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<title>Exception Handling in LLVM</title>
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2009-08-15 08:56:09 +00:00
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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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2012-04-19 20:20:34 +00:00
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<link rel="stylesheet" href="_static/llvm.css" type="text/css">
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<body>
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2011-04-18 23:59:50 +00:00
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<h1>Exception Handling in LLVM</h1>
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2007-03-14 19:29:42 +00:00
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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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2009-08-22 01:42:39 +00:00
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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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There is an impedance matching problem between LLVM and
gcc exception handling: if an exception unwinds through
an invoke, then execution must branch to the invoke's
unwind target. We previously tried to enforce this by
appending a cleanup action to every selector, however
this does not always work correctly due to an optimization
in the C++ unwinding runtime: if only cleanups would be
run while unwinding an exception, then the program just
terminates without actually executing the cleanups, as
invoke semantics would require. I was hoping this
wouldn't be a problem, but in fact it turns out to be the
cause of all the remaining failures in the LLVM testsuite
(these also fail with -enable-correct-eh-support, so turning
on -enable-eh didn't make things worse!). Instead we need
to append a full-blown catch-all to the end of each
selector. The correct way of doing this depends on the
personality function, i.e. it is language dependent, so
can only be done by gcc. Thus this patch which generalizes
the eh.selector intrinsic so that it can handle all possible
kinds of action table entries (before it didn't accomodate
cleanups): now 0 indicates a cleanup, and filters have to be
specified using the number of type infos plus one rather than
the number of type infos. Related gcc patches will cause
Ada to pass a cleanup (0) to force the selector to always
fire, while C++ will use a C++ catch-all (null).
llvm-svn: 41484
2007-08-27 15:47:50 +00:00
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<li><a href="#cleanups">Cleanups</a></li>
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2007-03-14 19:29:42 +00:00
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<li><a href="#throw_filters">Throw Filters</a></li>
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There is an impedance matching problem between LLVM and
gcc exception handling: if an exception unwinds through
an invoke, then execution must branch to the invoke's
unwind target. We previously tried to enforce this by
appending a cleanup action to every selector, however
this does not always work correctly due to an optimization
in the C++ unwinding runtime: if only cleanups would be
run while unwinding an exception, then the program just
terminates without actually executing the cleanups, as
invoke semantics would require. I was hoping this
wouldn't be a problem, but in fact it turns out to be the
cause of all the remaining failures in the LLVM testsuite
(these also fail with -enable-correct-eh-support, so turning
on -enable-eh didn't make things worse!). Instead we need
to append a full-blown catch-all to the end of each
selector. The correct way of doing this depends on the
personality function, i.e. it is language dependent, so
can only be done by gcc. Thus this patch which generalizes
the eh.selector intrinsic so that it can handle all possible
kinds of action table entries (before it didn't accomodate
cleanups): now 0 indicates a cleanup, and filters have to be
specified using the number of type infos plus one rather than
the number of type infos. Related gcc patches will cause
Ada to pass a cleanup (0) to force the selector to always
fire, while C++ will use a C++ catch-all (null).
