-.. _exception_handling:
-
==========================
Exception Handling in LLVM
==========================
A more complete description of the Itanium ABI exception handling runtime
support of can be found at `Itanium C++ ABI: Exception Handling
-<http://www.codesourcery.com/cxx-abi/abi-eh.html>`_. A description of the
+<http://mentorembedded.github.com/cxx-abi/abi-eh.html>`_. A description of the
exception frame format can be found at `Exception Frames
-<http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html>`_,
+<http://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html>`_,
with details of the DWARF 4 specification at `DWARF 4 Standard
<http://dwarfstd.org/Dwarf4Std.php>`_. A description for the C++ exception
table formats can be found at `Exception Handling Tables
-<http://www.codesourcery.com/cxx-abi/exceptions.pdf>`_.
+<http://mentorembedded.github.com/cxx-abi/exceptions.pdf>`_.
Setjmp/Longjmp Exception Handling
---------------------------------
exceptions are, by their nature, intended for uncommon code paths, DWARF
exception handling is generally preferred to SJLJ.
+Windows Runtime Exception Handling
+-----------------------------------
+
+Windows runtime based exception handling uses the same basic IR structure as
+Itanium ABI based exception handling, but it relies on the personality
+functions provided by the native Windows runtime library, ``__CxxFrameHandler3``
+for C++ exceptions: ``__C_specific_handler`` for 64-bit SEH or
+``_frame_handler3/4`` for 32-bit SEH. This results in a very different
+execution model and requires some minor modifications to the initial IR
+representation and a significant restructuring just before code generation.
+
+General information about the Windows x64 exception handling mechanism can be
+found at `MSDN Exception Handling (x64)
+<https://msdn.microsoft.com/en-us/library/1eyas8tf(v=vs.80).aspx>`_.
+
Overview
--------
#. where to continue if the call raises an exception, either by a throw or the
unwinding of a throw
-The term used to define a the place where an ``invoke`` continues after an
+The term used to define the place where an ``invoke`` continues after an
exception is called a *landing pad*. LLVM landing pads are conceptually
alternative function entry points where an exception structure reference and a
type info index are passed in as arguments. The landing pad saves the exception
structure reference and then proceeds to select the catch block that corresponds
to the type info of the exception object.
-The LLVM `landingpad instruction <LangRef.html#i_landingpad>`_ is used to convey
-information about the landing pad to the back end. For C++, the ``landingpad``
-instruction returns a pointer and integer pair corresponding to the pointer to
-the *exception structure* and the *selector value* respectively.
+The LLVM :ref:`i_landingpad` is used to convey information about the landing
+pad to the back end. For C++, the ``landingpad`` instruction returns a pointer
+and integer pair corresponding to the pointer to the *exception structure* and
+the *selector value* respectively.
The ``landingpad`` instruction takes a reference to the personality function to
be used for this ``try``/``catch`` sequence. The remainder of the instruction is
a list of *cleanup*, *catch*, and *filter* clauses. The exception is tested
-against the clauses sequentially from first to last. The selector value is a
-positive number if the exception matched a type info, a negative number if it
-matched a filter, and zero if it matched a cleanup. If nothing is matched, the
-behavior of the program is `undefined`_. If a type info matched, then the
-selector value is the index of the type info in the exception table, which can
-be obtained using the `llvm.eh.typeid.for`_ intrinsic.
+against the clauses sequentially from first to last. The clauses have the
+following meanings:
+
+- ``catch <type> @ExcType``
+
+ - This clause means that the landingpad block should be entered if the
+ exception being thrown is of type ``@ExcType`` or a subtype of
+ ``@ExcType``. For C++, ``@ExcType`` is a pointer to the ``std::type_info``
+ object (an RTTI object) representing the C++ exception type.
+
+ - If ``@ExcType`` is ``null``, any exception matches, so the landingpad
+ should always be entered. This is used for C++ catch-all blocks ("``catch
+ (...)``").
+
+ - When this clause is matched, the selector value will be equal to the value
+ returned by "``@llvm.eh.typeid.for(i8* @ExcType)``". This will always be a
+ positive value.
+
+- ``filter <type> [<type> @ExcType1, ..., <type> @ExcTypeN]``
+
+ - This clause means that the landingpad should be entered if the exception
+ being thrown does *not* match any of the types in the list (which, for C++,
+ are again specified as ``std::type_info`` pointers).
+
+ - C++ front-ends use this to implement C++ exception specifications, such as
+ "``void foo() throw (ExcType1, ..., ExcTypeN) { ... }``".
+
+ - When this clause is matched, the selector value will be negative.
