LLVM Assembly Language Reference Manual

The LLVM code representation is designed to be used in three different forms: as an in-memory compiler IR, as an on-disk bytecode 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 a 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.

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:

  %x = add int 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 bytecode. The violations pointed out by the verifier pass indicate bugs in transformation passes or input to the parser.

LLVM uses three different forms of identifiers, for different purposes:

Numeric constants are represented as you would expect: 12, -3 123.421, etc. Floating point constants have an optional hexadecimal notation.
Named values are represented as a string of characters with a '%' prefix. For example, %foo, %DivisionByZero, %a.really.long.identifier. The actual regular expression used is '%[a-zA-Z$._][a-zA-Z$._0-9]*'. Identifiers which require other characters in their names can be surrounded with quotes. In this way, anything except a " character can be used in a name.
Unnamed values are represented as an unsigned numeric value with a '%' prefix. For example, %12, %2, %44.

LLVM requires that values start with a '%' sign 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', 'cast', 'ret', etc...), for primitive type names ('void', 'uint', etc...), and others. These reserved words cannot conflict with variable names, because none of them start with a '%' character.

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

The easy way:

  %result = mul uint %X, 8

After strength reduction:

  %result = shl uint %X, ubyte 3

And the hard way:

  add uint %X, %X           ; yields {uint}:%0
  add uint %0, %0           ; yields {uint}:%1
  %result = add uint %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

...and it also show 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. Comments are shown in italic text.

The one non-intuitive notation for constants is the optional hexidecimal form of floating point constants. For example, the form 'double 0x432ff973cafa8000' is equivalent to (but harder to read than) 'double 4.5e+15' which is also supported by the parser. The only time hexadecimal floating point constants are useful (and the only time that they are generated by the disassembler) is when an FP constant has to be emitted that is not representable as a decimal floating point number exactly. For example, NaN's, infinities, and other special cases are represented in their IEEE hexadecimal format so that assembly and disassembly do not cause any bits to change in the constants.

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 IR 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.

The primitive types are the fundamental building blocks of the LLVM system. The current set of primitive types are as follows:

Type	Description
`void`	No value
`ubyte`	Unsigned 8 bit value
`ushort`	Unsigned 16 bit value
`uint`	Unsigned 32 bit value
`ulong`	Unsigned 64 bit value
`float`	32 bit floating point value
`label`	Branch destination

Type	Description
`bool`	True or False value
`sbyte`	Signed 8 bit value
`short`	Signed 16 bit value
`int`	Signed 32 bit value
`long`	Signed 64 bit value
`double`	64 bit floating point value

These different primitive types fall into a few useful classifications:

Classification	Types
signed	`sbyte, short, int, long, float, double`
unsigned	`ubyte, ushort, uint, ulong`
integer	`ubyte, sbyte, ushort, short, uint, int, ulong, long`
integral	`bool, ubyte, sbyte, ushort, short, uint, int, ulong, long`
floating point	`float, double`
first class	`bool, ubyte, sbyte, ushort, short, uint, int, ulong, long, float, double, pointer, packed`

The first class types are perhaps the most important. Values of these types are the only ones which can be produced by instructions, passed as arguments, or used as operands to instructions. This means that all structures and arrays must be manipulated either by pointer or by component.

The real power in LLVM comes from the derived types in the system. This is what allows a programmer to represent arrays, functions, pointers, and other useful types. Note that these derived types may be recursive: For example, it is possible to have a two dimensional array.

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 int ]
[41 x int ]
[40 x uint] Array of 40 integer values.
Array of 41 integer values.
Array of 40 unsigned integer values.

Here are some examples of multidimensional arrays:

[3 x [4 x int]]
[12 x [10 x float]]
[2 x [3 x [4 x uint]]] 3x4 array integer values.
12x10 array of single precision floating point values.
2x3x4 array of unsigned integer values.

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. Function types are usually used to build virtual function tables (which are structures of pointers to functions), for indirect function calls, and when defining a function.

The return type of a function type cannot be an aggregate type.

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 variable argument handling intrinsic functions.

Examples:

int (int)
float (int, int *) *
int (sbyte *, ...) function taking an int, returning an int
Pointer to a function that takes an int and a pointer to int, returning float.
A vararg function that takes at least one pointer to sbyte (signed char in C), which returns an integer. This is the signature for printf in LLVM.

Overview:

The structure type is used to represent a collection of data members together in memory. The packing of the field types is defined to match the ABI of the underlying processor. The elements of a structure may be any type that has a size.

Structures are accessed using 'load and 'store' by getting a pointer to a field with the 'getelementptr' instruction.

Syntax:

  { <type list> }

Examples:

{ int, int, int }
{ float, int (int) * } a triple of three int values
A pair, where the first element is a float and the second element is a pointer to a function that takes an int, returning an int.

Overview:

As in many languages, the pointer type represents a pointer or reference to another object, which must live in memory.

Syntax:

  <type> *

Examples:

[4x int]*
int (int *) * A pointer to array of four int values
A pointer to a function that takes an int, returning an int.

Overview:

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

Syntax:

  < <# elements> x <elementtype> >

The number of elements is a constant integer value, elementtype may be any integral or floating point type.

Examples:

<4 x int>
<8 x float>
<2 x uint> Packed vector of 4 integer values.
Packed vector of 8 floating-point values.
Packed vector of 2 unsigned integer values.

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:

; Declare the string constant as a global constant...
%.LC0 = internal constant [13 x sbyte] c"hello world\0A\00"          ; [13 x sbyte]*

; External declaration of the puts function
declare int %puts(sbyte*)                                            ; int(sbyte*)* 

; Definition of main function
int %main() {                                                        ; int()* 
        ; Convert [13x sbyte]* to sbyte *...
        %cast210 = getelementptr [13 x sbyte]* %.LC0, long 0, long 0 ; sbyte*

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

This example is made up of a global variable named ".LC0", an external declaration of the "puts" function, and a function definition for "main".

