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5 <title>Exception Handling in LLVM</title>
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10 <div class="doc_title">Exception Handling in LLVM</div>
12 <table class="layout" style="width:100%">
16 <li><a href="#introduction">Introduction</a>
18 <li><a href="#itanium">Itanium ABI Zero-cost Exception Handling</a></li>
19 <li><a href="#overview">Overview</a></li>
21 <li><a href="#codegen">LLVM Code Generation</a>
23 <li><a href="#throw">Throw</a></li>
24 <li><a href="#try_catch">Try/Catch</a></li>
25 <li><a href="#finallys">Finallys</a></li>
26 <li><a href="#throw_filters">Throw Filters</a></li>
28 <li><a href="#format_common_intrinsics">Exception Handling Intrinsics</a>
30 <li><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a></li>
31 <li><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a></li>
32 <li><a href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a></li>
34 <li><a href="#asm">Asm Table Formats</a>
36 <li><a href="#unwind_tables">Exception Handling Frame</a></li>
37 <li><a href="#exception_tables">Exception Tables</a></li>
39 <li><a href="#todo">ToDo</a></li>
44 <div class="doc_author">
45 <p>Written by <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
49 <!-- *********************************************************************** -->
50 <div class="doc_section"><a name="introduction">Introduction</a></div>
51 <!-- *********************************************************************** -->
53 <div class="doc_text">
55 <p>This document is the central repository for all information pertaining to
56 exception handling in LLVM. It describes the format that LLVM exception
57 handling information takes, which is useful for those interested in creating
58 front-ends or dealing directly with the information. Further, this document
59 provides specific examples of what exception handling information is used for
64 <!-- ======================================================================= -->
65 <div class="doc_subsection">
66 <a name="itanium">Itanium ABI Zero-cost Exception Handling</a>
69 <div class="doc_text">
71 <p>Exception handling for most programming languages is designed to recover from
72 conditions that rarely occur during general use of an application. To that end,
73 exception handling should not interfere with the main flow of an
74 application's algorithm by performing checkpointing tasks such as saving
75 the current pc or register state.</p>
77 <p>The Itanium ABI Exception Handling Specification defines a methodology for
78 providing outlying data in the form of exception tables without inlining
79 speculative exception handling code in the flow of an application's main
80 algorithm. Thus, the specification is said to add "zero-cost" to the normal
81 execution of an application.</p>
83 <p>A more complete description of the Itanium ABI exception handling runtime
84 support of can be found at <a
85 href="http://www.codesourcery.com/cxx-abi/abi-eh.html">Itanium C++ ABI:
86 Exception Handling.</a> A description of the exception frame format can be
88 href="http://refspecs.freestandards.org/LSB_3.0.0/LSB-Core-generic/LSB-
89 Core-generic/ehframechpt.html">Exception Frames</a>, with details of the Dwarf
90 specification at <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">Dwarf 3
91 Standard.</a> A description for the C++ exception table formats can be found at
92 <a href="http://www.codesourcery.com/cxx-abi/exceptions.pdf">Exception Handling
97 <!-- ======================================================================= -->
98 <div class="doc_subsection">
99 <a name="overview">Overview</a>
102 <div class="doc_text">
104 <p>When an exception is thrown in llvm code, the runtime does a best effort to
105 find a handler suited to process the circumstance.</p>
107 <p>The runtime first attempts to find an <i>exception frame</i> corresponding to
108 the function where the exception was thrown. If the programming language (ex.
109 C++) supports exception handling, the exception frame contains a reference to an
110 exception table describing how to process the exception. If the language (ex.
111 C) does not support exception handling or if the exception needs to be forwarded
112 to a prior activation, the exception frame contains information about how to
113 unwind the current activation and restore the state of the prior activation.
114 This process is repeated until the exception is handled. If the exception is
115 not handled and no activations remain, then the application is terminated with
116 an appropriate error message.</p>
118 <p>Since different programming languages have different behaviors when handling
119 exceptions, the exception handling ABI provides a mechanism for supplying
120 <i>personalities.</i> An exception handling personality is defined by way of a
121 <i>personality function</i> (ex. for C++ <tt>__gxx_personality_v0</tt>) which
122 receives the context of the exception, an <i>exception structure</i> containing
123 the exception object type and value, and a reference to the exception table for
124 the current function. The personality function for the current compile unit is
125 specified in a <i>common exception frame</i>.</p>
127 <p>The organization of an exception table is language dependent. For C++, an
128 exception table is organized as a series of code ranges defining what to do if
129 an exception occurs in that range. Typically, the information associated with a
130 range defines which types of exception objects (using C++ <i>type info</i>) that
131 are handled in that range, and an associated action that should take place.
