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5 <title>Exception Handling in LLVM</title>
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]6 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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
14<div class="doc_title">Exception Handling in LLVM</div>
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>
36 <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a></li>
37 <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a></li>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]38 <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]39 <li><a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a></li>
40 <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]41 <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]42 <li><a href="#llvm_eh_sjlj_callsite"><tt>llvm.eh.sjlj.callsite</tt></a></li>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]43 </ol></li>
44 <li><a href="#asm">Asm Table Formats</a>
45 <ol>
46 <li><a href="#unwind_tables">Exception Handling Frame</a></li>
47 <li><a href="#exception_tables">Exception Tables</a></li>
48 </ol></li>
49 <li><a href="#todo">ToDo</a></li>
50</ul>
51</td>
52</tr></table>
53
54<div class="doc_author">
55 <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
56</div>
57
58
59<!-- *********************************************************************** -->
Eric Christopher61f69782009-09-09 01:44:53 +0000[diff] [blame]60<div class="doc_section"><a name="introduction">Introduction</a></div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]61<!-- *********************************************************************** -->
62
63<div class="doc_text">
64
65<p>This document is the central repository for all information pertaining to
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]66 exception handling in LLVM. It describes the format that LLVM exception
67 handling information takes, which is useful for those interested in creating
68 front-ends or dealing directly with the information. Further, this document
69 provides specific examples of what exception handling information is used for
70 in C/C++.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]71
72</div>
73
74<!-- ======================================================================= -->
75<div class="doc_subsection">
76 <a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
77</div>
78
79<div class="doc_text">
80
81<p>Exception handling for most programming languages is designed to recover from
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]82 conditions that rarely occur during general use of an application. To that
83 end, exception handling should not interfere with the main flow of an
84 application's algorithm by performing checkpointing tasks, such as saving the
85 current pc or register state.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]86
87<p>The Itanium ABI Exception Handling Specification defines a methodology for
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]88 providing outlying data in the form of exception tables without inlining
89 speculative exception handling code in the flow of an application's main
90 algorithm. Thus, the specification is said to add "zero-cost" to the normal
91 execution of an application.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]92
93<p>A more complete description of the Itanium ABI exception handling runtime
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]94 support of can be found at
95 <a href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI:
96 Exception Handling</a>. A description of the exception frame format can be
97 found at
98 <a href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html">Exception
99 Frames</a>, with details of the DWARF 3 specification at
100 <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3 Standard</a>.
101 A description for the C++ exception table formats can be found at
102 <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
103 Tables</a>.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]104
105</div>
106
107<!-- ======================================================================= -->
108<div class="doc_subsection">
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]109 <a name="sjlj">Setjmp/Longjmp Exception Handling</a>
110</div>
111
112<div class="doc_text">
113
114<p>Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
115 <a href="#llvm_eh_sjlj_setjmp"><tt>llvm.eh.sjlj.setjmp</tt></a> and
116 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> to
117 handle control flow for exception handling.</p>
118
119<p>For each function which does exception processing, be it try/catch blocks
120 or cleanups, that function registers itself on a global frame list. When
121 exceptions are being unwound, the runtime uses this list to identify which
122 functions need processing.<p>
123
124<p>Landing pad selection is encoded in the call site entry of the function
125 context. The runtime returns to the function via
126 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>, where
127 a switch table transfers control to the appropriate landing pad based on
128 the index stored in the function context.</p>
129
130<p>In contrast to DWARF exception handling, which encodes exception regions
131 and frame information in out-of-line tables, SJLJ exception handling
132 builds and removes the unwind frame context at runtime. This results in
133 faster exception handling at the expense of slower execution when no
134 exceptions are thrown. As exceptions are, by their nature, intended for
135 uncommon code paths, DWARF exception handling is generally preferred to
136 SJLJ.</p>
137</div>
138
139<!-- ======================================================================= -->
140<div class="doc_subsection">
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]141 <a name="overview">Overview</a>
142</div>
143
144<div class="doc_text">
145
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]146<p>When an exception is thrown in LLVM code, the runtime does its best to find a
147 handler suited to processing the circumstance.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]148
149<p>The runtime first attempts to find an <i>exception frame</i> corresponding to
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]150 the function where the exception was thrown. If the programming language
151 (e.g. C++) supports exception handling, the exception frame contains a
152 reference to an exception table describing how to process the exception. If
153 the language (e.g. C) does not support exception handling, or if the
154 exception needs to be forwarded to a prior activation, the exception frame
155 contains information about how to unwind the current activation and restore
156 the state of the prior activation. This process is repeated until the
157 exception is handled. If the exception is not handled and no activations
