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gerrit-public.fairphone.software / toolchain / llvm-project / 1ad9bd9a4e43032b99a4dfcc3f23e8c24662ae5a / . / llvm / docs / LinkTimeOptimization.html
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NAKAMURA Takumib8004d92011-10-31 11:21:59 +0000[diff] [blame]5 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]6 <title>LLVM Link Time Optimization: Design and Implementation</title>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]7 <link rel="stylesheet" href="llvm.css" type="text/css">
8</head>
9
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]10<h1>
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]11 LLVM Link Time Optimization: Design and Implementation
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]12</h1>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]13
14<ul>
15 <li><a href="#desc">Description</a></li>
16 <li><a href="#design">Design Philosophy</a>
17 <ul>
18 <li><a href="#example1">Example of link time optimization</a></li>
19 <li><a href="#alternative_approaches">Alternative Approaches</a></li>
20 </ul></li>
Devang Patelc3b31162006-08-14 18:39:35 +0000[diff] [blame]21 <li><a href="#multiphase">Multi-phase communication between LLVM and linker</a>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]22 <ul>
Duncan Sandse9da6db2010-09-29 20:09:55 +0000[diff] [blame]23 <li><a href="#phase1">Phase 1 : Read LLVM Bitcode Files</a></li>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]24 <li><a href="#phase2">Phase 2 : Symbol Resolution</a></li>
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]25 <li><a href="#phase3">Phase 3 : Optimize Bitcode Files</a></li>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]26 <li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
27 </ul></li>
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]28 <li><a href="#lto">libLTO</a>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]29 <ul>
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]30 <li><a href="#lto_module_t">lto_module_t</a></li>
31 <li><a href="#lto_code_gen_t">lto_code_gen_t</a></li>
32 </ul>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]33</ul>
34
35<div class="doc_author">
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]36<p>Written by Devang Patel and Nick Kledzik</p>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]37</div>
38
39<!-- *********************************************************************** -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]40<h2>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]41<a name="desc">Description</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]42</h2>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]43<!-- *********************************************************************** -->
44
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]45<div>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]46<p>
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]47LLVM features powerful intermodular optimizations which can be used at link
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]48time. Link Time Optimization (LTO) is another name for intermodular optimization
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]49when performed during the link stage. This document describes the interface
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]50and design between the LTO optimizer and the linker.</p>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]51</div>
52
53<!-- *********************************************************************** -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]54<h2>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]55<a name="design">Design Philosophy</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]56</h2>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]57<!-- *********************************************************************** -->
58
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]59<div>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]60<p>
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]61The LLVM Link Time Optimizer provides complete transparency, while doing
62intermodular optimization, in the compiler tool chain. Its main goal is to let
63the developer take advantage of intermodular optimizations without making any
64significant changes to the developer's makefiles or build system. This is
65achieved through tight integration with the linker. In this model, the linker
Gabor Greifa54634a2007-07-06 22:07:22 +0000[diff] [blame]66treates LLVM bitcode files like native object files and allows mixing and
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]67matching among them. The linker uses <a href="#lto">libLTO</a>, a shared
68object, to handle LLVM bitcode files. This tight integration between
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]69the linker and LLVM optimizer helps to do optimizations that are not possible
70in other models. The linker input allows the optimizer to avoid relying on
71conservative escape analysis.
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]72</p>
73
74<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]75<h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]76 <a name="example1">Example of link time optimization</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]77</h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]78
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]79<div>
Duncan Sands24334a32007-04-07 17:43:25 +0000[diff] [blame]80 <p>The following example illustrates the advantages of LTO's integrated
81 approach and clean interface. This example requires a system linker which
82 supports LTO through the interface described in this document. Here,
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]83 clang transparently invokes system linker. </p>
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]84 <ul>
Gabor Greifa54634a2007-07-06 22:07:22 +0000[diff] [blame]85 <li> Input source file <tt>a.c</tt> is compiled into LLVM bitcode form.
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]86 <li> Input source file <tt>main.c</tt> is compiled into native object code.
