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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
9<div class="doc_title">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]10 LLVM Link Time Optimization: Design and Implementation
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]11</div>
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>
22 <li><a href="#phase1">Phase 1 : Read LLVM Bytecode Files</a></li>
23 <li><a href="#phase2">Phase 2 : Symbol Resolution</a></li>
24 <li><a href="#phase3">Phase 3 : Optimize Bytecode Files</a></li>
25 <li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
26 </ul></li>
Devang Patel2c1292f2006-08-14 18:39:35 +0000[diff] [blame]27 <li><a href="#lto">LLVMlto</a>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]28 <ul>
29 <li><a href="#llvmsymbol">LLVMSymbol</a></li>
30 <li><a href="#readllvmobjectfile">readLLVMObjectFile()</a></li>
31 <li><a href="#optimizemodules">optimizeModules()</a></li>
Devang Patelc1e6e132006-09-06 20:22:55 +0000[diff] [blame^]32 <li><a href="#gettargettriple">getTargetTriple()</a></li>
Devang Patel2c1292f2006-08-14 18:39:35 +0000[diff] [blame]33 </ul></li>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]34 <li><a href="#debug">Debugging Information</a></li>
35</ul>
36
37<div class="doc_author">
Devang Patel2c1292f2006-08-14 18:39:35 +0000[diff] [blame]38<p>Written by Devang Patel</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]39</div>
40
41<!-- *********************************************************************** -->
42<div class="doc_section">
43<a name="desc">Description</a>
44</div>
45<!-- *********************************************************************** -->
46
47<div class="doc_text">
48<p>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]49LLVM features powerful intermodular optimizations which can be used at link
50time. Link Time Optimization is another name for intermodular optimization
51when performed during the link stage. This document describes the interface
52and design between the LLVM intermodular optimizer and the linker.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]53</div>
54
55<!-- *********************************************************************** -->
56<div class="doc_section">
57<a name="design">Design Philosophy</a>
58</div>
59<!-- *********************************************************************** -->
60
61<div class="doc_text">
62<p>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]63The LLVM Link Time Optimizer provides complete transparency, while doing
64intermodular optimization, in the compiler tool chain. Its main goal is to let
65the developer take advantage of intermodular optimizations without making any
66significant changes to the developer's makefiles or build system. This is
67achieved through tight integration with the linker. In this model, the linker
68treates LLVM bytecode files like native object files and allows mixing and
69matching among them. The linker uses <a href="#lto">LLVMlto</a>, a dynamically
70loaded library, to handle LLVM bytecode files. This tight integration between
71the linker and LLVM optimizer helps to do optimizations that are not possible
72in other models. The linker input allows the optimizer to avoid relying on
73conservative escape analysis.
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]74</p>
Devang Patel2c1292f2006-08-14 18:39:35 +0000[diff] [blame]75</div>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]76
77<!-- ======================================================================= -->
78<div class="doc_subsection">
79 <a name="example1">Example of link time optimization</a>
80</div>
81
82<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]83 <p>The following example illustrates the advantages of LTO's integrated
84 approach and clean interface.</p>
85 <ul>
86 <li> Input source file <tt>a.c</tt> is compiled into LLVM byte code form.
87 <li> Input source file <tt>main.c</tt> is compiled into native object code.
88 </ul>
89<pre>
Devang Patel2c1292f2006-08-14 18:39:35 +0000[diff] [blame]90--- a.h ---
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]91extern int foo1(void);
92extern void foo2(void);
93extern void foo4(void);
94--- a.c ---
95#include "a.h"
96
97static signed int i = 0;
98
99void foo2(void) {
100 i = -1;
101}
102
103static int foo3() {
104foo4();
105return 10;
106}
107
108int foo1(void) {
109int data = 0;
110
111if (i &lt; 0) { data = foo3(); }
112
113data = data + 42;
114return data;
115}
116
117--- main.c ---
118#include &lt;stdio.h&gt;
119#include "a.h"
120
121void foo4(void) {
122 printf ("Hi\n");
123}
124
125int main() {
126 return foo1();
127}
128
129--- command lines ---
130$ llvm-gcc4 --emit-llvm -c a.c -o a.o # &lt;-- a.o is LLVM bytecode file
131$ llvm-gcc4 -c main.c -o main.o # &lt;-- main.o is native object file
132$ llvm-gcc4 a.o main.o -o main # &lt;-- standard link command without any modifications
133</pre>
134 <p>In this example, the linker recognizes that <tt>foo2()</tt> is an
135 externally visible symbol defined in LLVM byte code file. This information
136 is collected using <a href="#readllvmobjectfile"> readLLVMObjectFile()</a>.
