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LLVM 3.4
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Thu, 12 Dec 2013 13:56:28 +0900
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1 ==========================
2 Exception Handling in LLVM
3 ==========================
4
5 .. contents::
6 :local:
7
8 Introduction
9 ============
10
11 This document is the central repository for all information pertaining to
12 exception handling in LLVM. It describes the format that LLVM exception
13 handling information takes, which is useful for those interested in creating
14 front-ends or dealing directly with the information. Further, this document
15 provides specific examples of what exception handling information is used for in
16 C and C++.
17
18 Itanium ABI Zero-cost Exception Handling
19 ----------------------------------------
20
21 Exception handling for most programming languages is designed to recover from
22 conditions that rarely occur during general use of an application. To that end,
23 exception handling should not interfere with the main flow of an application's
24 algorithm by performing checkpointing tasks, such as saving the current pc or
25 register state.
26
27 The Itanium ABI Exception Handling Specification defines a methodology for
28 providing outlying data in the form of exception tables without inlining
29 speculative exception handling code in the flow of an application's main
30 algorithm. Thus, the specification is said to add "zero-cost" to the normal
31 execution of an application.
32
33 A more complete description of the Itanium ABI exception handling runtime
34 support of can be found at `Itanium C++ ABI: Exception Handling
35 <http://mentorembedded.github.com/cxx-abi/abi-eh.html>`_. A description of the
36 exception frame format can be found at `Exception Frames
37 <http://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html>`_,
38 with details of the DWARF 4 specification at `DWARF 4 Standard
39 <http://dwarfstd.org/Dwarf4Std.php>`_. A description for the C++ exception
40 table formats can be found at `Exception Handling Tables
41 <http://mentorembedded.github.com/cxx-abi/exceptions.pdf>`_.
42
43 Setjmp/Longjmp Exception Handling
44 ---------------------------------
45
46 Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
47 `llvm.eh.sjlj.setjmp`_ and `llvm.eh.sjlj.longjmp`_ to handle control flow for
48 exception handling.
49
50 For each function which does exception processing --- be it ``try``/``catch``
51 blocks or cleanups --- that function registers itself on a global frame
52 list. When exceptions are unwinding, the runtime uses this list to identify
53 which functions need processing.
54
55 Landing pad selection is encoded in the call site entry of the function
56 context. The runtime returns to the function via `llvm.eh.sjlj.longjmp`_, where
57 a switch table transfers control to the appropriate landing pad based on the
58 index stored in the function context.
59
60 In contrast to DWARF exception handling, which encodes exception regions and
61 frame information in out-of-line tables, SJLJ exception handling builds and
62 removes the unwind frame context at runtime. This results in faster exception
63 handling at the expense of slower execution when no exceptions are thrown. As
64 exceptions are, by their nature, intended for uncommon code paths, DWARF
65 exception handling is generally preferred to SJLJ.
66
67 Overview
68 --------
69
70 When an exception is thrown in LLVM code, the runtime does its best to find a
71 handler suited to processing the circumstance.
72
73 The runtime first attempts to find an *exception frame* corresponding to the
74 function where the exception was thrown. If the programming language supports
75 exception handling (e.g. C++), the exception frame contains a reference to an
76 exception table describing how to process the exception. If the language does
77 not support exception handling (e.g. C), or if the exception needs to be
78 forwarded to a prior activation, the exception frame contains information about
79 how to unwind the current activation and restore the state of the prior
80 activation. This process is repeated until the exception is handled. If the
81 exception is not handled and no activations remain, then the application is
82 terminated with an appropriate error message.
83
84 Because different programming languages have different behaviors when handling
85 exceptions, the exception handling ABI provides a mechanism for
86 supplying *personalities*. An exception handling personality is defined by
87 way of a *personality function* (e.g. ``__gxx_personality_v0`` in C++),
88 which receives the context of the exception, an *exception structure*
89 containing the exception object type and value, and a reference to the exception
90 table for the current function. The personality function for the current
91 compile unit is specified in a *common exception frame*.
92
93 The organization of an exception table is language dependent. For C++, an
94 exception table is organized as a series of code ranges defining what to do if
95 an exception occurs in that range. Typically, the information associated with a
96 range defines which types of exception objects (using C++ *type info*) that are
97 handled in that range, and an associated action that should take place. Actions
98 typically pass control to a *landing pad*.
99
100 A landing pad corresponds roughly to the code found in the ``catch`` portion of
101 a ``try``/``catch`` sequence. When execution resumes at a landing pad, it
102 receives an *exception structure* and a *selector value* corresponding to the
103 *type* of exception thrown. The selector is then used to determine which *catch*
104 should actually process the exception.
105
106 LLVM Code Generation
107 ====================
108
109 From a C++ developer's perspective, exceptions are defined in terms of the
110 ``throw`` and ``try``/``catch`` statements. In this section we will describe the
111 implementation of LLVM exception handling in terms of C++ examples.
112
113 Throw
114 -----
115
116 Languages that support exception handling typically provide a ``throw``
117 operation to initiate the exception process. Internally, a ``throw`` operation
118 breaks down into two steps.