+
+ - The array argument to ``filter`` may be empty; for example, "``[0 x i8**]
+ undef``". This means that the landingpad should always be entered. (Note
+ that such a ``filter`` would not be equivalent to "``catch i8* null``",
+ because ``filter`` and ``catch`` produce negative and positive selector
+ values respectively.)
+
+- ``cleanup``
+
+ - This clause means that the landingpad should always be entered.
+
+ - C++ front-ends use this for calling objects' destructors.
+
+ - When this clause is matched, the selector value will be zero.
+
+ - The runtime may treat "``cleanup``" differently from "``catch <type>
+ null``".
+
+ In C++, if an unhandled exception occurs, the language runtime will call
+ ``std::terminate()``, but it is implementation-defined whether the runtime
+ unwinds the stack and calls object destructors first. For example, the GNU
+ C++ unwinder does not call object destructors when an unhandled exception
+ occurs. The reason for this is to improve debuggability: it ensures that
+ ``std::terminate()`` is called from the context of the ``throw``, so that
+ this context is not lost by unwinding the stack. A runtime will typically
+ implement this by searching for a matching non-``cleanup`` clause, and
+ aborting if it does not find one, before entering any landingpad blocks.
Once the landing pad has the type info selector, the code branches to the code
for the first catch. The catch then checks the value of the type info selector
destructors are a typical example, but other languages and language extensions
provide a variety of different kinds of cleanups. In general, a landing pad may
need to run arbitrary amounts of cleanup code before actually entering a catch
-block. To indicate the presence of cleanups, a `landingpad
-instruction <LangRef.html#i_landingpad>`_ should have a *cleanup*
-clause. Otherwise, the unwinder will not stop at the landing pad if there are no
-catches or filters that require it to.
+block. To indicate the presence of cleanups, a :ref:`i_landingpad` should have
+a *cleanup* clause. Otherwise, the unwinder will not stop at the landing pad if
+there are no catches or filters that require it to.
.. note::
exceptions and throws a third.
When all cleanups are finished, if the exception is not handled by the current
-function, resume unwinding by calling the `resume
-instruction <LangRef.html#i_resume>`_, passing in the result of the
-``landingpad`` instruction for the original landing pad.
+function, resume unwinding by calling the :ref:`resume instruction <i_resume>`,
+passing in the result of the ``landingpad`` instruction for the original
+landing pad.
Throw Filters
-------------
C++ allows the specification of which exception types may be thrown from a
function. To represent this, a top level landing pad may exist to filter out
-invalid types. To express this in LLVM code the `landingpad
-instruction <LangRef.html#i_landingpad>`_ will have a filter clause. The clause
-consists of an array of type infos. ``landingpad`` will return a negative value
+invalid types. To express this in LLVM code the :ref:`i_landingpad` will have a
+filter clause. The clause consists of an array of type infos.
+``landingpad`` will return a negative value
if the exception does not match any of the type infos. If no match is found then
a call to ``__cxa_call_unexpected`` should be made, otherwise
``_Unwind_Resume``. Each of these functions requires a reference to the
the `resume instruction <LangRef.html#i_resume>`_ if none of the conditions
match.
+C++ Exception Handling using the Windows Runtime
+=================================================
+
+(Note: Windows C++ exception handling support is a work in progress and is
+ not yet fully implemented. The text below describes how it will work
+ when completed.)
+
+The Windows runtime function for C++ exception handling uses a multi-phase
+approach. When an exception occurs it searches the current callstack for a
+frame that has a handler for the exception. If a handler is found, it then
+calls the cleanup handler for each frame above the handler which has a
+cleanup handler before calling the catch handler. These calls are all made
+from a stack context different from the original frame in which the handler
+is defined. Therefore, it is necessary to outline these handlers from their
+original context before code generation.
+
+Catch handlers are called with a pointer to the handler itself as the first
+argument and a pointer to the parent function's stack frame as the second
+argument. The catch handler uses the `llvm.recoverframe
+<LangRef.html#llvm-frameallocate-and-llvm-framerecover-intrinsics>`_ to get a
+pointer to a frame allocation block that is created in the parent frame using
+the `llvm.allocateframe
+<LangRef.html#llvm-frameallocate-and-llvm-framerecover-intrinsics>`_ intrinsic.
+The ``WinEHPrepare`` pass will have created a structure definition for the
+contents of this block. The first two members of the structure will always be
+(1) a 32-bit integer that the runtime uses to track the exception state of the
+parent frame for the purposes of handling chained exceptions and (2) a pointer
+to the object associated with the exception (roughly, the parameter of the
+catch clause). These two members will be followed by any frame variables from
+the parent function which must be accessed in any of the functions unwind or
+catch handlers. The catch handler returns the address at which execution
+should continue.
