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author mir3636
date Fri, 16 Feb 2018 21:02:11 +0900
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children c2174574ed3a
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1 ================================
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2 Frequently Asked Questions (FAQ)
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3 ================================
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5 .. contents::
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6 :local:
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9 License
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10 =======
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12 Does the University of Illinois Open Source License really qualify as an "open source" license?
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13 -----------------------------------------------------------------------------------------------
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14 Yes, the license is `certified
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15 <http://www.opensource.org/licenses/UoI-NCSA.php>`_ by the Open Source
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16 Initiative (OSI).
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19 Can I modify LLVM source code and redistribute the modified source?
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20 -------------------------------------------------------------------
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21 Yes. The modified source distribution must retain the copyright notice and
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22 follow the three bulleted conditions listed in the `LLVM license
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23 <http://llvm.org/svn/llvm-project/llvm/trunk/LICENSE.TXT>`_.
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26 Can I modify the LLVM source code and redistribute binaries or other tools based on it, without redistributing the source?
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27 --------------------------------------------------------------------------------------------------------------------------
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28 Yes. This is why we distribute LLVM under a less restrictive license than GPL,
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29 as explained in the first question above.
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32 Source Code
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33 ===========
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34
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35 In what language is LLVM written?
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36 ---------------------------------
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37 All of the LLVM tools and libraries are written in C++ with extensive use of
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38 the STL.
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41 How portable is the LLVM source code?
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42 -------------------------------------
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43 The LLVM source code should be portable to most modern Unix-like operating
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44 systems. Most of the code is written in standard C++ with operating system
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45 services abstracted to a support library. The tools required to build and
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46 test LLVM have been ported to a plethora of platforms.
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47
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48 Some porting problems may exist in the following areas:
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49
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50 * The autoconf/makefile build system relies heavily on UNIX shell tools,
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51 like the Bourne Shell and sed. Porting to systems without these tools
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52 (MacOS 9, Plan 9) will require more effort.
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53
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54 What API do I use to store a value to one of the virtual registers in LLVM IR's SSA representation?
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55 ---------------------------------------------------------------------------------------------------
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56
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57 In short: you can't. It's actually kind of a silly question once you grok
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58 what's going on. Basically, in code like:
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59
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60 .. code-block:: llvm
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61
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62 %result = add i32 %foo, %bar
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63
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64 , ``%result`` is just a name given to the ``Value`` of the ``add``
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65 instruction. In other words, ``%result`` *is* the add instruction. The
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66 "assignment" doesn't explicitly "store" anything to any "virtual register";
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67 the "``=``" is more like the mathematical sense of equality.
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68
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69 Longer explanation: In order to generate a textual representation of the
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70 IR, some kind of name has to be given to each instruction so that other
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71 instructions can textually reference it. However, the isomorphic in-memory
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72 representation that you manipulate from C++ has no such restriction since
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73 instructions can simply keep pointers to any other ``Value``'s that they
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74 reference. In fact, the names of dummy numbered temporaries like ``%1`` are
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75 not explicitly represented in the in-memory representation at all (see
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76 ``Value::getName()``).
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77
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78
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79 Source Languages
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80 ================
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81
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82 What source languages are supported?
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83 ------------------------------------
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84
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85 LLVM currently has full support for C and C++ source languages through
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86 `Clang <http://clang.llvm.org/>`_. Many other language frontends have
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87 been written using LLVM, and an incomplete list is available at
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88 `projects with LLVM <http://llvm.org/ProjectsWithLLVM/>`_.
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90
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91 I'd like to write a self-hosting LLVM compiler. How should I interface with the LLVM middle-end optimizers and back-end code generators?
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92 ----------------------------------------------------------------------------------------------------------------------------------------
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93 Your compiler front-end will communicate with LLVM by creating a module in the
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94 LLVM intermediate representation (IR) format. Assuming you want to write your
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95 language's compiler in the language itself (rather than C++), there are 3
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96 major ways to tackle generating LLVM IR from a front-end:
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97
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98 1. **Call into the LLVM libraries code using your language's FFI (foreign
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99 function interface).**
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100
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101 * *for:* best tracks changes to the LLVM IR, .ll syntax, and .bc format
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102
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103 * *for:* enables running LLVM optimization passes without a emit/parse
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104 overhead
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105
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106 * *for:* adapts well to a JIT context
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107
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108 * *against:* lots of ugly glue code to write
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109
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110 2. **Emit LLVM assembly from your compiler's native language.**
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111
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112 * *for:* very straightforward to get started
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113
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114 * *against:* the .ll parser is slower than the bitcode reader when
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115 interfacing to the middle end
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116
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117 * *against:* it may be harder to track changes to the IR
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118
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119 3. **Emit LLVM bitcode from your compiler's native language.**
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120
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121 * *for:* can use the more-efficient bitcode reader when interfacing to the
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122 middle end
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123
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124 * *against:* you'll have to re-engineer the LLVM IR object model and bitcode
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125 writer in your language
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126
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127 * *against:* it may be harder to track changes to the IR
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128
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129 If you go with the first option, the C bindings in include/llvm-c should help
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130 a lot, since most languages have strong support for interfacing with C. The
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131 most common hurdle with calling C from managed code is interfacing with the
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132 garbage collector. The C interface was designed to require very little memory
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133 management, and so is straightforward in this regard.