87 </ul>
Misha Brukman7fda6f62008-12-16 03:07:49 +0000[diff] [blame]88<pre class="doc_code">
Devang Patelc3b31162006-08-14 18:39:35 +0000[diff] [blame]89--- a.h ---
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]90extern int foo1(void);
91extern void foo2(void);
92extern void foo4(void);
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]93
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]94--- a.c ---
95#include "a.h"
96
97static signed int i = 0;
98
99void foo2(void) {
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]100 i = -1;
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]101}
102
103static int foo3() {
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]104 foo4();
105 return 10;
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]106}
107
108int foo1(void) {
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]109 int data = 0;
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]110
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]111 if (i &lt; 0)
112 data = foo3();
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]113
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]114 data = data + 42;
115 return data;
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]116}
117
118--- main.c ---
119#include &lt;stdio.h&gt;
120#include "a.h"
121
122void foo4(void) {
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]123 printf("Hi\n");
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]124}
125
126int main() {
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]127 return foo1();
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]128}
129
130--- command lines ---
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]131$ clang -emit-llvm -c a.c -o a.o # &lt;-- a.o is LLVM bitcode file
132$ clang -c main.c -o main.o # &lt;-- main.o is native object file
133$ clang a.o main.o -o main # &lt;-- standard link command without any modifications
Misha Brukman7fda6f62008-12-16 03:07:49 +0000[diff] [blame]134</pre>
Bill Wendling1e7126d2011-09-18 12:51:05 +0000[diff] [blame]135
136<ul>
137 <li>In this example, the linker recognizes that <tt>foo2()</tt> is an
138 externally visible symbol defined in LLVM bitcode file. The linker
139 completes its usual symbol resolution pass and finds that <tt>foo2()</tt>
140 is not used anywhere. This information is used by the LLVM optimizer and
141 it removes <tt>foo2()</tt>.</li>
142 <li>As soon as <tt>foo2()</tt> is removed, the optimizer recognizes that condition
143 <tt>i &lt; 0</tt> is always false, which means <tt>foo3()</tt> is never
144 used. Hence, the optimizer also removes <tt>foo3()</tt>.</li>
145 <li>And this in turn, enables linker to remove <tt>foo4()</tt>.</li>
146</ul>
147
148<p>This example illustrates the advantage of tight integration with the
149 linker. Here, the optimizer can not remove <tt>foo3()</tt> without the
150 linker's input.</p>
151
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]152</div>
153
154<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]155<h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]156 <a name="alternative_approaches">Alternative Approaches</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]157</h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]158
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]159<div>
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]160 <dl>
161 <dt><b>Compiler driver invokes link time optimizer separately.</b></dt>
162 <dd>In this model the link time optimizer is not able to take advantage of
163 information collected during the linker's normal symbol resolution phase.
164 In the above example, the optimizer can not remove <tt>foo2()</tt> without
165 the linker's input because it is externally visible. This in turn prohibits
166 the optimizer from removing <tt>foo3()</tt>.</dd>
167 <dt><b>Use separate tool to collect symbol information from all object
168 files.</b></dt>
169 <dd>In this model, a new, separate, tool or library replicates the linker's
170 capability to collect information for link time optimization. Not only is
171 this code duplication difficult to justify, but it also has several other
172 disadvantages. For example, the linking semantics and the features
173 provided by the linker on various platform are not unique. This means,
174 this new tool needs to support all such features and platforms in one
175 super tool or a separate tool per platform is required. This increases
Benjamin Kramer0f420382009-10-12 14:46:08 +0000[diff] [blame]176 maintenance cost for link time optimizer significantly, which is not
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]177 necessary. This approach also requires staying synchronized with linker
178 developements on various platforms, which is not the main focus of the link
179 time optimizer. Finally, this approach increases end user's build time due
180 to the duplication of work done by this separate tool and the linker itself.
181 </dd>
182 </dl>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]183</div>
184
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]185</div>
186
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]187<!-- *********************************************************************** -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]188<h2>
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]189 <a name="multiphase">Multi-phase communication between libLTO and linker</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]190</h2>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]191
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]192<div>
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]193 <p>The linker collects information about symbol defininitions and uses in
194 various link objects which is more accurate than any information collected
195 by other tools during typical build cycles. The linker collects this
196 information by looking at the definitions and uses of symbols in native .o
197 files and using symbol visibility information. The linker also uses
198 user-supplied information, such as a list of exported symbols. LLVM
199 optimizer collects control flow information, data flow information and knows
200 much more about program structure from the optimizer's point of view.
