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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.">
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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>
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 Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]59<h2><a name="introduction">Introduction</a></h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]60<!-- *********************************************************************** -->
61
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]62<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]63
64<p>This document is the central repository for all information pertaining to
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]70
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]71<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]72<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]73 <a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]74</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]75
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]76<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]77
78<p>Exception handling for most programming languages is designed to recover from
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]83
84<p>The Itanium ABI Exception Handling Specification defines a methodology for
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]89
90<p>A more complete description of the Itanium ABI exception handling runtime
Bill Wendlingc4f661e2009-08-15 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 Wendling4a065dd2011-09-20 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 Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]101
102</div>
103
104<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]105<h3>
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]106 <a name="sjlj">Setjmp/Longjmp Exception Handling</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]107</h3>
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]108
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]109<div>
Jim Grosbach00484d12009-08-22 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 Wendling4a065dd2011-09-20 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 Grosbach00484d12009-08-22 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 Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]134
Jim Grosbach00484d12009-08-22 01:42:39 +0000[diff] [blame]135</div>
136
137<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]138<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]139 <a name="overview">Overview</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]140</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]141
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]142<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]143
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]146
147<p>The runtime first attempts to find an <i>exception frame</i> corresponding to
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]158
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]168
169<p>The organization of an exception table is language dependent. For C++, an
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]176
Bill Wendling4a065dd2011-09-20 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 Wendlingc4f661e2009-08-15 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 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
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]194<p>From the C++ developers 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 Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]207 operation to initiate the exception process. Internally, a throw operation
Bill Wendling4a065dd2011-09-20 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]216
Bill Wendling4a065dd2011-09-20 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]221
222</div>
223
224<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]225<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]226 <a name="try_catch">Try/Catch</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]227</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]228
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]229<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]230
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]237
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]244
Bill Wendling4a065dd2011-09-20 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]250
Bill Wendling4a065dd2011-09-20 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]262
263<p>Once the landing pad has the type info selector, the code branches to the
Bill Wendling4a065dd2011-09-20 01:08:53 +0000[diff] [blame]264 code for the first catch. The catch then checks the value of the type info
Bill Wendlingc4f661e2009-08-15 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 Wendling4a065dd2011-09-20 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]275
Bill Wendlingc4f661e2009-08-15 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 Wendling169e1b02009-09-10 22:14:16 +0000[diff] [blame]283 <li><tt>__cxa_end_catch</tt> takes no arguments. This function:<br><br>
284 <ol>
Bill Wendling808b9ce2009-09-10 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 Wendling169e1b02009-09-10 22:14:16 +0000[diff] [blame]291 </ol>
Bill Wendling808b9ce2009-09-10 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 Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]294</ul>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]295
296</div>
297
298<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]299<h3>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]300 <a name="cleanups">Cleanups</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]301</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]302
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]303<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]304
Bill Wendling4a065dd2011-09-20 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 McCalld7c10862011-05-28 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 Wendling4a065dd2011-09-20 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]325
326</div>
327
328<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]329<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]330 <a name="throw_filters">Throw Filters</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]331</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]332
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]333<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]334
Bill Wendlingc4f661e2009-08-15 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 Wendling4a065dd2011-09-20 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]350
351</div>
352
353<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]354<h3>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]355 <a name="restrictions">Restrictions</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]356</h3>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]357
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]358<div>
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]359
Bill Wendling4a065dd2011-09-20 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 McCalld7c10862011-05-28 07:45:59 +0000[diff] [blame]368 rather than just cleanups.</p>
369
Bill Wendling4a065dd2011-09-20 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 Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]381
382</div>
383
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]384</div>
385
Duncan Sands6590b042007-08-27 15:47:50 +0000[diff] [blame]386<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]387<h2>
Duncan Sands8036ca42007-03-30 12:22:09 +0000[diff] [blame]388 <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]389</h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]390
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]391<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]392
