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5 <title>Exception Handling in LLVM</title>
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Eric Christopher98e0a592009-09-09 01:44:53 +0000[diff] [blame]7 <meta name="description"
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]8 content="Exception Handling in LLVM.">
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]9 <link rel="stylesheet" href="llvm.css" type="text/css">
10</head>
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]11
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]12<body>
13
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]14<h1>Exception Handling in LLVM</h1>
Jim Laskey69567732007-03-14 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 Grosbach8a2cfcb2009-08-22 01:42:39 +0000[diff] [blame]23 <li><a href="#sjlj">Setjmp/Longjmp Exception Handling</a></li>
Jim Laskey69567732007-03-14 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 Sandsef5a6542007-08-27 15:47:50 +0000[diff] [blame]30 <li><a href="#cleanups">Cleanups</a></li>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]31 <li><a href="#throw_filters">Throw Filters</a></li>
Duncan Sandsef5a6542007-08-27 15:47:50 +0000[diff] [blame]32 <li><a href="#restrictions">Restrictions</a></li>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]33 </ol></li>
Duncan Sands16f122e2007-03-30 12:22:09 +0000[diff] [blame]34 <li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]35 <ol>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]36 <li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li>
Jim Grosbach06928192009-05-14 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 Grosbach693e36a2009-08-11 00:09:57 +0000[diff] [blame]39 <li><a href="#llvm_eh_sjlj_lsda"><tt>llvm.eh.sjlj.lsda</tt></a></li>
Jim Grosbach54c05302010-01-28 01:45:32 +0000[diff] [blame]40 <li><a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a></li>
Jim Grosbachbbdc5d22010-10-19 23:27:08 +0000[diff] [blame]41 <li><a href="#llvm_eh_sjlj_dispatchsetup"><tt>llvm.eh.sjlj.dispatchsetup</tt></a></li>
Jim Laskey69567732007-03-14 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>
48 <li><a href="#todo">ToDo</a></li>
49</ul>
50</td>
51</tr></table>
52
53<div class="doc_author">
54 <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
55</div>
56
57
58<!-- *********************************************************************** -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]59<h2><a name="introduction">Introduction</a></h2>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]60<!-- *********************************************************************** -->
61
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]62<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]63
64<p>This document is the central repository for all information pertaining to
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]65 exception handling in LLVM. It describes the format that LLVM exception
66 handling information takes, which is useful for those interested in creating
67 front-ends or dealing directly with the information. Further, this document
68 provides specific examples of what exception handling information is used for
69 in C/C++.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]70
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]71<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]72<h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]73 <a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]74</h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]75
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]76<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]77
78<p>Exception handling for most programming languages is designed to recover from
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]79 conditions that rarely occur during general use of an application. To that
80 end, exception handling should not interfere with the main flow of an
81 application's algorithm by performing checkpointing tasks, such as saving the
82 current pc or register state.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]83
84<p>The Itanium ABI Exception Handling Specification defines a methodology for
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]85 providing outlying data in the form of exception tables without inlining
86 speculative exception handling code in the flow of an application's main
87 algorithm. Thus, the specification is said to add "zero-cost" to the normal
88 execution of an application.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]89
90<p>A more complete description of the Itanium ABI exception handling runtime
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]91 support of can be found at
92 <a href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI:
93 Exception Handling</a>. A description of the exception frame format can be
94 found at
95 <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]96 Frames</a>, with details of the DWARF 4 specification at
97 <a href="http://dwarfstd.org/Dwarf4Std.php">DWARF 4 Standard</a>.
