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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>Exception Handling in LLVM</title>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<meta name="description"
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content="Exception Handling in LLVM.">
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<h1>Exception Handling in LLVM</h1>
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<table class="layout" style="width:100%">
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<tr class="layout">
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<td class="left">
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<ul>
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<li><a href="#introduction">Introduction</a>
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<ol>
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<li><a href="#itanium">Itanium ABI Zero-cost Exception Handling</a></li>
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<li><a href="#sjlj">Setjmp/Longjmp Exception Handling</a></li>
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<li><a href="#overview">Overview</a></li>
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</ol></li>
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<li><a href="#codegen">LLVM Code Generation</a>
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<ol>
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<li><a href="#throw">Throw</a></li>
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<li><a href="#try_catch">Try/Catch</a></li>
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<li><a href="#cleanups">Cleanups</a></li>
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<li><a href="#throw_filters">Throw Filters</a></li>
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<li><a href="#restrictions">Restrictions</a></li>
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</ol></li>
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<li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>
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<ol>
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<li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li>
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<li><a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a></li>
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<li><a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a></li>
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<li><a href="#llvm_eh_sjlj_lsda"><tt>llvm.eh.sjlj.lsda</tt></a></li>
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<li><a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a></li>
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</ol></li>
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<li><a href="#asm">Asm Table Formats</a>
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<ol>
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<li><a href="#unwind_tables">Exception Handling Frame</a></li>
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<li><a href="#exception_tables">Exception Tables</a></li>
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</ol></li>
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</ul>
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</td>
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</tr></table>
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<div class="doc_author">
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<p>Written by the <a href="http://llvm.org/">LLVM Team</a></p>
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</div>
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<!-- *********************************************************************** -->
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<h2><a name="introduction">Introduction</a></h2>
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<!-- *********************************************************************** -->
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<div>
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<p>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
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in C and C++.</p>
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<!-- ======================================================================= -->
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<h3>
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<a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
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</h3>
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<div>
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<p>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
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end, exception handling should not interfere with the main flow of an
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application's algorithm by performing checkpointing tasks, such as saving the
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current pc or register state.</p>
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<p>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.</p>
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<p>A more complete description of the Itanium ABI exception handling runtime
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support of can be found at
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<a href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI:
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Exception Handling</a>. A description of the exception frame format can be
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found at
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<a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception
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Frames</a>, with details of the DWARF 4 specification at
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<a href="http://dwarfstd.org/Dwarf4Std.php">DWARF 4 Standard</a>.
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A description for the C++ exception table formats can be found at
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<a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
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Tables</a>.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="sjlj">Setjmp/Longjmp Exception Handling</a>
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</h3>
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<div>
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<p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
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<a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and
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<a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to
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handle control flow for exception handling.</p>
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<p>For each function which does exception processing — be
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it <tt>try</tt>/<tt>catch</tt> blocks or cleanups — that function
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registers itself on a global frame list. When exceptions are unwinding, the
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runtime uses this list to identify which functions need processing.<p>
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<p>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
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<a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where
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a switch table transfers control to the appropriate landing pad based on
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the index stored in the function context.</p>
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<p>In contrast to DWARF exception handling, which encodes exception regions
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and frame information in out-of-line tables, SJLJ exception handling
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builds and removes the unwind frame context at runtime. This results in
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faster exception handling at the expense of slower execution when no
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exceptions are thrown. As exceptions are, by their nature, intended for
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uncommon code paths, DWARF exception handling is generally preferred to
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SJLJ.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="overview">Overview</a>
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</h3>
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<div>
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<p>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.</p>
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<p>The runtime first attempts to find an <i>exception frame</i> corresponding to