llvm-svn: 41484
2007-08-27 15:47:50 +00:00
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<li><a href="#restrictions">Restrictions</a></li>
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</ol></li>
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2007-03-30 12:22:09 +00:00
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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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2009-05-14 00:46:35 +00:00
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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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2009-08-11 00:09:57 +00:00
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<li><a href="#llvm_eh_sjlj_lsda"><tt>llvm.eh.sjlj.lsda</tt></a></li>
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2010-01-28 01:45:32 +00:00
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<li><a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a></li>
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2007-03-14 19:29:42 +00:00
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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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2011-11-27 21:02:12 +00:00
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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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2011-04-18 23:59:50 +00:00
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<h2><a name="introduction">Introduction</a></h2>
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2007-03-14 19:29:42 +00:00
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<!-- *********************************************************************** -->
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2011-04-23 00:30:22 +00:00
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<div>
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2007-03-14 19:29:42 +00:00
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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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2011-09-26 21:06:33 +00:00
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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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2011-04-18 23:59:50 +00:00
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</h3>
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2007-03-14 19:29:42 +00:00
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2011-04-23 00:30:22 +00:00
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<div>
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2007-03-14 19:29:42 +00:00
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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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2007-03-14 19:29:42 +00:00
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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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2007-03-14 19:29:42 +00:00
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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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2011-09-20 01:08:53 +00:00
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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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2007-03-14 19:29:42 +00:00
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</div>
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2009-08-22 01:42:39 +00:00
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<!-- ======================================================================= -->
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2011-04-18 23:59:50 +00:00
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<h3>
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2009-08-22 01:42:39 +00:00
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<a name="sjlj">Setjmp/Longjmp Exception Handling</a>
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2011-04-18 23:59:50 +00:00
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</h3>
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2009-08-22 01:42:39 +00:00
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2011-04-23 00:30:22 +00:00
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<div>
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2009-08-22 01:42:39 +00:00
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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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2011-09-20 01:08:53 +00:00
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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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2009-08-22 01:42:39 +00:00
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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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2011-09-20 01:08:53 +00:00
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2009-08-22 01:42:39 +00:00
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</div>
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2007-03-14 19:29:42 +00:00
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<!-- ======================================================================= -->
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2011-04-18 23:59:50 +00:00
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<h3>
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2007-03-14 19:29:42 +00:00
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<a name="overview">Overview</a>
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2011-04-18 23:59:50 +00:00
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</h3>
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2007-03-14 19:29:42 +00:00
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2011-04-23 00:30:22 +00:00
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<div>
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2007-03-14 19:29:42 +00:00
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2009-08-15 08:56:09 +00:00
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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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2007-03-14 19:29:42 +00:00
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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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2011-09-26 21:06:33 +00:00
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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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2011-09-26 21:06:33 +00:00
