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gerrit-public.fairphone.software / fp2-dev / platform / external / llvm / 4917ec9ec775d7bc7fbc3362b4c576c32b7617f4 / . / docs / LinkTimeOptimization.html
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Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]1<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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3<html>
4<head>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]5 <title>LLVM Link Time Optimization: Design and Implementation</title>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]6 <link rel="stylesheet" href="llvm.css" type="text/css">
7</head>
8
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]9<h1>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]10 LLVM Link Time Optimization: Design and Implementation
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]11</h1>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]12
13<ul>
14 <li><a href="#desc">Description</a></li>
15 <li><a href="#design">Design Philosophy</a>
16 <ul>
17 <li><a href="#example1">Example of link time optimization</a></li>
18 <li><a href="#alternative_approaches">Alternative Approaches</a></li>
19 </ul></li>
Devang Patel2c1292f2006-08-14 18:39:35 +0000[diff] [blame]20 <li><a href="#multiphase">Multi-phase communication between LLVM and linker</a>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]21 <ul>
Duncan Sands18d52f22010-09-29 20:09:55 +0000[diff] [blame]22 <li><a href="#phase1">Phase 1 : Read LLVM Bitcode Files</a></li>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]23 <li><a href="#phase2">Phase 2 : Symbol Resolution</a></li>
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]24 <li><a href="#phase3">Phase 3 : Optimize Bitcode Files</a></li>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]25 <li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
26 </ul></li>
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]27 <li><a href="#lto">libLTO</a>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]28 <ul>
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]29 <li><a href="#lto_module_t">lto_module_t</a></li>
30 <li><a href="#lto_code_gen_t">lto_code_gen_t</a></li>
31 </ul>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]32</ul>
33
34<div class="doc_author">
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]35<p>Written by Devang Patel and Nick Kledzik</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]36</div>
37
38<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]39<h2>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]40<a name="desc">Description</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]41</h2>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]42<!-- *********************************************************************** -->
43
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]44<div>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]45<p>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]46LLVM features powerful intermodular optimizations which can be used at link
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]47time. Link Time Optimization (LTO) is another name for intermodular optimization
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]48when performed during the link stage. This document describes the interface
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]49and design between the LTO optimizer and the linker.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]50</div>
51
52<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]53<h2>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]54<a name="design">Design Philosophy</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]55</h2>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]56<!-- *********************************************************************** -->
57
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]58<div>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]59<p>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]60The LLVM Link Time Optimizer provides complete transparency, while doing
61intermodular optimization, in the compiler tool chain. Its main goal is to let
62the developer take advantage of intermodular optimizations without making any
63significant changes to the developer's makefiles or build system. This is
64achieved through tight integration with the linker. In this model, the linker
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]65treates LLVM bitcode files like native object files and allows mixing and
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]66matching among them. The linker uses <a href="#lto">libLTO</a>, a shared
67object, to handle LLVM bitcode files. This tight integration between
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]68the linker and LLVM optimizer helps to do optimizations that are not possible
69in other models. The linker input allows the optimizer to avoid relying on
70conservative escape analysis.
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]71</p>
72
73<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]74<h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]75 <a name="example1">Example of link time optimization</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]76</h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]77
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]78<div>
Duncan Sands6db928a2007-04-07 17:43:25 +0000[diff] [blame]79 <p>The following example illustrates the advantages of LTO's integrated
80 approach and clean interface. This example requires a system linker which
81 supports LTO through the interface described in this document. Here,
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]82 clang transparently invokes system linker. </p>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]83 <ul>
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]84 <li> Input source file <tt>a.c</tt> is compiled into LLVM bitcode form.
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]85 <li> Input source file <tt>main.c</tt> is compiled into native object code.
86 </ul>
Misha Brukman64722e52008-12-16 03:07:49 +0000[diff] [blame]87<pre class="doc_code">
Devang Patel2c1292f2006-08-14 18:39:35 +0000[diff] [blame]88--- a.h ---
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]89extern int foo1(void);
90extern void foo2(void);
91extern void foo4(void);
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]92
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]93--- a.c ---
94#include "a.h"
95
96static signed int i = 0;
97
98void foo2(void) {
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]99 i = -1;
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]100}
101
102static int foo3() {
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]103 foo4();
104 return 10;
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]105}
106
107int foo1(void) {
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]108 int data = 0;
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]109
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]110 if (i &lt; 0)
111 data = foo3();
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]112
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]113 data = data + 42;
114 return data;
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]115}
116
117--- main.c ---
118#include &lt;stdio.h&gt;
119#include "a.h"
120
121void foo4(void) {
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]122 printf("Hi\n");
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]123}
124
125int main() {
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]126 return foo1();
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]127}
128
129--- command lines ---
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]130$ clang -emit-llvm -c a.c -o a.o # &lt;-- a.o is LLVM bitcode file
131$ clang -c main.c -o main.o # &lt;-- main.o is native object file
132$ clang a.o main.o -o main # &lt;-- standard link command without any modifications
Misha Brukman64722e52008-12-16 03:07:49 +0000[diff] [blame]133</pre>
Bill Wendling88816ec2011-09-18 12:51:05 +0000[diff] [blame]134
135<ul>
136 <li>In this example, the linker recognizes that <tt>foo2()</tt> is an
137 externally visible symbol defined in LLVM bitcode file. The linker
138 completes its usual symbol resolution pass and finds that <tt>foo2()</tt>
139 is not used anywhere. This information is used by the LLVM optimizer and
140 it removes <tt>foo2()</tt>.</li>
141 <li>As soon as <tt>foo2()</tt> is removed, the optimizer recognizes that condition
142 <tt>i &lt; 0</tt> is always false, which means <tt>foo3()</tt> is never
143 used. Hence, the optimizer also removes <tt>foo3()</tt>.</li>
144 <li>And this in turn, enables linker to remove <tt>foo4()</tt>.</li>
145</ul>
146
147<p>This example illustrates the advantage of tight integration with the
148 linker. Here, the optimizer can not remove <tt>foo3()</tt> without the
149 linker's input.</p>
150
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]151</div>
152
153<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]154<h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]155 <a name="alternative_approaches">Alternative Approaches</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]156</h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]157
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]158<div>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]159 <dl>
160 <dt><b>Compiler driver invokes link time optimizer separately.</b></dt>
161 <dd>In this model the link time optimizer is not able to take advantage of
162 information collected during the linker's normal symbol resolution phase.