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the function where the exception was thrown. If the programming language
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supports exception handling (e.g. C++), the exception frame contains a
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reference to an exception table describing how to process the exception. If
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the language does not support exception handling (e.g. C), or if the
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exception needs to be forwarded to a prior activation, the exception frame
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contains information about how to unwind the current activation and restore
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the state of the prior activation. This process is repeated until the
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exception is handled. If the exception is not handled and no activations
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remain, then the application is terminated with an appropriate error
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message.</p>
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<p>Because different programming languages have different behaviors when
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handling exceptions, the exception handling ABI provides a mechanism for
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supplying <i>personalities</i>. An exception handling personality is defined
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by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt>
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in C++), which receives the context of the exception, an <i>exception
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structure</i> containing the exception object type and value, and a reference
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to the exception table for the current function. The personality function
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for the current compile unit is specified in a <i>common exception
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frame</i>.</p>
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<p>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
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if an exception occurs in that range. Typically, the information associated
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with a range defines which types of exception objects (using C++ <i>type
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info</i>) that are handled in that range, and an associated action that
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should take place. Actions typically pass control to a <i>landing
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pad</i>.</p>
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<p>A landing pad corresponds roughly to the code found in the <tt>catch</tt>
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portion of a <tt>try</tt>/<tt>catch</tt> sequence. When execution resumes at
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a landing pad, it receives an <i>exception structure</i> and a
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<i>selector value</i> corresponding to the <i>type</i> of exception
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thrown. The selector is then used to determine which <i>catch</i> should
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actually process the exception.</p>
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</div>
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</div>
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<!-- ======================================================================= -->
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<h2>
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<a name="codegen">LLVM Code Generation</a>
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</h2>
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<div>
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<p>From a C++ developer's perspective, exceptions are defined in terms of the
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<tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section
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we will describe the implementation of LLVM exception handling in terms of
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C++ examples.</p>
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<!-- ======================================================================= -->
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<h3>
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<a name="throw">Throw</a>
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</h3>
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<div>
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<p>Languages that support exception handling typically provide a <tt>throw</tt>
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operation to initiate the exception process. Internally, a <tt>throw</tt>
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operation breaks down into two steps.</p>
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<ol>
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<li>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
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structure will contain the type and value of the object being thrown.</li>
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<li>A call is made to the runtime to raise the exception, passing the
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exception structure as an argument.</li>
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</ol>
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<p>In C++, the allocation of the exception structure is done by the
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<tt>__cxa_allocate_exception</tt> runtime function. The exception raising is
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handled by <tt>__cxa_throw</tt>. The type of the exception is represented
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using a C++ RTTI structure.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="try_catch">Try/Catch</a>
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</h3>
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<div>
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<p>A call within the scope of a <i>try</i> statement can potentially raise an
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exception. In those circumstances, the LLVM C++ front-end replaces the call
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with an <tt>invoke</tt> instruction. Unlike a call, the <tt>invoke</tt> has
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two potential continuation points:</p>
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<ol>
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<li>where to continue when the call succeeds as per normal, and</li>
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<li>where to continue if the call raises an exception, either by a throw or
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the unwinding of a throw</li>
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</ol>
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<p>The term used to define a the place where an <tt>invoke</tt> continues after
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an exception is called a <i>landing pad</i>. LLVM landing pads are
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conceptually alternative function entry points where an exception structure
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reference and a type info index are passed in as arguments. The landing pad
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saves the exception structure reference and then proceeds to select the catch
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block that corresponds to the type info of the exception object.</p>
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<p>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
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instruction</a> is used to convey information about the landing pad to the
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back end. For C++, the <tt>landingpad</tt> instruction returns a pointer and
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integer pair corresponding to the pointer to the <i>exception structure</i>
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and the <i>selector value</i> respectively.</p>
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<p>The <tt>landingpad</tt> instruction takes a reference to the personality
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function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The
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remainder of the instruction is a list of <i>cleanup</i>, <i>catch</i>,