+
+Cleanup handlers perform any cleanup necessary as the frame goes out of scope,
+such as calling object destructors. The runtime handles the actual unwinding
+of the stack. If an exception occurs in a cleanup handler the runtime manages
+termination of the process. Cleanup handlers are called with the same arguments
+as catch handlers (a pointer to the handler and a pointer to the parent stack
+frame) and use the same mechanism described above to access frame variables
+in the parent function. Cleanup handlers do not return a value.
+
+The IR generated for Windows runtime based C++ exception handling is initially
+very similar to the ``landingpad`` mechanism described above. Calls to
+libc++abi functions (such as ``__cxa_begin_catch``/``__cxa_end_catch`` and
+``__cxa_throw_exception`` are replaced with calls to intrinsics or Windows
+runtime functions (such as ``llvm.eh.begincatch``/``llvm.eh.endcatch`` and
+``__CxxThrowException``).
+
+During the WinEHPrepare pass, the handler functions are outlined into handler
+functions and the original landing pad code is replaced with a call to the
+``llvm.eh.actions`` intrinsic that describes the order in which handlers will
+be processed from the logical location of the landing pad and an indirect
+branch to the return value of the ``llvm.eh.actions`` intrinsic. The
+``llvm.eh.actions`` intrinsic is defined as returning the address at which
+execution will continue. This is a temporary construct which will be removed
+before code generation, but it allows for the accurate tracking of control
+flow until then.
+
+A typical landing pad will look like this after outlining:
+
+.. code-block:: llvm
+
+ lpad:
+ %vals = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @__CxxFrameHandler3 to i8*)
+ cleanup
+ catch i8* bitcast (i8** @_ZTIi to i8*)
+ catch i8* bitcast (i8** @_ZTIf to i8*)
+ %recover = call i8* (...)* @llvm.eh.actions(
+ i32 3, i8* bitcast (i8** @_ZTIi to i8*), i8* (i8*, i8*)* @_Z4testb.catch.1)
+ i32 2, i8* null, void (i8*, i8*)* @_Z4testb.cleanup.1)
+ i32 1, i8* bitcast (i8** @_ZTIf to i8*), i8* (i8*, i8*)* @_Z4testb.catch.0)
+ i32 0, i8* null, void (i8*, i8*)* @_Z4testb.cleanup.0)
+ indirectbr i8* %recover, [label %try.cont1, label %try.cont2]
+
+In this example, the landing pad represents an exception handling context with
+two catch handlers and a cleanup handler that have been outlined. If an
+exception is thrown with a type that matches ``_ZTIi``, the ``_Z4testb.catch.1``
+handler will be called an no clean-up is needed. If an exception is thrown
+with a type that matches ``_ZTIf``, first the ``_Z4testb.cleanup.1`` handler
+will be called to perform unwind-related cleanup, then the ``_Z4testb.catch.1``
+handler will be called. If an exception is throw which does not match either
+of these types and the exception is handled by another frame further up the
+call stack, first the ``_Z4testb.cleanup.1`` handler will be called, then the
+``_Z4testb.cleanup.0`` handler (which corresponds to a different scope) will be
+called, and exception handling will continue at the next frame in the call
+stack will be called. One of the catch handlers will return the address of
+``%try.cont1`` in the parent function and the other will return the address of
+``%try.cont2``, meaning that execution continues at one of those blocks after
+an exception is caught.
+
+
Exception Handling Intrinsics
=============================
.. _llvm.eh.typeid.for:
-llvm.eh.typeid.for
-------------------
+``llvm.eh.typeid.for``
+----------------------
.. code-block:: llvm
function. This value can be used to compare against the result of
``landingpad`` instruction. The single argument is a reference to a type info.
+Uses of this intrinsic are generated by the C++ front-end.
+
+.. _llvm.eh.begincatch:
+
+``llvm.eh.begincatch``
+----------------------
+
+.. code-block:: llvm
+
+ void @llvm.eh.begincatch(i8* %ehptr, i8* %ehobj)
+
+
+This intrinsic marks the beginning of catch handling code within the blocks
+following a ``landingpad`` instruction. The exact behavior of this function
+depends on the compilation target and the personality function associated
+with the ``landingpad`` instruction.
+
+The first argument to this intrinsic is a pointer that was previously extracted
+from the aggregate return value of the ``landingpad`` instruction. The second
+argument to the intrinsic is a pointer to stack space where the exception object
+should be stored. The runtime handles the details of copying the exception
+object into the slot. If the second parameter is null, no copy occurs.
+
+Uses of this intrinsic are generated by the C++ front-end. Many targets will
+use implementation-specific functions (such as ``__cxa_begin_catch``) instead
+of this intrinsic. The intrinsic is provided for targets that require a more
+abstract interface.