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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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2011-09-26 21:06:33 +00:00
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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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2007-03-14 19:29:42 +00:00
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2011-09-26 21:06:33 +00:00
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<p>The organization of an exception table is language dependent. For C++, an
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2009-08-15 08:56:09 +00:00
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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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2011-09-26 21:06:33 +00:00
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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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2007-03-14 19:29:42 +00:00
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2011-09-26 21:06:33 +00:00
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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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2007-03-14 19:29:42 +00:00
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</div>
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2011-04-23 00:30:22 +00:00
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</div>
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2007-03-14 19:29:42 +00:00
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<!-- ======================================================================= -->
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2011-04-18 23:59:50 +00:00
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<h2>
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2007-03-14 19:29:42 +00:00
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<a name="codegen">LLVM Code Generation</a>
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2011-04-18 23:59:50 +00:00
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</h2>
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2007-03-14 19:29:42 +00:00
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2011-04-23 00:30:22 +00:00
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<div>
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2007-03-14 19:29:42 +00:00
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2011-09-26 21:06:33 +00:00
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<p>From a C++ developer's perspective, exceptions are defined in terms of the
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2011-09-20 01:08:53 +00:00
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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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2011-04-18 23:59:50 +00:00
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<h3>
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<a name="throw">Throw</a>
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2011-04-18 23:59:50 +00:00
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</h3>
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2007-03-14 19:29:42 +00:00
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2011-04-23 00:30:22 +00:00
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<div>
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2007-03-14 19:29:42 +00:00
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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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2011-09-20 01:08:53 +00:00
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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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2007-03-14 19:29:42 +00:00
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2011-09-26 21:06:33 +00:00
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<p>In C++, the allocation of the exception structure is done by the
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2011-09-20 01:08:53 +00:00
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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
|
|
|
|
|
using a C++ RTTI structure.</p>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
|
|
|
<h3>
|
2007-03-14 19:29:42 +00:00
|
|
|
<a name="try_catch">Try/Catch</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h3>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
<div>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2009-08-15 08:56:09 +00:00
|
|
|
<p>A call within the scope of a <i>try</i> statement can potentially raise an
|
2011-09-26 21:06:33 +00:00
|
|
|
exception. In those circumstances, the LLVM C++ front-end replaces the call
|
|
|
|
|
with an <tt>invoke</tt> instruction. Unlike a call, the <tt>invoke</tt> has
|
|
|
|
|
two potential continuation points:</p>
|
|
|
|
|
|
|
|
|
|
<ol>
|
|
|
|
|
<li>where to continue when the call succeeds as per normal, and</li>
|
|
|
|
|
|
|
|
|
|
<li>where to continue if the call raises an exception, either by a throw or
|
|
|
|
|
the unwinding of a throw</li>
|
|
|
|
|
</ol>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2009-08-15 08:56:09 +00:00
|
|
|
<p>The term used to define a the place where an <tt>invoke</tt> continues after
|
2011-09-26 21:06:33 +00:00
|
|
|
an exception is called a <i>landing pad</i>. LLVM landing pads are
|
2009-08-15 08:56:09 +00:00
|
|
|
conceptually alternative function entry points where an exception structure
|
2011-09-26 21:06:33 +00:00
|
|
|
reference and a type info index are passed in as arguments. The landing pad
|
2009-08-15 08:56:09 +00:00
|
|
|
saves the exception structure reference and then proceeds to select the catch
|
|
|
|
|
block that corresponds to the type info of the exception object.</p>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-09-20 01:08:53 +00:00
|
|
|
<p>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
|
|
|
|
|
instruction</a> is used to convey information about the landing pad to the
|
|
|
|
|
back end. For C++, the <tt>landingpad</tt> instruction returns a pointer and
|
2011-09-26 21:06:33 +00:00
|
|
|
integer pair corresponding to the pointer to the <i>exception structure</i>
|
|
|
|
|
and the <i>selector value</i> respectively.</p>
|
2011-09-20 01:08:53 +00:00
|
|
|
|
|
|
|
|
<p>The <tt>landingpad</tt> instruction takes a reference to the personality
|
|
|
|
|
function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The
|
2011-09-26 21:06:33 +00:00
|
|
|
remainder of the instruction is a list of <i>cleanup</i>, <i>catch</i>,
|
|
|
|
|
and <i>filter</i> clauses. The exception is tested against the clauses
