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gerrit-public.fairphone.software / fp2-dev / platform / external / llvm / 24771028a07f7cff91af537b15ac74b3452818d8 / . / docs / ExceptionHandling.html
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5 <title>Exception Handling in LLVM</title>
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Eric Christopher61f69782009-09-09 01:44:53 +0000[diff] [blame]7 <meta name="description"
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]8 content="Exception Handling in LLVM.">
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]9 <link rel="stylesheet" href="llvm.css" type="text/css">
10</head>
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]11
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]12<body>
13
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]14<h1>Exception Handling in LLVM</h1>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]15
16<table class="layout" style="width:100%">
17 <tr class="layout">
18 <td class="left">
19<ul>
20 <li><a href="#introduction">Introduction</a>
21 <ol>
22 <li><a href="#itanium">Itanium ABI Zero-cost Exception Handling</a></li>
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]23 <li><a href="#sjlj">Setjmp/Longjmp Exception Handling</a></li>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]24 <li><a href="#overview">Overview</a></li>
25 </ol></li>
26 <li><a href="#codegen">LLVM Code Generation</a>
27 <ol>
28 <li><a href="#throw">Throw</a></li>
29 <li><a href="#try_catch">Try/Catch</a></li>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]30 <li><a href="#cleanups">Cleanups</a></li>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]31 <li><a href="#throw_filters">Throw Filters</a></li>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]32 <li><a href="#restrictions">Restrictions</a></li>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]33 </ol></li>
Duncan Sands8036ca42007-03-30 12:22:09 +0000[diff] [blame]34 <li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]35 <ol>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]36 <li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li>
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]37 <li><a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a></li>
38 <li><a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a></li>
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]39 <li><a href="#llvm_eh_sjlj_lsda"><tt>llvm.eh.sjlj.lsda</tt></a></li>
Jim Grosbachca752c92010-01-28 01:45:32 +0000[diff] [blame]40 <li><a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a></li>
Jim Grosbache4ad3872010-10-19 23:27:08 +0000[diff] [blame]41 <li><a href="#llvm_eh_sjlj_dispatchsetup"><tt>llvm.eh.sjlj.dispatchsetup</tt></a></li>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]42 </ol></li>
43 <li><a href="#asm">Asm Table Formats</a>
44 <ol>
45 <li><a href="#unwind_tables">Exception Handling Frame</a></li>
46 <li><a href="#exception_tables">Exception Tables</a></li>
47 </ol></li>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]48</ul>
49</td>
50</tr></table>
51
52<div class="doc_author">
53 <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
54</div>
55
56
57<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]58<h2><a name="introduction">Introduction</a></h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]59<!-- *********************************************************************** -->
60
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]61<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]62
63<p>This document is the central repository for all information pertaining to
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]64 exception handling in LLVM. It describes the format that LLVM exception
65 handling information takes, which is useful for those interested in creating
66 front-ends or dealing directly with the information. Further, this document
67 provides specific examples of what exception handling information is used for
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]68 in C and C++.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]69
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]70<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]71<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]72 <a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]73</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]74
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]75<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]76
77<p>Exception handling for most programming languages is designed to recover from
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]78 conditions that rarely occur during general use of an application. To that
79 end, exception handling should not interfere with the main flow of an
80 application's algorithm by performing checkpointing tasks, such as saving the
81 current pc or register state.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]82
83<p>The Itanium ABI Exception Handling Specification defines a methodology for
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]84 providing outlying data in the form of exception tables without inlining
85 speculative exception handling code in the flow of an application's main
86 algorithm. Thus, the specification is said to add "zero-cost" to the normal
87 execution of an application.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]88
89<p>A more complete description of the Itanium ABI exception handling runtime
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]90 support of can be found at
91 <a href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI:
92 Exception Handling</a>. A description of the exception frame format can be
93 found at
94 <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]95 Frames</a>, with details of the DWARF 4 specification at
96 <a href="http://dwarfstd.org/Dwarf4Std.php">DWARF 4 Standard</a>.