158 remain, then the application is terminated with an appropriate error
159 message.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]160
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]161<p>Because different programming languages have different behaviors when
162 handling exceptions, the exception handling ABI provides a mechanism for
163 supplying <i>personalities.</i> An exception handling personality is defined
164 by way of a <i>personality function</i> (e.g. <tt>__gxx_personality_v0</tt>
165 in C++), which receives the context of the exception, an <i>exception
166 structure</i> containing the exception object type and value, and a reference
167 to the exception table for the current function. The personality function
168 for the current compile unit is specified in a <i>common exception
169 frame</i>.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]170
171<p>The organization of an exception table is language dependent. For C++, an
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]172 exception table is organized as a series of code ranges defining what to do
173 if an exception occurs in that range. Typically, the information associated
174 with a range defines which types of exception objects (using C++ <i>type
175 info</i>) that are handled in that range, and an associated action that
176 should take place. Actions typically pass control to a <i>landing
177 pad</i>.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]178
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]179<p>A landing pad corresponds to the code found in the <i>catch</i> portion of
180 a <i>try</i>/<i>catch</i> sequence. When execution resumes at a landing
181 pad, it receives the exception structure and a selector corresponding to
182 the <i>type</i> of exception thrown. The selector is then used to determine
183 which <i>catch</i> should actually process the exception.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]184
185</div>
186
187<!-- ======================================================================= -->
188<div class="doc_section">
189 <a name="codegen">LLVM Code Generation</a>
190</div>
191
192<div class="doc_text">
193
194<p>At the time of this writing, only C++ exception handling support is available
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]195 in LLVM. So the remainder of this document will be somewhat C++-centric.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]196
197<p>From the C++ developers perspective, exceptions are defined in terms of the
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]198 <tt>throw</tt> and <tt>try</tt>/<tt>catch</tt> statements. In this section
199 we will describe the implementation of LLVM exception handling in terms of
200 C++ examples.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]201
202</div>
203
204<!-- ======================================================================= -->
205<div class="doc_subsection">
206 <a name="throw">Throw</a>
207</div>
208
209<div class="doc_text">
210
211<p>Languages that support exception handling typically provide a <tt>throw</tt>
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]212 operation to initiate the exception process. Internally, a throw operation
213 breaks down into two steps. First, a request is made to allocate exception
214 space for an exception structure. This structure needs to survive beyond the
215 current activation. This structure will contain the type and value of the
216 object being thrown. Second, a call is made to the runtime to raise the
217 exception, passing the exception structure as an argument.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]218
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]219<p>In C++, the allocation of the exception structure is done by
220 the <tt>__cxa_allocate_exception</tt> runtime function. The exception
221 raising is handled by <tt>__cxa_throw</tt>. The type of the exception is
222 represented using a C++ RTTI structure.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]223
224</div>
225
226<!-- ======================================================================= -->
227<div class="doc_subsection">
228 <a name="try_catch">Try/Catch</a>
229</div>
230
231<div class="doc_text">
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
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: where to continue when the call succeeds
237 as per normal; and where to continue if the call raises an exception, either
238 by a throw or the unwinding of a throw.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]239
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]240<p>The term used to define a the place where an <tt>invoke</tt> continues after
241 an exception is called a <i>landing pad</i>. LLVM landing pads are
242 conceptually alternative function entry points where an exception structure
243 reference and a type info index are passed in as arguments. The landing pad
244 saves the exception structure reference and then proceeds to select the catch
245 block that corresponds to the type info of the exception object.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]246
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]247<p>Two LLVM intrinsic functions are used to convey information about the landing
248 pad to the back end.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]249
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]250<ol>
251 <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no
252 arguments and returns a pointer to the exception structure. This only
253 returns a sensible value if called after an <tt>invoke</tt> has branched
254 to a landing pad. Due to code generation limitations, it must currently
255 be called in the landing pad itself.</li>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]256
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]257 <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum
258 of three arguments. The first argument is the reference to the exception
259 structure. The second argument is a reference to the personality function
260 to be used for this <tt>try</tt>/<tt>catch</tt> sequence. Each of the
261 remaining arguments is either a reference to the type info for
262 a <tt>catch</tt> statement, a <a href="#throw_filters">filter</a>
263 expression, or the number zero (<tt>0</tt>) representing
264 a <a href="#cleanups">cleanup</a>. The exception is tested against the
265 arguments sequentially from first to last. The result of
266 the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a
267 positive number if the exception matched a type info, a negative number if
268 it matched a filter, and zero if it matched a cleanup. If nothing is
269 matched, the behaviour of the program
270 is <a href="#restrictions">undefined</a>. This only returns a sensible
271 value if called after an <tt>invoke</tt> has branched to a landing pad.
