit signed integer.
- ``Z``: An immediate 32-bit unsigned integer.
- ``o``, ``v``: Treated the same as ``m``, at the moment.
- ``q``: An 8, 16, 32, or 64-bit register which can be accessed as an 8-bit
  ``l`` integer register. On X86-32, this is the ``a``, ``b``, ``c``, and ``d``
  registers, and on X86-64, it is all of the integer registers.
- ``Q``: An 8, 16, 32, or 64-bit register which can be accessed as an 8-bit
  ``h`` integer register. This is the ``a``, ``b``, ``c``, and ``d`` registers.
- ``r`` or ``l``: An 8, 16, 32, or 64-bit integer register.
- ``R``: An 8, 16, 32, or 64-bit "legacy" integer register -- one which has
  existed since i386, and can be accessed without the REX prefix.
- ``f``: A 32, 64, or 80-bit '387 FPU stack pseudo-register.
- ``y``: A 64-bit MMX register, if MMX is enabled.
- ``x``: If SSE is enabled: a 32 or 64-bit scalar operand, or 128-bit vector
  operand in a SSE register. If AVX is also enabled, can also be a 256-bit
  vector operand in an AVX register. If AVX-512 is also enabled, can also be a
  512-bit vector operand in an AVX512 register, Otherwise, an error.
- ``Y``: The same as ``x``, if *SSE2* is enabled, otherwise an error.
- ``A``: Special case: allocates EAX first, then EDX, for a single operand (in
  32-bit mode, a 64-bit integer operand will get split into two registers). It
  is not recommended to use this constraint, as in 64-bit mode, the 64-bit
  operand will get allocated only to RAX -- if two 32-bit operands are needed,
  you're better off splitting it yourself, before passing it to the asm
  statement.

XCore:

- ``r``: A 32-bit integer register.


.. _inline-asm-modifiers:

Asm template argument modifiers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

In the asm template string, modifiers can be used on the operand reference, like
"``${0:n}``".

The modifiers are, in general, expected to behave the same way they do in
GCC. LLVM's support is often implemented on an 'as-needed' basis, to support C
inline asm code which was supported by GCC. A mismatch in behavior between LLVM
and GCC likely indicates a bug in LLVM.

Target-independent:

- ``c``: Print an immediate integer constant unadorned, without
  the target-specific immediate punctuation (e.g. no ``$`` prefix).
- ``n``: Negate and print immediate integer constant unadorned, without the
  target-specific immediate punctuation (e.g. no ``$`` prefix).
- ``l``: Print as an unadorned label, without the target-specific label
  punctuation (e.g. no ``$`` prefix).

AArch64:

- ``w``: Print a GPR register with a ``w*`` name instead of ``x*`` name. E.g.,
  instead of ``x30``, print ``w30``.
- ``x``: Print a GPR register with a ``x*`` name. (this is the default, anyhow).
- ``b``, ``h``, ``s``, ``d``, ``q``: Print a floating-point/SIMD register with a
  ``b*``, ``h*``, ``s*``, ``d*``, or ``q*`` name, rather than the default of
  ``v*``.

AMDGPU:

- ``r``: No effect.

ARM:

- ``a``: Print an operand as an address (with ``[`` and ``]`` surrounding a
  register).
- ``P``: No effect.
- ``q``: No effect.
- ``y``: Print a VFP single-precision register as an indexed double (e.g. print
  as ``d4[1]`` instead of ``s9``)
- ``B``: Bitwise invert and print an immediate integer constant without ``#``
  prefix.
- ``L``: Print the low 16-bits of an immediate integer constant.
- ``M``: Print as a register set suitable for ldm/stm. Also prints *all*
  register operands subsequent to the specified one (!), so use carefully.
- ``Q``: Print the low-order register of a register-pair, or the low-order
  register of a two-register operand.
- ``R``: Print the high-order register of a register-pair, or the high-order
  register of a two-register operand.
- ``H``: Print the second register of a register-pair. (On a big-endian system,
  ``H`` is equivalent to ``Q``, and on little-endian system, ``H`` is equivalent
  to ``R``.)

  .. FIXME: H doesn't currently support printing the second register
     of a two-register operand.

- ``e``: Print the low doubleword register of a NEON quad register.
- ``f``: Print the high doubleword register of a NEON quad register.
- ``m``: Print the base register of a memory operand without the ``[`` and ``]``
  adornment.

Hexagon:

- ``L``: Print the second register of a two-register operand. Requires that it
  has been allocated consecutively to the first.

  .. FIXME: why is it restricted to consecutive ones? And there's
     nothing that ensures that happens, is there?

- ``I``: Print the letter 'i' if the operand is an integer constant, otherwise
  nothing. Used to print 'addi' vs 'add' instructions.

MSP430:

No additional modifiers.

MIPS:

- ``X``: Print an immediate integer as hexadecimal
- ``x``: Print the low 16 bits of an immediate integer as hexadecimal.
- ``d``: Print an immediate integer as decimal.
- ``m``: Subtract one and print an immediate integer as decimal.
- ``z``: Print $0 if an immediate zero, otherwise print normally.
- ``L``: Print the low-order register of a two-register operand, or prints the
  address of the low-order word of a double-word memory operand.

  .. FIXME: L seems to be missing memory operand support.

- ``M``: Print the high-order register of a two-register operand, or prints the
  address of the high-order word of a double-word memory operand.

  .. FIXME: M seems to be missing memory operand support.

- ``D``: Print the second register of a two-register operand, or prints the
  second word of a double-word memory operand. (On a big-endian system, ``D`` is
  equivalent to ``L``, and on little-endian system, ``D`` is equivalent to
  ``M``.)
- ``w``: No effect. Provided for compatibility with GCC which requires this
  modifier in order to print MSA registers (``W0-W31``) with the ``f``
  constraint.

NVPTX:

- ``r``: No effect.

PowerPC:

- ``L``: Print the second register of a two-register operand. Requires that it
  has been allocated consecutively to the first.

  .. FIXME: why is it restricted to consecutive ones? And there's
     nothing that ensures that happens, is there?

- ``I``: Print the letter 'i' if the operand is an integer constant, otherwise
  nothing. Used to print 'addi' vs 'add' instructions.
- ``y``: For a memory operand, prints formatter for a two-register X-form
  instruction. (Currently always prints ``r0,OPERAND``).
- ``U``: Prints 'u' if the memory operand is an update form, and nothing
  otherwise. (NOTE: LLVM does not support update form, so this will currently
  always print nothing)
- ``X``: Prints 'x' if the memory operand is