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]97 A description for the C++ exception table formats can be found at
98 <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
99 Tables</a>.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]100
101</div>
102
103<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]104<h3>
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]105 <a name="sjlj">Setjmp/Longjmp Exception Handling</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]106</h3>
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]107
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]108<div>
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]109
110<p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
111 <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and
112 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to
113 handle control flow for exception handling.</p>
114
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]115<p>For each function which does exception processing &mdash; be
116 it <tt>try</tt>/<tt>catch</tt> blocks or cleanups &mdash; that function
117 registers itself on a global frame list. When exceptions are unwinding, the
118 runtime uses this list to identify which functions need processing.<p>
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]119
120<p>Landing pad selection is encoded in the call site entry of the function
121 context. The runtime returns to the function via
122 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where
123 a switch table transfers control to the appropriate landing pad based on
124 the index stored in the function context.</p>
125
126<p>In contrast to DWARF exception handling, which encodes exception regions
127 and frame information in out-of-line tables, SJLJ exception handling
128 builds and removes the unwind frame context at runtime. This results in
129 faster exception handling at the expense of slower execution when no
130 exceptions are thrown. As exceptions are, by their nature, intended for
131 uncommon code paths, DWARF exception handling is generally preferred to
132 SJLJ.</p>
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]133
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]134</div>
135
136<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]137<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]138 <a name="overview">Overview</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]139</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]140
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]141<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]142
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]143<p>When an exception is thrown in LLVM code, the runtime does its best to find a
144 handler suited to processing the circumstance.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]145
146<p>The runtime first attempts to find an <i>exception frame</i> corresponding to
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]147 the function where the exception was thrown. If the programming language
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]148 supports exception handling (e.g. C++), the exception frame contains a
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]149 reference to an exception table describing how to process the exception. If
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]150 the language does not support exception handling (e.g. C), or if the
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]151 exception needs to be forwarded to a prior activation, the exception frame
152 contains information about how to unwind the current activation and restore
153 the state of the prior activation. This process is repeated until the
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]154 exception is handled. If the exception is not handled and no activations
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]155 remain, then the application is terminated with an appropriate error
156 message.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]157
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]158<p>Because different programming languages have different behaviors when
159 handling exceptions, the exception handling ABI provides a mechanism for
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]160 supplying <i>personalities</i>. An exception handling personality is defined
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]161 by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt>
162 in C++), which receives the context of the exception, an <i>exception
163 structure</i> containing the exception object type and value, and a reference
164 to the exception table for the current function. The personality function
165 for the current compile unit is specified in a <i>common exception
166 frame</i>.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]167
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]168<p>The organization of an exception table is language dependent. For C++, an
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]169 exception table is organized as a series of code ranges defining what to do
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]170 if an exception occurs in that range. Typically, the information associated
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]171 with a range defines which types of exception objects (using C++ <i>type
172 info</i>) that are handled in that range, and an associated action that
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]173 should take place. Actions typically pass control to a <i>landing
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]174 pad</i>.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]175
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]176<p>A landing pad corresponds roughly to the code found in the <tt>catch</tt>
177 portion of a <tt>try</tt>/<tt>catch</tt> sequence. When execution resumes at
178 a landing pad, it receives an <i>exception structure</i> and a
179 <i>selector value</i> corresponding to the <i>type</i> of exception
180 thrown. The selector is then used to determine which <i>catch</i> should
181 actually process the exception.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]182
183</div>
184
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]185</div>
186
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]187<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]188<h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]189 <a name="codegen">LLVM Code Generation</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]190</h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]191
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]192<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]193
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]194<p>From a C++ developer's perspective, exceptions are defined in terms of the
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]195 <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]196 we will describe the implementation of LLVM exception handling in terms of
197 C++ examples.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]198
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]199<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]200<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]201 <a name="throw">Throw</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]202</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]203
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]204<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]205
206<p>Languages that support exception handling typically provide a <tt>throw</tt>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]207 operation to initiate the exception process. Internally, a <tt>throw</tt>
208 operation breaks down into two steps.</p>
209
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]210<ol>
211 <li>A request is made to allocate exception space for an exception structure.