272 Due to codegen limitations, it must currently be called in the landing pad
273 itself. If a type info matched, then the selector value is the index of
274 the type info in the exception table, which can be obtained using the
275 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a>
276 intrinsic.</li>
277</ol>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]278
279<p>Once the landing pad has the type info selector, the code branches to the
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]280 code for the first catch. The catch then checks the value of the type info
281 selector against the index of type info for that catch. Since the type info
282 index is not known until all the type info have been gathered in the backend,
283 the catch code will call the
284 <a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic
285 to determine the index for a given type info. If the catch fails to match
286 the selector then control is passed on to the next catch. Note: Since the
287 landing pad will not be used if there is no match in the list of type info on
288 the call to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>, then
289 neither the last catch nor <i>catch all</i> need to perform the check
290 against the selector.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]291
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]292<p>Finally, the entry and exit of catch code is bracketed with calls
293 to <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.</p>
294
295<ul>
296 <li><tt>__cxa_begin_catch</tt> takes a exception structure reference as an
297 argument and returns the value of the exception object.</li>
298
Bill Wendling169e1b02009-09-10 22:14:16 +0000[diff] [blame]299 <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>
300 <ol>
Bill Wendling808b9ce2009-09-10 22:12:50 +0000[diff] [blame]301 <li>Locates the most recently caught exception and decrements its handler
302 count,</li>
303 <li>Removes the exception from the "caught" stack if the handler count
304 goes to zero, and</li>
305 <li>Destroys the exception if the handler count goes to zero, and the
306 exception was not re-thrown by throw.</li>
Bill Wendling169e1b02009-09-10 22:14:16 +0000[diff] [blame]307 </ol>
Bill Wendling808b9ce2009-09-10 22:12:50 +0000[diff] [blame]308 <p>Note: a rethrow from within the catch may replace this call with
309 a <tt>__cxa_rethrow</tt>.</p></li>
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]310</ul>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]311
312</div>
313
314<!-- ======================================================================= -->
315<div class="doc_subsection">
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]316 <a name="cleanups">Cleanups</a>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]317</div>
318
319<div class="doc_text">
320
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]321<p>To handle destructors and cleanups in <tt>try</tt> code, control may not run
322 directly from a landing pad to the first catch. Control may actually flow
323 from the landing pad to clean up code and then to the first catch. Since the
324 required clean up for each <tt>invoke</tt> in a <tt>try</tt> may be different
325 (e.g. intervening constructor), there may be several landing pads for a given
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]326 try. If cleanups need to be run, an <tt>i32 0</tt> should be passed as the
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]327 last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]328 However, when using DWARF exception handling with C++, a <tt>i8* null</tt>
329 <a href="#restrictions">must</a> be passed instead.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]330
331</div>
332
333<!-- ======================================================================= -->
334<div class="doc_subsection">
335 <a name="throw_filters">Throw Filters</a>
336</div>
337
338<div class="doc_text">
339
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]340<p>C++ allows the specification of which exception types can be thrown from a
341 function. To represent this a top level landing pad may exist to filter out
342 invalid types. To express this in LLVM code the landing pad will
343 call <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The
344 arguments are a reference to the exception structure, a reference to the
345 personality function, the length of the filter expression (the number of type
346 infos plus one), followed by the type infos themselves.
347 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> will return a
348 negative value if the exception does not match any of the type infos. If no
349 match is found then a call to <tt>__cxa_call_unexpected</tt> should be made,
350 otherwise <tt>_Unwind_Resume</tt>. Each of these functions requires a
351 reference to the exception structure. Note that the most general form of an
352 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> call can contain
353 any number of type infos, filter expressions and cleanups (though having more
354 than one cleanup is pointless). The LLVM C++ front-end can generate such
355 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls due to
356 inlining creating nested exception handling scopes.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]357
358</div>
359
360<!-- ======================================================================= -->
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]361<div class="doc_subsection">
362 <a name="restrictions">Restrictions</a>
363</div>
364
365<div class="doc_text">
366
367<p>The semantics of the invoke instruction require that any exception that
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]368 unwinds through an invoke call should result in a branch to the invoke's
369 unwind label. However such a branch will only happen if the
370 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> matches. Thus in
371 order to ensure correct operation, the front-end must only generate
372 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> calls that are
373 guaranteed to always match whatever exception unwinds through the invoke.