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]98 A description for the C++ exception table formats can be found at
99 <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
100 Tables</a>.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]101
102</div>
103
104<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]105<h3>
Jim Grosbach8a2cfcb2009-08-22 01:42:39 +0000[diff] [blame]106 <a name="sjlj">Setjmp/Longjmp Exception Handling</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]107</h3>
Jim Grosbach8a2cfcb2009-08-22 01:42:39 +0000[diff] [blame]108
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]109<div>
Jim Grosbach8a2cfcb2009-08-22 01:42:39 +0000[diff] [blame]110
111<p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
112 <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and
113 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to
114 handle control flow for exception handling.</p>
115
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]116<p>For each function which does exception processing &mdash; be
117 it <tt>try</tt>/<tt>catch</tt> blocks or cleanups &mdash; that function
118 registers itself on a global frame list. When exceptions are unwinding, the
119 runtime uses this list to identify which functions need processing.<p>
Jim Grosbach8a2cfcb2009-08-22 01:42:39 +0000[diff] [blame]120
121<p>Landing pad selection is encoded in the call site entry of the function
122 context. The runtime returns to the function via
123 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where
124 a switch table transfers control to the appropriate landing pad based on
125 the index stored in the function context.</p>
126
127<p>In contrast to DWARF exception handling, which encodes exception regions
128 and frame information in out-of-line tables, SJLJ exception handling
129 builds and removes the unwind frame context at runtime. This results in
130 faster exception handling at the expense of slower execution when no
131 exceptions are thrown. As exceptions are, by their nature, intended for
132 uncommon code paths, DWARF exception handling is generally preferred to
133 SJLJ.</p>
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]134
Jim Grosbach8a2cfcb2009-08-22 01:42:39 +0000[diff] [blame]135</div>
136
137<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]138<h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]139 <a name="overview">Overview</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]140</h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]141
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]142<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]143
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]144<p>When an exception is thrown in LLVM code, the runtime does its best to find a
145 handler suited to processing the circumstance.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]146
147<p>The runtime first attempts to find an <i>exception frame</i> corresponding to
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]148 the function where the exception was thrown. If the programming language
149 (e.g. C++) supports exception handling, the exception frame contains a
150 reference to an exception table describing how to process the exception. If
151 the language (e.g. C) does not support exception handling, or if the
152 exception needs to be forwarded to a prior activation, the exception frame
153 contains information about how to unwind the current activation and restore
154 the state of the prior activation. This process is repeated until the
155 exception is handled. If the exception is not handled and no activations
156 remain, then the application is terminated with an appropriate error
157 message.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]158
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]159<p>Because different programming languages have different behaviors when
160 handling exceptions, the exception handling ABI provides a mechanism for
161 supplying <i>personalities.</i> An exception handling personality is defined
162 by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt>
163 in C++), which receives the context of the exception, an <i>exception
164 structure</i> containing the exception object type and value, and a reference
165 to the exception table for the current function. The personality function
166 for the current compile unit is specified in a <i>common exception
167 frame</i>.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]168
169<p>The organization of an exception table is language dependent. For C++, an
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]170 exception table is organized as a series of code ranges defining what to do
171 if an exception occurs in that range. Typically, the information associated
172 with a range defines which types of exception objects (using C++ <i>type
173 info</i>) that are handled in that range, and an associated action that
174 should take place. Actions typically pass control to a <i>landing
175 pad</i>.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]176
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]177<p>A landing pad corresponds to the code found in the <tt>catch</tt> portion of
178 a <tt>try</tt>/<tt>catch</tt> sequence. When execution resumes at a landing
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]179 pad, it receives the exception structure and a selector corresponding to
180 the <i>type</i> of exception thrown. The selector is then used to determine
181 which <i>catch</i> should actually process the exception.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]182
183</div>
184
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]185</div>
186
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]187<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]188<h2>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]189 <a name="codegen">LLVM Code Generation</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]190</h2>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]191
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]192<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]193
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]194<p>From the C++ developers perspective, exceptions are defined in terms of the
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]195 <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]196 we will describe the implementation of LLVM exception handling in terms of
197 C++ examples.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]198
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]199<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]200<h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]201 <a name="throw">Throw</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]202</h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]203
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]204<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]205
206<p>Languages that support exception handling typically provide a <tt>throw</tt>
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]207 operation to initiate the exception process. Internally, a throw operation
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]208 breaks down into two steps.</p>
209<ol>
210 <li>A request is made to allocate exception space for an exception structure.