212 This structure needs to survive beyond the current activation. This
213 structure will contain the type and value of the object being thrown.</li>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]214
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]215 <li>A call is made to the runtime to raise the exception, passing the
216 exception structure as an argument.</li>
217</ol>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]218
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]219<p>In C++, the allocation of the exception structure is done by the
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]220 <tt>__cxa_allocate_exception</tt> runtime function. The exception raising is
221 handled by <tt>__cxa_throw</tt>. The type of the exception is represented
222 using a C++ RTTI structure.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]223
224</div>
225
226<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]227<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]228 <a name="try_catch">Try/Catch</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]229</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]230
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]231<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]232
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]233<p>A call within the scope of a <i>try</i> statement can potentially raise an
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]234 exception. In those circumstances, the LLVM C++ front-end replaces the call
235 with an <tt>invoke</tt> instruction. Unlike a call, the <tt>invoke</tt> has
236 two potential continuation points:</p>
237
238<ol>
239 <li>where to continue when the call succeeds as per normal, and</li>
240
241 <li>where to continue if the call raises an exception, either by a throw or
242 the unwinding of a throw</li>
243</ol>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]244
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]245<p>The term used to define a the place where an <tt>invoke</tt> continues after
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]246 an exception is called a <i>landing pad</i>. LLVM landing pads are
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]247 conceptually alternative function entry points where an exception structure
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]248 reference and a type info index are passed in as arguments. The landing pad
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]249 saves the exception structure reference and then proceeds to select the catch
250 block that corresponds to the type info of the exception object.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]251
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]252<p>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
253 instruction</a> is used to convey information about the landing pad to the
254 back end. For C++, the <tt>landingpad</tt> instruction returns a pointer and
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]255 integer pair corresponding to the pointer to the <i>exception structure</i>
256 and the <i>selector value</i> respectively.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]257
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]258<p>The <tt>landingpad</tt> instruction takes a reference to the personality
259 function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]260 remainder of the instruction is a list of <i>cleanup</i>, <i>catch</i>,
261 and <i>filter</i> clauses. The exception is tested against the clauses
262 sequentially from first to last. The selector value is a positive number if
263 the exception matched a type info, a negative number if it matched a filter,
Bill Wendling24771022011-09-27 10:37:28 +0000[diff] [blame^]264 and zero if it matched a cleanup. If nothing is matched, the behavior of
Bill Wendlinga69d9982011-09-26 21:10:31 +0000[diff] [blame]265 the program is <a href="#restrictions">undefined</a>. If a type info matched,
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]266 then the selector value is the index of the type info in the exception table,
267 which can be obtained using the
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]268 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]269
270<p>Once the landing pad has the type info selector, the code branches to the
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]271 code for the first catch. The catch then checks the value of the type info
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]272 selector against the index of type info for that catch. Since the type info
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]273 index is not known until all the type infos have been gathered in the
274 backend, the catch code must call the
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]275 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to
276 determine the index for a given type info. If the catch fails to match the
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]277 selector then control is passed on to the next catch.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]278
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]279<p>Finally, the entry and exit of catch code is bracketed with calls to
280 <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]281
282<ul>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]283 <li><tt>__cxa_begin_catch</tt> takes an exception structure reference as an
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]284 argument and returns the value of the exception object.</li>
285
Bill Wendling169e1b02009-09-10 22:14:16 +0000[diff] [blame]286 <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>
287 <ol>
Bill Wendling808b9ce2009-09-10 22:12:50 +0000[diff] [blame]288 <li>Locates the most recently caught exception and decrements its handler
289 count,</li>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]290 <li>Removes the exception from the <i>caught</i> stack if the handler
291 count goes to zero, and</li>
292 <li>Destroys the exception if the handler count goes to zero and the
Bill Wendling808b9ce2009-09-10 22:12:50 +0000[diff] [blame]293 exception was not re-thrown by throw.</li>
Bill Wendling169e1b02009-09-10 22:14:16 +0000[diff] [blame]294 </ol>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]295 <p><b>Note:</b> a rethrow from within the catch may replace this call with
Bill Wendling808b9ce2009-09-10 22:12:50 +0000[diff] [blame]296 a <tt>__cxa_rethrow</tt>.</p></li>
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]297</ul>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]298
299</div>
300
301<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]302<h3>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]303 <a name="cleanups">Cleanups</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]304</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]305
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]306<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]307
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]308<p>A cleanup is extra code which needs to be run as part of unwinding a scope.
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]309 C++ destructors are a typical example, but other languages and language
310 extensions provide a variety of different kinds of cleanups. In general, a
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]311 landing pad may need to run arbitrary amounts of cleanup code before actually
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]312 entering a catch block. To indicate the presence of cleanups, a
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]313 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
314 should have a <i>cleanup</i> clause. Otherwise, the unwinder will not stop at
315 the landing pad if there are no catches or filters that require it to.</p>
John McCalld7c10862011-05-28 07:45:59 +0000[diff] [blame]316
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]317<p><b>Note:</b> Do not allow a new exception to propagate out of the execution
318 of a cleanup. This can corrupt the internal state of the unwinder.
319 Different languages describe different high-level semantics for these
320 situations: for example, C++ requires that the process be terminated, whereas
321 Ada cancels both exceptions and throws a third.</p>
John McCalld7c10862011-05-28 07:45:59 +0000[diff] [blame]322
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]323<p>When all cleanups are finished, if the exception is not handled by the
324 current function, resume unwinding by calling the
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]325 <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]326 the result of the <tt>landingpad</tt> instruction for the original landing
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]327 pad.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]328
329</div>
330
331<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]332<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]333 <a name="throw_filters">Throw Filters</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]334</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]335
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]336<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]337
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]338<p>C++ allows the specification of which exception types may be thrown from a
339 function. To represent this, a top level landing pad may exist to filter out
340 invalid types. To express this in LLVM code the
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]341 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will
342 have a filter clause. The clause consists of an array of type infos.