119
120 #. A request is made to allocate exception space for an exception structure.
121 This structure needs to survive beyond the current activation. This structure
122 will contain the type and value of the object being thrown.
123
124 #. A call is made to the runtime to raise the exception, passing the exception
125 structure as an argument.
126
127 In C++, the allocation of the exception structure is done by the
128 ``__cxa_allocate_exception`` runtime function. The exception raising is handled
129 by ``__cxa_throw``. The type of the exception is represented using a C++ RTTI
130 structure.
131
132 Try/Catch
133 ---------
134
135 A call within the scope of a *try* statement can potentially raise an
136 exception. In those circumstances, the LLVM C++ front-end replaces the call with
137 an ``invoke`` instruction. Unlike a call, the ``invoke`` has two potential
138 continuation points:
139
140 #. where to continue when the call succeeds as per normal, and
141
142 #. where to continue if the call raises an exception, either by a throw or the
143 unwinding of a throw
144
145 The term used to define a the place where an ``invoke`` continues after an
146 exception is called a *landing pad*. LLVM landing pads are conceptually
147 alternative function entry points where an exception structure reference and a
148 type info index are passed in as arguments. The landing pad saves the exception
149 structure reference and then proceeds to select the catch block that corresponds
150 to the type info of the exception object.
151
152 The LLVM :ref:`i_landingpad` is used to convey information about the landing
153 pad to the back end. For C++, the ``landingpad`` instruction returns a pointer
154 and integer pair corresponding to the pointer to the *exception structure* and
155 the *selector value* respectively.
156
157 The ``landingpad`` instruction takes a reference to the personality function to
158 be used for this ``try``/``catch`` sequence. The remainder of the instruction is
159 a list of *cleanup*, *catch*, and *filter* clauses. The exception is tested
160 against the clauses sequentially from first to last. The selector value is a
161 positive number if the exception matched a type info, a negative number if it
162 matched a filter, and zero if it matched a cleanup. If nothing is matched, the
163 behavior of the program is `undefined`_. If a type info matched, then the
164 selector value is the index of the type info in the exception table, which can
165 be obtained using the `llvm.eh.typeid.for`_ intrinsic.
166
167 Once the landing pad has the type info selector, the code branches to the code
168 for the first catch. The catch then checks the value of the type info selector
169 against the index of type info for that catch. Since the type info index is not
170 known until all the type infos have been gathered in the backend, the catch code
171 must call the `llvm.eh.typeid.for`_ intrinsic to determine the index for a given
172 type info. If the catch fails to match the selector then control is passed on to
173 the next catch.
174
175 Finally, the entry and exit of catch code is bracketed with calls to
176 ``__cxa_begin_catch`` and ``__cxa_end_catch``.
177
178 * ``__cxa_begin_catch`` takes an exception structure reference as an argument
179 and returns the value of the exception object.
180
181 * ``__cxa_end_catch`` takes no arguments. This function:
182
183 #. Locates the most recently caught exception and decrements its handler
184 count,
185
186 #. Removes the exception from the *caught* stack if the handler count goes to
187 zero, and
188
189 #. Destroys the exception if the handler count goes to zero and the exception
190 was not re-thrown by throw.
191
192 .. note::
193
194 a rethrow from within the catch may replace this call with a
195 ``__cxa_rethrow``.
196
197 Cleanups
198 --------
199
200 A cleanup is extra code which needs to be run as part of unwinding a scope. C++
201 destructors are a typical example, but other languages and language extensions
202 provide a variety of different kinds of cleanups. In general, a landing pad may
203 need to run arbitrary amounts of cleanup code before actually entering a catch
204 block. To indicate the presence of cleanups, a :ref:`i_landingpad` should have
205 a *cleanup* clause. Otherwise, the unwinder will not stop at the landing pad if
206 there are no catches or filters that require it to.
207
208 .. note::
209
210 Do not allow a new exception to propagate out of the execution of a
211 cleanup. This can corrupt the internal state of the unwinder. Different
212 languages describe different high-level semantics for these situations: for
213 example, C++ requires that the process be terminated, whereas Ada cancels both
214 exceptions and throws a third.
215
216 When all cleanups are finished, if the exception is not handled by the current
217 function, resume unwinding by calling the `resume
218 instruction <LangRef.html#i_resume>`_, passing in the result of the
219 ``landingpad`` instruction for the original landing pad.
220
221 Throw Filters
222 -------------
223
224 C++ allows the specification of which exception types may be thrown from a
225 function. To represent this, a top level landing pad may exist to filter out
226 invalid types. To express this in LLVM code the :ref:`i_landingpad` will have a
227 filter clause. The clause consists of an array of type infos.
228 ``landingpad`` will return a negative value
229 if the exception does not match any of the type infos. If no match is found then
230 a call to ``__cxa_call_unexpected`` should be made, otherwise
231 ``_Unwind_Resume``. Each of these functions requires a reference to the
232 exception structure. Note that the most general form of a ``landingpad``
233 instruction can have any number of catch, cleanup, and filter clauses (though
234 having more than one cleanup is pointless). The LLVM C++ front-end can generate
235 such ``landingpad`` instructions due to inlining creating nested exception
236 handling scopes.