+
+When used in the native Windows C++ exception handling implementation, this
+intrinsic serves as a placeholder to delimit code before a catch handler is
+outlined. When the handler is is outlined, this intrinsic will be replaced
+by instructions that retrieve the exception object pointer from the frame
+allocation block.
+
+
+.. _llvm.eh.endcatch:
+
+``llvm.eh.endcatch``
+----------------------
+
+.. code-block:: llvm
+
+ void @llvm.eh.endcatch()
+
+
+This intrinsic marks the end of catch handling code within the current block,
+which will be a successor of a block which called ``llvm.eh.begincatch''.
+The exact behavior of this function depends on the compilation target and the
+personality function associated with the corresponding ``landingpad``
+instruction.
+
+There may be more than one call to ``llvm.eh.endcatch`` for any given call to
+``llvm.eh.begincatch`` with each ``llvm.eh.endcatch`` call corresponding to the
+end of a different control path. All control paths following a call to
+``llvm.eh.begincatch`` must reach a call to ``llvm.eh.endcatch``.
+
+Uses of this intrinsic are generated by the C++ front-end. Many targets will
+use implementation-specific functions (such as ``__cxa_begin_catch``) instead
+of this intrinsic. The intrinsic is provided for targets that require a more
+abstract interface.
+
+When used in the native Windows C++ exception handling implementation, this
+intrinsic serves as a placeholder to delimit code before a catch handler is
+outlined. After the handler is outlined, this intrinsic is simply removed.
+
+.. _llvm.eh.actions:
+
+``llvm.eh.actions``
+----------------------
+
+.. code-block:: llvm
+
+ void @llvm.eh.actions()
+
+This intrinsic represents the list of actions to take when an exception is
+thrown. It is typically used by Windows exception handling schemes where cleanup
+outlining is required by the runtime. The arguments are a sequence of ``i32``
+sentinels indicating the action type followed by some pre-determined number of
+arguments required to implement that action.
+
+A code of ``i32 0`` indicates a cleanup action, which expects one additional
+argument. The argument is a pointer to a function that implements the cleanup
+action.
+
+A code of ``i32 1`` indicates a catch action, which expects three additional
+arguments. Different EH schemes give different meanings to the three arguments,
+but the first argument indicates whether the catch should fire, the second is
+the frameescape index of the exception object, and the third is the code to run
+to catch the exception.
+
+For Windows C++ exception handling, the first argument for a catch handler is a
+pointer to the RTTI type descriptor for the object to catch. The second
+argument is an index into the argument list of the ``llvm.frameescape`` call in
+the main function. The exception object will be copied into the provided stack
+object. If the exception object is not required, this argument should be -1.
+The third argument is a pointer to a function implementing the catch. This
+function returns the address of the basic block where execution should resume
+after handling the exception.
+
+For Windows SEH, the first argument is a pointer to the filter function, which
+indicates if the exception should be caught or not. The second argument is
+typically negative one. The third argument is the address of a basic block
+where the exception will be handled. In other words, catch handlers are not
+outlined in SEH. After running cleanups, execution immediately resumes at this
+PC.
+
+In order to preserve the structure of the CFG, a call to '``llvm.eh.actions``'
+must be followed by an ':ref:`indirectbr <i_indirectbr>`' instruction that
+jumps to the result of the intrinsic call.
+
+
+SJLJ Intrinsics
+---------------
+
+The ``llvm.eh.sjlj`` intrinsics are used internally within LLVM's
+backend. Uses of them are generated by the backend's
+``SjLjEHPrepare`` pass.
+
.. _llvm.eh.sjlj.setjmp:
-llvm.eh.sjlj.setjmp
--------------------
+``llvm.eh.sjlj.setjmp``
+~~~~~~~~~~~~~~~~~~~~~~~
.. code-block:: llvm
.. _llvm.eh.sjlj.longjmp:
-llvm.eh.sjlj.longjmp
---------------------
+``llvm.eh.sjlj.longjmp``
+~~~~~~~~~~~~~~~~~~~~~~~~
.. code-block:: llvm
pointer are restored from the buffer, then control is transferred to the
destination address.
-llvm.eh.sjlj.lsda
------------------
+``llvm.eh.sjlj.lsda``
+~~~~~~~~~~~~~~~~~~~~~
.. code-block:: llvm
function. The SJLJ front-end code stores this address in the exception handling
function context for use by the runtime.
-llvm.eh.sjlj.callsite
----------------------
+``llvm.eh.sjlj.callsite``
+~~~~~~~~~~~~~~~~~~~~~~~~~
.. code-block:: llvm
projects / oota-llvm.git / blobdiff