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 linkage types:

internal: Global values with internal linkage are only directly accessible by objects in the current module. In particular, linking code into a module with an internal global value may cause the internal to be renamed as necessary to avoid collisions. Because the symbol is internal to the module, all references can be updated. This corresponds to the notion of the 'static' keyword in C, or the idea of "anonymous namespaces" in C++.
linkonce:: "linkonce" linkage is similar to internal linkage, with the twist that linking together two modules defining the same linkonce globals will cause one of the globals to be discarded. This is typically used to implement inline functions. Unreferenced linkonce globals are allowed to be discarded.
weak:: "weak" linkage is exactly the same as linkonce linkage, except that unreferenced weak globals may not be discarded. This is used to implement constructs in C such as "int X;" at global scope.
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.
externally visible:: 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.

For example, since the ".LC0" variable is defined to be internal, if another module defined a ".LC0" variable and was linked with this one, one of the two would be renamed, preventing a collision. Since "main" and "puts" are external (i.e., lacking any linkage declarations), they are accessible outside of the current module. It is illegal for a function declaration to have any linkage type other than "externally visible".

Global variables define regions of memory allocated at compilation time instead of run-time. Global variables may optionally be initialized. A variable may be defined as a global "constant", which indicates that the contents of the variable will never be modified (opening options for optimization).

As SSA values, global variables define pointer values that are in scope (i.e. they dominate) for 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.

LLVM function definitions are composed of a (possibly empty) argument list, an opening curly brace, a list of basic blocks, and a closing curly brace. LLVM function declarations are defined with the "declare" keyword, a function name, and a function signature.

A function definition contains a list of basic blocks, forming the CFG 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 terminator instruction (such as a branch or function return).

The first basic block in program 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 PHI nodes.

LLVM functions are identified by their name and type signature. Hence, two functions with the same name but different parameter lists or return values are considered different functions, and LLVM will resolves references to each appropriately.

The LLVM instruction set consists of several different classifications of instructions: terminator instructions, binary instructions, memory instructions, and other instructions.

As mentioned previously, every basic block in a program ends with a "Terminator" instruction, which indicates which block should be executed after the current block is finished. These terminator instructions typically yield a 'void' value: they produce control flow, not values (the one exception being the 'invoke' instruction).

There are five different terminator instructions: the 'ret' instruction, the 'br' instruction, the 'switch' instruction, the 'invoke' instruction, the 'unwind' instruction, and the 'unreachable' instruction.

Syntax:

  ret <type> <value>       ; Return a value from a non-void function
  ret void                 ; Return from void function

Overview:

The 'ret' instruction is used to return control flow (and a value) from a function, back to the caller.

There are two forms of the 'ret' instruction: one that returns a value and then causes control flow, and one that just causes control flow to occur.

Arguments:

The 'ret' instruction may return any 'first class' type. Notice that a function is not well formed if there exists a 'ret' instruction inside of the function that returns a value that does not match the return type of the function.

Semantics:

When the 'ret' instruction is executed, control flow returns back to the calling function's context. If the caller is a "call" instruction, execution continues at the instruction after the call. If the caller was an "invoke" instruction, execution continues at the beginning "normal" of the destination block. If the instruction returns a value, that value shall set the call or invoke instruction's return value.

Example:

  ret int 5                       ; Return an integer value of 5
  ret void                        ; Return from a void function

Syntax:

  br bool <cond>, label <iftrue>, label <iffalse>
  br label <dest>          ; Unconditional branch

Overview:

The 'br' instruction is used to cause control flow to transfer to a different basic block in the current function. There are two forms of this instruction, corresponding to a conditional branch and an unconditional branch.

Arguments:

The conditional branch form of the 'br' instruction takes a single 'bool' value and two 'label' values. The unconditional form of the 'br' instruction takes a single 'label' value as a target.

Semantics:

Upon execution of a conditional 'br' instruction, the 'bool' argument is evaluated. If the value is true, control flows to the 'iftrue' label argument. If "cond" is false, control flows to the 'iffalse' label argument.

Example:

Test:
  %cond = seteq int %a, %b
  br bool %cond, label %IfEqual, label %IfUnequal
IfEqual:
  ret int 1
IfUnequal:
  ret int 0

Syntax:

  switch <intty> <value>, label <defaultdest> [ <intty> <val>, label <dest> ... ]

Overview:

The 'switch' instruction is used to transfer control flow to one of several different places. It is a generalization of the 'br' instruction, allowing a branch to occur to one of many possible destinations.

Arguments:

The 'switch' instruction uses three parameters: an integer comparison value 'value', a default 'label' destination, and an array of pairs of comparison value constants and 'label's. The table is not allowed to contain duplicate constant entries.

Semantics:

The switch instruction specifies a table of values and destinations. When the 'switch' instruction is executed, this table is searched for the given value. If the value is found, control flow is transfered to the corresponding destination; otherwise, control flow is transfered to the default destination.

Implementation:

Depending on properties of the target machine and the particular switch instruction, this instruction may be code generated in different ways. For example, it could be generated as a series of chained conditional branches or with a lookup table.

Example:

 ; Emulate a conditional br instruction
 %Val = cast bool %value to int
 switch int %Val, label %truedest [int 0, label %falsedest ]

 ; Emulate an unconditional br instruction
 switch uint 0, label %dest [ ]

 ; Implement a jump table:
 switch uint %val, label %otherwise [ uint 0, label %onzero 
                                      uint 1, label %onone 
                                      uint 2, label %ontwo ]

Syntax:

  <result> = invoke <ptr to function ty> %<function ptr val>(<function args>)
                 to label <normal label> except label <exception label>

Overview:

The 'invoke' instruction causes control to transfer to a specified function, with the possibility of control flow transfer to either the 'normal' label label or the 'exception'label. If the callee function returns with the "ret" instruction, control flow will return to the "normal" label. If the callee (or any indirect callees) returns with the "unwind" instruction, control is interrupted, and continued at the dynamically nearest "except" label.