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and <i>filter</i> clauses. The exception is tested against the clauses
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sequentially from first to last. The selector value is a positive number if
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the exception matched a type info, a negative number if it matched a filter,
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and zero if it matched a cleanup. If nothing is matched, the behavior of
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the program is <a href="#restrictions">undefined</a>. If a type info matched,
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then the selector value is the index of the type info in the exception table,
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which can be obtained using the
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<a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
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<p>Once the landing pad has the type info selector, the code branches to the
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code for the first catch. The catch then checks the value of the type info
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selector against the index of type info for that catch. Since the type info
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index is not known until all the type infos have been gathered in the
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backend, the catch code must call the
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<a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to
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determine the index for a given type info. If the catch fails to match the
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selector then control is passed on to the next catch.</p>
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<p>Finally, the entry and exit of catch code is bracketed with calls to
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<tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>
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<ul>
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<li><tt>__cxa_begin_catch</tt> takes an exception structure reference as an
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argument and returns the value of the exception object.</li>
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<li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>
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<ol>
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<li>Locates the most recently caught exception and decrements its handler
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count,</li>
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<li>Removes the exception from the <i>caught</i> stack if the handler
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count goes to zero, and</li>
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<li>Destroys the exception if the handler count goes to zero and the
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exception was not re-thrown by throw.</li>
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</ol>
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<p><b>Note:</b> a rethrow from within the catch may replace this call with
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a <tt>__cxa_rethrow</tt>.</p></li>
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</ul>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="cleanups">Cleanups</a>
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</h3>
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<div>
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<p>A cleanup is extra code which needs to be run as part of unwinding a scope.
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C++ destructors are a typical example, but other languages and language
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extensions provide a variety of different kinds of cleanups. In general, a
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landing pad may need to run arbitrary amounts of cleanup code before actually
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entering a catch block. To indicate the presence of cleanups, a
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<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
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should have a <i>cleanup</i> clause. Otherwise, the unwinder will not stop at
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the landing pad if there are no catches or filters that require it to.</p>
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<p><b>Note:</b> Do not allow a new exception to propagate out of the execution
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of a cleanup. This can corrupt the internal state of the unwinder.
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Different languages describe different high-level semantics for these
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situations: for example, C++ requires that the process be terminated, whereas
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Ada cancels both exceptions and throws a third.</p>
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<p>When all cleanups are finished, if the exception is not handled by the
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current function, resume unwinding by calling the
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<a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in
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the result of the <tt>landingpad</tt> instruction for the original landing
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pad.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="throw_filters">Throw Filters</a>
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</h3>
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<div>
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<p>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
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<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will
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have a filter clause. The clause consists of an array of type infos.
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<tt>landingpad</tt> will return a negative value if the exception does not
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match any of the type infos. If no match is found then a call
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to <tt>__cxa_call_unexpected</tt> should be made, otherwise
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<tt>_Unwind_Resume</tt>. Each of these functions requires a reference to the
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exception structure. Note that the most general form of a
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<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can
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have any number of catch, cleanup, and filter clauses (though having more
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than one cleanup is pointless). The LLVM C++ front-end can generate such
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<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due
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to inlining creating nested exception handling scopes.</p>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="restrictions">Restrictions</a>
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</h3>
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<div>
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<p>The unwinder delegates the decision of whether to stop in a call frame to
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that call frame's language-specific personality function. Not all unwinders
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guarantee that they will stop to perform cleanups. For example, the GNU C++
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unwinder doesn't do so unless the exception is actually caught somewhere
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further up the stack.</p>
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<p>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
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a function catches exceptions of type <tt>A</tt>, and it's inlined into a
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function that catches exceptions of type <tt>B</tt>. The inliner will update
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the <tt>landingpad</tt> instruction for the inlined landing pad to include
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the fact that <tt>B</tt> is also caught. If that landing pad assumes that it
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will only be entered to catch an <tt>A</tt>, it's in for a rude awakening.