137 Based on this information, the linker completes its usual symbol resolution
138 pass and finds that <tt>foo2()</tt> is not used anywhere. This information
139 is used by the LLVM optimizer and it removes <tt>foo2()</tt>. As soon as
140 <tt>foo2()</tt> is removed, the optimizer recognizes that condition
141 <tt>i &lt; 0</tt> is always false, which means <tt>foo3()</tt> is never
142 used. Hence, the optimizer removes <tt>foo3()</tt>, also. And this in turn,
143 enables linker to remove <tt>foo4()</tt>. This example illustrates the
144 advantage of tight integration with the linker. Here, the optimizer can not
145 remove <tt>foo3()</tt> without the linker's input.
146 </p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]147</div>
148
149<!-- ======================================================================= -->
150<div class="doc_subsection">
151 <a name="alternative_approaches">Alternative Approaches</a>
152</div>
153
154<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]155 <dl>
156 <dt><b>Compiler driver invokes link time optimizer separately.</b></dt>
157 <dd>In this model the link time optimizer is not able to take advantage of
158 information collected during the linker's normal symbol resolution phase.
159 In the above example, the optimizer can not remove <tt>foo2()</tt> without
160 the linker's input because it is externally visible. This in turn prohibits
161 the optimizer from removing <tt>foo3()</tt>.</dd>
162 <dt><b>Use separate tool to collect symbol information from all object
163 files.</b></dt>
164 <dd>In this model, a new, separate, tool or library replicates the linker's
165 capability to collect information for link time optimization. Not only is
166 this code duplication difficult to justify, but it also has several other
167 disadvantages. For example, the linking semantics and the features
168 provided by the linker on various platform are not unique. This means,
169 this new tool needs to support all such features and platforms in one
170 super tool or a separate tool per platform is required. This increases
171 maintance cost for link time optimizer significantly, which is not
172 necessary. This approach also requires staying synchronized with linker
173 developements on various platforms, which is not the main focus of the link
174 time optimizer. Finally, this approach increases end user's build time due
175 to the duplication of work done by this separate tool and the linker itself.
176 </dd>
177 </dl>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]178</div>
179
180<!-- *********************************************************************** -->
181<div class="doc_section">
182 <a name="multiphase">Multi-phase communication between LLVM and linker</a>
183</div>
184
185<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]186 <p>The linker collects information about symbol defininitions and uses in
187 various link objects which is more accurate than any information collected
188 by other tools during typical build cycles. The linker collects this
189 information by looking at the definitions and uses of symbols in native .o
190 files and using symbol visibility information. The linker also uses
191 user-supplied information, such as a list of exported symbols. LLVM
192 optimizer collects control flow information, data flow information and knows
193 much more about program structure from the optimizer's point of view.
194 Our goal is to take advantage of tight intergration between the linker and
195 the optimizer by sharing this information during various linking phases.
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]196</p>
197</div>
198
199<!-- ======================================================================= -->
200<div class="doc_subsection">
201 <a name="phase1">Phase 1 : Read LLVM Bytecode Files</a>
202</div>
203
204<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]205 <p>The linker first reads all object files in natural order and collects
206 symbol information. This includes native object files as well as LLVM byte
207 code files. In this phase, the linker uses
208 <a href="#readllvmobjectfile"> readLLVMObjectFile() </a> to collect symbol
209 information from each LLVM bytecode files and updates its internal global
210 symbol table accordingly. The intent of this interface is to avoid overhead
211 in the non LLVM case, where all input object files are native object files,
212 by putting this code in the error path of the linker. When the linker sees
213 the first llvm .o file, it <tt>dlopen()</tt>s the dynamic library. This is
214 to allow changes to the LLVM LTO code without relinking the linker.
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]215</p>
216</div>
217
218<!-- ======================================================================= -->
219<div class="doc_subsection">
220 <a name="phase2">Phase 2 : Symbol Resolution</a>
221</div>
222
223<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]224 <p>In this stage, the linker resolves symbols using global symbol table
225 information to report undefined symbol errors, read archive members, resolve
226 weak symbols, etc. The linker is able to do this seamlessly even though it
227 does not know the exact content of input LLVM bytecode files because it uses
228 symbol information provided by
229 <a href="#readllvmobjectfile">readLLVMObjectFile()</a>. If dead code
230 stripping is enabled then the linker collects the list of live symbols.
231 </p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]232</div>
233
234<!-- ======================================================================= -->
235<div class="doc_subsection">
236 <a name="phase3">Phase 3 : Optimize Bytecode Files</a>
237</div>
238<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]239 <p>After symbol resolution, the linker updates symbol information supplied
240 by LLVM bytecode files appropriately. For example, whether certain LLVM
241 bytecode supplied symbols are used or not. In the example above, the linker
242 reports that <tt>foo2()</tt> is not used anywhere in the program, including
243 native <tt>.o</tt> files. This information is used by the LLVM interprocedural
244 optimizer. The linker uses <a href="#optimizemodules">optimizeModules()</a>
245 and requests an optimized native object file of the LLVM portion of the
246 program.
