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ReStructuredText
==========================
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Exception Handling in LLVM
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==========================
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.. contents::
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:local:
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Introduction
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============
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This document is the central repository for all information pertaining to
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exception handling in LLVM. It describes the format that LLVM exception
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handling information takes, which is useful for those interested in creating
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front-ends or dealing directly with the information. Further, this document
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provides specific examples of what exception handling information is used for in
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C and C++.
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Itanium ABI Zero-cost Exception Handling
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----------------------------------------
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Exception handling for most programming languages is designed to recover from
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conditions that rarely occur during general use of an application. To that end,
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exception handling should not interfere with the main flow of an application's
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algorithm by performing checkpointing tasks, such as saving the current pc or
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register state.
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The Itanium ABI Exception Handling Specification defines a methodology for
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providing outlying data in the form of exception tables without inlining
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speculative exception handling code in the flow of an application's main
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algorithm. Thus, the specification is said to add "zero-cost" to the normal
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execution of an application.
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A more complete description of the Itanium ABI exception handling runtime
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support of can be found at `Itanium C++ ABI: Exception Handling
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<http://mentorembedded.github.com/cxx-abi/abi-eh.html>`_. A description of the
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exception frame format can be found at `Exception Frames
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<http://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html>`_,
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with details of the DWARF 4 specification at `DWARF 4 Standard
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<http://dwarfstd.org/Dwarf4Std.php>`_. A description for the C++ exception
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table formats can be found at `Exception Handling Tables
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<http://mentorembedded.github.com/cxx-abi/exceptions.pdf>`_.
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Setjmp/Longjmp Exception Handling
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---------------------------------
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Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
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`llvm.eh.sjlj.setjmp`_ and `llvm.eh.sjlj.longjmp`_ to handle control flow for
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exception handling.
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For each function which does exception processing --- be it ``try``/``catch``
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blocks or cleanups --- that function registers itself on a global frame
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list. When exceptions are unwinding, the runtime uses this list to identify
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which functions need processing.
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Landing pad selection is encoded in the call site entry of the function
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context. The runtime returns to the function via `llvm.eh.sjlj.longjmp`_, where
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a switch table transfers control to the appropriate landing pad based on the
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index stored in the function context.
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In contrast to DWARF exception handling, which encodes exception regions and
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frame information in out-of-line tables, SJLJ exception handling builds and
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removes the unwind frame context at runtime. This results in faster exception
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handling at the expense of slower execution when no exceptions are thrown. As
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exceptions are, by their nature, intended for uncommon code paths, DWARF
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exception handling is generally preferred to SJLJ.
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Windows Runtime Exception Handling
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-----------------------------------
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Windows runtime based exception handling uses the same basic IR structure as
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Itanium ABI based exception handling, but it relies on the personality
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functions provided by the native Windows runtime library, ``__CxxFrameHandler3``
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for C++ exceptions: ``__C_specific_handler`` for 64-bit SEH or
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``_frame_handler3/4`` for 32-bit SEH. This results in a very different
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execution model and requires some minor modifications to the initial IR
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representation and a significant restructuring just before code generation.
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General information about the Windows x64 exception handling mechanism can be
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found at `MSDN Exception Handling (x64)
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<https://msdn.microsoft.com/en-us/library/1eyas8tf(v=vs.80).aspx>`_.
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Overview
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--------
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When an exception is thrown in LLVM code, the runtime does its best to find a
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handler suited to processing the circumstance.
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The runtime first attempts to find an *exception frame* corresponding to the
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function where the exception was thrown. If the programming language supports
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exception handling (e.g. C++), the exception frame contains a reference to an
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exception table describing how to process the exception. If the language does
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not support exception handling (e.g. C), or if the exception needs to be
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forwarded to a prior activation, the exception frame contains information about
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how to unwind the current activation and restore the state of the prior
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activation. This process is repeated until the exception is handled. If the
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exception is not handled and no activations remain, then the application is
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terminated with an appropriate error message.
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Because different programming languages have different behaviors when handling
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exceptions, the exception handling ABI provides a mechanism for
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supplying *personalities*. An exception handling personality is defined by
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way of a *personality function* (e.g. ``__gxx_personality_v0`` in C++),
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which receives the context of the exception, an *exception structure*
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containing the exception object type and value, and a reference to the exception
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table for the current function. The personality function for the current
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compile unit is specified in a *common exception frame*.