Benjamin Kramer0f420382009-10-12 14:46:08 +0000[diff] [blame]201 Our goal is to take advantage of tight integration between the linker and
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]202 the optimizer by sharing this information during various linking phases.
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]203</p>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]204
205<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]206<h3>
Gabor Greifa54634a2007-07-06 22:07:22 +0000[diff] [blame]207 <a name="phase1">Phase 1 : Read LLVM Bitcode Files</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]208</h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]209
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]210<div>
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]211 <p>The linker first reads all object files in natural order and collects
Gabor Greifa54634a2007-07-06 22:07:22 +0000[diff] [blame]212 symbol information. This includes native object files as well as LLVM bitcode
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]213 files. To minimize the cost to the linker in the case that all .o files
214 are native object files, the linker only calls <tt>lto_module_create()</tt>
215 when a supplied object file is found to not be a native object file. If
216 <tt>lto_module_create()</tt> returns that the file is an LLVM bitcode file,
217 the linker
218 then iterates over the module using <tt>lto_module_get_symbol_name()</tt> and
219 <tt>lto_module_get_symbol_attribute()</tt> to get all symbols defined and
220 referenced.
221 This information is added to the linker's global symbol table.
222</p>
223 <p>The lto* functions are all implemented in a shared object libLTO. This
224 allows the LLVM LTO code to be updated independently of the linker tool.
225 On platforms that support it, the shared object is lazily loaded.
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]226</p>
227</div>
228
229<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]230<h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]231 <a name="phase2">Phase 2 : Symbol Resolution</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]232</h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]233
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]234<div>
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]235 <p>In this stage, the linker resolves symbols using global symbol table.
236 It may report undefined symbol errors, read archive members, replace
237 weak symbols, etc. The linker is able to do this seamlessly even though it
238 does not know the exact content of input LLVM bitcode files. If dead code
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]239 stripping is enabled then the linker collects the list of live symbols.
240 </p>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]241</div>
242
243<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]244<h3>
Gabor Greifa54634a2007-07-06 22:07:22 +0000[diff] [blame]245 <a name="phase3">Phase 3 : Optimize Bitcode Files</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]246</h3>
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]247<div>
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]248 <p>After symbol resolution, the linker tells the LTO shared object which
249 symbols are needed by native object files. In the example above, the linker
250 reports that only <tt>foo1()</tt> is used by native object files using
251 <tt>lto_codegen_add_must_preserve_symbol()</tt>. Next the linker invokes
252 the LLVM optimizer and code generators using <tt>lto_codegen_compile()</tt>
253 which returns a native object file creating by merging the LLVM bitcode files
254 and applying various optimization passes.
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]255</p>
256</div>
257
258<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]259<h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]260 <a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]261</h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]262
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]263<div>
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]264 <p>In this phase, the linker reads optimized a native object file and
265 updates the internal global symbol table to reflect any changes. The linker
266 also collects information about any changes in use of external symbols by
Nick Lewycky9a7459a2010-04-24 22:01:40 +0000[diff] [blame]267 LLVM bitcode files. In the example above, the linker notes that
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]268 <tt>foo4()</tt> is not used any more. If dead code stripping is enabled then
269 the linker refreshes the live symbol information appropriately and performs
270 dead code stripping.</p>
271 <p>After this phase, the linker continues linking as if it never saw LLVM
Gabor Greifa54634a2007-07-06 22:07:22 +0000[diff] [blame]272 bitcode files.</p>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]273</div>
274
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]275</div>
276
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]277<!-- *********************************************************************** -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]278<h2>
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]279<a name="lto">libLTO</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]280</h2>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]281
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]282<div>
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]283 <p><tt>libLTO</tt> is a shared object that is part of the LLVM tools, and
284 is intended for use by a linker. <tt>libLTO</tt> provides an abstract C
Reid Spencer105780f2006-08-14 19:19:55 +0000[diff] [blame]285 interface to use the LLVM interprocedural optimizer without exposing details
286 of LLVM's internals. The intention is to keep the interface as stable as
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]287 possible even when the LLVM optimizer continues to evolve. It should even
288 be possible for a completely different compilation technology to provide
289 a different libLTO that works with their object files and the standard
290 linker tool.</p>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]291
292<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]293<h3>
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]294 <a name="lto_module_t">lto_module_t</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]295</h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]296
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]297<div>
Misha Brukman7fda6f62008-12-16 03:07:49 +0000[diff] [blame]298
299<p>A non-native object file is handled via an <tt>lto_module_t</tt>.