163 In the above example, the optimizer can not remove <tt>foo2()</tt> without
164 the linker's input because it is externally visible. This in turn prohibits
165 the optimizer from removing <tt>foo3()</tt>.</dd>
166 <dt><b>Use separate tool to collect symbol information from all object
167 files.</b></dt>
168 <dd>In this model, a new, separate, tool or library replicates the linker's
169 capability to collect information for link time optimization. Not only is
170 this code duplication difficult to justify, but it also has several other
171 disadvantages. For example, the linking semantics and the features
172 provided by the linker on various platform are not unique. This means,
173 this new tool needs to support all such features and platforms in one
174 super tool or a separate tool per platform is required. This increases
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]175 maintenance cost for link time optimizer significantly, which is not
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]176 necessary. This approach also requires staying synchronized with linker
177 developements on various platforms, which is not the main focus of the link
178 time optimizer. Finally, this approach increases end user's build time due
179 to the duplication of work done by this separate tool and the linker itself.
180 </dd>
181 </dl>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]182</div>
183
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]184</div>
185
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]186<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]187<h2>
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]188 <a name="multiphase">Multi-phase communication between libLTO and linker</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]189</h2>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]190
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]191<div>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]192 <p>The linker collects information about symbol defininitions and uses in
193 various link objects which is more accurate than any information collected
194 by other tools during typical build cycles. The linker collects this
195 information by looking at the definitions and uses of symbols in native .o
196 files and using symbol visibility information. The linker also uses
197 user-supplied information, such as a list of exported symbols. LLVM
198 optimizer collects control flow information, data flow information and knows
199 much more about program structure from the optimizer's point of view.
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]200 Our goal is to take advantage of tight integration between the linker and
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]201 the optimizer by sharing this information during various linking phases.
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]202</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]203
204<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]205<h3>
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]206 <a name="phase1">Phase 1 : Read LLVM Bitcode Files</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]207</h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]208
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]209<div>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]210 <p>The linker first reads all object files in natural order and collects
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]211 symbol information. This includes native object files as well as LLVM bitcode
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]212 files. To minimize the cost to the linker in the case that all .o files
213 are native object files, the linker only calls <tt>lto_module_create()</tt>
214 when a supplied object file is found to not be a native object file. If
215 <tt>lto_module_create()</tt> returns that the file is an LLVM bitcode file,
216 the linker
217 then iterates over the module using <tt>lto_module_get_symbol_name()</tt> and
218 <tt>lto_module_get_symbol_attribute()</tt> to get all symbols defined and
219 referenced.
220 This information is added to the linker's global symbol table.
221</p>
222 <p>The lto* functions are all implemented in a shared object libLTO. This
223 allows the LLVM LTO code to be updated independently of the linker tool.
224 On platforms that support it, the shared object is lazily loaded.
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]225</p>
226</div>
227
228<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]229<h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]230 <a name="phase2">Phase 2 : Symbol Resolution</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]231</h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]232
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]233<div>
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]234 <p>In this stage, the linker resolves symbols using global symbol table.
235 It may report undefined symbol errors, read archive members, replace
236 weak symbols, etc. The linker is able to do this seamlessly even though it
237 does not know the exact content of input LLVM bitcode files. If dead code
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]238 stripping is enabled then the linker collects the list of live symbols.
239 </p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]240</div>
241
242<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]243<h3>
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]244 <a name="phase3">Phase 3 : Optimize Bitcode Files</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]245</h3>
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]246<div>
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]247 <p>After symbol resolution, the linker tells the LTO shared object which
248 symbols are needed by native object files. In the example above, the linker
249 reports that only <tt>foo1()</tt> is used by native object files using
250 <tt>lto_codegen_add_must_preserve_symbol()</tt>. Next the linker invokes
251 the LLVM optimizer and code generators using <tt>lto_codegen_compile()</tt>
252 which returns a native object file creating by merging the LLVM bitcode files
253 and applying various optimization passes.
