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annotate docs/ExceptionHandling.rst @ 65:35ab9e3560be

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remove null pointer access bug which occured if the callee codesegment is accessed by pointer on LowerCallTo().
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Tue, 11 Feb 2014 02:09:46 +0900
parents 95c75e76d11b
children 54457678186b
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1 ==========================
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2 Exception Handling in LLVM
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3 ==========================
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4
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5 .. contents::
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6 :local:
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7
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8 Introduction
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9 ============
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10
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11 This document is the central repository for all information pertaining to
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12 exception handling in LLVM. It describes the format that LLVM exception
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13 handling information takes, which is useful for those interested in creating
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14 front-ends or dealing directly with the information. Further, this document
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15 provides specific examples of what exception handling information is used for in
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16 C and C++.
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17
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18 Itanium ABI Zero-cost Exception Handling
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19 ----------------------------------------
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20
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21 Exception handling for most programming languages is designed to recover from
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22 conditions that rarely occur during general use of an application. To that end,
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23 exception handling should not interfere with the main flow of an application's
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24 algorithm by performing checkpointing tasks, such as saving the current pc or
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25 register state.
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26
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27 The Itanium ABI Exception Handling Specification defines a methodology for
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28 providing outlying data in the form of exception tables without inlining
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29 speculative exception handling code in the flow of an application's main
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30 algorithm. Thus, the specification is said to add "zero-cost" to the normal
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31 execution of an application.
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32
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33 A more complete description of the Itanium ABI exception handling runtime
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34 support of can be found at `Itanium C++ ABI: Exception Handling
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35 <http://mentorembedded.github.com/cxx-abi/abi-eh.html>`_. A description of the
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36 exception frame format can be found at `Exception Frames
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37 <http://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/ehframechpt.html>`_,
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38 with details of the DWARF 4 specification at `DWARF 4 Standard
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39 <http://dwarfstd.org/Dwarf4Std.php>`_. A description for the C++ exception
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40 table formats can be found at `Exception Handling Tables
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41 <http://mentorembedded.github.com/cxx-abi/exceptions.pdf>`_.
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42
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43 Setjmp/Longjmp Exception Handling
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44 ---------------------------------
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45
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46 Setjmp/Longjmp (SJLJ) based exception handling uses LLVM intrinsics
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47 `llvm.eh.sjlj.setjmp`_ and `llvm.eh.sjlj.longjmp`_ to handle control flow for
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48 exception handling.
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49
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50 For each function which does exception processing --- be it ``try``/``catch``
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51 blocks or cleanups --- that function registers itself on a global frame
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52 list. When exceptions are unwinding, the runtime uses this list to identify
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53 which functions need processing.
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54
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55 Landing pad selection is encoded in the call site entry of the function
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56 context. The runtime returns to the function via `llvm.eh.sjlj.longjmp`_, where
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57 a switch table transfers control to the appropriate landing pad based on the
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58 index stored in the function context.
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59
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60 In contrast to DWARF exception handling, which encodes exception regions and
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61 frame information in out-of-line tables, SJLJ exception handling builds and
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62 removes the unwind frame context at runtime. This results in faster exception
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63 handling at the expense of slower execution when no exceptions are thrown. As
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64 exceptions are, by their nature, intended for uncommon code paths, DWARF
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65 exception handling is generally preferred to SJLJ.
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66
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67 Overview
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68 --------
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69
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70 When an exception is thrown in LLVM code, the runtime does its best to find a
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71 handler suited to processing the circumstance.
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72
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73 The runtime first attempts to find an *exception frame* corresponding to the
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74 function where the exception was thrown. If the programming language supports
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75 exception handling (e.g. C++), the exception frame contains a reference to an
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76 exception table describing how to process the exception. If the language does
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77 not support exception handling (e.g. C), or if the exception needs to be
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78 forwarded to a prior activation, the exception frame contains information about
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79 how to unwind the current activation and restore the state of the prior
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80 activation. This process is repeated until the exception is handled. If the
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81 exception is not handled and no activations remain, then the application is
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82 terminated with an appropriate error message.