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The organization of an exception table is language dependent. For C++, an
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exception table is organized as a series of code ranges defining what to do if
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an exception occurs in that range. Typically, the information associated with a
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range defines which types of exception objects (using C++ *type info*) that are
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handled in that range, and an associated action that should take place. Actions
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typically pass control to a *landing pad*.
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A landing pad corresponds roughly to the code found in the ``catch`` portion of
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a ``try``/``catch`` sequence. When execution resumes at a landing pad, it
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receives an *exception structure* and a *selector value* corresponding to the
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*type* of exception thrown. The selector is then used to determine which *catch*
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should actually process the exception.
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LLVM Code Generation
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====================
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From a C++ developer's perspective, exceptions are defined in terms of the
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``throw`` and ``try``/``catch`` statements. In this section we will describe the
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implementation of LLVM exception handling in terms of C++ examples.
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Throw
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-----
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Languages that support exception handling typically provide a ``throw``
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operation to initiate the exception process. Internally, a ``throw`` operation
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breaks down into two steps.
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#. A request is made to allocate exception space for an exception structure.
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This structure needs to survive beyond the current activation. This structure
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will contain the type and value of the object being thrown.
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#. A call is made to the runtime to raise the exception, passing the exception
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structure as an argument.
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In C++, the allocation of the exception structure is done by the
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``__cxa_allocate_exception`` runtime function. The exception raising is handled
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by ``__cxa_throw``. The type of the exception is represented using a C++ RTTI
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structure.
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Try/Catch
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---------
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A call within the scope of a *try* statement can potentially raise an
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exception. In those circumstances, the LLVM C++ front-end replaces the call with
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an ``invoke`` instruction. Unlike a call, the ``invoke`` has two potential
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continuation points:
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#. where to continue when the call succeeds as per normal, and
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#. where to continue if the call raises an exception, either by a throw or the
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unwinding of a throw
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The term used to define the place where an ``invoke`` continues after an
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exception is called a *landing pad*. LLVM landing pads are conceptually
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alternative function entry points where an exception structure reference and a
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type info index are passed in as arguments. The landing pad saves the exception
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structure reference and then proceeds to select the catch block that corresponds
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to the type info of the exception object.
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The LLVM :ref:`i_landingpad` is used to convey information about the landing
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pad to the back end. For C++, the ``landingpad`` instruction returns a pointer
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and integer pair corresponding to the pointer to the *exception structure* and
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the *selector value* respectively.
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The ``landingpad`` instruction takes a reference to the personality function to
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be used for this ``try``/``catch`` sequence. The remainder of the instruction is
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a list of *cleanup*, *catch*, and *filter* clauses. The exception is tested
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against the clauses sequentially from first to last. The clauses have the
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following meanings:
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- ``catch <type> @ExcType``
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- This clause means that the landingpad block should be entered if the
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exception being thrown is of type ``@ExcType`` or a subtype of
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``@ExcType``. For C++, ``@ExcType`` is a pointer to the ``std::type_info``
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object (an RTTI object) representing the C++ exception type.
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- If ``@ExcType`` is ``null``, any exception matches, so the landingpad
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should always be entered. This is used for C++ catch-all blocks ("``catch
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(...)``").
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- When this clause is matched, the selector value will be equal to the value
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returned by "``@llvm.eh.typeid.for(i8* @ExcType)``". This will always be a
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positive value.
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- ``filter <type> [<type> @ExcType1, ..., <type> @ExcTypeN]``
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- This clause means that the landingpad should be entered if the exception
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being thrown does *not* match any of the types in the list (which, for C++,
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are again specified as ``std::type_info`` pointers).
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- C++ front-ends use this to implement C++ exception specifications, such as
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"``void foo() throw (ExcType1, ..., ExcTypeN) { ... }``".
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- When this clause is matched, the selector value will be negative.
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- The array argument to ``filter`` may be empty; for example, "``[0 x i8**]
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undef``". This means that the landingpad should always be entered. (Note
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that such a ``filter`` would not be equivalent to "``catch i8* null``",
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because ``filter`` and ``catch`` produce negative and positive selector
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values respectively.)
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- ``cleanup``
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- This clause means that the landingpad should always be entered.
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- C++ front-ends use this for calling objects' destructors.
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- When this clause is matched, the selector value will be zero.
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- The runtime may treat "``cleanup``" differently from "``catch <type>
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null``".