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]254</p>
255</div>
256
257<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]258<h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]259 <a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]260</h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]261
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]262<div>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]263 <p>In this phase, the linker reads optimized a native object file and
264 updates the internal global symbol table to reflect any changes. The linker
265 also collects information about any changes in use of external symbols by
Nick Lewycky1f048642010-04-24 22:01:40 +0000[diff] [blame]266 LLVM bitcode files. In the example above, the linker notes that
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]267 <tt>foo4()</tt> is not used any more. If dead code stripping is enabled then
268 the linker refreshes the live symbol information appropriately and performs
269 dead code stripping.</p>
270 <p>After this phase, the linker continues linking as if it never saw LLVM
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]271 bitcode files.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]272</div>
273
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]274</div>
275
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]276<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]277<h2>
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]278<a name="lto">libLTO</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]279</h2>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]280
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]281<div>
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]282 <p><tt>libLTO</tt> is a shared object that is part of the LLVM tools, and
283 is intended for use by a linker. <tt>libLTO</tt> provides an abstract C
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]284 interface to use the LLVM interprocedural optimizer without exposing details
285 of LLVM's internals. The intention is to keep the interface as stable as
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]286 possible even when the LLVM optimizer continues to evolve. It should even
287 be possible for a completely different compilation technology to provide
288 a different libLTO that works with their object files and the standard
289 linker tool.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]290
291<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]292<h3>
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]293 <a name="lto_module_t">lto_module_t</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]294</h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]295
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]296<div>
Misha Brukman64722e52008-12-16 03:07:49 +0000[diff] [blame]297
298<p>A non-native object file is handled via an <tt>lto_module_t</tt>.
299The following functions allow the linker to check if a file (on disk
300or in a memory buffer) is a file which libLTO can process:</p>
301
302<pre class="doc_code">
303lto_module_is_object_file(const char*)
304lto_module_is_object_file_for_target(const char*, const char*)
305lto_module_is_object_file_in_memory(const void*, size_t)
306lto_module_is_object_file_in_memory_for_target(const void*, size_t, const char*)
307</pre>
308
309<p>If the object file can be processed by libLTO, the linker creates a
310<tt>lto_module_t</tt> by using one of</p>
311
312<pre class="doc_code">
313lto_module_create(const char*)
314lto_module_create_from_memory(const void*, size_t)
315</pre>
316
317<p>and when done, the handle is released via</p>
318
319<pre class="doc_code">
320lto_module_dispose(lto_module_t)
321</pre>
322
323<p>The linker can introspect the non-native object file by getting the number of
324symbols and getting the name and attributes of each symbol via:</p>
325
326<pre class="doc_code">
327lto_module_get_num_symbols(lto_module_t)
328lto_module_get_symbol_name(lto_module_t, unsigned int)
329lto_module_get_symbol_attribute(lto_module_t, unsigned int)
330</pre>
331
332<p>The attributes of a symbol include the alignment, visibility, and kind.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]333</div>
334
335<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]336<h3>
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]337 <a name="lto_code_gen_t">lto_code_gen_t</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]338</h3>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]339
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]340<div>
Misha Brukman64722e52008-12-16 03:07:49 +0000[diff] [blame]341
342<p>Once the linker has loaded each non-native object files into an
343<tt>lto_module_t</tt>, it can request libLTO to process them all and
344generate a native object file. This is done in a couple of steps.
345First, a code generator is created with:</p>
346
347<pre class="doc_code">lto_codegen_create()</pre>
348
349<p>Then, each non-native object file is added to the code generator with:</p>
350
351<pre class="doc_code">
352lto_codegen_add_module(lto_code_gen_t, lto_module_t)
353</pre>
354
355<p>The linker then has the option of setting some codegen options. Whether or
356not to generate DWARF debug info is set with:</p>
357
358<pre class="doc_code">lto_codegen_set_debug_model(lto_code_gen_t)</pre>
359
360<p>Which kind of position independence is set with:</p>
361
362<pre class="doc_code">lto_codegen_set_pic_model(lto_code_gen_t) </pre>
363
364<p>And each symbol that is referenced by a native object file or otherwise must
365not be optimized away is set with:</p>
366
367<pre class="doc_code">
368lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*)
369</pre>
370
371<p>After all these settings are done, the linker requests that a native object
372file be created from the modules with the settings using:</p>
373
374<pre class="doc_code">lto_codegen_compile(lto_code_gen_t, size*)</pre>
375
376<p>which returns a pointer to a buffer containing the generated native
377object file. The linker then parses that and links it with the rest
378of the native object files.</p>
379
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]380</div>
381
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]382</div>
383
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]384<!-- *********************************************************************** -->
385
386<hr>
387<address>
388 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]392
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]393 Devang Patel and Nick Kledzik<br>
NAKAMURA Takumib9a33632011-04-09 02:13:37 +0000[diff] [blame]394 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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396</address>
397
398</body>
399</html>
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