Arguments:

This instruction requires several arguments:

'ptr to function ty': shall be the signature of the pointer to function value being invoked. In most cases, this is a direct function invocation, but indirect invokes are just as possible, branching off an arbitrary pointer to function value.
'function ptr val': An LLVM value containing a pointer to a function to be invoked.
'function args': argument list whose types match the function signature argument types. If the function signature indicates the function accepts a variable number of arguments, the extra arguments can be specified.
'normal label': the label reached when the called function executes a 'ret' instruction.
'exception label': the label reached when a callee returns with the unwind instruction.

Semantics:

This instruction is designed to operate as a standard 'call' instruction in most regards. The primary difference is that it establishes an association with a label, which is used by the runtime library to unwind the stack.

This instruction is used in languages with destructors to ensure that proper cleanup is performed in the case of either a longjmp or a thrown exception. Additionally, this is important for implementation of 'catch' clauses in high-level languages that support them.

Example:

  %retval = invoke int %Test(int 15)
              to label %Continue
              except label %TestCleanup     ; {int}:retval set

Syntax:

  unwind

Overview:

The 'unwind' instruction unwinds the stack, continuing control flow at the first callee in the dynamic call stack which used an invoke instruction to perform the call. This is primarily used to implement exception handling.

Semantics:

The 'unwind' intrinsic causes execution of the current function to immediately halt. The dynamic call stack is then searched for the first invoke instruction on the call stack. Once found, execution continues at the "exceptional" destination block specified by the invoke instruction. If there is no invoke instruction in the dynamic call chain, undefined behavior results.

Syntax:

  unreachable

Overview:

The 'unreachable' instruction has no defined semantics. This instruction is used to inform the optimizer that a particular portion of the code is not reachable. This can be used to indicate that the code after a no-return function cannot be reached, and other facts.

Semantics:

The 'unreachable' instruction has no defined semantics.

Binary operators are used to do most of the computation in a program. They require two operands, execute an operation on them, and produce a single value. Although, that single value might represent multiple data, as is the case with the packed data type. The result value of a binary operator is not necessarily the same type as its operands.

There are several different binary operators:

Syntax:

  <result> = add <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'add' instruction returns the sum of its two operands.

Arguments:

The two arguments to the 'add' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.

Semantics:

The value produced is the integer or floating point sum of the two operands.

Example:

  <result> = add int 4, %var          ; yields {int}:result = 4 + %var

Syntax:

  <result> = sub <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'sub' instruction returns the difference of its two operands.

Note that the 'sub' instruction is used to represent the 'neg' instruction present in most other intermediate representations.

Arguments:

The two arguments to the 'sub' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.

Semantics:

The value produced is the integer or floating point difference of the two operands.

Example:

  <result> = sub int 4, %var          ; yields {int}:result = 4 - %var
  <result> = sub int 0, %val          ; yields {int}:result = -%var

Syntax:

  <result> = mul <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'mul' instruction returns the product of its two operands.

Arguments:

The two arguments to the 'mul' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.

Semantics:

The value produced is the integer or floating point product of the two operands.

There is no signed vs unsigned multiplication. The appropriate action is taken based on the type of the operand.

Example:

  <result> = mul int 4, %var          ; yields {int}:result = 4 * %var

Syntax:

  <result> = div <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'div' instruction returns the quotient of its two operands.

Arguments:

The two arguments to the 'div' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.

Semantics:

The value produced is the integer or floating point quotient of the two operands.

Example:

  <result> = div int 4, %var          ; yields {int}:result = 4 / %var

Syntax:

  <result> = rem <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'rem' instruction returns the remainder from the division of its two operands.

Arguments:

The two arguments to the 'rem' instruction must be either integer or floating point values. This instruction can also take packed versions of the values. Both arguments must have identical types.

Semantics:

This returns the remainder of a division (where the result has the same sign as the divisor), not the modulus (where the result has the same sign as the dividend) of a value. For more information about the difference, see: The Math Forum.

Example:

  <result> = rem int 4, %var          ; yields {int}:result = 4 % %var

Syntax:

  <result> = seteq <ty> <var1>, <var2>   ; yields {bool}:result
  <result> = setne <ty> <var1>, <var2>   ; yields {bool}:result
  <result> = setlt <ty> <var1>, <var2>   ; yields {bool}:result
  <result> = setgt <ty> <var1>, <var2>   ; yields {bool}:result
  <result> = setle <ty> <var1>, <var2>   ; yields {bool}:result
  <result> = setge <ty> <var1>, <var2>   ; yields {bool}:result

Overview:

The 'setcc' family of instructions returns a boolean value based on a comparison of their two operands.

Arguments:

The two arguments to the 'setcc' instructions must be of first class type (it is not possible to compare 'label's, 'array's, 'structure' or 'void' values, etc...). Both arguments must have identical types.

Semantics:

The 'seteq' instruction yields a true 'bool' value if both operands are equal.
The 'setne' instruction yields a true 'bool' value if both operands are unequal.
The 'setlt' instruction yields a true 'bool' value if the first operand is less than the second operand.
The 'setgt' instruction yields a true 'bool' value if the first operand is greater than the second operand.
The 'setle' instruction yields a true 'bool' value if the first operand is less than or equal to the second operand.
The 'setge' instruction yields a true 'bool' value if the first operand is greater than or equal to the second operand.