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83
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84 Because different programming languages have different behaviors when handling
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85 exceptions, the exception handling ABI provides a mechanism for
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86 supplying *personalities*. An exception handling personality is defined by
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87 way of a *personality function* (e.g. ``__gxx_personality_v0`` in C++),
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88 which receives the context of the exception, an *exception structure*
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89 containing the exception object type and value, and a reference to the exception
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90 table for the current function. The personality function for the current
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91 compile unit is specified in a *common exception frame*.
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92
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93 The organization of an exception table is language dependent. For C++, an
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94 exception table is organized as a series of code ranges defining what to do if
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95 an exception occurs in that range. Typically, the information associated with a
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96 range defines which types of exception objects (using C++ *type info*) that are
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97 handled in that range, and an associated action that should take place. Actions
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98 typically pass control to a *landing pad*.
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99
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100 A landing pad corresponds roughly to the code found in the ``catch`` portion of
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101 a ``try``/``catch`` sequence. When execution resumes at a landing pad, it
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102 receives an *exception structure* and a *selector value* corresponding to the
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103 *type* of exception thrown. The selector is then used to determine which *catch*
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104 should actually process the exception.
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105
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106 LLVM Code Generation
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107 ====================
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108
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109 From a C++ developer's perspective, exceptions are defined in terms of the
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110 ``throw`` and ``try``/``catch`` statements. In this section we will describe the
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111 implementation of LLVM exception handling in terms of C++ examples.
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112
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113 Throw
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114 -----
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115
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116 Languages that support exception handling typically provide a ``throw``
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117 operation to initiate the exception process. Internally, a ``throw`` operation
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118 breaks down into two steps.
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119
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120 #. A request is made to allocate exception space for an exception structure.
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121 This structure needs to survive beyond the current activation. This structure
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122 will contain the type and value of the object being thrown.
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123
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124 #. A call is made to the runtime to raise the exception, passing the exception
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125 structure as an argument.
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126
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127 In C++, the allocation of the exception structure is done by the
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
128 ``__cxa_allocate_exception`` runtime function. The exception raising is handled
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
129 by ``__cxa_throw``. The type of the exception is represented using a C++ RTTI
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
130 structure.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
131
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
132 Try/Catch
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
133 ---------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
134
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
135 A call within the scope of a *try* statement can potentially raise an
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
136 exception. In those circumstances, the LLVM C++ front-end replaces the call with
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
137 an ``invoke`` instruction. Unlike a call, the ``invoke`` has two potential
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
138 continuation points:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
139
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
140 #. where to continue when the call succeeds as per normal, and
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
141
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
142 #. where to continue if the call raises an exception, either by a throw or the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
143 unwinding of a throw
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
144
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
145 The term used to define a the place where an ``invoke`` continues after an
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
146 exception is called a *landing pad*. LLVM landing pads are conceptually
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
147 alternative function entry points where an exception structure reference and a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
148 type info index are passed in as arguments. The landing pad saves the exception
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
149 structure reference and then proceeds to select the catch block that corresponds
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
150 to the type info of the exception object.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
151
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
152 The LLVM :ref:`i_landingpad` is used to convey information about the landing
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
153 pad to the back end. For C++, the ``landingpad`` instruction returns a pointer
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
154 and integer pair corresponding to the pointer to the *exception structure* and
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
155 the *selector value* respectively.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
156
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
157 The ``landingpad`` instruction takes a reference to the personality function to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
158 be used for this ``try``/``catch`` sequence. The remainder of the instruction is
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
159 a list of *cleanup*, *catch*, and *filter* clauses. The exception is tested
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
160 against the clauses sequentially from first to last. The selector value is a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
161 positive number if the exception matched a type info, a negative number if it
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
162 matched a filter, and zero if it matched a cleanup. If nothing is matched, the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
163 behavior of the program is `undefined`_. If a type info matched, then the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
164 selector value is the index of the type info in the exception table, which can
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
165 be obtained using the `llvm.eh.typeid.for`_ intrinsic.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
166
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
167 Once the landing pad has the type info selector, the code branches to the code
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
168 for the first catch. The catch then checks the value of the type info selector
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