374 For most languages it is enough to pass zero, indicating the presence of
375 a <a href="#cleanups">cleanup</a>, as the
376 last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> argument.
377 However for C++ this is not sufficient, because the C++ personality function
378 will terminate the program if it detects that unwinding the exception only
379 results in matches with cleanups. For C++ a <tt>null i8*</tt> should be
380 passed as the last <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>
381 argument instead. This is interpreted as a catch-all by the C++ personality
382 function, and will always match.</p>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]383
384</div>
385
386<!-- ======================================================================= -->
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]387<div class="doc_section">
Duncan Sands8036ca42007-03-30 12:22:09 +0000[diff] [blame]388 <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]389</div>
390
391<div class="doc_text">
392
393<p>LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]394 provide exception handling information at various points in generated
395 code.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]396
397</div>
398
399<!-- ======================================================================= -->
400<div class="doc_subsubsection">
401 <a name="llvm_eh_exception">llvm.eh.exception</a>
402</div>
403
404<div class="doc_text">
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]405
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]406<pre>
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]407 i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>( )
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]408</pre>
409
Duncan Sands6531d472008-12-29 15:27:32 +0000[diff] [blame]410<p>This intrinsic returns a pointer to the exception structure.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]411
412</div>
413
414<!-- ======================================================================= -->
415<div class="doc_subsubsection">
416 <a name="llvm_eh_selector">llvm.eh.selector</a>
417</div>
418
419<div class="doc_text">
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]420
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]421<pre>
Duncan Sandsb01bbdc2009-10-14 16:11:37 +0000[diff] [blame]422 i32 %<a href="#llvm_eh_selector">llvm.eh.selector</a>(i8*, i8*, i8*, ...)
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]423</pre>
424
Duncan Sands6531d472008-12-29 15:27:32 +0000[diff] [blame]425<p>This intrinsic is used to compare the exception with the given type infos,
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]426 filters and cleanups.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]427
428<p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]429 three arguments. The first argument is the reference to the exception
430 structure. The second argument is a reference to the personality function to
431 be used for this try catch sequence. Each of the remaining arguments is
432 either a reference to the type info for a catch statement,
433 a <a href="#throw_filters">filter</a> expression, or the number zero
434 representing a <a href="#cleanups">cleanup</a>. The exception is tested
435 against the arguments sequentially from first to last. The result of
436 the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a positive
437 number if the exception matched a type info, a negative number if it matched
438 a filter, and zero if it matched a cleanup. If nothing is matched, the
439 behaviour of the program is <a href="#restrictions">undefined</a>. If a type
440 info matched then the selector value is the index of the type info in the
441 exception table, which can be obtained using the
442 <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]443
444</div>
445
446<!-- ======================================================================= -->
447<div class="doc_subsubsection">
448 <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
449</div>
450
451<div class="doc_text">
Bill Wendlingbf230bf2009-08-15 20:08:04 +0000[diff] [blame]452
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]453<pre>
Duncan Sandsb01bbdc2009-10-14 16:11:37 +0000[diff] [blame]454 i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for</a>(i8*)
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]455</pre>
456
457<p>This intrinsic returns the type info index in the exception table of the
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]458 current function. This value can be used to compare against the result
459 of <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The single
460 argument is a reference to a type info.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]461
462</div>
463
464<!-- ======================================================================= -->
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]465<div class="doc_subsubsection">
466 <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
467</div>
468
469<div class="doc_text">
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]470
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]471<pre>
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]472 i32 %<a href="#llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>(i8*)
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]473</pre>
474
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]475<p>The SJLJ exception handling uses this intrinsic to force register saving for
476 the current function and to store the address of the following instruction
477 for use as a destination address by <a href="#llvm_eh_sjlj_longjmp">
478 <tt>llvm.eh.sjlj.longjmp</tt></a>. The buffer format and the overall
479 functioning of this intrinsic is compatible with the GCC
480 <tt>__builtin_setjmp</tt> implementation, allowing code built with the
481 two compilers to interoperate.</p>
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]482