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134
95c75e76d11b LLVM 3.4
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diff changeset
135 What support is there for a higher level source language constructs for building a compiler?
95c75e76d11b LLVM 3.4
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136 --------------------------------------------------------------------------------------------
95c75e76d11b LLVM 3.4
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137 Currently, there isn't much. LLVM supports an intermediate representation
95c75e76d11b LLVM 3.4
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138 which is useful for code representation but will not support the high level
95c75e76d11b LLVM 3.4
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139 (abstract syntax tree) representation needed by most compilers. There are no
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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140 facilities for lexical nor semantic analysis.
95c75e76d11b LLVM 3.4
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141
95c75e76d11b LLVM 3.4
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142
95c75e76d11b LLVM 3.4
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143 I don't understand the ``GetElementPtr`` instruction. Help!
95c75e76d11b LLVM 3.4
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144 -----------------------------------------------------------
95c75e76d11b LLVM 3.4
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145 See `The Often Misunderstood GEP Instruction <GetElementPtr.html>`_.
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146
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147
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148 Using the C and C++ Front Ends
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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149 ==============================
95c75e76d11b LLVM 3.4
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150
95c75e76d11b LLVM 3.4
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151 Can I compile C or C++ code to platform-independent LLVM bitcode?
95c75e76d11b LLVM 3.4
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152 -----------------------------------------------------------------
95c75e76d11b LLVM 3.4
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153 No. C and C++ are inherently platform-dependent languages. The most obvious
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
154 example of this is the preprocessor. A very common way that C code is made
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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155 portable is by using the preprocessor to include platform-specific code. In
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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diff changeset
156 practice, information about other platforms is lost after preprocessing, so
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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157 the result is inherently dependent on the platform that the preprocessing was
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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158 targeting.
95c75e76d11b LLVM 3.4
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159
95c75e76d11b LLVM 3.4
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parents:
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160 Another example is ``sizeof``. It's common for ``sizeof(long)`` to vary
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
161 between platforms. In most C front-ends, ``sizeof`` is expanded to a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
162 constant immediately, thus hard-wiring a platform-specific detail.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
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163
95c75e76d11b LLVM 3.4
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164 Also, since many platforms define their ABIs in terms of C, and since LLVM is
95c75e76d11b LLVM 3.4
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parents:
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165 lower-level than C, front-ends currently must emit platform-specific IR in
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
166 order to have the result conform to the platform ABI.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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167
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168
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
169 Questions about code generated by the demo page
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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170 ===============================================
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171
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parents:
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172 What is this ``llvm.global_ctors`` and ``_GLOBAL__I_a...`` stuff that happens when I ``#include <iostream>``?
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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173 -------------------------------------------------------------------------------------------------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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174 If you ``#include`` the ``<iostream>`` header into a C++ translation unit,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
175 the file will probably use the ``std::cin``/``std::cout``/... global objects.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
176 However, C++ does not guarantee an order of initialization between static
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
177 objects in different translation units, so if a static ctor/dtor in your .cpp
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
178 file used ``std::cout``, for example, the object would not necessarily be
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
179 automatically initialized before your use.
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 To make ``std::cout`` and friends work correctly in these scenarios, the STL
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
182 that we use declares a static object that gets created in every translation
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
183 unit that includes ``<iostream>``. This object has a static constructor
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
184 and destructor that initializes and destroys the global iostream objects
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
185 before they could possibly be used in the file. The code that you see in the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
186 ``.ll`` file corresponds to the constructor and destructor registration code.
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
187
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
188 If you would like to make it easier to *understand* the LLVM code generated
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
189 by the compiler in the demo page, consider using ``printf()`` instead of
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
190 ``iostream``\s to print values.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
191
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parents:
diff changeset
192
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
193 Where did all of my code go??
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
194 -----------------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
195 If you are using the LLVM demo page, you may often wonder what happened to
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
196 all of the code that you typed in. Remember that the demo script is running
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
197 the code through the LLVM optimizers, so if your code doesn't actually do
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
198 anything useful, it might all be deleted.
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 To prevent this, make sure that the code is actually needed. For example, if
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
201 you are computing some expression, return the value from the function instead
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
202 of leaving it in a local variable. If you really want to constrain the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
203 optimizer, you can read from and assign to ``volatile`` global variables.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
204
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diff changeset
205
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
206 What is this "``undef``" thing that shows up in my code?
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 ``undef`` is the LLVM way of representing a value that is not defined. You
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
209 can get these if you do not initialize a variable before you use it. For
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
210 example, the C function:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
211
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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212 .. code-block:: c
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
213
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
214 int X() { int i; return i; }
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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 Is compiled to "``ret i32 undef``" because "``i``" never has a value specified
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
217 for it.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
218
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
219
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
220 Why does instcombine + simplifycfg turn a call to a function with a mismatched calling convention into "unreachable"? Why not make the verifier reject it?