211 This structure needs to survive beyond the current activation. This
212 structure will contain the type and value of the object being thrown.</li>
213 <li>A call is made to the runtime to raise the exception, passing the
214 exception structure as an argument.</li>
215</ol>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]216
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]217<p>In C++, the allocation of the exception structure is done by then
218 <tt>__cxa_allocate_exception</tt> runtime function. The exception raising is
219 handled by <tt>__cxa_throw</tt>. The type of the exception is represented
220 using a C++ RTTI structure.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]221
222</div>
223
224<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]225<h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]226 <a name="try_catch">Try/Catch</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]227</h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]228
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]229<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]230
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]231<p>A call within the scope of a <i>try</i> statement can potentially raise an
232 exception. In those circumstances, the LLVM C++ front-end replaces the call
233 with an <tt>invoke</tt> instruction. Unlike a call, the <tt>invoke</tt> has
234 two potential continuation points: where to continue when the call succeeds
235 as per normal; and where to continue if the call raises an exception, either
236 by a throw or the unwinding of a throw.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]237
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]238<p>The term used to define a the place where an <tt>invoke</tt> continues after
239 an exception is called a <i>landing pad</i>. LLVM landing pads are
240 conceptually alternative function entry points where an exception structure
241 reference and a type info index are passed in as arguments. The landing pad
242 saves the exception structure reference and then proceeds to select the catch
243 block that corresponds to the type info of the exception object.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]244
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]245<p>The LLVM <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
246 instruction</a> is used to convey information about the landing pad to the
247 back end. For C++, the <tt>landingpad</tt> instruction returns a pointer and
248 integer pair corresponding to the pointer to the exception structure and the
249 "selector value" respectively.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]250
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]251<p>The <tt>landingpad</tt> instruction takes a reference to the personality
252 function to be used for this <tt>try</tt>/<tt>catch</tt> sequence. The
253 remainder of the instruction is a list of <i>catch</i> and <i>filter</i>
254 clauses. The exception is tested against the clauses sequentially from first
255 to last. The selector value is a positive number if the exception matched a
256 type info, a negative number if it matched a filter, and zero if it matched a
257 cleanup. If nothing is matched, the behaviour of the program
258 is <a href="#restrictions">undefined</a>. If a type info matched, then the
259 selector value is the index of the type info in the exception table, which
260 can be obtained using the
261 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]262
263<p>Once the landing pad has the type info selector, the code branches to the
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]264 code for the first catch. The catch then checks the value of the type info
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]265 selector against the index of type info for that catch. Since the type info
266 index is not known until all the type info have been gathered in the backend,
267 the catch code will call the
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]268 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to
269 determine the index for a given type info. If the catch fails to match the
270 selector then control is passed on to the next catch. Note: Since the landing
271 pad will not be used if there is no match in the list of type info on the
272 call to the <a href="LangRef.html#i_landingpad"><tt>landingpad</tt>
273 instruction</a>, then neither the last catch nor <i>catch all</i> need to
274 perform the check against the selector.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]275
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]276<p>Finally, the entry and exit of catch code is bracketed with calls
277 to <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>
278
279<ul>
280 <li><tt>__cxa_begin_catch</tt> takes a exception structure reference as an
281 argument and returns the value of the exception object.</li>
282
Bill Wendlingfd272012009-09-10 22:14:16 +0000[diff] [blame]283 <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>
284 <ol>
Bill Wendling33b693f2009-09-10 22:12:50 +0000[diff] [blame]285 <li>Locates the most recently caught exception and decrements its handler
286 count,</li>
287 <li>Removes the exception from the "caught" stack if the handler count
288 goes to zero, and</li>
289 <li>Destroys the exception if the handler count goes to zero, and the
290 exception was not re-thrown by throw.</li>
Bill Wendlingfd272012009-09-10 22:14:16 +0000[diff] [blame]291 </ol>
Bill Wendling33b693f2009-09-10 22:12:50 +0000[diff] [blame]292 <p>Note: a rethrow from within the catch may replace this call with
293 a <tt>__cxa_rethrow</tt>.</p></li>
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]294</ul>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]295
296</div>
297
298<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]299<h3>
Duncan Sandsef5a6542007-08-27 15:47:50 +0000[diff] [blame]300 <a name="cleanups">Cleanups</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]301</h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]302
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]303<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]304
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]305<p>A cleanup is extra code which needs to be run as part of unwinding a scope.