169 against the index of type info for that catch. Since the type info index is not
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
170 known until all the type infos have been gathered in the backend, the catch code
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
171 must call the `llvm.eh.typeid.for`_ intrinsic to determine the index for a given
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
172 type info. If the catch fails to match the selector then control is passed on to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
173 the next catch.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
174
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
175 Finally, the entry and exit of catch code is bracketed with calls to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
176 ``__cxa_begin_catch`` and ``__cxa_end_catch``.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
177
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
178 * ``__cxa_begin_catch`` takes an exception structure reference as an argument
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
179 and returns the value of the exception object.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
180
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
181 * ``__cxa_end_catch`` takes no arguments. This function:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
182
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
183 #. Locates the most recently caught exception and decrements its handler
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
184 count,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
185
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
186 #. Removes the exception from the *caught* stack if the handler count goes to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
187 zero, and
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
188
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
189 #. Destroys the exception if the handler count goes to zero and the exception
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
190 was not re-thrown by throw.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
191
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
192 .. note::
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
193
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
194 a rethrow from within the catch may replace this call with a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
195 ``__cxa_rethrow``.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
196
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
197 Cleanups
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
198 --------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
199
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
200 A cleanup is extra code which needs to be run as part of unwinding a scope. C++
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
201 destructors are a typical example, but other languages and language extensions
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
202 provide a variety of different kinds of cleanups. In general, a landing pad may
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
203 need to run arbitrary amounts of cleanup code before actually entering a catch
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
204 block. To indicate the presence of cleanups, a :ref:`i_landingpad` should have
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
205 a *cleanup* clause. Otherwise, the unwinder will not stop at the landing pad if
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
206 there are no catches or filters that require it to.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
207
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
208 .. note::
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
209
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
210 Do not allow a new exception to propagate out of the execution of a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
211 cleanup. This can corrupt the internal state of the unwinder. Different
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
212 languages describe different high-level semantics for these situations: for
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
213 example, C++ requires that the process be terminated, whereas Ada cancels both
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
214 exceptions and throws a third.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
215
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
216 When all cleanups are finished, if the exception is not handled by the current
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
217 function, resume unwinding by calling the `resume
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
218 instruction <LangRef.html#i_resume>`_, passing in the result of the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
219 ``landingpad`` instruction for the original landing pad.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
220
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
221 Throw Filters
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
222 -------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
223
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
224 C++ allows the specification of which exception types may be thrown from a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
225 function. To represent this, a top level landing pad may exist to filter out
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
226 invalid types. To express this in LLVM code the :ref:`i_landingpad` will have a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
227 filter clause. The clause consists of an array of type infos.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
228 ``landingpad`` will return a negative value
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
229 if the exception does not match any of the type infos. If no match is found then
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
230 a call to ``__cxa_call_unexpected`` should be made, otherwise
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
231 ``_Unwind_Resume``. Each of these functions requires a reference to the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
232 exception structure. Note that the most general form of a ``landingpad``
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
233 instruction can have any number of catch, cleanup, and filter clauses (though
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
234 having more than one cleanup is pointless). The LLVM C++ front-end can generate
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
235 such ``landingpad`` instructions due to inlining creating nested exception
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
236 handling scopes.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
237
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
238 .. _undefined:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
239
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
240 Restrictions
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
241 ------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
242
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
243 The unwinder delegates the decision of whether to stop in a call frame to that
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
244 call frame's language-specific personality function. Not all unwinders guarantee
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
245 that they will stop to perform cleanups. For example, the GNU C++ unwinder
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
246 doesn't do so unless the exception is actually caught somewhere further up the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
247 stack.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
248
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
249 In order for inlining to behave correctly, landing pads must be prepared to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
250 handle selector results that they did not originally advertise. Suppose that a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
251 function catches exceptions of type ``A``, and it's inlined into a function that
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
252 catches exceptions of type ``B``. The inliner will update the ``landingpad``
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
253 instruction for the inlined landing pad to include the fact that ``B`` is also
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
254 caught. If that landing pad assumes that it will only be entered to catch an
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
255 ``A``, it's in for a rude awakening. Consequently, landing pads must test for
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
256 the selector results they understand and then resume exception propagation with
95c75e76d11b LLVM 3.4
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diff changeset
257 the `resume instruction <LangRef.html#i_resume>`_ if none of the conditions
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diff changeset
258 match.