Bill Wendling4a065dd2011-09-20 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 Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]397 provide exception handling information at various points in generated
398 code.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]399
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]400<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]401<h4>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]402 <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]403</h4>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]404
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]405<div>
Bill Wendlingbf230bf2009-08-15 20:08:04 +0000[diff] [blame]406
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]407<pre>
Duncan Sandsb01bbdc2009-10-14 16:11:37 +0000[diff] [blame]408 i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for</a>(i8*)
Jim Laskeyd0d39b62007-03-14 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 Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]412 current function. This value can be used to compare against the result
Bill Wendling3c051962011-09-20 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]415
416</div>
417
418<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]419<h4>
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]420 <a name="llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]421</h4>
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]422
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]423<div>
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]424
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]425<pre>
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]426 i32 %<a href="#llvm_eh_sjlj_setjmp">llvm.eh.sjlj.setjmp</a>(i8*)
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]427</pre>
428
Bill Wendlingc4f661e2009-08-15 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 Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]436
Bill Wendlingc4f661e2009-08-15 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 Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]444
Benjamin Kramere15192b2009-08-05 15:42:44 +0000[diff] [blame]445</div>
446
Jim Grosbachf9570122009-05-14 00:46:35 +0000[diff] [blame]447<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]448<h4>
Jim Grosbach90346e22010-05-26 16:21:41 +0000[diff] [blame]449 <a name="llvm_eh_sjlj_longjmp">llvm.eh.sjlj.longjmp</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]450</h4>
Jim Grosbach90346e22010-05-26 16:21:41 +0000[diff] [blame]451
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]452<div>
Jim Grosbach90346e22010-05-26 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 Lattner7a2bdde2011-04-15 05:18:47 +0000[diff] [blame]463 are restored from the buffer, then control is transferred to the
Jim Grosbach90346e22010-05-26 16:21:41 +0000[diff] [blame]464 destination address.</p>
465
466</div>
467<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]468<h4>
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]469 <a name="llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]470</h4>
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]471
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]472<div>
Bill Wendling3cf4ffd2009-08-15 20:07:42 +0000[diff] [blame]473
Jim Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]474<pre>
Dan Gohman3dfb3cf2010-05-28 17:07:41 +0000[diff] [blame]475 i8* %<a href="#llvm_eh_sjlj_lsda">llvm.eh.sjlj.lsda</a>()
Jim Grosbach1b747ad2009-08-11 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 Wendlingc4f661e2009-08-15 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 Grosbach1b747ad2009-08-11 00:09:57 +0000[diff] [blame]483
484</div>
485
486<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]487<h4>
Jim Grosbachca752c92010-01-28 01:45:32 +0000[diff] [blame]488 <a name="llvm_eh_sjlj_callsite">llvm.eh.sjlj.callsite</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]489</h4>
Jim Grosbachca752c92010-01-28 01:45:32 +0000[diff] [blame]490
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]491<div>
Jim Grosbachca752c92010-01-28 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 Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]505<h4>
Jim Grosbache4ad3872010-10-19 23:27:08 +0000[diff] [blame]506 <a name="llvm_eh_sjlj_dispatchsetup">llvm.eh.sjlj.dispatchsetup</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]507</h4>
Jim Grosbache4ad3872010-10-19 23:27:08 +0000[diff] [blame]508
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]509<div>
Jim Grosbache4ad3872010-10-19 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 Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]521</div>
522
Jim Grosbache4ad3872010-10-19 23:27:08 +0000[diff] [blame]523<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]524<h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]525 <a name="asm">Asm Table Formats</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]526</h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]527
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]528<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]529
530<p>There are two tables that are used by the exception handling runtime to
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]531 determine which actions should take place when an exception is thrown.</p>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]532
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]533<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]534<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]535 <a name="unwind_tables">Exception Handling Frame</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]536</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]537
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]538<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]539
540<p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]546
547<p>Todo - Table details here.</p>
548
549</div>
550
551<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]552<h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]553 <a name="exception_tables">Exception Tables</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]554</h3>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]555
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]556<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]557
558<p>An exception table contains information about what actions to take when an
Bill Wendlingc4f661e2009-08-15 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 Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]562
563<p>Todo - Table details here.</p>
564
565</div>
566
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]567</div>
568
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]569<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]570<h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]571 <a name="todo">ToDo</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]572</h2>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]573
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]574<div>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]575
576<ol>
577
Bill Wendlingc4f661e2009-08-15 08:56:09 +0000[diff] [blame]578 <li>Testing/Testing/Testing.</li>
Jim Laskeyd0d39b62007-03-14 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 Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]589 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]590 <a href="http://validator.w3.org/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]591 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]592
593 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
NAKAMURA Takumib9a33632011-04-09 02:13:37 +0000[diff] [blame]594 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
Jim Laskeyd0d39b62007-03-14 19:29:42 +0000[diff] [blame]595 Last modified: $Date$
596</address>
597
598</body>
599</html>