306 C++ destructors are a prominent example, but other languages and language
307 extensions provide a variety of different kinds of cleanup. In general, a
308 landing pad may need to run arbitrary amounts of cleanup code before actually
309 entering a catch block. To indicate the presence of cleanups, a
310 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
311 should have a <i>cleanup</i> clause. Otherwise, the unwinder will not stop at
312 the landing pad if there are no catches or filters that require it to.</p>
John McCall046c47e2011-05-28 07:45:59 +0000[diff] [blame]313
314<p>Do not allow a new exception to propagate out of the execution of a
315 cleanup. This can corrupt the internal state of the unwinder.
316 Different languages describe different high-level semantics for
317 these situations: for example, C++ requires that the process be
318 terminated, whereas Ada cancels both exceptions and throws a third.</p>
319
320<p>When all cleanups have completed, if the exception is not handled
321 by the current function, resume unwinding by calling the
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]322 <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a>, passing in
323 the results of the <tt>landingpad</tt> instruction for the original landing
324 pad.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]325
326</div>
327
328<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]329<h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]330 <a name="throw_filters">Throw Filters</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]331</h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]332
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]333<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]334
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]335<p>C++ allows the specification of which exception types can be thrown from a
336 function. To represent this a top level landing pad may exist to filter out
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]337 invalid types. To express this in LLVM code the
338 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> will
339 have a filter clause. The clause consists of an array of type infos.
340 <tt>landingpad</tt> will return a negative value if the exception does not
341 match any of the type infos. If no match is found then a call
342 to <tt>__cxa_call_unexpected</tt> should be made, otherwise
343 <tt>_Unwind_Resume</tt>. Each of these functions requires a reference to the
344 exception structure. Note that the most general form of a
345 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a> can
346 have any number of catch, cleanup, and filter clauses (though having more
347 than one cleanup is pointless). The LLVM C++ front-end can generate such
348 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instructions</a> due
349 to inlining creating nested exception handling scopes.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]350
351</div>
352
353<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]354<h3>
Duncan Sandsef5a6542007-08-27 15:47:50 +0000[diff] [blame]355 <a name="restrictions">Restrictions</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]356</h3>
Duncan Sandsef5a6542007-08-27 15:47:50 +0000[diff] [blame]357
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]358<div>
Duncan Sandsef5a6542007-08-27 15:47:50 +0000[diff] [blame]359
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]360<p>The unwinder delegates the decision of whether to stop in a call frame to
361 that call frame's language-specific personality function. Not all
362 personalities functions guarantee that they will stop to perform
363 cleanups. For example, the GNU C++ personality doesn't do so unless the
364 exception is actually caught somewhere further up the stack. When using this
365 personality to implement EH for a language that guarantees that cleanups will
366 always be run, be sure to indicate a catch-all in the
367 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>
John McCall046c47e2011-05-28 07:45:59 +0000[diff] [blame]368 rather than just cleanups.</p>
369
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]370<p>In order for inlining to behave correctly, landing pads must be prepared to
371 handle selector results that they did not originally advertise. Suppose that
372 a function catches exceptions of type <tt>A</tt>, and it's inlined into a
373 function that catches exceptions of type <tt>B</tt>. The inliner will update
374 the <tt>landingpad</tt> instruction for the inlined landing pad to include
375 the fact that <tt>B</tt> is caught. If that landing pad assumes that it will
376 only be entered to catch an <tt>A</tt>, it's in for a rude surprise.
377 Consequently, landing pads must test for the selector results they understand
378 and then resume exception propagation with the
379 <a href="LangRef.html#i_resume"><tt>resume</tt> instruction</a> if none of
380 the conditions match.</p>
Duncan Sandsef5a6542007-08-27 15:47:50 +0000[diff] [blame]381
382</div>
383
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]384</div>
385
Duncan Sandsef5a6542007-08-27 15:47:50 +0000[diff] [blame]386<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]387<h2>
Duncan Sands16f122e2007-03-30 12:22:09 +0000[diff] [blame]388 <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]389</h2>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]390
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]391<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]392
Bill Wendling01161842011-09-20 01:08:53 +0000[diff] [blame]393<p>In addition to the
394 <a href="LangRef.html#i_landingpad"><tt>landingpad</tt></a> and
395 <a href="LangRef.html#i_resume"><tt>resume</tt></a> instructions, LLVM uses
396 several intrinsic functions (name prefixed with "<tt>llvm.eh</tt>") to
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]397 provide exception handling information at various points in generated
398 code.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]399
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]400<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]401<h4>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]402 <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]403</h4>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]404
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]405<div>
Bill Wendling4c1b9b02009-08-15 20:08:04 +0000[diff] [blame]406
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]407<pre>
Duncan Sands8e6ccb62009-10-14 16:11:37 +0000[diff] [blame]408 i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for</a>(i8*)
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]409</pre>
410
411<p>This intrinsic returns the type info index in the exception table of the
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]412 current function. This value can be used to compare against the result
Bill Wendling12e58f92011-09-20 01:14:54 +0000[diff] [blame]413 of <a href="LangRef.html#i_landingpad"><tt>landingpad</tt> instruction</a>.