Example:

  <result> = seteq int   4, 5        ; yields {bool}:result = false
  <result> = setne float 4, 5        ; yields {bool}:result = true
  <result> = setlt uint  4, 5        ; yields {bool}:result = true
  <result> = setgt sbyte 4, 5        ; yields {bool}:result = false
  <result> = setle sbyte 4, 5        ; yields {bool}:result = true
  <result> = setge sbyte 4, 5        ; yields {bool}:result = false

Bitwise binary operators are used to do various forms of bit-twiddling in a program. They are generally very efficient instructions, and can commonly be strength reduced from other instructions. They require two operands, execute an operation on them, and produce a single value. The resulting value of the bitwise binary operators is always the same type as its first operand.

Syntax:

  <result> = and <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'and' instruction returns the bitwise logical and of its two operands.

Arguments:

The two arguments to the 'and' instruction must be integral values. Both arguments must have identical types.

Semantics:

The truth table used for the 'and' instruction is:

In0	In1	Out
0	0	0
0	1	0
1	0	0
1	1	1

Example:

  <result> = and int 4, %var         ; yields {int}:result = 4 & %var
  <result> = and int 15, 40          ; yields {int}:result = 8
  <result> = and int 4, 8            ; yields {int}:result = 0

Syntax:

  <result> = or <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'or' instruction returns the bitwise logical inclusive or of its two operands.

Arguments:

The two arguments to the 'or' instruction must be integral values. Both arguments must have identical types.

Semantics:

The truth table used for the 'or' instruction is:

In0	In1	Out
0	0	0
0	1	1
1	0	1
1	1	1

Example:

  <result> = or int 4, %var         ; yields {int}:result = 4 | %var
  <result> = or int 15, 40          ; yields {int}:result = 47
  <result> = or int 4, 8            ; yields {int}:result = 12

Syntax:

  <result> = xor <ty> <var1>, <var2>   ; yields {ty}:result

Overview:

The 'xor' instruction returns the bitwise logical exclusive or of its two operands. The xor is used to implement the "one's complement" operation, which is the "~" operator in C.

Arguments:

The two arguments to the 'xor' instruction must be integral values. Both arguments must have identical types.

Semantics:

The truth table used for the 'xor' instruction is:

In0	In1	Out
0	0	0
0	1	1
1	0	1
1	1	0

Example:

  <result> = xor int 4, %var         ; yields {int}:result = 4 ^ %var
  <result> = xor int 15, 40          ; yields {int}:result = 39
  <result> = xor int 4, 8            ; yields {int}:result = 12
  <result> = xor int %V, -1          ; yields {int}:result = ~%V

Syntax:

  <result> = shl <ty> <var1>, ubyte <var2>   ; yields {ty}:result

Overview:

The 'shl' instruction returns the first operand shifted to the left a specified number of bits.

Arguments:

The first argument to the 'shl' instruction must be an integer type. The second argument must be an 'ubyte' type.

Semantics:

The value produced is var1 * 2^var2.

Example:

  <result> = shl int 4, ubyte %var   ; yields {int}:result = 4 << %var
  <result> = shl int 4, ubyte 2      ; yields {int}:result = 16
  <result> = shl int 1, ubyte 10     ; yields {int}:result = 1024

Syntax:

  <result> = shr <ty> <var1>, ubyte <var2>   ; yields {ty}:result

Overview:

The 'shr' instruction returns the first operand shifted to the right a specified number of bits.

Arguments:

The first argument to the 'shr' instruction must be an integer type. The second argument must be an 'ubyte' type.

Semantics:

If the first argument is a signed type, the most significant bit is duplicated in the newly free'd bit positions. If the first argument is unsigned, zero bits shall fill the empty positions.

Example:

  <result> = shr int 4, ubyte %var   ; yields {int}:result = 4 >> %var
  <result> = shr uint 4, ubyte 1     ; yields {uint}:result = 2
  <result> = shr int 4, ubyte 2      ; yields {int}:result = 1
  <result> = shr sbyte 4, ubyte 3    ; yields {sbyte}:result = 0
  <result> = shr sbyte -2, ubyte 1   ; yields {sbyte}:result = -1

A key design point of an SSA-based representation is how it represents memory. In LLVM, no memory locations are in SSA form, which makes things very simple. This section describes how to read, write, allocate and free memory in LLVM.

Syntax:

  <result> = malloc <type>, uint <NumElements>     ; yields {type*}:result
  <result> = malloc <type>                         ; yields {type*}:result

Overview:

The 'malloc' instruction allocates memory from the system heap and returns a pointer to it.

Arguments:

The 'malloc' instruction allocates sizeof(<type>)*NumElements bytes of memory from the operating system and returns a pointer of the appropriate type to the program. The second form of the instruction is a shorter version of the first instruction that defaults to allocating one element.

'type' must be a sized type.

Semantics:

Memory is allocated using the system "malloc" function, and a pointer is returned.

Example:

  %array  = malloc [4 x ubyte ]                    ; yields {[%4 x ubyte]*}:array

  %size   = add uint 2, 2                          ; yields {uint}:size = uint 4
  %array1 = malloc ubyte, uint 4                   ; yields {ubyte*}:array1
  %array2 = malloc [12 x ubyte], uint %size        ; yields {[12 x ubyte]*}:array2

Syntax:

  free <type> <value>                              ; yields {void}

Overview:

The 'free' instruction returns memory back to the unused memory heap, to be reallocated in the future.

Arguments:

'value' shall be a pointer value that points to a value that was allocated with the 'malloc' instruction.

Semantics:

Access to the memory pointed to by the pointer is not longer defined after this instruction executes.

Example:

  %array  = malloc [4 x ubyte]                    ; yields {[4 x ubyte]*}:array
            free   [4 x ubyte]* %array

Syntax:

  <result> = alloca <type>, uint <NumElements>  ; yields {type*}:result
  <result> = alloca <type>                      ; yields {type*}:result

Overview:

The 'alloca' instruction allocates memory on the current stack frame of the procedure that is live until the current function returns to its caller.