132 Actions typically pass control to a <i>landing pad</i>.</p>
134 <p>A landing pad corresponds to the code found in the catch portion of a
135 try/catch sequence. When execution resumes at a landing pad, it receives the
136 exception structure and a selector corresponding to the <i>type</i> of exception
137 thrown. The selector is then used to determine which catch should actually
138 process the exception.</p>
142 <!-- ======================================================================= -->
143 <div class="doc_section">
144 <a name="codegen">LLVM Code Generation</a>
147 <div class="doc_text">
149 <p>At the time of this writing, only C++ exception handling support is available
150 in LLVM. So the remainder of this document will be somewhat C++-centric.</p>
152 <p>From the C++ developers perspective, exceptions are defined in terms of the
153 <tt>throw</tt> and <tt>try/catch</tt> statements. In this section we will
154 describe the implementation of llvm exception handling in terms of C++
159 <!-- ======================================================================= -->
160 <div class="doc_subsection">
161 <a name="throw">Throw</a>
164 <div class="doc_text">
166 <p>Languages that support exception handling typically provide a <tt>throw</tt>
167 operation to initiate the exception process. Internally, a throw operation
168 breaks down into two steps. First, a request is made to allocate exception
169 space for an exception structure. This structure needs to survive beyond the
170 current activation. This structure will contain the type and value of the
171 object being thrown. Second, a call is made to the runtime to raise the
172 exception, passing the exception structure as an argument.</p>
174 <p>In C++, the allocation of the exception structure is done by the
175 <tt>__cxa_allocate_exception</tt> runtime function. The exception raising is
176 handled by <tt>__cxa_throw</tt>. The type of the exception is represented using
177 a C++ RTTI type info structure.</p>
181 <!-- ======================================================================= -->
182 <div class="doc_subsection">
183 <a name="try_catch">Try/Catch</a>
186 <div class="doc_text">
188 <p>A call within the scope of a try statement can potentially raise an exception.
189 In those circumstances, the LLVM C++ front-end replaces the call with an
190 <tt>invoke</tt> instruction. Unlike a call, the invoke has two potential
191 continuation points; where to continue when the call succeeds as per normal, and
192 where to continue if the call raises an exception, either by a throw or the
193 unwinding of a throw.</p>
195 <p>The term used to define a the place where an invoke continues after an
196 exception is called a <i>landing pad</i>. LLVM landing pads are conceptually
197 alternative function entry points where a exception structure reference and a type
198 info index are passed in as arguments. The landing pad saves the exception
199 structure reference and then proceeds to select the catch block that corresponds
200 to the type info of the exception object.</p>
202 <p>Two llvm intrinsic functions are used convey information about the landing
203 pad to the back end.</p>
205 <p><a href="#llvm_eh_exception"><tt>llvm.eh.exception</tt></a> takes no
206 arguments and returns the exception structure reference. The backend replaces
207 this intrinsic with the code that accesses the first argument of a call. The
208 LLVM C++ front end generates code to save this value in an alloca location for
209 further use in the landing pad and catch code.</p>
211 <p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
212 three arguments. The first argument is the reference to the exception
213 structure. The second argument is a reference to the personality function to be
214 used for this try catch sequence. Each of the remaining arguments is either a
215 reference to the type info for a catch statement, or a non-negative integer
216 followed by that many type info references, representing a
217 <a href="#throw_filters">filter</a>.
218 The exception is tested against the arguments sequentially from first to last.
219 The <i>catch all</i> (...) is represented with a <tt>null i8*</tt>. The result
220 of the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a positive
221 number if the exception matched a type info, a negative number if it matched a
222 filter, and zero if it didn't match anything. If a type info matched then the
223 returned value is the index of the type info in the exception table.
224 The LLVM C++ front end generates code to save this value in an alloca location
225 for further use in the landing pad and catch code.</p>
227 <p>Once the landing pad has the type info selector, the code branches to the
228 code for the first catch. The catch then checks the value of the type info
229 selector against the index of type info for that catch. Since the type info
230 index is not known until all the type info have been gathered in the backend,
231 the catch code will call the <a
232 href="#llvm_eh_typeid_for"><tt>llvm.eh.typeid.for</tt></a> intrinsic to
233 determine the index for a given type info. If the catch fails to match the
234 selector then control is passed on to the next catch. Note: Since the landing
235 pad will not be used if there is no match in the list of type info on the call
236 to <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>, then neither the
237 last catch nor <i>catch all</i> need to perform the the check against the
240 <p>Finally, the entry and exit of catch code is bracketed with calls to
241 <tt>__cxa_begin_catch</tt> and <tt>__cxa_end_catch</tt>.