414 The single argument is a reference to a type info.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]415
416</div>
417
418<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]419<h4>
Jim Grosbach06928192009-05-14 00:46:35 +0000[diff] [blame]420 <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]421</h4>
Jim Grosbach06928192009-05-14 00:46:35 +0000[diff] [blame]422
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]423<div>
Bill Wendling527241b2009-08-15 20:07:42 +0000[diff] [blame]424
Jim Grosbach06928192009-05-14 00:46:35 +0000[diff] [blame]425<pre>
Bill Wendling527241b2009-08-15 20:07:42 +0000[diff] [blame]426 i32 %<a href="#llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>(i8*)
Jim Grosbach06928192009-05-14 00:46:35 +0000[diff] [blame]427</pre>
428
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]429<p>The SJLJ exception handling uses this intrinsic to force register saving for
430 the current function and to store the address of the following instruction
431 for use as a destination address by <a href="#llvm_eh_sjlj_longjmp">
432 <tt>llvm.eh.sjlj.longjmp</tt></a>. The buffer format and the overall
433 functioning of this intrinsic is compatible with the GCC
434 <tt>__builtin_setjmp</tt> implementation, allowing code built with the
435 two compilers to interoperate.</p>
Jim Grosbach06928192009-05-14 00:46:35 +0000[diff] [blame]436
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]437<p>The single parameter is a pointer to a five word buffer in which the calling
438 context is saved. The front end places the frame pointer in the first word,
439 and the target implementation of this intrinsic should place the destination
440 address for a
441 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the
442 second word. The following three words are available for use in a
443 target-specific manner.</p>
Jim Grosbach06928192009-05-14 00:46:35 +0000[diff] [blame]444
Benjamin Kramereaccdd32009-08-05 15:42:44 +0000[diff] [blame]445</div>
446
Jim Grosbach06928192009-05-14 00:46:35 +0000[diff] [blame]447<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]448<h4>
Jim Grosbach172aaad2010-05-26 16:21:41 +0000[diff] [blame]449 <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]450</h4>
Jim Grosbach172aaad2010-05-26 16:21:41 +0000[diff] [blame]451
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]452<div>
Jim Grosbach172aaad2010-05-26 16:21:41 +0000[diff] [blame]453
454<pre>
455 void %<a href="#llvm_eh_sjlj_longjmp">llvm.eh.sjlj.setjmp</a>(i8*)
456</pre>
457
458<p>The <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>
459 intrinsic is used to implement <tt>__builtin_longjmp()</tt> for SJLJ
460 style exception handling. The single parameter is a pointer to a
461 buffer populated by <a href="#llvm_eh_sjlj_setjmp">
462 <tt>llvm.eh.sjlj.setjmp</tt></a>. The frame pointer and stack pointer
Chris Lattner0ab5e2c2011-04-15 05:18:47 +0000[diff] [blame]463 are restored from the buffer, then control is transferred to the
Jim Grosbach172aaad2010-05-26 16:21:41 +0000[diff] [blame]464 destination address.</p>
465
466</div>
467<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]468<h4>
Jim Grosbach693e36a2009-08-11 00:09:57 +0000[diff] [blame]469 <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]470</h4>
Jim Grosbach693e36a2009-08-11 00:09:57 +0000[diff] [blame]471
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]472<div>
Bill Wendling527241b2009-08-15 20:07:42 +0000[diff] [blame]473
Jim Grosbach693e36a2009-08-11 00:09:57 +0000[diff] [blame]474<pre>
Dan Gohmand6a6f612010-05-28 17:07:41 +0000[diff] [blame]475 i8* %<a href="#llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>()
Jim Grosbach693e36a2009-08-11 00:09:57 +0000[diff] [blame]476</pre>
477
478<p>Used for SJLJ based exception handling, the <a href="#llvm_eh_sjlj_lsda">
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]479 <tt>llvm.eh.sjlj.lsda</tt></a> intrinsic returns the address of the Language
480 Specific Data Area (LSDA) for the current function. The SJLJ front-end code
481 stores this address in the exception handling function context for use by the
482 runtime.</p>
Jim Grosbach693e36a2009-08-11 00:09:57 +0000[diff] [blame]483
484</div>
485
486<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]487<h4>
Jim Grosbach54c05302010-01-28 01:45:32 +0000[diff] [blame]488 <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]489</h4>
Jim Grosbach54c05302010-01-28 01:45:32 +0000[diff] [blame]490
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]491<div>
Jim Grosbach54c05302010-01-28 01:45:32 +0000[diff] [blame]492
493<pre>
494 void %<a href="#llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>(i32)
495</pre>
496
497<p>For SJLJ based exception handling, the <a href="#llvm_eh_sjlj_callsite">
498 <tt>llvm.eh.sjlj.callsite</tt></a> intrinsic identifies the callsite value
499 associated with the following invoke instruction. This is used to ensure