237
238 .. _undefined:
239
240 Restrictions
241 ------------
242
243 The unwinder delegates the decision of whether to stop in a call frame to that
244 call frame's language-specific personality function. Not all unwinders guarantee
245 that they will stop to perform cleanups. For example, the GNU C++ unwinder
246 doesn't do so unless the exception is actually caught somewhere further up the
247 stack.
248
249 In order for inlining to behave correctly, landing pads must be prepared to
250 handle selector results that they did not originally advertise. Suppose that a
251 function catches exceptions of type ``A``, and it's inlined into a function that
252 catches exceptions of type ``B``. The inliner will update the ``landingpad``
253 instruction for the inlined landing pad to include the fact that ``B`` is also
254 caught. If that landing pad assumes that it will only be entered to catch an
255 ``A``, it's in for a rude awakening. Consequently, landing pads must test for
256 the selector results they understand and then resume exception propagation with
257 the `resume instruction <LangRef.html#i_resume>`_ if none of the conditions
258 match.
259
260 Exception Handling Intrinsics
261 =============================
262
263 In addition to the ``landingpad`` and ``resume`` instructions, LLVM uses several
264 intrinsic functions (name prefixed with ``llvm.eh``) to provide exception
265 handling information at various points in generated code.
266
267 .. _llvm.eh.typeid.for:
268
269 ``llvm.eh.typeid.for``
270 ----------------------
271
272 .. code-block:: llvm
273
274 i32 @llvm.eh.typeid.for(i8* %type_info)
275
276
277 This intrinsic returns the type info index in the exception table of the current
278 function. This value can be used to compare against the result of
279 ``landingpad`` instruction. The single argument is a reference to a type info.
280
281 .. _llvm.eh.sjlj.setjmp:
282
283 ``llvm.eh.sjlj.setjmp``
284 -----------------------
285
286 .. code-block:: llvm
287
288 i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf)
289
290 For SJLJ based exception handling, this intrinsic forces register saving for the
291 current function and stores the address of the following instruction for use as
292 a destination address by `llvm.eh.sjlj.longjmp`_. The buffer format and the
293 overall functioning of this intrinsic is compatible with the GCC
294 ``__builtin_setjmp`` implementation allowing code built with the clang and GCC
295 to interoperate.
296
297 The single parameter is a pointer to a five word buffer in which the calling
298 context is saved. The front end places the frame pointer in the first word, and
299 the target implementation of this intrinsic should place the destination address
300 for a `llvm.eh.sjlj.longjmp`_ in the second word. The following three words are
301 available for use in a target-specific manner.
302
303 .. _llvm.eh.sjlj.longjmp:
304
305 ``llvm.eh.sjlj.longjmp``
306 ------------------------
307
308 .. code-block:: llvm
309
310 void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf)
311
312 For SJLJ based exception handling, the ``llvm.eh.sjlj.longjmp`` intrinsic is
313 used to implement ``__builtin_longjmp()``. The single parameter is a pointer to
314 a buffer populated by `llvm.eh.sjlj.setjmp`_. The frame pointer and stack
315 pointer are restored from the buffer, then control is transferred to the
316 destination address.
317
318 ``llvm.eh.sjlj.lsda``
319 ---------------------
320
321 .. code-block:: llvm
322
323 i8* @llvm.eh.sjlj.lsda()
324
325 For SJLJ based exception handling, the ``llvm.eh.sjlj.lsda`` intrinsic returns
326 the address of the Language Specific Data Area (LSDA) for the current
327 function. The SJLJ front-end code stores this address in the exception handling
328 function context for use by the runtime.
329
330 ``llvm.eh.sjlj.callsite``
331 -------------------------
332
333 .. code-block:: llvm
334
335 void @llvm.eh.sjlj.callsite(i32 %call_site_num)
336
337 For SJLJ based exception handling, the ``llvm.eh.sjlj.callsite`` intrinsic
338 identifies the callsite value associated with the following ``invoke``
339 instruction. This is used to ensure that landing pad entries in the LSDA are
340 generated in matching order.
341
342 Asm Table Formats
343 =================
344
345 There are two tables that are used by the exception handling runtime to
346 determine which actions should be taken when an exception is thrown.
347
348 Exception Handling Frame
349 ------------------------
350
351 An exception handling frame ``eh_frame`` is very similar to the unwind frame
352 used by DWARF debug info. The frame contains all the information necessary to
353 tear down the current frame and restore the state of the prior frame. There is
354 an exception handling frame for each function in a compile unit, plus a common
355 exception handling frame that defines information common to all functions in the
356 unit.
357
358 Exception Tables
359 ----------------
360
361 An exception table contains information about what actions to take when an
362 exception is thrown in a particular part of a function's code. There is one
363 exception table per function, except leaf functions and functions that have
364 calls only to non-throwing functions. They do not need an exception table.