|
|
|
|
|
sequentially from first to last. The selector value is a positive number if
|
|
|
|
|
the exception matched a type info, a negative number if it matched a filter,
|
2011-09-27 10:37:28 +00:00
|
|
|
and zero if it matched a cleanup. If nothing is matched, the behavior of
|
2011-09-26 21:10:31 +00:00
|
|
|
the program is <a href="#restrictions">undefined</a>. If a type info matched,
|
2011-09-26 21:06:33 +00:00
|
|
|
then the selector value is the index of the type info in the exception table,
|
|
|
|
|
which can be obtained using the
|
2011-09-20 01:08:53 +00:00
|
|
|
<a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
<p>Once the landing pad has the type info selector, the code branches to the
|
2011-09-20 01:08:53 +00:00
|
|
|
code for the first catch. The catch then checks the value of the type info
|
2009-08-15 08:56:09 +00:00
|
|
|
selector against the index of type info for that catch. Since the type info
|
2011-09-26 21:06:33 +00:00
|
|
|
index is not known until all the type infos have been gathered in the
|
|
|
|
|
backend, the catch code must call the
|
2011-09-20 01:08:53 +00:00
|
|
|
<a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to
|
|
|
|
|
determine the index for a given type info. If the catch fails to match the
|
2011-09-26 21:06:33 +00:00
|
|
|
selector then control is passed on to the next catch.</p>
|
|
|
|
|
|
|
|
|
|
<p>Finally, the entry and exit of catch code is bracketed with calls to
|
|
|
|
|
<tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>
|
2009-08-15 08:56:09 +00:00
|
|
|
|
|
|
|
|
<ul>
|
2011-09-26 21:06:33 +00:00
|
|
|
<li><tt>__cxa_begin_catch</tt> takes an exception structure reference as an
|
2009-08-15 08:56:09 +00:00
|
|
|
argument and returns the value of the exception object.</li>
|
|
|
|
|
|
2009-09-10 22:14:16 +00:00
|
|
|
<li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>
|
|
|
|
|
<ol>
|
2009-09-10 22:12:50 +00:00
|
|
|
<li>Locates the most recently caught exception and decrements its handler
|
|
|
|
|
count,</li>
|
2011-09-26 21:06:33 +00:00
|
|
|
<li>Removes the exception from the <i>caught</i> stack if the handler
|
|
|
|
|
count goes to zero, and</li>
|
|
|
|
|
<li>Destroys the exception if the handler count goes to zero and the
|
2009-09-10 22:12:50 +00:00
|
|
|
exception was not re-thrown by throw.</li>
|
2009-09-10 22:14:16 +00:00
|
|
|
</ol>
|
2011-09-26 21:06:33 +00:00
|
|
|
<p><b>Note:</b> a rethrow from within the catch may replace this call with
|
2009-09-10 22:12:50 +00:00
|
|
|
a <tt>__cxa_rethrow</tt>.</p></li>
|
2009-08-15 08:56:09 +00:00
|
|
|
</ul>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
|
|
|
<h3>
|
There is an impedance matching problem between LLVM and
gcc exception handling: if an exception unwinds through
an invoke, then execution must branch to the invoke's
unwind target. We previously tried to enforce this by
appending a cleanup action to every selector, however
this does not always work correctly due to an optimization
in the C++ unwinding runtime: if only cleanups would be
run while unwinding an exception, then the program just
terminates without actually executing the cleanups, as
invoke semantics would require. I was hoping this
wouldn't be a problem, but in fact it turns out to be the
cause of all the remaining failures in the LLVM testsuite
(these also fail with -enable-correct-eh-support, so turning
on -enable-eh didn't make things worse!). Instead we need
to append a full-blown catch-all to the end of each
selector. The correct way of doing this depends on the
personality function, i.e. it is language dependent, so
can only be done by gcc. Thus this patch which generalizes
the eh.selector intrinsic so that it can handle all possible
kinds of action table entries (before it didn't accomodate
cleanups): now 0 indicates a cleanup, and filters have to be
specified using the number of type infos plus one rather than
the number of type infos. Related gcc patches will cause
Ada to pass a cleanup (0) to force the selector to always
fire, while C++ will use a C++ catch-all (null).
llvm-svn: 41484
2007-08-27 15:47:50 +00:00
|
|
|
<a name="cleanups">Cleanups</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h3>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
<div>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-09-20 01:08:53 +00:00
|
|
|
<p>A cleanup is extra code which needs to be run as part of unwinding a scope.
|
2011-09-26 21:06:33 +00:00
|
|
|
C++ destructors are a typical example, but other languages and language
|
|
|
|
|
extensions provide a variety of different kinds of cleanups. In general, a
|
2011-09-20 01:08:53 +00:00
|
|
|
landing pad may need to run arbitrary amounts of cleanup code before actually
|
2011-09-26 21:06:33 +00:00
|
|
|
entering a catch block. To indicate the presence of cleanups, a
|
2011-09-20 01:08:53 +00:00
|
|
|
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
|
|
|
|
|
should have a <i>cleanup</i> clause. Otherwise, the unwinder will not stop at
|
|
|
|
|
the landing pad if there are no catches or filters that require it to.</p>
|
2011-05-28 07:45:59 +00:00
|
|
|
|
2011-09-26 21:06:33 +00:00
|
|
|
<p><b>Note:</b> Do not allow a new exception to propagate out of the execution
|
|
|
|
|
of a cleanup. This can corrupt the internal state of the unwinder.
|
|
|
|
|
Different languages describe different high-level semantics for these
|
|
|
|
|
situations: for example, C++ requires that the process be terminated, whereas
|
|
|
|
|
Ada cancels both exceptions and throws a third.</p>
|
2011-05-28 07:45:59 +00:00
|
|
|
|
2011-09-26 21:06:33 +00:00
|
|
|
<p>When all cleanups are finished, if the exception is not handled by the
|
|
|
|
|
current function, resume unwinding by calling the
|
2011-09-20 01:08:53 +00:00
|
|
|
<a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in
|
2011-09-26 21:06:33 +00:00
|
|
|
the result of the <tt>landingpad</tt> instruction for the original landing
|
2011-09-20 01:08:53 +00:00
|
|
|
pad.</p>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
|
|
|
<h3>
|
2007-03-14 19:29:42 +00:00
|
|
|
<a name="throw_filters">Throw Filters</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h3>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
<div>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-09-26 21:06:33 +00:00
|
|
|
<p>C++ allows the specification of which exception types may be thrown from a
|
|
|
|
|
function. To represent this, a top level landing pad may exist to filter out
|
|
|
|
|
invalid types. To express this in LLVM code the
|
2011-09-20 01:08:53 +00:00
|
|
|
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will
|
|
|
|
|
have a filter clause. The clause consists of an array of type infos.