500 that landing pad entries in the LSDA are generated in the matching order.</p>
501
502</div>
503
504<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]505<h4>
Jim Grosbachbbdc5d22010-10-19 23:27:08 +0000[diff] [blame]506 <a name="llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]507</h4>
Jim Grosbachbbdc5d22010-10-19 23:27:08 +0000[diff] [blame]508
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]509<div>
Jim Grosbachbbdc5d22010-10-19 23:27:08 +0000[diff] [blame]510
511<pre>
512 void %<a href="#llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>(i32)
513</pre>
514
515<p>For SJLJ based exception handling, the <a href="#llvm_eh_sjlj_dispatchsetup">
516 <tt>llvm.eh.sjlj.dispatchsetup</tt></a> intrinsic is used by targets to do
517 any unwind-edge setup they need. By default, no action is taken. </p>
518
519</div>
520
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]521</div>
522
Jim Grosbachbbdc5d22010-10-19 23:27:08 +0000[diff] [blame]523<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]524<h2>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]525 <a name="asm">Asm Table Formats</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]526</h2>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]527
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]528<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]529
530<p>There are two tables that are used by the exception handling runtime to
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]531 determine which actions should take place when an exception is thrown.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]532
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]533<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]534<h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]535 <a name="unwind_tables">Exception Handling Frame</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]536</h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]537
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]538<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]539
540<p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]541 frame used by dwarf debug info. The frame contains all the information
542 necessary to tear down the current frame and restore the state of the prior
543 frame. There is an exception handling frame for each function in a compile
544 unit, plus a common exception handling frame that defines information common
545 to all functions in the unit.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]546
547<p>Todo - Table details here.</p>
548
549</div>
550
551<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]552<h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]553 <a name="exception_tables">Exception Tables</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]554</h3>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]555
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]556<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]557
558<p>An exception table contains information about what actions to take when an
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]559 exception is thrown in a particular part of a function's code. There is one
560 exception table per function except leaf routines and functions that have
561 only calls to non-throwing functions will not need an exception table.</p>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]562
563<p>Todo - Table details here.</p>
564
565</div>
566
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]567</div>
568
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]569<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]570<h2>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]571 <a name="todo">ToDo</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]572</h2>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]573
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]574<div>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]575
576<ol>
577
Bill Wendling365ab602009-08-15 08:56:09 +0000[diff] [blame]578 <li>Testing/Testing/Testing.</li>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]579
580</ol>
581
582</div>
583
584<!-- *********************************************************************** -->
585
586<hr>
587<address>
588 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman86242e12008-12-11 17:34:48 +0000[diff] [blame]589 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]590 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman86242e12008-12-11 17:34:48 +0000[diff] [blame]591 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]592
593 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
NAKAMURA Takumica46f5a2011-04-09 02:13:37 +0000[diff] [blame]594 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Jim Laskey69567732007-03-14 19:29:42 +0000[diff] [blame]595 Last modified: $Date$
596</address>
597
598</body>
599</html>