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author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Sun, 31 Jan 2016 17:34:49 +0900
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children 1172e4bd9c6f
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1 ======================================================
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2 LLVM Link Time Optimization: Design and Implementation
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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 Description
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9 ===========
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10
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11 LLVM features powerful intermodular optimizations which can be used at link
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12 time. Link Time Optimization (LTO) is another name for intermodular
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13 optimization when performed during the link stage. This document describes the
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14 interface and design between the LTO optimizer and the linker.
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15
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16 Design Philosophy
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17 =================
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18
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19 The LLVM Link Time Optimizer provides complete transparency, while doing
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20 intermodular optimization, in the compiler tool chain. Its main goal is to let
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21 the developer take advantage of intermodular optimizations without making any
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22 significant changes to the developer's makefiles or build system. This is
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23 achieved through tight integration with the linker. In this model, the linker
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24 treates LLVM bitcode files like native object files and allows mixing and
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25 matching among them. The linker uses `libLTO`_, a shared object, to handle LLVM
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26 bitcode files. This tight integration between the linker and LLVM optimizer
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27 helps to do optimizations that are not possible in other models. The linker
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28 input allows the optimizer to avoid relying on conservative escape analysis.
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29
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30 .. _libLTO-example:
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31
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32 Example of link time optimization
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33 ---------------------------------
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34
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35 The following example illustrates the advantages of LTO's integrated approach
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36 and clean interface. This example requires a system linker which supports LTO
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37 through the interface described in this document. Here, clang transparently
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38 invokes system linker.
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39
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40 * Input source file ``a.c`` is compiled into LLVM bitcode form.
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41 * Input source file ``main.c`` is compiled into native object code.
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42
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43 .. code-block:: c++
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44
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45 --- a.h ---
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46 extern int foo1(void);
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47 extern void foo2(void);
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48 extern void foo4(void);
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49
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50 --- a.c ---
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51 #include "a.h"
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52
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53 static signed int i = 0;
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54
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55 void foo2(void) {
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56 i = -1;
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57 }
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58
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59 static int foo3() {
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60 foo4();
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61 return 10;
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62 }
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63
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64 int foo1(void) {
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65 int data = 0;
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66
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67 if (i < 0)
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68 data = foo3();
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69
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70 data = data + 42;
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71 return data;
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72 }
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73
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74 --- main.c ---
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75 #include <stdio.h>
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76 #include "a.h"
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77
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78 void foo4(void) {
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79 printf("Hi\n");
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80 }
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81
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82 int main() {
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83 return foo1();
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84 }
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85
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86 To compile, run:
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87
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88 .. code-block:: console
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89
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90 % clang -emit-llvm -c a.c -o a.o # <-- a.o is LLVM bitcode file
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91 % clang -c main.c -o main.o # <-- main.o is native object file
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92 % clang a.o main.o -o main # <-- standard link command without modifications
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93
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94 * In this example, the linker recognizes that ``foo2()`` is an externally
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95 visible symbol defined in LLVM bitcode file. The linker completes its usual
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96 symbol resolution pass and finds that ``foo2()`` is not used
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97 anywhere. This information is used by the LLVM optimizer and it
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98 removes ``foo2()``.
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99
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100 * As soon as ``foo2()`` is removed, the optimizer recognizes that condition ``i
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101 < 0`` is always false, which means ``foo3()`` is never used. Hence, the
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102 optimizer also removes ``foo3()``.
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103
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104 * And this in turn, enables linker to remove ``foo4()``.
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105
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106 This example illustrates the advantage of tight integration with the
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107 linker. Here, the optimizer can not remove ``foo3()`` without the linker's
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108 input.
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109
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110 Alternative Approaches
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111 ----------------------
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112
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113 **Compiler driver invokes link time optimizer separately.**
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114 In this model the link time optimizer is not able to take advantage of
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115 information collected during the linker's normal symbol resolution phase.
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116 In the above example, the optimizer can not remove ``foo2()`` without the
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117 linker's input because it is externally visible. This in turn prohibits the
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118 optimizer from removing ``foo3()``.