Arguments:

The the 'alloca' instruction allocates sizeof(<type>)*NumElements bytes of memory on the runtime stack, returning a pointer of the appropriate type to the program. The second form of the instruction is a shorter version of the first that defaults to allocating one element.

'type' may be any sized type.

Semantics:

Memory is allocated, a pointer is returned. 'alloca'd memory is automatically released when the function returns. The 'alloca' instruction is commonly used to represent automatic variables that must have an address available. When the function returns (either with the ret or invoke instructions), the memory is reclaimed.

Example:

  %ptr = alloca int                              ; yields {int*}:ptr
  %ptr = alloca int, uint 4                      ; yields {int*}:ptr

Syntax:

  <result> = load <ty>* <pointer>
  <result> = volatile load <ty>* <pointer>

Overview:

The 'load' instruction is used to read from memory.

Arguments:

The argument to the 'load' instruction specifies the memory address to load from. The pointer must point to a first class type. If the load is marked as volatile then the optimizer is not allowed to modify the number or order of execution of this load with other volatile load and store instructions.

Semantics:

The location of memory pointed to is loaded.

Examples:

  %ptr = alloca int                               ; yields {int*}:ptr
  store int 3, int* %ptr                          ; yields {void}
  %val = load int* %ptr                           ; yields {int}:val = int 3

Syntax:

Overview:

Arguments:

There are two arguments to the 'store' instruction: a value to store and an address to store it into. The type of the '<pointer>' operand must be a pointer to the type of the '<value>' operand. If the store is marked as volatile then the optimizer is not allowed to modify the number or order of execution of this store with other volatile load and store instructions.

Semantics:

The contents of memory are updated to contain '<value>' at the location specified by the '<pointer>' operand.

Example:

Syntax:

  <result> = getelementptr <ty>* <ptrval>{, <ty> <idx>}*

Overview:

The 'getelementptr' instruction is used to get the address of a subelement of an aggregate data structure.

Arguments:

This instruction takes a list of integer constants that indicate what elements of the aggregate object to index to. The actual types of the arguments provided depend on the type of the first pointer argument. The 'getelementptr' instruction is used to index down through the type levels of a structure. When indexing into a structure, only uint integer constants are allowed. When indexing into an array or pointer int and long indexes are allowed of any sign.

For example, let's consider a C code fragment and how it gets compiled to LLVM:

  struct RT {
    char A;
    int B[10][20];
    char C;
  };
  struct ST {
    int X;
    double Y;
    struct RT Z;
  };

  int *foo(struct ST *s) {
    return &s[1].Z.B[5][13];
  }

The LLVM code generated by the GCC frontend is:

  %RT = type { sbyte, [10 x [20 x int]], sbyte }
  %ST = type { int, double, %RT }

  implementation

  int* %foo(%ST* %s) {
  entry:
    %reg = getelementptr %ST* %s, int 1, uint 2, uint 1, int 5, int 13
    ret int* %reg
  }

Semantics:

The index types specified for the 'getelementptr' instruction depend on the pointer type that is being index into. Pointer and array types require uint, int, ulong, or long values, and structure types require uint constants.

In the example above, the first index is indexing into the '%ST*' type, which is a pointer, yielding a '%ST' = '{ int, double, %RT }' type, a structure. The second index indexes into the third element of the structure, yielding a '%RT' = '{ sbyte, [10 x [20 x int]], sbyte }' type, another structure. The third index indexes into the second element of the structure, yielding a '[10 x [20 x int]]' type, an array. The two dimensions of the array are subscripted into, yielding an 'int' type. The 'getelementptr' instruction return a pointer to this element, thus computing a value of 'int*' type.

Note that it is perfectly legal to index partially through a structure, returning a pointer to an inner element. Because of this, the LLVM code for the given testcase is equivalent to:

  int* "foo"(%ST* %s) {
    %t1 = getelementptr %ST* %s, int 1                        ; yields %ST*:%t1
    %t2 = getelementptr %ST* %t1, int 0, uint 2               ; yields %RT*:%t2
    %t3 = getelementptr %RT* %t2, int 0, uint 1               ; yields [10 x [20 x int]]*:%t3
    %t4 = getelementptr [10 x [20 x int]]* %t3, int 0, int 5  ; yields [20 x int]*:%t4
    %t5 = getelementptr [20 x int]* %t4, int 0, int 13        ; yields int*:%t5
    ret int* %t5
  }

Example:

    ; yields [12 x ubyte]*:aptr
    %aptr = getelementptr {int, [12 x ubyte]}* %sptr, long 0, uint 1

The instructions in this category are the "miscellaneous" instructions, which defy better classification.

Syntax:

  <result> = phi <ty> [ <val0>, <label0>], ...

Overview:

The 'phi' instruction is used to implement the φ node in the SSA graph representing the function.

Arguments:

The type of the incoming values are specified with the first type field. After this, the 'phi' instruction takes a list of pairs as arguments, with one pair for each predecessor basic block of the current block. Only values of first class type may be used as the value arguments to the PHI node. Only labels may be used as the label arguments.

There must be no non-phi instructions between the start of a basic block and the PHI instructions: i.e. PHI instructions must be first in a basic block.

Semantics:

At runtime, the 'phi' instruction logically takes on the value specified by the parameter, depending on which basic block we came from in the last terminator instruction.

Example:

Loop:       ; Infinite loop that counts from 0 on up...
  %indvar = phi uint [ 0, %LoopHeader ], [ %nextindvar, %Loop ]
  %nextindvar = add uint %indvar, 1
  br label %Loop

Syntax:

  <result> = cast <ty> <value> to <ty2>             ; yields ty2

Overview:

The 'cast' instruction is used as the primitive means to convert integers to floating point, change data type sizes, and break type safety (by casting pointers).