343 <tt>landingpad</tt> will return a negative value if the exception does not
344 match any of the type infos. If no match is found then a call
345 to <tt>__cxa_call_unexpected</tt> should be made, otherwise
346 <tt>_Unwind_Resume</tt>. Each of these functions requires a reference to the
347 exception structure. Note that the most general form of a
348 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can
349 have any number of catch, cleanup, and filter clauses (though having more
350 than one cleanup is pointless). The LLVM C++ front-end can generate such
351 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due
352 to inlining creating nested exception handling scopes.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]353
354</div>
355
356<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]357<h3>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]358 <a name="restrictions">Restrictions</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]359</h3>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]360
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]361<div>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]362
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]363<p>The unwinder delegates the decision of whether to stop in a call frame to
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]364 that call frame's language-specific personality function. Not all personality
365 functions guarantee that they will stop to perform cleanups. For example, the
366 GNU C++ personality function doesn't do so unless the exception is actually
367 caught somewhere further up the stack. When using this personality to
368 implement EH for a language that guarantees that cleanups will always be run
369 (e.g. Ada), be sure to indicate a catch-all in the
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]370 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
John McCalld7c10862011-05-28 07:45:59 +0000[diff] [blame]371 rather than just cleanups.</p>
372
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]373<p>In order for inlining to behave correctly, landing pads must be prepared to
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]374 handle selector results that they did not originally advertise. Suppose that
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]375 a function catches exceptions of type <tt>A</tt>, and it's inlined into a
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]376 function that catches exceptions of type <tt>B</tt>. The inliner will update
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]377 the <tt>landingpad</tt> instruction for the inlined landing pad to include
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]378 the fact that <tt>B</tt> is also caught. If that landing pad assumes that it
379 will only be entered to catch an <tt>A</tt>, it's in for a rude awakening.
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]380 Consequently, landing pads must test for the selector results they understand
381 and then resume exception propagation with the
382 <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of
383 the conditions match.</p>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]384
385</div>
386
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]387</div>
388
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]389<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]390<h2>
Duncan Sands8036ca42007-03-30 12:22:09 +0000[diff] [blame]391 <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]392</h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]393
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]394<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]395
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]396<p>In addition to the
397 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt></a> and
398 <a href="LangRef.html#i_resume"><tt>resume</tt></a> instructions, LLVM uses
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]399 several intrinsic functions (name prefixed with <i><tt>llvm.eh</tt></i>) to
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]400 provide exception handling information at various points in generated
401 code.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]402
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]403<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]404<h4>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]405 <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]406</h4>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]407
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]408<div>
Bill Wendlingbf230bf2009-08-15 20:08:04 +0000[diff] [blame]409
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]410<pre>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]411 i32 @llvm.eh.typeid.for(i8* %type_info)
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]412</pre>
413
414<p>This intrinsic returns the type info index in the exception table of the
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]415 current function. This value can be used to compare against the result
Bill Wendling3c051962011-09-20 01:14:54 +0000[diff] [blame]416 of <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>.