|
|
|
|
|
<tt>landingpad</tt> will return a negative value if the exception does not
|
|
|
|
|
match any of the type infos. If no match is found then a call
|
|
|
|
|
to <tt>__cxa_call_unexpected</tt> should be made, otherwise
|
|
|
|
|
<tt>_Unwind_Resume</tt>. Each of these functions requires a reference to the
|
|
|
|
|
exception structure. Note that the most general form of a
|
|
|
|
|
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can
|
|
|
|
|
have any number of catch, cleanup, and filter clauses (though having more
|
|
|
|
|
than one cleanup is pointless). The LLVM C++ front-end can generate such
|
|
|
|
|
<a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due
|
|
|
|
|
to inlining creating nested exception handling scopes.</p>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
There is an impedance matching problem between LLVM and
gcc exception handling: if an exception unwinds through
an invoke, then execution must branch to the invoke's
unwind target. We previously tried to enforce this by
appending a cleanup action to every selector, however
this does not always work correctly due to an optimization
in the C++ unwinding runtime: if only cleanups would be
run while unwinding an exception, then the program just
terminates without actually executing the cleanups, as
invoke semantics would require. I was hoping this
wouldn't be a problem, but in fact it turns out to be the
cause of all the remaining failures in the LLVM testsuite
(these also fail with -enable-correct-eh-support, so turning
on -enable-eh didn't make things worse!). Instead we need
to append a full-blown catch-all to the end of each
selector. The correct way of doing this depends on the
personality function, i.e. it is language dependent, so
can only be done by gcc. Thus this patch which generalizes
the eh.selector intrinsic so that it can handle all possible
kinds of action table entries (before it didn't accomodate
cleanups): now 0 indicates a cleanup, and filters have to be
specified using the number of type infos plus one rather than
the number of type infos. Related gcc patches will cause
Ada to pass a cleanup (0) to force the selector to always
fire, while C++ will use a C++ catch-all (null).
llvm-svn: 41484
2007-08-27 15:47:50 +00:00
|
|
|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
|
|
|
<h3>
|
There is an impedance matching problem between LLVM and
gcc exception handling: if an exception unwinds through
an invoke, then execution must branch to the invoke's
unwind target. We previously tried to enforce this by
appending a cleanup action to every selector, however
this does not always work correctly due to an optimization
in the C++ unwinding runtime: if only cleanups would be
run while unwinding an exception, then the program just
terminates without actually executing the cleanups, as
invoke semantics would require. I was hoping this
wouldn't be a problem, but in fact it turns out to be the
cause of all the remaining failures in the LLVM testsuite
(these also fail with -enable-correct-eh-support, so turning
on -enable-eh didn't make things worse!). Instead we need
to append a full-blown catch-all to the end of each
selector. The correct way of doing this depends on the
personality function, i.e. it is language dependent, so
can only be done by gcc. Thus this patch which generalizes
the eh.selector intrinsic so that it can handle all possible
kinds of action table entries (before it didn't accomodate
cleanups): now 0 indicates a cleanup, and filters have to be
specified using the number of type infos plus one rather than
the number of type infos. Related gcc patches will cause
Ada to pass a cleanup (0) to force the selector to always
fire, while C++ will use a C++ catch-all (null).
llvm-svn: 41484
2007-08-27 15:47:50 +00:00
|
|
|
<a name="restrictions">Restrictions</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h3>
|
There is an impedance matching problem between LLVM and
gcc exception handling: if an exception unwinds through
an invoke, then execution must branch to the invoke's
unwind target. We previously tried to enforce this by
appending a cleanup action to every selector, however
this does not always work correctly due to an optimization
in the C++ unwinding runtime: if only cleanups would be
run while unwinding an exception, then the program just
terminates without actually executing the cleanups, as
invoke semantics would require. I was hoping this
wouldn't be a problem, but in fact it turns out to be the
cause of all the remaining failures in the LLVM testsuite
(these also fail with -enable-correct-eh-support, so turning
on -enable-eh didn't make things worse!). Instead we need
to append a full-blown catch-all to the end of each
selector. The correct way of doing this depends on the
personality function, i.e. it is language dependent, so
can only be done by gcc. Thus this patch which generalizes
the eh.selector intrinsic so that it can handle all possible
kinds of action table entries (before it didn't accomodate
cleanups): now 0 indicates a cleanup, and filters have to be
specified using the number of type infos plus one rather than
the number of type infos. Related gcc patches will cause
Ada to pass a cleanup (0) to force the selector to always
fire, while C++ will use a C++ catch-all (null).