242 <tt>__cxa_begin_catch</tt> takes a exception structure reference as an argument
243 and returns the value of the exception object.</tt> <tt>__cxa_end_catch</tt>
244 takes a exception structure reference as an argument. This function clears the
245 exception from the exception space. Note: a rethrow from within the catch may
246 replace this call with a <tt>__cxa_rethrow</tt>.</p>
250 <!-- ======================================================================= -->
251 <div class="doc_subsection">
252 <a name="finallys">Finallys</a>
255 <div class="doc_text">
257 <p>To handle destructors and cleanups in try code, control may not run directly
258 from a landing pad to the first catch. Control may actually flow from the
259 landing pad to clean up code and then to the first catch. Since the required
260 clean up for each invoke in a try may be different (ex., intervening
261 constructor), there may be several landing pads for a given try.</p>
265 <!-- ======================================================================= -->
266 <div class="doc_subsection">
267 <a name="throw_filters">Throw Filters</a>
270 <div class="doc_text">
272 <p>C++ allows the specification of which exception types that can be thrown from
273 a function. To represent this a top level landing pad may exist to filter out
274 invalid types. To express this in LLVM code the landing pad will call <a
275 href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The arguments are the
276 number of different type infos the function may throw, followed by the type
278 <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> will return a negative
279 value if the exception does not match any of the type infos. If no match is
280 found then a call to <tt>__cxa_call_unexpected</tt> should be made, otherwise
281 <tt>_Unwind_Resume</tt>. Each of these functions require a reference to the
282 exception structure.</p>
286 <!-- ======================================================================= -->
287 <div class="doc_section">
288 <a name="format_common_intrinsics">Exception Handling Intrinsics</a>
291 <div class="doc_text">
293 <p>LLVM uses several intrinsic functions (name prefixed with "llvm.eh") to
294 provide exception handling information at various points in generated code.</p>
298 <!-- ======================================================================= -->
299 <div class="doc_subsubsection">
300 <a name="llvm_eh_exception">llvm.eh.exception</a>
303 <div class="doc_text">
305 i8* %<a href="#llvm_eh_exception">llvm.eh.exception</a>( )
308 <p>This intrinsic indicates that the exception structure is available at this
309 point in the code. The backend will replace this intrinsic with code to fetch
310 the first argument of a call. The effect is that the intrinsic result is the
311 exception structure reference.</p>
315 <!-- ======================================================================= -->
316 <div class="doc_subsubsection">
317 <a name="llvm_eh_selector">llvm.eh.selector</a>
320 <div class="doc_text">
322 i32 %<a href="#llvm_eh_selector">llvm.eh.selector</a>(i8*, i8*, i8*, ...)
325 <p>This intrinsic indicates that the exception selector is available at this
326 point in the code. The backend will replace this intrinsic with code to fetch
327 the second argument of a call. The effect is that the intrinsic result is the
328 exception selector.</p>
330 <p><a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> takes a minimum of
331 three arguments. The first argument is the reference to the exception
332 structure. The second argument is a reference to the personality function to be
333 used for this try catch sequence. Each of the remaining arguments is either a
334 reference to the type info for a catch statement, or a non-negative integer
335 followed by that many type info references, representing a
336 <a href="#throw_filters">filter</a>.
337 The exception is tested against the arguments sequentially from first to last.
338 The <i>catch all</i> (...) is represented with a <tt>null i8*</tt>. The result
339 of the <a href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a> is a positive
340 number if the exception matched a type info, a negative number if it matched a
341 filter, and zero if it didn't match anything. If a type info matched then the
342 returned value is the index of the type info in the exception table.</p>
346 <!-- ======================================================================= -->
347 <div class="doc_subsubsection">
348 <a name="llvm_eh_typeid_for">llvm.eh.typeid.for</a>
351 <div class="doc_text">
353 i32 %<a href="#llvm_eh_typeid_for">llvm.eh.typeid.for</a>(i8*)
356 <p>This intrinsic returns the type info index in the exception table of the
357 current function. This value can be used to compare against the result of <a
358 href="#llvm_eh_selector"><tt>llvm.eh.selector</tt></a>. The single argument is
359 a reference to a type info.</p>
363 <!-- ======================================================================= -->
364 <div class="doc_section">
365 <a name="asm">Asm Table Formats</a>
368 <div class="doc_text">
370 <p>There are two tables that are used by the exception handling runtime to
371 determine which actions should take place when an exception is thrown.</p>
375 <!-- ======================================================================= -->
376 <div class="doc_subsection">
377 <a name="unwind_tables">Exception Handling Frame</a>
380 <div class="doc_text">
382 <p>An exception handling frame <tt>eh_frame</tt> is very similar to the unwind
383 frame used by dwarf debug info. The frame contains all the information
384 necessary to tear down the current frame and restore the state of the prior
385 frame. There is an exception handling frame for each function in a compile
386 unit, plus a common exception handling frame that defines information common to
387 all functions in the unit.</p>
389 <p>Todo - Table details here.</p>
393 <!-- ======================================================================= -->
394 <div class="doc_subsection">
395 <a name="exception_tables">Exception Tables</a>
398 <div class="doc_text">
400 <p>An exception table contains information about what actions to take when an
401 exception is thrown in a particular part of a function's code. There is
402 one exception table per function except leaf routines and functions that have
403 only calls to non-throwing functions will not need an exception table.</p>
405 <p>Todo - Table details here.</p>
409 <!-- ======================================================================= -->
410 <div class="doc_section">
411 <a name="todo">ToDo</a>
414 <div class="doc_text">
418 <li><p>Testing/Testing/Testing.</li></p>
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