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In C++, if an unhandled exception occurs, the language runtime will call
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``std::terminate()``, but it is implementation-defined whether the runtime
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unwinds the stack and calls object destructors first. For example, the GNU
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C++ unwinder does not call object destructors when an unhandled exception
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occurs. The reason for this is to improve debuggability: it ensures that
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``std::terminate()`` is called from the context of the ``throw``, so that
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this context is not lost by unwinding the stack. A runtime will typically
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implement this by searching for a matching non-``cleanup`` clause, and
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aborting if it does not find one, before entering any landingpad blocks.
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Once the landing pad has the type info selector, the code branches to the code
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for the first catch. The catch then checks the value of the type info selector
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against the index of type info for that catch. Since the type info index is not
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known until all the type infos have been gathered in the backend, the catch code
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must call the `llvm.eh.typeid.for`_ intrinsic to determine the index for a given
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type info. If the catch fails to match the selector then control is passed on to
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the next catch.
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Finally, the entry and exit of catch code is bracketed with calls to
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``__cxa_begin_catch`` and ``__cxa_end_catch``.
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* ``__cxa_begin_catch`` takes an exception structure reference as an argument
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and returns the value of the exception object.
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* ``__cxa_end_catch`` takes no arguments. This function:
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#. Locates the most recently caught exception and decrements its handler
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count,
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#. Removes the exception from the *caught* stack if the handler count goes to
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zero, and
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#. Destroys the exception if the handler count goes to zero and the exception
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was not re-thrown by throw.
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.. note::
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a rethrow from within the catch may replace this call with a
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``__cxa_rethrow``.
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Cleanups
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--------
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A cleanup is extra code which needs to be run as part of unwinding a scope. C++
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destructors are a typical example, but other languages and language extensions
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provide a variety of different kinds of cleanups. In general, a landing pad may
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need to run arbitrary amounts of cleanup code before actually entering a catch
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block. To indicate the presence of cleanups, a :ref:`i_landingpad` should have
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a *cleanup* clause. Otherwise, the unwinder will not stop at the landing pad if
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there are no catches or filters that require it to.
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.. note::
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Do not allow a new exception to propagate out of the execution of a
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cleanup. This can corrupt the internal state of the unwinder. Different
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languages describe different high-level semantics for these situations: for
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example, C++ requires that the process be terminated, whereas Ada cancels both
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exceptions and throws a third.
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When all cleanups are finished, if the exception is not handled by the current
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function, resume unwinding by calling the :ref:`resume instruction <i_resume>`,
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passing in the result of the ``landingpad`` instruction for the original
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landing pad.
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Throw Filters
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-------------
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C++ allows the specification of which exception types may be thrown from a
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function. To represent this, a top level landing pad may exist to filter out
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invalid types. To express this in LLVM code the :ref:`i_landingpad` will have a
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filter clause. The clause consists of an array of type infos.
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``landingpad`` will return a negative value
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if the exception does not match any of the type infos. If no match is found then
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a call to ``__cxa_call_unexpected`` should be made, otherwise
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``_Unwind_Resume``. Each of these functions requires a reference to the
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exception structure. Note that the most general form of a ``landingpad``
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instruction can have any number of catch, cleanup, and filter clauses (though
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having more than one cleanup is pointless). The LLVM C++ front-end can generate
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such ``landingpad`` instructions due to inlining creating nested exception
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handling scopes.
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.. _undefined:
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Restrictions
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------------
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The unwinder delegates the decision of whether to stop in a call frame to that
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call frame's language-specific personality function. Not all unwinders guarantee
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that they will stop to perform cleanups. For example, the GNU C++ unwinder
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doesn't do so unless the exception is actually caught somewhere further up the
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stack.
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In order for inlining to behave correctly, landing pads must be prepared to
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handle selector results that they did not originally advertise. Suppose that a
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function catches exceptions of type ``A``, and it's inlined into a function that
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catches exceptions of type ``B``. The inliner will update the ``landingpad``
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instruction for the inlined landing pad to include the fact that ``B`` is also
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caught. If that landing pad assumes that it will only be entered to catch an
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``A``, it's in for a rude awakening. Consequently, landing pads must test for
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the selector results they understand and then resume exception propagation with
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the `resume instruction <LangRef.html#i_resume>`_ if none of the conditions
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match.
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C++ Exception Handling using the Windows Runtime
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=================================================
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(Note: Windows C++ exception handling support is a work in progress and is
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not yet fully implemented. The text below describes how it will work
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when completed.)