llvm-svn: 41484
2007-08-27 15:47:50 +00:00
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
<div>
|
There is an impedance matching problem between LLVM and
gcc exception handling: if an exception unwinds through
an invoke, then execution must branch to the invoke's
unwind target. We previously tried to enforce this by
appending a cleanup action to every selector, however
this does not always work correctly due to an optimization
in the C++ unwinding runtime: if only cleanups would be
run while unwinding an exception, then the program just
terminates without actually executing the cleanups, as
invoke semantics would require. I was hoping this
wouldn't be a problem, but in fact it turns out to be the
cause of all the remaining failures in the LLVM testsuite
(these also fail with -enable-correct-eh-support, so turning
on -enable-eh didn't make things worse!). Instead we need
to append a full-blown catch-all to the end of each
selector. The correct way of doing this depends on the
personality function, i.e. it is language dependent, so
can only be done by gcc. Thus this patch which generalizes
the eh.selector intrinsic so that it can handle all possible
kinds of action table entries (before it didn't accomodate
cleanups): now 0 indicates a cleanup, and filters have to be
specified using the number of type infos plus one rather than
the number of type infos. Related gcc patches will cause
Ada to pass a cleanup (0) to force the selector to always
fire, while C++ will use a C++ catch-all (null).
llvm-svn: 41484
2007-08-27 15:47:50 +00:00
|
|
|
|
2011-09-20 01:08:53 +00:00
|
|
|
<p>The unwinder delegates the decision of whether to stop in a call frame to
|
2011-09-27 20:16:57 +00:00
|
|
|
that call frame's language-specific personality function. Not all unwinders
|
|
|
|
|
guarantee that they will stop to perform cleanups. For example, the GNU C++
|
|
|
|
|
unwinder doesn't do so unless the exception is actually caught somewhere
|
|
|
|
|
further up the stack.</p>
|
2011-05-28 07:45:59 +00:00
|
|
|
|
2011-09-20 01:08:53 +00:00
|
|
|
<p>In order for inlining to behave correctly, landing pads must be prepared to
|
2011-09-26 21:06:33 +00:00
|
|
|
handle selector results that they did not originally advertise. Suppose that
|
2011-09-20 01:08:53 +00:00
|
|
|
a function catches exceptions of type <tt>A</tt>, and it's inlined into a
|
2011-09-26 21:06:33 +00:00
|
|
|
function that catches exceptions of type <tt>B</tt>. The inliner will update
|
2011-09-20 01:08:53 +00:00
|
|
|
the <tt>landingpad</tt> instruction for the inlined landing pad to include
|
2011-09-26 21:06:33 +00:00
|
|
|
the fact that <tt>B</tt> is also caught. If that landing pad assumes that it
|
|
|
|
|
will only be entered to catch an <tt>A</tt>, it's in for a rude awakening.
|
2011-09-20 01:08:53 +00:00
|
|
|
Consequently, landing pads must test for the selector results they understand
|
|
|
|
|
and then resume exception propagation with the
|
|
|
|
|
<a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of
|
|
|
|
|
the conditions match.</p>
|
There is an impedance matching problem between LLVM and
gcc exception handling: if an exception unwinds through
an invoke, then execution must branch to the invoke's
unwind target. We previously tried to enforce this by
appending a cleanup action to every selector, however
this does not always work correctly due to an optimization
in the C++ unwinding runtime: if only cleanups would be
run while unwinding an exception, then the program just
terminates without actually executing the cleanups, as
invoke semantics would require. I was hoping this
wouldn't be a problem, but in fact it turns out to be the
cause of all the remaining failures in the LLVM testsuite
(these also fail with -enable-correct-eh-support, so turning
on -enable-eh didn't make things worse!). Instead we need
to append a full-blown catch-all to the end of each
selector. The correct way of doing this depends on the
personality function, i.e. it is language dependent, so
can only be done by gcc. Thus this patch which generalizes
the eh.selector intrinsic so that it can handle all possible
kinds of action table entries (before it didn't accomodate
cleanups): now 0 indicates a cleanup, and filters have to be
specified using the number of type infos plus one rather than
the number of type infos. Related gcc patches will cause
Ada to pass a cleanup (0) to force the selector to always
fire, while C++ will use a C++ catch-all (null).