Arguments:

The 'cast' instruction takes a value to cast, which must be a first class value, and a type to cast it to, which must also be a first class type.

Semantics:

This instruction follows the C rules for explicit casts when determining how the data being cast must change to fit in its new container.

When casting to bool, any value that would be considered true in the context of a C 'if' condition is converted to the boolean 'true' values, all else are 'false'.

When extending an integral value from a type of one signness to another (for example 'sbyte' to 'ulong'), the value is sign-extended if the source value is signed, and zero-extended if the source value is unsigned. bool values are always zero extended into either zero or one.

Example:

  %X = cast int 257 to ubyte              ; yields ubyte:1
  %Y = cast int 123 to bool               ; yields bool:true

Syntax:

  <result> = select bool <cond>, <ty> <val1>, <ty> <val2>             ; yields ty

Overview:

The 'select' instruction is used to choose one value based on a condition, without branching.

Arguments:

The 'select' instruction requires a boolean value indicating the condition, and two values of the same first class type.

Semantics:

If the boolean condition evaluates to true, the instruction returns the first value argument, otherwise it returns the second value argument.

Example:

  %X = select bool true, ubyte 17, ubyte 42          ; yields ubyte:17

Syntax:

  <result> = call <ty>* <fnptrval>(<param list>)

Overview:

The 'call' instruction represents a simple function call.

Arguments:

This instruction requires several arguments:

'ty': shall be the signature of the pointer to function value being invoked. The argument types must match the types implied by this signature.
'fnptrval': An LLVM value containing a pointer to a function to be invoked. In most cases, this is a direct function invocation, but indirect calls are just as possible, calling an arbitrary pointer to function values.
'function args': argument list whose types match the function signature argument types. If the function signature indicates the function accepts a variable number of arguments, the extra arguments can be specified.

Semantics:

The 'call' instruction is used to cause control flow to transfer to a specified function, with its incoming arguments bound to the specified values. Upon a 'ret' instruction in the called function, control flow continues with the instruction after the function call, and the return value of the function is bound to the result argument. This is a simpler case of the invoke instruction.

Example:

  %retval = call int %test(int %argc)
  call int(sbyte*, ...) *%printf(sbyte* %msg, int 12, sbyte 42);

Syntax:

  <resultarglist> = vanext <va_list> <arglist>, <argty>

Overview:

The 'vanext' instruction is used to access arguments passed through the "variable argument" area of a function call. It is used to implement the va_arg macro in C.

Arguments:

This instruction takes a va_list value and the type of the argument. It returns another va_list. The actual type of va_list may be defined differently for different targets. Most targets use a va_list type of sbyte* or some other pointer type.

Semantics:

The 'vanext' instruction advances the specified va_list past an argument of the specified type. In conjunction with the vaarg instruction, it is used to implement the va_arg macro available in C. For more information, see the variable argument handling Intrinsic Functions.

It is legal for this instruction to be called in a function which does not take a variable number of arguments, for example, the vfprintf function.

vanext is an LLVM instruction instead of an intrinsic function because it takes a type as an argument. The type refers to the current argument in the va_list, it tells the compiler how far on the stack it needs to advance to find the next argument

Example:

See the variable argument processing section.

Syntax:

  <resultval> = vaarg <va_list> <arglist>, <argty>

Overview:

The 'vaarg' instruction is used to access arguments passed through the "variable argument" area of a function call. It is used to implement the va_arg macro in C.

Arguments:

This instruction takes a va_list value and the type of the argument. It returns a value of the specified argument type. Again, the actual type of va_list is target specific.

Semantics:

The 'vaarg' instruction loads an argument of the specified type from the specified va_list. In conjunction with the vanext instruction, it is used to implement the va_arg macro available in C. For more information, see the variable argument handling Intrinsic Functions.

It is legal for this instruction to be called in a function which does not take a variable number of arguments, for example, the vfprintf function.

vaarg is an LLVM instruction instead of an intrinsic function because it takes an type as an argument.

Example:

See the variable argument processing section.

LLVM supports the notion of an "intrinsic function". These functions have well known names and semantics, and are required to follow certain restrictions. Overall, these instructions represent an extension mechanism for the LLVM language that does not require changing all of the transformations in LLVM to add to the language (or the bytecode reader/writer, the parser, etc...).

Intrinsic function names must all start with an "llvm." prefix, this prefix is reserved in LLVM for intrinsic names, thus functions may not be named this. Intrinsic functions must always be external functions: you cannot define the body of intrinsic functions. Intrinsic functions may only be used in call or invoke instructions: it is illegal to take the address of an intrinsic function. Additionally, because intrinsic functions are part of the LLVM language, it is required that they all be documented here if any are added.

Adding an intrinsic to LLVM is straight-forward if it is possible to express the concept in LLVM directly (ie, code generator support is not _required_). To do this, extend the default implementation of the IntrinsicLowering class to handle the intrinsic. Code generators use this class to lower intrinsics they do not understand to raw LLVM instructions that they do.

Variable argument support is defined in LLVM with the vanext instruction and these three intrinsic functions. These functions are related to the similarly named macros defined in the <stdarg.h> header file.

All of these functions operate on arguments that use a target-specific value type "va_list". The LLVM assembly language reference manual does not define what this type is, so all transformations should be prepared to handle intrinsics with any type used.