300The following functions allow the linker to check if a file (on disk
301or in a memory buffer) is a file which libLTO can process:</p>
302
303<pre class="doc_code">
304lto_module_is_object_file(const char*)
305lto_module_is_object_file_for_target(const char*, const char*)
306lto_module_is_object_file_in_memory(const void*, size_t)
307lto_module_is_object_file_in_memory_for_target(const void*, size_t, const char*)
308</pre>
309
310<p>If the object file can be processed by libLTO, the linker creates a
311<tt>lto_module_t</tt> by using one of</p>
312
313<pre class="doc_code">
314lto_module_create(const char*)
315lto_module_create_from_memory(const void*, size_t)
316</pre>
317
318<p>and when done, the handle is released via</p>
319
320<pre class="doc_code">
321lto_module_dispose(lto_module_t)
322</pre>
323
324<p>The linker can introspect the non-native object file by getting the number of
325symbols and getting the name and attributes of each symbol via:</p>
326
327<pre class="doc_code">
328lto_module_get_num_symbols(lto_module_t)
329lto_module_get_symbol_name(lto_module_t, unsigned int)
330lto_module_get_symbol_attribute(lto_module_t, unsigned int)
331</pre>
332
333<p>The attributes of a symbol include the alignment, visibility, and kind.</p>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]334</div>
335
336<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]337<h3>
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]338 <a name="lto_code_gen_t">lto_code_gen_t</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]339</h3>
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]340
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]341<div>
Misha Brukman7fda6f62008-12-16 03:07:49 +0000[diff] [blame]342
343<p>Once the linker has loaded each non-native object files into an
344<tt>lto_module_t</tt>, it can request libLTO to process them all and
345generate a native object file. This is done in a couple of steps.
346First, a code generator is created with:</p>
347
348<pre class="doc_code">lto_codegen_create()</pre>
349
350<p>Then, each non-native object file is added to the code generator with:</p>
351
352<pre class="doc_code">
353lto_codegen_add_module(lto_code_gen_t, lto_module_t)
354</pre>
355
356<p>The linker then has the option of setting some codegen options. Whether or
357not to generate DWARF debug info is set with:</p>
358
359<pre class="doc_code">lto_codegen_set_debug_model(lto_code_gen_t)</pre>
360
361<p>Which kind of position independence is set with:</p>
362
363<pre class="doc_code">lto_codegen_set_pic_model(lto_code_gen_t) </pre>
364
365<p>And each symbol that is referenced by a native object file or otherwise must
366not be optimized away is set with:</p>
367
368<pre class="doc_code">
369lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*)
370</pre>
371
372<p>After all these settings are done, the linker requests that a native object
373file be created from the modules with the settings using:</p>
374
375<pre class="doc_code">lto_codegen_compile(lto_code_gen_t, size*)</pre>
376
377<p>which returns a pointer to a buffer containing the generated native
378object file. The linker then parses that and links it with the rest
379of the native object files.</p>
380
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]381</div>
382
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]383</div>
384
Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]385<!-- *********************************************************************** -->
386
387<hr>
388<address>
389 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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Devang Patelb15c0072006-08-14 18:03:40 +0000[diff] [blame]393
Nick Kledzik65556232008-02-29 19:34:52 +0000[diff] [blame]394 Devang Patel and Nick Kledzik<br>
NAKAMURA Takumica46f5a2011-04-09 02:13:37 +0000[diff] [blame]395 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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397</address>
398
399</body>
400</html>
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