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119
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120 **Use separate tool to collect symbol information from all object files.**
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121 In this model, a new, separate, tool or library replicates the linker's
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122 capability to collect information for link time optimization. Not only is
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123 this code duplication difficult to justify, but it also has several other
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124 disadvantages. For example, the linking semantics and the features provided
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125 by the linker on various platform are not unique. This means, this new tool
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126 needs to support all such features and platforms in one super tool or a
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127 separate tool per platform is required. This increases maintenance cost for
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128 link time optimizer significantly, which is not necessary. This approach
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129 also requires staying synchronized with linker developements on various
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130 platforms, which is not the main focus of the link time optimizer. Finally,
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131 this approach increases end user's build time due to the duplication of work
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132 done by this separate tool and the linker itself.
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parents:
diff changeset
133
95c75e76d11b LLVM 3.4
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parents:
diff changeset
134 Multi-phase communication between ``libLTO`` and linker
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
135 =======================================================
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
136
83
60c9769439b8 LLVM 3.7
Tatsuki IHA <e125716@ie.u-ryukyu.ac.jp>
parents: 0
diff changeset
137 The linker collects information about symbol definitions and uses in various
0
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
138 link objects which is more accurate than any information collected by other
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
139 tools during typical build cycles. The linker collects this information by
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
140 looking at the definitions and uses of symbols in native .o files and using
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
141 symbol visibility information. The linker also uses user-supplied information,
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
142 such as a list of exported symbols. LLVM optimizer collects control flow
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
143 information, data flow information and knows much more about program structure
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
144 from the optimizer's point of view. Our goal is to take advantage of tight
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
145 integration between the linker and the optimizer by sharing this information
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
146 during various linking phases.
95c75e76d11b LLVM 3.4
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parents:
diff changeset
147
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
148 Phase 1 : Read LLVM Bitcode Files
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
149 ---------------------------------
95c75e76d11b LLVM 3.4
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parents:
diff changeset
150
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
151 The linker first reads all object files in natural order and collects symbol
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
152 information. This includes native object files as well as LLVM bitcode files.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
153 To minimize the cost to the linker in the case that all .o files are native
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
154 object files, the linker only calls ``lto_module_create()`` when a supplied
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
155 object file is found to not be a native object file. If ``lto_module_create()``
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
156 returns that the file is an LLVM bitcode file, the linker then iterates over the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
157 module using ``lto_module_get_symbol_name()`` and
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
158 ``lto_module_get_symbol_attribute()`` to get all symbols defined and referenced.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
159 This information is added to the linker's global symbol table.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
160
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
161
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
162 The lto* functions are all implemented in a shared object libLTO. This allows
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
163 the LLVM LTO code to be updated independently of the linker tool. On platforms
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
164 that support it, the shared object is lazily loaded.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
165
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
166 Phase 2 : Symbol Resolution
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
167 ---------------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
168
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
169 In this stage, the linker resolves symbols using global symbol table. It may
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
170 report undefined symbol errors, read archive members, replace weak symbols, etc.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
171 The linker is able to do this seamlessly even though it does not know the exact
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
172 content of input LLVM bitcode files. If dead code stripping is enabled then the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
173 linker collects the list of live symbols.
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 Phase 3 : Optimize Bitcode Files
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
176 --------------------------------
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 After symbol resolution, the linker tells the LTO shared object which symbols
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
179 are needed by native object files. In the example above, the linker reports
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
180 that only ``foo1()`` is used by native object files using
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
181 ``lto_codegen_add_must_preserve_symbol()``. Next the linker invokes the LLVM
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
182 optimizer and code generators using ``lto_codegen_compile()`` which returns a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
183 native object file creating by merging the LLVM bitcode files and applying
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
184 various optimization passes.