95c75e76d11b LLVM 3.4
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diff changeset
259
95c75e76d11b LLVM 3.4
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diff changeset
260 Exception Handling Intrinsics
95c75e76d11b LLVM 3.4
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diff changeset
261 =============================
95c75e76d11b LLVM 3.4
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262
95c75e76d11b LLVM 3.4
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263 In addition to the ``landingpad`` and ``resume`` instructions, LLVM uses several
95c75e76d11b LLVM 3.4
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parents:
diff changeset
264 intrinsic functions (name prefixed with ``llvm.eh``) to provide exception
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
265 handling information at various points in generated code.
95c75e76d11b LLVM 3.4
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parents:
diff changeset
266
95c75e76d11b LLVM 3.4
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diff changeset
267 .. _llvm.eh.typeid.for:
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parents:
diff changeset
268
95c75e76d11b LLVM 3.4
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diff changeset
269 ``llvm.eh.typeid.for``
95c75e76d11b LLVM 3.4
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parents:
diff changeset
270 ----------------------
95c75e76d11b LLVM 3.4
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271
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272 .. code-block:: llvm
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273
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274 i32 @llvm.eh.typeid.for(i8* %type_info)
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diff changeset
275
95c75e76d11b LLVM 3.4
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276
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diff changeset
277 This intrinsic returns the type info index in the exception table of the current
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
278 function. This value can be used to compare against the result of
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parents:
diff changeset
279 ``landingpad`` instruction. The single argument is a reference to a type info.
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diff changeset
280
95c75e76d11b LLVM 3.4
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diff changeset
281 .. _llvm.eh.sjlj.setjmp:
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diff changeset
282
95c75e76d11b LLVM 3.4
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diff changeset
283 ``llvm.eh.sjlj.setjmp``
95c75e76d11b LLVM 3.4
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parents:
diff changeset
284 -----------------------
95c75e76d11b LLVM 3.4
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parents:
diff changeset
285
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parents:
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286 .. code-block:: llvm
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parents:
diff changeset
287
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diff changeset
288 i32 @llvm.eh.sjlj.setjmp(i8* %setjmp_buf)
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diff changeset
289
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diff changeset
290 For SJLJ based exception handling, this intrinsic forces register saving for the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
291 current function and stores the address of the following instruction for use as
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
292 a destination address by `llvm.eh.sjlj.longjmp`_. The buffer format and the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
293 overall functioning of this intrinsic is compatible with the GCC
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
294 ``__builtin_setjmp`` implementation allowing code built with the clang and GCC
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
295 to interoperate.
95c75e76d11b LLVM 3.4
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parents:
diff changeset
296
95c75e76d11b LLVM 3.4
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parents:
diff changeset
297 The single parameter is a pointer to a five word buffer in which the calling
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parents:
diff changeset
298 context is saved. The front end places the frame pointer in the first word, and
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
299 the target implementation of this intrinsic should place the destination address
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
300 for a `llvm.eh.sjlj.longjmp`_ in the second word. The following three words are
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
301 available for use in a target-specific manner.
