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365 lines
16 KiB
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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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 a 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 selector value is a
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positive number if the exception matched a type info, a negative number if it
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matched a filter, and zero if it matched a cleanup. If nothing is matched, the
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behavior of the program is `undefined`_. If a type info matched, then the
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selector value is the index of the type info in the exception table, which can
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be obtained using the `llvm.eh.typeid.for`_ intrinsic.
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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 `resume
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instruction <LangRef.html#i_resume>`_, passing in the result of the
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``landingpad`` instruction for the original 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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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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.. _llvm.eh.sjlj.setjmp:
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``llvm.eh.sjlj.setjmp``
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-----------------------
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.. code-block:: llvm
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i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf)
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For SJLJ based exception handling, this intrinsic forces register saving for the
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current function and stores the address of the following instruction for use as
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a destination address by `llvm.eh.sjlj.longjmp`_. The buffer format and the
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overall functioning of this intrinsic is compatible with the GCC
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``__builtin_setjmp`` implementation allowing code built with the clang and GCC
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to interoperate.
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The single parameter is a pointer to a five word buffer in which the calling
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context is saved. The front end places the frame pointer in the first word, and
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the target implementation of this intrinsic should place the destination address
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for a `llvm.eh.sjlj.longjmp`_ in the second word. The following three words are
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available for use in a target-specific manner.
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.. _llvm.eh.sjlj.longjmp:
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``llvm.eh.sjlj.longjmp``
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------------------------
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.. code-block:: llvm
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void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf)
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For SJLJ based exception handling, the ``llvm.eh.sjlj.longjmp`` intrinsic is
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used to implement ``__builtin_longjmp()``. The single parameter is a pointer to
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a buffer populated by `llvm.eh.sjlj.setjmp`_. The frame pointer and stack
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pointer are restored from the buffer, then control is transferred to the
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destination address.
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``llvm.eh.sjlj.lsda``
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---------------------
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.. code-block:: llvm
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i8* @llvm.eh.sjlj.lsda()
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For SJLJ based exception handling, the ``llvm.eh.sjlj.lsda`` intrinsic returns
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the address of the Language Specific Data Area (LSDA) for the current
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function. The SJLJ front-end code stores this address in the exception handling
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function context for use by the runtime.
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``llvm.eh.sjlj.callsite``
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-------------------------
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.. code-block:: llvm
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void @llvm.eh.sjlj.callsite(i32 %call_site_num)
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For SJLJ based exception handling, the ``llvm.eh.sjlj.callsite`` intrinsic
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identifies the callsite value associated with the following ``invoke``
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instruction. This is used to ensure that landing pad entries in the LSDA are
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generated in matching order.
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Asm Table Formats
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=================
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There are two tables that are used by the exception handling runtime to
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determine which actions should be taken when an exception is thrown.
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Exception Handling Frame
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------------------------
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An exception handling frame ``eh_frame`` is very similar to the unwind frame
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used by DWARF debug info. The frame contains all the information necessary to
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tear down the current frame and restore the state of the prior frame. There is
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an exception handling frame for each function in a compile unit, plus a common
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exception handling frame that defines information common to all functions in the
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unit.
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Exception Tables
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----------------
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An exception table contains information about what actions to take when an
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exception is thrown in a particular part of a function's code. There is one
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exception table per function, except leaf functions and functions that have
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calls only to non-throwing functions. They do not need an exception table.
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