This example shows how the vanext instruction and the variable argument handling intrinsic functions are used.

int %test(int %X, ...) {
  ; Initialize variable argument processing
  %ap = call sbyte* %llvm.va_start()

  ; Read a single integer argument
  %tmp = vaarg sbyte* %ap, int

  ; Advance to the next argument
  %ap2 = vanext sbyte* %ap, int

  ; Demonstrate usage of llvm.va_copy and llvm.va_end
  %aq = call sbyte* %llvm.va_copy(sbyte* %ap2)
  call void %llvm.va_end(sbyte* %aq)

  ; Stop processing of arguments.
  call void %llvm.va_end(sbyte* %ap2)
  ret int %tmp
}

Syntax:

  call <va_list> ()* %llvm.va_start()

Overview:

The 'llvm.va_start' intrinsic returns a new <arglist> for subsequent use by the variable argument intrinsics.

Semantics:

The 'llvm.va_start' intrinsic works just like the va_start macro available in C. In a target-dependent way, it initializes and returns a va_list element, so that the next vaarg will produce the first variable argument passed to the function. Unlike the C va_start macro, this intrinsic does not need to know the last argument of the function, the compiler can figure that out.

Note that this intrinsic function is only legal to be called from within the body of a variable argument function.

Syntax:

  call void (<va_list>)* %llvm.va_end(<va_list> <arglist>)

Overview:

The 'llvm.va_end' intrinsic destroys <arglist> which has been initialized previously with llvm.va_start or llvm.va_copy.

Arguments:

The argument is a va_list to destroy.

Semantics:

The 'llvm.va_end' intrinsic works just like the va_end macro available in C. In a target-dependent way, it destroys the va_list. Calls to llvm.va_start and llvm.va_copy must be matched exactly with calls to llvm.va_end.

Syntax:

  call  <va_list> (<va_list>)* %llvm.va_copy(<va_list> <destarglist>)

Overview:

The 'llvm.va_copy' intrinsic copies the current argument position from the source argument list to the destination argument list.

Arguments:

The argument is the va_list to copy.

Semantics:

The 'llvm.va_copy' intrinsic works just like the va_copy macro available in C. In a target-dependent way, it copies the source va_list element into the returned list. This intrinsic is necessary because the llvm.va_start intrinsic may be arbitrarily complex and require memory allocation, for example.

LLVM support for Accurate Garbage Collection requires the implementation and generation of these intrinsics. These intrinsics allow identification of GC roots on the stack, as well as garbage collector implementations that require read and write barriers. Front-ends for type-safe garbage collected languages should generate these intrinsics to make use of the LLVM garbage collectors. For more details, see Accurate Garbage Collection with LLVM.

Syntax:

  call void (<ty>**, <ty2>*)* %llvm.gcroot(<ty>** %ptrloc, <ty2>* %metadata)

Overview:

The 'llvm.gcroot' intrinsic declares the existance of a GC root to the code generator, and allows some metadata to be associated with it.

Arguments:

The first argument specifies the address of a stack object that contains the root pointer. The second pointer (which must be either a constant or a global value address) contains the meta-data to be associated with the root.

Semantics:

At runtime, a call to this intrinsics stores a null pointer into the "ptrloc" location. At compile-time, the code generator generates information to allow the runtime to find the pointer at GC safe points.

Syntax:

  call sbyte* (sbyte**)* %llvm.gcread(sbyte** %Ptr)

Overview:

The 'llvm.gcread' intrinsic identifies reads of references from heap locations, allowing garbage collector implementations that require read barriers.

Arguments:

The argument is the address to read from, which should be an address allocated from the garbage collector.

Semantics:

The 'llvm.gcread' intrinsic has the same semantics as a load instruction, but may be replaced with substantially more complex code by the garbage collector runtime, as needed.

Syntax:

  call void (sbyte*, sbyte**)* %llvm.gcwrite(sbyte* %P1, sbyte** %P2)

Overview:

The 'llvm.gcwrite' intrinsic identifies writes of references to heap locations, allowing garbage collector implementations that require write barriers (such as generational or reference counting collectors).

Arguments:

The first argument is the reference to store, and the second is the heap location to store to.

Semantics:

The 'llvm.gcwrite' intrinsic has the same semantics as a store instruction, but may be replaced with substantially more complex code by the garbage collector runtime, as needed.

These intrinsics are provided by LLVM to expose special features that may only be implemented with code generator support.

Syntax:

  call void* ()* %llvm.returnaddress(uint <level>)

Overview:

The 'llvm.returnaddress' intrinsic returns a target-specific value indicating the return address of the current function or one of its callers.

Arguments:

The argument to this intrinsic indicates which function to return the address for. Zero indicates the calling function, one indicates its caller, etc. The argument is required to be a constant integer value.

Semantics:

The 'llvm.returnaddress' intrinsic either returns a pointer indicating the return address of the specified call frame, or zero if it cannot be identified. The value returned by this intrinsic is likely to be incorrect or 0 for arguments other than zero, so it should only be used for debugging purposes.

Note that calling this intrinsic does not prevent function inlining or other aggressive transformations, so the value returned may not that of the obvious source-language caller.

Syntax:

  call void* ()* %llvm.frameaddress(uint <level>)

Overview:

The 'llvm.frameaddress' intrinsic returns the target-specific frame pointer value for the specified stack frame.

Arguments:

The argument to this intrinsic indicates which function to return the frame pointer for. Zero indicates the calling function, one indicates its caller, etc. The argument is required to be a constant integer value.

Semantics:

The 'llvm.frameaddress' intrinsic either returns a pointer indicating the frame address of the specified call frame, or zero if it cannot be identified. The value returned by this intrinsic is likely to be incorrect or 0 for arguments other than zero, so it should only be used for debugging purposes.

Note that calling this intrinsic does not prevent function inlining or other aggressive transformations, so the value returned may not that of the obvious source-language caller.

These intrinsics are provided by LLVM to support the implementation of operating system level code.

Syntax:

  call <integer type> (<integer type>)* %llvm.readport (<integer type> <address>)

Overview:

The 'llvm.readport' intrinsic reads data from the specified hardware I/O port.