llvm-svn: 41484
2007-08-27 15:47:50 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
</div>
|
|
|
|
|
|
2007-03-14 19:29:42 +00:00
|
|
|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
|
|
|
<h2>
|
2007-03-30 12:22:09 +00:00
|
|
|
<a name="format_common_intrinsics">Exception Handling Intrinsics</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h2>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
<div>
|
2007-03-14 19:29:42 +00:00
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2011-09-20 01:08:53 +00:00
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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
|
2011-09-26 21:06:33 +00:00
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several intrinsic functions (name prefixed with <i><tt>llvm.eh</tt></i>) to
|
2009-08-15 08:56:09 +00:00
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provide exception handling information at various points in generated
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code.</p>
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2007-03-14 19:29:42 +00:00
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<!-- ======================================================================= -->
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2011-04-18 23:59:50 +00:00
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<h4>
|
2007-03-14 19:29:42 +00:00
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<a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
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2011-04-18 23:59:50 +00:00
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</h4>
|
2007-03-14 19:29:42 +00:00
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2011-04-23 00:30:22 +00:00
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<div>
|
2009-08-15 20:08:04 +00:00
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2007-03-14 19:29:42 +00:00
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<pre>
|
2011-09-26 21:06:33 +00:00
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i32 @llvm.eh.typeid.for(i8* %type_info)
|
2007-03-14 19:29:42 +00:00
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</pre>
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<p>This intrinsic returns the type info index in the exception table of the
|
2009-08-15 08:56:09 +00:00
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current function. This value can be used to compare against the result
|
2011-09-20 01:14:54 +00:00
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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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2007-03-14 19:29:42 +00:00
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</div>
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2009-05-14 00:46:35 +00:00
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<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
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<h4>
|
2009-05-14 00:46:35 +00:00
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<a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
|
2011-04-18 23:59:50 +00:00
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</h4>
|
2009-05-14 00:46:35 +00:00
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2011-04-23 00:30:22 +00:00
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<div>
|
2009-08-15 20:07:42 +00:00
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2009-05-14 00:46:35 +00:00
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<pre>
|
2011-09-26 21:06:33 +00:00
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i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf)
|
2009-05-14 00:46:35 +00:00
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</pre>
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2011-09-26 21:06:33 +00:00
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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>
|
2009-05-14 00:46:35 +00:00
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|
2009-08-15 08:56:09 +00:00
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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
|
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target-specific manner.</p>
|
2009-05-14 00:46:35 +00:00
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|
2009-08-05 15:42:44 +00:00
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</div>
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|
2010-05-26 16:21:41 +00:00
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|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
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|
<h4>
|
2010-05-26 16:21:41 +00:00
|
|
|
<a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h4>
|
2010-05-26 16:21:41 +00:00
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|
2011-04-23 00:30:22 +00:00
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|
<div>
|
2010-05-26 16:21:41 +00:00
|
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|
|
<pre>
|
2011-09-26 21:06:33 +00:00
|
|
|
void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf)
|
2010-05-26 16:21:41 +00:00
|
|
|