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]247</p>
248</div>
249
250<!-- ======================================================================= -->
251<div class="doc_subsection">
252 <a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
253</div>
254
255<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]256 <p>In this phase, the linker reads optimized a native object file and
257 updates the internal global symbol table to reflect any changes. The linker
258 also collects information about any changes in use of external symbols by
259 LLVM bytecode files. In the examle above, the linker notes that
260 <tt>foo4()</tt> is not used any more. If dead code stripping is enabled then
261 the linker refreshes the live symbol information appropriately and performs
262 dead code stripping.</p>
263 <p>After this phase, the linker continues linking as if it never saw LLVM
264 bytecode files.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]265</div>
266
267<!-- *********************************************************************** -->
268<div class="doc_section">
269<a name="lto">LLVMlto</a>
270</div>
271
272<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]273 <p><tt>LLVMlto</tt> is a dynamic library that is part of the LLVM tools, and
274 is intended for use by a linker. <tt>LLVMlto</tt> provides an abstract C++
275 interface to use the LLVM interprocedural optimizer without exposing details
276 of LLVM's internals. The intention is to keep the interface as stable as
277 possible even when the LLVM optimizer continues to evolve.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]278</div>
279
280<!-- ======================================================================= -->
281<div class="doc_subsection">
282 <a name="llvmsymbol">LLVMSymbol</a>
283</div>
284
285<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]286 <p>The <tt>LLVMSymbol</tt> class is used to describe the externally visible
287 functions and global variables, defined in LLVM bytecode files, to the linker.
288 This includes symbol visibility information. This information is used by
289 the linker to do symbol resolution. For example: function <tt>foo2()</tt> is
290 defined inside an LLVM bytecode module and it is an externally visible symbol.
291 This helps the linker connect the use of <tt>foo2()</tt> in native object
292 files with a future definition of the symbol <tt>foo2()</tt>. The linker
293 will see the actual definition of <tt>foo2()</tt> when it receives the
294 optimized native object file in
295 <a href="#phase4">Symbol Resolution after optimization</a> phase. If the
296 linker does not find any uses of <tt>foo2()</tt>, it updates LLVMSymbol
297 visibility information to notify LLVM intermodular optimizer that it is dead.
298 The LLVM intermodular optimizer takes advantage of such information to
299 generate better code.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]300</div>
301
302<!-- ======================================================================= -->
303<div class="doc_subsection">
304 <a name="readllvmobjectfile">readLLVMObjectFile()</a>
305</div>
306
307<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]308 <p>The <tt>readLLVMObjectFile()</tt> function is used by the linker to read
Chris Lattner29a965d2006-08-14 20:07:50 +0000[diff] [blame]309 LLVM bytecode files and collect LLVMSymbol information. This routine also
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]310 supplies a list of externally defined symbols that are used by LLVM bytecode
311 files. The linker uses this symbol information to do symbol resolution.
312 Internally, <a href="#lto">LLVMlto</a> maintains LLVM bytecode modules in
313 memory. This function also provides a list of external references used by
314 bytecode files.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]315</div>
316
317<!-- ======================================================================= -->
318<div class="doc_subsection">
319 <a name="optimizemodules">optimizeModules()</a>
320</div>
321
322<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]323 <p>The linker invokes <tt>optimizeModules</tt> to optimize already read
324 LLVM bytecode files by applying LLVM intermodular optimization techniques.
325 This function runs the LLVM intermodular optimizer and generates native
326 object code as <tt>.o</tt> files at the name and location provided by the
327 linker.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]328</div>
329
Devang Patelc1e6e132006-09-06 20:22:55 +0000[diff] [blame^]330<!-- ======================================================================= -->
331<div class="doc_subsection">
332 <a name="gettargettriple">getTargetTriple()</a>
333</div>
334
335<div class="doc_text">
336 <p>The linker may use <tt>getTargetTriple()</tt> to query target architecture
337 while validating LLVM bytecode file.</p>
338</div>
339
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]340<!-- *********************************************************************** -->
341<div class="doc_section">
342 <a name="debug">Debugging Information</a>
343</div>
344<!-- *********************************************************************** -->
345
346<div class="doc_text">
347
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]348<p><tt> ... To be completed ... </tt></p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]349
350</div>
351
352<!-- *********************************************************************** -->
353
354<hr>
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Devang Patel2c1292f2006-08-14 18:39:35 +0000[diff] [blame]361 Devang Patel<br>
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