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]483<p>The single parameter is a pointer to a five word buffer in which the calling
484 context is saved. The front end places the frame pointer in the first word,
485 and the target implementation of this intrinsic should place the destination
486 address for a
487 <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a> in the
488 second word. The following three words are available for use in a
489 target-specific manner.</p>
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]490
Benjamin Kramere15192b2009-08-05 15:42:44 +0000[diff] [blame]491</div>
492
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]493<!-- ======================================================================= -->
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]494<div class="doc_subsubsection">
Jim Grosbach90346e22010-05-26 16:21:41 +0000[diff] [blame^]495 <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>
496</div>
497
498<div class="doc_text">
499
500<pre>
501 void %<a href="#llvm_eh_sjlj_longjmp">llvm.eh.sjlj.setjmp</a>(i8*)
502</pre>
503
504<p>The <a href="#llvm_eh_sjlj_longjmp"><tt>llvm.eh.sjlj.longjmp</tt></a>
505 intrinsic is used to implement <tt>__builtin_longjmp()</tt> for SJLJ
506 style exception handling. The single parameter is a pointer to a
507 buffer populated by <a href="#llvm_eh_sjlj_setjmp">
508 <tt>llvm.eh.sjlj.setjmp</tt></a>. The frame pointer and stack pointer
509 are restored from the buffer, then control is transfered to the
510 destination address.</p>
511
512</div>
513<!-- ======================================================================= -->
514<div class="doc_subsubsection">
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]515 <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
516</div>
517
518<div class="doc_text">
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]519
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]520<pre>
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]521 i8* %<a href="#llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>( )
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]522</pre>
523
524<p>Used for SJLJ based exception handling, the <a href="#llvm_eh_sjlj_lsda">
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]525 <tt>llvm.eh.sjlj.lsda</tt></a> intrinsic returns the address of the Language
526 Specific Data Area (LSDA) for the current function. The SJLJ front-end code
527 stores this address in the exception handling function context for use by the
528 runtime.</p>
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]529
530</div>
531
532<!-- ======================================================================= -->
Jim Grosbachca752c92010-01-28 01:45:32 +0000[diff] [blame]533<div class="doc_subsubsection">
534 <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
535</div>
536
537<div class="doc_text">
538
539<pre>
540 void %<a href="#llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>(i32)
541</pre>
542
543<p>For SJLJ based exception handling, the <a href="#llvm_eh_sjlj_callsite">
544 <tt>llvm.eh.sjlj.callsite</tt></a> intrinsic identifies the callsite value
545 associated with the following invoke instruction. This is used to ensure
546 that landing pad entries in the LSDA are generated in the matching order.</p>
547
548</div>
549
550<!-- ======================================================================= -->
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]551<div class="doc_section">
552 <a name="asm">Asm Table Formats</a>
553</div>
554
555<div class="doc_text">
556
557<p>There are two tables that are used by the exception handling runtime to
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]558 determine which actions should take place when an exception is thrown.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]559
560</div>
561
562<!-- ======================================================================= -->
563<div class="doc_subsection">
564 <a name="unwind_tables">Exception Handling Frame</a>
565</div>
566
567<div class="doc_text">
568
569<p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]570 frame used by dwarf debug info. The frame contains all the information
571 necessary to tear down the current frame and restore the state of the prior
572 frame. There is an exception handling frame for each function in a compile
573 unit, plus a common exception handling frame that defines information common
574 to all functions in the unit.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]575
576<p>Todo - Table details here.</p>
577
578</div>
579
580<!-- ======================================================================= -->
581<div class="doc_subsection">
582 <a name="exception_tables">Exception Tables</a>
583</div>
584
585<div class="doc_text">
586
587<p>An exception table contains information about what actions to take when an
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]588 exception is thrown in a particular part of a function's code. There is one
589 exception table per function except leaf routines and functions that have
590 only calls to non-throwing functions will not need an exception table.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]591
592<p>Todo - Table details here.</p>
593
594</div>
595
596<!-- ======================================================================= -->
597<div class="doc_section">
598 <a name="todo">ToDo</a>
599</div>
600
601<div class="doc_text">
602
603<ol>
604
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]605 <li>Testing/Testing/Testing.</li>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]606
607</ol>
608
609</div>
610
611<!-- *********************************************************************** -->
612
613<hr>
614<address>
615 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]619
620 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
621 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
622 Last modified: $Date$
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