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Consequently, landing pads must test for the selector results they understand
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and then resume exception propagation with the
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<a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of
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the conditions match.</p>
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</div>
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</div>
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<!-- ======================================================================= -->
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<h2>
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<a name="format_common_intrinsics">Exception Handling Intrinsics</a>
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</h2>
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<div>
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<p>In addition to the
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<a href="LangRef.html#i_landingpad"><tt>landingpad</tt></a> and
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<a href="LangRef.html#i_resume"><tt>resume</tt></a> instructions, LLVM uses
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several intrinsic functions (name prefixed with <i><tt>llvm.eh</tt></i>) to
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provide exception handling information at various points in generated
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code.</p>
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<!-- ======================================================================= -->
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<h4>
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<a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
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</h4>
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<div>
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<pre>
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i32 @llvm.eh.typeid.for(i8* %type_info)
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</pre>
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<p>This intrinsic returns the type info index in the exception table of the
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current function. This value can be used to compare against the result
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of <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>.
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The single argument is a reference to a type info.</p>
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</div>
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<!-- ======================================================================= -->
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<h4>
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<a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
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</h4>
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<div>
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<pre>
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i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf)
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</pre>
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<p>For SJLJ based exception handling, this intrinsic forces register saving for
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the current function and stores the address of the following instruction for
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use as a destination address
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by <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>. The
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buffer format and the overall functioning of this intrinsic is compatible
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with the GCC <tt>__builtin_setjmp</tt> implementation allowing code built
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with the clang and GCC to interoperate.</p>
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<p>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,
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and the target implementation of this intrinsic should place the destination
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address for a
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<a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the
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second word. The following three words are available for use in a
|
|
target-specific manner.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h4>
|
|
<a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>
|
|
</h4>
|
|
|
|
<div>
|
|
|
|
<pre>
|
|
void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf)
|
|
</pre>
|
|
|
|
<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.longjmp</tt>
|
|
intrinsic is used to implement <tt>__builtin_longjmp()</tt>. The single
|
|
parameter is a pointer to a buffer populated
|
|
by <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a>. The frame
|
|
pointer and stack pointer are restored from the buffer, then control is
|
|
transferred to the destination address.</p>
|
|
|
|
</div>
|
|
<!-- ======================================================================= -->
|
|
<h4>
|
|
<a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
|
|
</h4>
|
|
|
|
<div>
|
|
|
|
<pre>
|
|
i8* @llvm.eh.sjlj.lsda()
|
|
</pre>
|
|
|
|
<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.lsda</tt> 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.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h4>
|
|
<a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
|
|
</h4>
|
|
|
|
<div>
|
|
|
|
<pre>
|
|
void @llvm.eh.sjlj.callsite(i32 %call_site_num)
|
|
</pre>
|
|
|
|
<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.callsite</tt>
|
|
intrinsic identifies the callsite value associated with the
|
|
following <tt>invoke</tt> instruction. This is used to ensure that landing
|
|
pad entries in the LSDA are generated in matching order.</p>
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h2>
|
|
<a name="asm">Asm Table Formats</a>
|
|
</h2>
|
|
|
|
<div>
|
|
|
|
<p>There are two tables that are used by the exception handling runtime to
|
|
determine which actions should be taken when an exception is thrown.</p>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3>
|
|
<a name="unwind_tables">Exception Handling Frame</a>
|
|
</h3>
|
|
|
|
<div>
|
|
|
|
<p>An exception handling frame <tt>eh_frame</tt> 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.</p>
|
|
|
|
<!-- Todo - Table details here. -->
|
|
|
|
</div>
|
|
|
|
<!-- ======================================================================= -->
|
|
<h3>
|
|
<a name="exception_tables">Exception Tables</a>
|
|
</h3>
|
|
|
|
<div>
|
|
|
|
<p>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.</p>
|
|
|
|
<!-- Todo - Table details here. -->
|
|
|
|
</div>
|
|
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
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|
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<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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