417 The single argument is a reference to a type info.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]418
419</div>
420
421<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]422<h4>
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]423 <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]424</h4>
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]425
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]426<div>
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]427
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]428<pre>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]429 i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf)
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]430</pre>
431
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]432<p>For SJLJ based exception handling, this intrinsic forces register saving for
433 the current function and stores the address of the following instruction for
434 use as a destination address
435 by <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>. The
436 buffer format and the overall functioning of this intrinsic is compatible
437 with the GCC <tt>__builtin_setjmp</tt> implementation allowing code built
438 with the clang and GCC to interoperate.</p>
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]439
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]440<p>The single parameter is a pointer to a five word buffer in which the calling
441 context is saved. The front end places the frame pointer in the first word,
442 and the target implementation of this intrinsic should place the destination
443 address for a
444 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the
445 second word. The following three words are available for use in a
446 target-specific manner.</p>
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]447
Benjamin Kramere15192b2009-08-05 15:42:44 +0000[diff] [blame]448</div>
449
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]450<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]451<h4>
Jim Grosbach90346e22010-05-26 16:21:41 +0000[diff] [blame]452 <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]453</h4>
Jim Grosbach90346e22010-05-26 16:21:41 +0000[diff] [blame]454
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]455<div>
Jim Grosbach90346e22010-05-26 16:21:41 +0000[diff] [blame]456
457<pre>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]458 void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf)
Jim Grosbach90346e22010-05-26 16:21:41 +0000[diff] [blame]459</pre>
460
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]461<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.longjmp</tt>
462 intrinsic is used to implement <tt>__builtin_longjmp()</tt>. The single
463 parameter is a pointer to a buffer populated
464 by <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a>. The frame
465 pointer and stack pointer are restored from the buffer, then control is
466 transferred to the destination address.</p>
Jim Grosbach90346e22010-05-26 16:21:41 +0000[diff] [blame]467
468</div>
469<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]470<h4>
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]471 <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]472</h4>
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]473
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]474<div>
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]475
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]476<pre>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]477 i8* @llvm.eh.sjlj.lsda()
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]478</pre>
479
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]480<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.lsda</tt> intrinsic
481 returns the address of the Language Specific Data Area (LSDA) for the current
482 function. The SJLJ front-end code stores this address in the exception
483 handling function context for use by the runtime.</p>
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]484
485</div>
486
487<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]488<h4>
Jim Grosbachca752c92010-01-28 01:45:32 +0000[diff] [blame]489 <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]490</h4>
Jim Grosbachca752c92010-01-28 01:45:32 +0000[diff] [blame]491
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]492<div>
Jim Grosbachca752c92010-01-28 01:45:32 +0000[diff] [blame]493
494<pre>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]495 void @llvm.eh.sjlj.callsite(i32 %call_site_num)
Jim Grosbachca752c92010-01-28 01:45:32 +0000[diff] [blame]496</pre>
497
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]498<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.callsite</tt>
499 intrinsic identifies the callsite value associated with the
500 following <tt>invoke</tt> instruction. This is used to ensure that landing
501 pad entries in the LSDA are generated in matching order.</p>
Jim Grosbachca752c92010-01-28 01:45:32 +0000[diff] [blame]502
503</div>
504
505<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]506<h4>
Jim Grosbache4ad3872010-10-19 23:27:08 +0000[diff] [blame]507 <a name="llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]508</h4>
Jim Grosbache4ad3872010-10-19 23:27:08 +0000[diff] [blame]509
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]510<div>
Jim Grosbache4ad3872010-10-19 23:27:08 +0000[diff] [blame]511
512<pre>
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]513 void @llvm.eh.sjlj.dispatchsetup(i32 %dispatch_value)
Jim Grosbache4ad3872010-10-19 23:27:08 +0000[diff] [blame]514</pre>
515
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]516<p>For SJLJ based exception handling, the <tt>llvm.eh.sjlj.dispatchsetup</tt>
517 intrinsic is used by targets to do any unwind edge setup they need. By
518 default, no action is taken.</p>
Jim Grosbache4ad3872010-10-19 23:27:08 +0000[diff] [blame]519
520</div>
521
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]522</div>
523
Jim Grosbache4ad3872010-10-19 23:27:08 +0000[diff] [blame]524<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]525<h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]526 <a name="asm">Asm Table Formats</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]527</h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]528
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]529<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]530
531<p>There are two tables that are used by the exception handling runtime to
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]532 determine which actions should be taken when an exception is thrown.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]533
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]534<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]535<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]536 <a name="unwind_tables">Exception Handling Frame</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]537</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]538
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]539<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]540
541<p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]542 frame used by DWARF debug info. The frame contains all the information
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]543 necessary to tear down the current frame and restore the state of the prior
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]544 frame. There is an exception handling frame for each function in a compile
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]545 unit, plus a common exception handling frame that defines information common
546 to all functions in the unit.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]547
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]548<!-- Todo - Table details here. -->
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]549
550</div>
551
552<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]553<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]554 <a name="exception_tables">Exception Tables</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]555</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]556
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]557<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]558
559<p>An exception table contains information about what actions to take when an
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]560 exception is thrown in a particular part of a function's code. There is one
561 exception table per function, except leaf functions and functions that have
562 calls only to non-throwing functions. They do not need an exception
563 table.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]564
Bill Wendling2a6b0732011-09-26 21:06:33 +0000[diff] [blame]565<!-- Todo - Table details here. -->
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]566
567</div>
568
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]569</div>
570
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]571<!-- *********************************************************************** -->
572
573<hr>
574<address>
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Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]579
580 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
NAKAMURA Takumib9a33632011-04-09 02:13:37 +0000[diff] [blame]581 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]582 Last modified: $Date$
583</address>
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