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 Phase 4 : Symbol Resolution after optimization
95c75e76d11b LLVM 3.4
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
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
189 In this phase, the linker reads optimized a native object file and updates the
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
190 internal global symbol table to reflect any changes. The linker also collects
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
191 information about any changes in use of external symbols by LLVM bitcode
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
192 files. In the example above, the linker notes that ``foo4()`` is not used any
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
193 more. If dead code stripping is enabled then the linker refreshes the live
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
194 symbol information appropriately and performs dead code stripping.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
195
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
196 After this phase, the linker continues linking as if it never saw LLVM bitcode
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
197 files.
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 .. _libLTO:
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
200
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
201 ``libLTO``
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
202 ==========
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
203
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
204 ``libLTO`` is a shared object that is part of the LLVM tools, and is intended
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
205 for use by a linker. ``libLTO`` provides an abstract C interface to use the LLVM
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
206 interprocedural optimizer without exposing details of LLVM's internals. The
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
207 intention is to keep the interface as stable as possible even when the LLVM
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
208 optimizer continues to evolve. It should even be possible for a completely
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
209 different compilation technology to provide a different libLTO that works with
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
210 their object files and the standard linker tool.
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:
diff changeset
212 ``lto_module_t``
95c75e76d11b LLVM 3.4
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
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
215 A non-native object file is handled via an ``lto_module_t``. The following
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
216 functions allow the linker to check if a file (on disk or in a memory buffer) is
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
217 a file which libLTO can process:
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 .. code-block:: c
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 lto_module_is_object_file(const char*)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
222 lto_module_is_object_file_for_target(const char*, const char*)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
223 lto_module_is_object_file_in_memory(const void*, size_t)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
224 lto_module_is_object_file_in_memory_for_target(const void*, size_t, const char*)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
225
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
226 If the object file can be processed by ``libLTO``, the linker creates a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
227 ``lto_module_t`` by using one of:
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 .. code-block:: c
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
230
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
231 lto_module_create(const char*)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
232 lto_module_create_from_memory(const void*, size_t)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
233
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
234 and when done, the handle is released via
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 .. code-block:: c
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 lto_module_dispose(lto_module_t)
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
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
241 The linker can introspect the non-native object file by getting the number of
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
242 symbols and getting the name and attributes of each symbol via:
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 .. code-block:: c
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
245
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
246 lto_module_get_num_symbols(lto_module_t)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
247 lto_module_get_symbol_name(lto_module_t, unsigned int)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
248 lto_module_get_symbol_attribute(lto_module_t, unsigned int)
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
249
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
250 The attributes of a symbol include the alignment, visibility, and kind.
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
251
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
252 ``lto_code_gen_t``
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
253 ------------------
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
254
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
255 Once the linker has loaded each non-native object files into an
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
256 ``lto_module_t``, it can request ``libLTO`` to process them all and generate a
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
257 native object file. This is done in a couple of steps. First, a code generator
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
258 is created with:
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:: c
95c75e76d11b LLVM 3.4
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 lto_codegen_create()
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
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263
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264 Then, each non-native object file is added to the code generator with:
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265
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266 .. code-block:: c
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267
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268 lto_codegen_add_module(lto_code_gen_t, lto_module_t)
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269
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270 The linker then has the option of setting some codegen options. Whether or not
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271 to generate DWARF debug info is set with:
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272
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273 .. code-block:: c
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274
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275 lto_codegen_set_debug_model(lto_code_gen_t)
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276
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277 Which kind of position independence is set with:
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278
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279 .. code-block:: c
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280
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281 lto_codegen_set_pic_model(lto_code_gen_t)
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282
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283 And each symbol that is referenced by a native object file or otherwise must not
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284 be optimized away is set with:
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285
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286 .. code-block:: c
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287
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288 lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*)
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289
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290 After all these settings are done, the linker requests that a native object file
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291 be created from the modules with the settings using:
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292
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293 .. code-block:: c
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294
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295 lto_codegen_compile(lto_code_gen_t, size*)
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296
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297 which returns a pointer to a buffer containing the generated native object file.
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298 The linker then parses that and links it with the rest of the native object
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299 files.