95c75e76d11b LLVM 3.4
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parents:
diff changeset
302
95c75e76d11b LLVM 3.4
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parents:
diff changeset
303 .. _llvm.eh.sjlj.longjmp:
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parents:
diff changeset
304
95c75e76d11b LLVM 3.4
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parents:
diff changeset
305 ``llvm.eh.sjlj.longjmp``
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
306 ------------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
307
95c75e76d11b LLVM 3.4
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parents:
diff changeset
308 .. code-block:: llvm
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parents:
diff changeset
309
95c75e76d11b LLVM 3.4
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parents:
diff changeset
310 void @llvm.eh.sjlj.longjmp(i8* %setjmp_buf)
95c75e76d11b LLVM 3.4
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parents:
diff changeset
311
95c75e76d11b LLVM 3.4
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parents:
diff changeset
312 For SJLJ based exception handling, the ``llvm.eh.sjlj.longjmp`` intrinsic is
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
313 used to implement ``__builtin_longjmp()``. The single parameter is a pointer to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
314 a buffer populated by `llvm.eh.sjlj.setjmp`_. The frame pointer and stack
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
315 pointer are restored from the buffer, then control is transferred to the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
316 destination address.
95c75e76d11b LLVM 3.4
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parents:
diff changeset
317
95c75e76d11b LLVM 3.4
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parents:
diff changeset
318 ``llvm.eh.sjlj.lsda``
95c75e76d11b LLVM 3.4
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parents:
diff changeset
319 ---------------------
95c75e76d11b LLVM 3.4
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parents:
diff changeset
320
95c75e76d11b LLVM 3.4
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parents:
diff changeset
321 .. code-block:: llvm
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parents:
diff changeset
322
95c75e76d11b LLVM 3.4
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parents:
diff changeset
323 i8* @llvm.eh.sjlj.lsda()
95c75e76d11b LLVM 3.4
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parents:
diff changeset
324
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
325 For SJLJ based exception handling, the ``llvm.eh.sjlj.lsda`` intrinsic returns
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
326 the address of the Language Specific Data Area (LSDA) for the current
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
327 function. The SJLJ front-end code stores this address in the exception handling
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
328 function context for use by the runtime.
95c75e76d11b LLVM 3.4
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parents:
diff changeset
329
95c75e76d11b LLVM 3.4
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parents:
diff changeset
330 ``llvm.eh.sjlj.callsite``
95c75e76d11b LLVM 3.4
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parents:
diff changeset
331 -------------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
332
95c75e76d11b LLVM 3.4
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parents:
diff changeset
333 .. code-block:: llvm
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
334
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
335 void @llvm.eh.sjlj.callsite(i32 %call_site_num)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
336
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
337 For SJLJ based exception handling, the ``llvm.eh.sjlj.callsite`` intrinsic
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
338 identifies the callsite value associated with the following ``invoke``
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
339 instruction. This is used to ensure that landing pad entries in the LSDA are
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
340 generated in matching order.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
341
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
342 Asm Table Formats
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
343 =================
95c75e76d11b LLVM 3.4
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parents:
diff changeset
344
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
345 There are two tables that are used by the exception handling runtime to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
346 determine which actions should be taken when an exception is thrown.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
347
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
348 Exception Handling Frame
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
349 ------------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
350
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
351 An exception handling frame ``eh_frame`` is very similar to the unwind frame
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
352 used by DWARF debug info. The frame contains all the information necessary to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
353 tear down the current frame and restore the state of the prior frame. There is
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
354 an exception handling frame for each function in a compile unit, plus a common
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
355 exception handling frame that defines information common to all functions in the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
356 unit.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
357
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
358 Exception Tables
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
359 ----------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
360
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
361 An exception table contains information about what actions to take when an
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
362 exception is thrown in a particular part of a function's code. There is one
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
363 exception table per function, except leaf functions and functions that have
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
364 calls only to non-throwing functions. They do not need an exception table.