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
221 ----------------------------------------------------------------------------------------------------------------------------------------------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
222 This is a common problem run into by authors of front-ends that are using
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
223 custom calling conventions: you need to make sure to set the right calling
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
224 convention on both the function and on each call to the function. For
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
225 example, this code:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
226
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
227 .. code-block:: llvm
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
228
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
229 define fastcc void @foo() {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
230 ret void
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
231 }
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
232 define void @bar() {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
233 call void @foo()
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
234 ret void
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
235 }
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
236
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
237 Is optimized to:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
238
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
239 .. code-block:: llvm
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
240
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
241 define fastcc void @foo() {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
242 ret void
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
243 }
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
244 define void @bar() {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
245 unreachable
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
246 }
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
247
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
248 ... with "``opt -instcombine -simplifycfg``". This often bites people because
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
249 "all their code disappears". Setting the calling convention on the caller and
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
250 callee is required for indirect calls to work, so people often ask why not
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
251 make the verifier reject this sort of thing.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
252
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
253 The answer is that this code has undefined behavior, but it is not illegal.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
254 If we made it illegal, then every transformation that could potentially create
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
255 this would have to ensure that it doesn't, and there is valid code that can
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
256 create this sort of construct (in dead code). The sorts of things that can
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
257 cause this to happen are fairly contrived, but we still need to accept them.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
258 Here's an example:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
259
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
260 .. code-block:: llvm
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
261
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
262 define fastcc void @foo() {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
263 ret void
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
264 }
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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265 define internal void @bar(void()* %FP, i1 %cond) {
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266 br i1 %cond, label %T, label %F
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267 T:
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268 call void %FP()
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269 ret void
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270 F:
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271 call fastcc void %FP()
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272 ret void
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273 }
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274 define void @test() {
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275 %X = or i1 false, false
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276 call void @bar(void()* @foo, i1 %X)
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277 ret void
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278 }
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279
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280 In this example, "test" always passes ``@foo``/``false`` into ``bar``, which
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281 ensures that it is dynamically called with the right calling conv (thus, the
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282 code is perfectly well defined). If you run this through the inliner, you
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283 get this (the explicit "or" is there so that the inliner doesn't dead code
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284 eliminate a bunch of stuff):
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285
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286 .. code-block:: llvm
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287
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288 define fastcc void @foo() {
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289 ret void
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290 }
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291 define void @test() {
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292 %X = or i1 false, false
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293 br i1 %X, label %T.i, label %F.i
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294 T.i:
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295 call void @foo()
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296 br label %bar.exit
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297 F.i:
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298 call fastcc void @foo()
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299 br label %bar.exit
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300 bar.exit:
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301 ret void
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302 }
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303
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304 Here you can see that the inlining pass made an undefined call to ``@foo``
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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305 with the wrong calling convention. We really don't want to make the inliner
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parents:
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306 have to know about this sort of thing, so it needs to be valid code. In this
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parents:
diff changeset
307 case, dead code elimination can trivially remove the undefined code. However,
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parents:
diff changeset
308 if ``%X`` was an input argument to ``@test``, the inliner would produce this:
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parents:
diff changeset
309
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310 .. code-block:: llvm
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parents:
diff changeset
311
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312 define fastcc void @foo() {
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313 ret void
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314 }
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315
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316 define void @test(i1 %X) {
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317 br i1 %X, label %T.i, label %F.i
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318 T.i:
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319 call void @foo()
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320 br label %bar.exit
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321 F.i:
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322 call fastcc void @foo()
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parents:
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323 br label %bar.exit
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diff changeset
324 bar.exit:
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325 ret void
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parents:
diff changeset
326 }
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parents:
diff changeset
327
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parents:
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328 The interesting thing about this is that ``%X`` *must* be false for the
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parents:
diff changeset
329 code to be well-defined, but no amount of dead code elimination will be able
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parents:
diff changeset
330 to delete the broken call as unreachable. However, since
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parents:
diff changeset
331 ``instcombine``/``simplifycfg`` turns the undefined call into unreachable, we
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
332 end up with a branch on a condition that goes to unreachable: a branch to
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parents:
diff changeset
333 unreachable can never happen, so "``-inline -instcombine -simplifycfg``" is
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parents:
diff changeset
334 able to produce:
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parents:
diff changeset
335
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parents:
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336 .. code-block:: llvm
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parents:
diff changeset
337
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338 define fastcc void @foo() {
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parents:
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339 ret void
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parents:
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340 }
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
341 define void @test(i1 %X) {
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parents:
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342 F.i:
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parents:
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343 call fastcc void @foo()
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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344 ret void
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parents:
diff changeset
345 }