Arguments:

The argument to this intrinsic indicates the hardware I/O address from which to read the data. The address is in the hardware I/O address namespace (as opposed to being a memory location for memory mapped I/O).

Semantics:

The 'llvm.readport' intrinsic reads data from the hardware I/O port specified by address and returns the value. The address and return value must be integers, but the size is dependent upon the platform upon which the program is code generated. For example, on x86, the address must be an unsigned 16 bit value, and the return value must be 8, 16, or 32 bits.

Syntax:

  call void (<integer type>, <integer type>)* %llvm.writeport (<integer type> <value>, <integer type> <address>)

Overview:

The 'llvm.writeport' intrinsic writes data to the specified hardware I/O port.

Arguments:

The first argument is the value to write to the I/O port.

The second argument indicates the hardware I/O address to which data should be written. The address is in the hardware I/O address namespace (as opposed to being a memory location for memory mapped I/O).

Semantics:

The 'llvm.writeport' intrinsic writes value to the I/O port specified by address. The address and value must be integers, but the size is dependent upon the platform upon which the program is code generated. For example, on x86, the address must be an unsigned 16 bit value, and the value written must be 8, 16, or 32 bits in length.

Syntax:

  call <result> (<ty>*)* %llvm.readio (<ty> * <pointer>)

Overview:

The 'llvm.readio' intrinsic reads data from a memory mapped I/O address.

Arguments:

The argument to this intrinsic is a pointer indicating the memory address from which to read the data. The data must be a first class type.

Semantics:

The 'llvm.readio' intrinsic reads data from a memory mapped I/O location specified by pointer and returns the value. The argument must be a pointer, and the return value must be a first class type. However, certain architectures may not support I/O on all first class types. For example, 32 bit processors may only support I/O on data types that are 32 bits or less.

This intrinsic enforces an in-order memory model for llvm.readio and llvm.writeio calls on machines that use dynamic scheduling. Dynamically scheduled processors may execute loads and stores out of order, re-ordering at run time accesses to memory mapped I/O registers. Using these intrinsics ensures that accesses to memory mapped I/O registers occur in program order.

Syntax:

  call void (<ty1>, <ty2>*)* %llvm.writeio (<ty1> <value>, <ty2> * <pointer>)

Overview:

The 'llvm.writeio' intrinsic writes data to the specified memory mapped I/O address.

Arguments:

The first argument is the value to write to the memory mapped I/O location. The second argument is a pointer indicating the memory address to which the data should be written.

Semantics:

The 'llvm.writeio' intrinsic writes value to the memory mapped I/O address specified by pointer. The value must be a first class type. However, certain architectures may not support I/O on all first class types. For example, 32 bit processors may only support I/O on data types that are 32 bits or less.

LLVM provides intrinsics for a few important standard C library functions. These intrinsics allow source-language front-ends to pass information about the alignment of the pointer arguments to the code generator, providing opportunity for more efficient code generation.

Syntax:

  call void (sbyte*, sbyte*, uint, uint)* %llvm.memcpy(sbyte* <dest>, sbyte* <src>,
                                                       uint <len>, uint <align>)

Overview:

The 'llvm.memcpy' intrinsic copies a block of memory from the source location to the destination location.

Note that, unlike the standard libc function, the llvm.memcpy intrinsic does not return a value, and takes an extra alignment argument.

Arguments:

The first argument is a pointer to the destination, the second is a pointer to the source. The third argument is an (arbitrarily sized) integer argument specifying the number of bytes to copy, and the fourth argument is the alignment of the source and destination locations.

If the call to this intrinisic has an alignment value that is not 0 or 1, then the caller guarantees that the size of the copy is a multiple of the alignment and that both the source and destination pointers are aligned to that boundary.

Semantics:

The 'llvm.memcpy' intrinsic copies a block of memory from the source location to the destination location, which are not allowed to overlap. It copies "len" bytes of memory over. If the argument is known to be aligned to some boundary, this can be specified as the fourth argument, otherwise it should be set to 0 or 1.

Syntax:

  call void (sbyte*, sbyte*, uint, uint)* %llvm.memmove(sbyte* <dest>, sbyte* <src>,
                                                       uint <len>, uint <align>)

Overview:

The 'llvm.memmove' intrinsic moves a block of memory from the source location to the destination location. It is similar to the 'llvm.memcpy' intrinsic but allows the two memory locations to overlap.

Note that, unlike the standard libc function, the llvm.memmove intrinsic does not return a value, and takes an extra alignment argument.

Arguments:

Semantics:

The 'llvm.memmove' intrinsic copies a block of memory from the source location to the destination location, which may overlap. It copies "len" bytes of memory over. If the argument is known to be aligned to some boundary, this can be specified as the fourth argument, otherwise it should be set to 0 or 1.

Syntax:

  call void (sbyte*, ubyte, uint, uint)* %llvm.memset(sbyte* <dest>, ubyte <val>,
                                                      uint <len>, uint <align>)

Overview:

The 'llvm.memset' intrinsic fills a block of memory with a particular byte value.

Note that, unlike the standard libc function, the llvm.memset intrinsic does not return a value, and takes an extra alignment argument.

Arguments:

The first argument is a pointer to the destination to fill, the second is the byte value to fill it with, the third argument is an (arbitrarily sized) integer argument specifying the number of bytes to fill, and the fourth argument is the known alignment of destination location.

If the call to this intrinisic has an alignment value that is not 0 or 1, then the caller guarantees that the size of the copy is a multiple of the alignment and that the destination pointer is aligned to that boundary.

Semantics:

The 'llvm.memset' intrinsic fills "len" bytes of memory starting at the destination location. If the argument is known to be aligned to some boundary, this can be specified as the fourth argument, otherwise it should be set to 0 or 1.