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The Windows runtime function for C++ exception handling uses a multi-phase
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approach. When an exception occurs it searches the current callstack for a
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frame that has a handler for the exception. If a handler is found, it then
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calls the cleanup handler for each frame above the handler which has a
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cleanup handler before calling the catch handler. These calls are all made
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from a stack context different from the original frame in which the handler
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is defined. Therefore, it is necessary to outline these handlers from their
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original context before code generation.
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Catch handlers are called with a pointer to the handler itself as the first
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argument and a pointer to the parent function's stack frame as the second
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argument. The catch handler uses the `llvm.localrecover
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<LangRef.html#llvm-localescape-and-llvm-localrecover-intrinsics>`_ to get a
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pointer to a frame allocation block that is created in the parent frame using
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the `llvm.localescape
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<LangRef.html#llvm-localescape-and-llvm-localrecover-intrinsics>`_ intrinsic.
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The ``WinEHPrepare`` pass will have created a structure definition for the
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contents of this block. The first two members of the structure will always be
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(1) a 32-bit integer that the runtime uses to track the exception state of the
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parent frame for the purposes of handling chained exceptions and (2) a pointer
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to the object associated with the exception (roughly, the parameter of the
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catch clause). These two members will be followed by any frame variables from
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the parent function which must be accessed in any of the functions unwind or
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catch handlers. The catch handler returns the address at which execution
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should continue.
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Cleanup handlers perform any cleanup necessary as the frame goes out of scope,
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such as calling object destructors. The runtime handles the actual unwinding
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of the stack. If an exception occurs in a cleanup handler the runtime manages
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termination of the process. Cleanup handlers are called with the same arguments
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as catch handlers (a pointer to the handler and a pointer to the parent stack
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frame) and use the same mechanism described above to access frame variables
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in the parent function. Cleanup handlers do not return a value.
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The IR generated for Windows runtime based C++ exception handling is initially
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very similar to the ``landingpad`` mechanism described above. Calls to
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libc++abi functions (such as ``__cxa_begin_catch``/``__cxa_end_catch`` and
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``__cxa_throw_exception`` are replaced with calls to intrinsics or Windows
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runtime functions (such as ``llvm.eh.begincatch``/``llvm.eh.endcatch`` and
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``__CxxThrowException``).
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During the WinEHPrepare pass, the handler functions are outlined into handler
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functions and the original landing pad code is replaced with a call to the
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``llvm.eh.actions`` intrinsic that describes the order in which handlers will
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be processed from the logical location of the landing pad and an indirect
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branch to the return value of the ``llvm.eh.actions`` intrinsic. The
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``llvm.eh.actions`` intrinsic is defined as returning the address at which
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execution will continue. This is a temporary construct which will be removed
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before code generation, but it allows for the accurate tracking of control
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flow until then.
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A typical landing pad will look like this after outlining:
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.. code-block:: llvm
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lpad:
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%vals = landingpad { i8*, i32 } personality i8* bitcast (i32 (...)* @__CxxFrameHandler3 to i8*)
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cleanup
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catch i8* bitcast (i8** @_ZTIi to i8*)
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catch i8* bitcast (i8** @_ZTIf to i8*)
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%recover = call i8* (...)* @llvm.eh.actions(
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i32 3, i8* bitcast (i8** @_ZTIi to i8*), i8* (i8*, i8*)* @_Z4testb.catch.1)
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i32 2, i8* null, void (i8*, i8*)* @_Z4testb.cleanup.1)
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i32 1, i8* bitcast (i8** @_ZTIf to i8*), i8* (i8*, i8*)* @_Z4testb.catch.0)
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i32 0, i8* null, void (i8*, i8*)* @_Z4testb.cleanup.0)
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indirectbr i8* %recover, [label %try.cont1, label %try.cont2]
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In this example, the landing pad represents an exception handling context with
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two catch handlers and a cleanup handler that have been outlined. If an
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exception is thrown with a type that matches ``_ZTIi``, the ``_Z4testb.catch.1``
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handler will be called an no clean-up is needed. If an exception is thrown
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with a type that matches ``_ZTIf``, first the ``_Z4testb.cleanup.1`` handler
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will be called to perform unwind-related cleanup, then the ``_Z4testb.catch.1``
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handler will be called. If an exception is throw which does not match either
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of these types and the exception is handled by another frame further up the
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call stack, first the ``_Z4testb.cleanup.1`` handler will be called, then the
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``_Z4testb.cleanup.0`` handler (which corresponds to a different scope) will be
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called, and exception handling will continue at the next frame in the call
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stack will be called. One of the catch handlers will return the address of
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``%try.cont1`` in the parent function and the other will return the address of
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``%try.cont2``, meaning that execution continues at one of those blocks after
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an exception is caught.