</pre>
|
|
|
|
|
|
2011-09-26 21:06:33 +00:00
|
|
|
<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.longjmp</tt>
|
|
|
|
|
intrinsic is used to implement <tt>__builtin_longjmp()</tt>. The single
|
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|
parameter is a pointer to a buffer populated
|
|
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|
|
by <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a>. The frame
|
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|
|
|
pointer and stack pointer are restored from the buffer, then control is
|
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|
|
transferred to the destination address.</p>
|
2010-05-26 16:21:41 +00:00
|
|
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|
|
|
|
|
</div>
|
2009-08-11 00:09:57 +00:00
|
|
|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
|
|
|
<h4>
|
2009-08-11 00:09:57 +00:00
|
|
|
<a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h4>
|
2009-08-11 00:09:57 +00:00
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
<div>
|
2009-08-15 20:07:42 +00:00
|
|
|
|
2009-08-11 00:09:57 +00:00
|
|
|
<pre>
|
2011-09-26 21:06:33 +00:00
|
|
|
i8* @llvm.eh.sjlj.lsda()
|
2009-08-11 00:09:57 +00:00
|
|
|
</pre>
|
|
|
|
|
|
2011-09-26 21:06:33 +00:00
|
|
|
<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>
|
2009-08-11 00:09:57 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
2010-01-28 01:45:32 +00:00
|
|
|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
|
|
|
<h4>
|
2010-01-28 01:45:32 +00:00
|
|
|
<a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h4>
|
2010-01-28 01:45:32 +00:00
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
<div>
|
2010-01-28 01:45:32 +00:00
|
|
|
|
|
|
|
|
<pre>
|
2011-09-26 21:06:33 +00:00
|
|
|
void @llvm.eh.sjlj.callsite(i32 %call_site_num)
|
2010-01-28 01:45:32 +00:00
|
|
|
</pre>
|
|
|
|
|
|
2011-09-26 21:06:33 +00:00
|
|
|
<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>
|
2010-01-28 01:45:32 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
2012-03-27 11:25:16 +00:00
|
|
|
</div>
|
|
|
|
|
|
2007-03-14 19:29:42 +00:00
|
|
|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
|
|
|
<h2>
|
2007-03-14 19:29:42 +00:00
|
|
|
<a name="asm">Asm Table Formats</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h2>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
<div>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
<p>There are two tables that are used by the exception handling runtime to
|
2011-09-26 21:06:33 +00:00
|
|
|
determine which actions should be taken when an exception is thrown.</p>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
|
|
|
<h3>
|
2007-03-14 19:29:42 +00:00
|
|
|
<a name="unwind_tables">Exception Handling Frame</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h3>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
<div>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
<p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
|
2011-09-26 21:06:33 +00:00
|
|
|
frame used by DWARF debug info. The frame contains all the information
|
2009-08-15 08:56:09 +00:00
|
|
|
necessary to tear down the current frame and restore the state of the prior
|
2011-09-26 21:06:33 +00:00
|
|
|
frame. There is an exception handling frame for each function in a compile
|
2009-08-15 08:56:09 +00:00
|
|
|
unit, plus a common exception handling frame that defines information common
|
|
|
|
|
to all functions in the unit.</p>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-09-26 21:06:33 +00:00
|
|
|
<!-- Todo - Table details here. -->
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- ======================================================================= -->
|
2011-04-18 23:59:50 +00:00
|
|
|
<h3>
|
2007-03-14 19:29:42 +00:00
|
|
|
<a name="exception_tables">Exception Tables</a>
|
2011-04-18 23:59:50 +00:00
|
|
|
</h3>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
<div>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
<p>An exception table contains information about what actions to take when an
|
2011-09-26 21:06:33 +00:00
|
|
|
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>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-09-26 21:06:33 +00:00
|
|
|
<!-- Todo - Table details here. -->
|
2007-03-14 19:29:42 +00:00
|
|
|
|
|
|
|
|
</div>
|
|
|
|
|
|
2011-04-23 00:30:22 +00:00
|
|
|
</div>
|
|
|
|
|
|
2007-03-14 19:29:42 +00:00
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<hr>
|
|
|
|
|
<address>
|
|
|
|
|
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
|
2008-12-11 17:34:48 +00:00
|
|
|
src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
|
2007-03-14 19:29:42 +00:00
|
|
|
<a href="http://validator.w3.org/check/referer"><img
|
2008-12-11 17:34:48 +00:00
|
|
|
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
|
2007-03-14 19:29:42 +00:00
|
|
|
|
2011-04-09 02:13:37 +00:00
|
|
|
<a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
|
2007-03-14 19:29:42 +00:00
|
|
|
Last modified: $Date$
|
|
|
|
|
</address>
|
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|
|
|
|
|
|
</body>
|
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|
|
</html>
|