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Exception Handling Intrinsics
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=============================
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In addition to the ``landingpad`` and ``resume`` instructions, LLVM uses several
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intrinsic functions (name prefixed with ``llvm.eh``) to provide exception
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handling information at various points in generated code.
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.. _llvm.eh.typeid.for:
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``llvm.eh.typeid.for``
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----------------------
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.. code-block:: llvm
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i32 @llvm.eh.typeid.for(i8* %type_info)
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This intrinsic returns the type info index in the exception table of the current
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function. This value can be used to compare against the result of
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``landingpad`` instruction. The single argument is a reference to a type info.
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Uses of this intrinsic are generated by the C++ front-end.
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.. _llvm.eh.begincatch:
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``llvm.eh.begincatch``
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----------------------
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.. code-block:: llvm
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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 localescape 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.localescape`` 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``
|
|
~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. code-block:: llvm
|
|
|
|
i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf)
|
|
|
|
For SJLJ based exception handling, this intrinsic forces register saving for the
|
|
current function and stores the address of the following instruction for use as
|
|
a destination address by `llvm.eh.sjlj.longjmp`_. The buffer format and the
|
|
overall functioning of this intrinsic is compatible with the GCC
|
|
``__builtin_setjmp`` implementation allowing code built with the clang and GCC
|
|
to interoperate.
|
|
|
|
The single parameter is a pointer to a five word buffer in which the calling
|
|
context is saved. The front end places the frame pointer in the first word, and
|
|
the target implementation of this intrinsic should place the destination address
|
|
for a `llvm.eh.sjlj.longjmp`_ in the second word. The following three words are
|
|
available for use in a target-specific manner.
|
|
|
|
.. _llvm.eh.sjlj.longjmp:
|
|
|
|
``llvm.eh.sjlj.longjmp``
|
|
~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. code-block:: llvm
|
|
|
|
void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf)
|
|
|
|
For SJLJ based exception handling, the ``llvm.eh.sjlj.longjmp`` intrinsic is
|
|
used to implement ``__builtin_longjmp()``. The single parameter is a pointer to
|
|
a buffer populated by `llvm.eh.sjlj.setjmp`_. The frame pointer and stack
|
|
pointer are restored from the buffer, then control is transferred to the
|
|
destination address.
|
|
|
|
``llvm.eh.sjlj.lsda``
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. code-block:: llvm
|
|
|
|
i8* @llvm.eh.sjlj.lsda()
|
|
|
|
For SJLJ based exception handling, the ``llvm.eh.sjlj.lsda`` intrinsic returns
|
|
the address of the Language Specific Data Area (LSDA) for the current
|
|
function. The SJLJ front-end code stores this address in the exception handling
|
|
function context for use by the runtime.
|
|
|
|
``llvm.eh.sjlj.callsite``
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
.. code-block:: llvm
|
|
|
|
void @llvm.eh.sjlj.callsite(i32 %call_site_num)
|
|
|
|
For SJLJ based exception handling, the ``llvm.eh.sjlj.callsite`` intrinsic
|
|
identifies the callsite value associated with the following ``invoke``
|
|
instruction. This is used to ensure that landing pad entries in the LSDA are
|
|
generated in matching order.
|
|
|
|
Asm Table Formats
|
|
=================
|
|
|
|
There are two tables that are used by the exception handling runtime to
|
|
determine which actions should be taken when an exception is thrown.
|
|
|
|
Exception Handling Frame
|
|
------------------------
|
|
|
|
An exception handling frame ``eh_frame`` is very similar to the unwind frame
|
|
used by DWARF debug info. The frame contains all the information necessary to
|
|
tear down the current frame and restore the state of the prior frame. There is
|
|
an exception handling frame for each function in a compile unit, plus a common
|
|
exception handling frame that defines information common to all functions in the
|
|
unit.
|
|
|
|
Exception Tables
|
|
----------------
|
|
|
|
An exception table contains information about what actions to take when an
|
|
exception is thrown in a particular part of a function's code. There is one
|
|
exception table per function, except leaf functions and functions that have
|
|
calls only to non-throwing functions. They do not need an exception table.
|