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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>
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<!-- *********************************************************************** -->
39<div class="doc_section">
40<a name="desc">Description</a>
41</div>
42<!-- *********************************************************************** -->
43
44<div class="doc_text">
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<!-- *********************************************************************** -->
53<div class="doc_section">
54<a name="design">Design Philosophy</a>
55</div>
56<!-- *********************************************************************** -->
57
58<div class="doc_text">
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>
Devang Patel2c1292f2006-08-14 18:39:35 +0000[diff] [blame]72</div>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]73
74<!-- ======================================================================= -->
75<div class="doc_subsection">
76 <a name="example1">Example of link time optimization</a>
77</div>
78
79<div class="doc_text">
Duncan Sands6db928a2007-04-07 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,
Tanya Lattner5efa7e92007-08-24 23:23:23 +0000[diff] [blame]83 llvm-gcc transparently invokes system linker. </p>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]84 <ul>
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]85 <li> Input source file <tt>a.c</tt> is compiled into LLVM bitcode form.
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]86 <li> Input source file <tt>main.c</tt> is compiled into native object code.
87 </ul>
Misha Brukman64722e52008-12-16 03:07:49 +0000[diff] [blame]88<pre class="doc_code">
Devang Patel2c1292f2006-08-14 18:39:35 +0000[diff] [blame]89--- a.h ---
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]90extern int foo1(void);
91extern void foo2(void);
92extern void foo4(void);
93--- a.c ---
94#include "a.h"
95
96static signed int i = 0;
97
98void foo2(void) {
99 i = -1;
100}
101
102static int foo3() {
103foo4();
104return 10;
105}
106
107int foo1(void) {
108int data = 0;
109
110if (i &lt; 0) { data = foo3(); }
111
112data = data + 42;
113return data;
114}
115
116--- main.c ---
117#include &lt;stdio.h&gt;
118#include "a.h"
119
120void foo4(void) {
121 printf ("Hi\n");
122}
123
124int main() {
125 return foo1();
126}
127
128--- command lines ---
Tanya Lattner5efa7e92007-08-24 23:23:23 +0000[diff] [blame]129$ llvm-gcc --emit-llvm -c a.c -o a.o # &lt;-- a.o is LLVM bitcode file
130$ llvm-gcc -c main.c -o main.o # &lt;-- main.o is native object file
131$ llvm-gcc a.o main.o -o main # &lt;-- standard link command without any modifications
Misha Brukman64722e52008-12-16 03:07:49 +0000[diff] [blame]132</pre>
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]133 <p>In this example, the linker recognizes that <tt>foo2()</tt> is an
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]134 externally visible symbol defined in LLVM bitcode file. The linker completes
135 its usual symbol resolution
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]136 pass and finds that <tt>foo2()</tt> is not used anywhere. This information
137 is used by the LLVM optimizer and it removes <tt>foo2()</tt>. As soon as
138 <tt>foo2()</tt> is removed, the optimizer recognizes that condition
139 <tt>i &lt; 0</tt> is always false, which means <tt>foo3()</tt> is never
140 used. Hence, the optimizer removes <tt>foo3()</tt>, also. And this in turn,
141 enables linker to remove <tt>foo4()</tt>. This example illustrates the
142 advantage of tight integration with the linker. Here, the optimizer can not
143 remove <tt>foo3()</tt> without the linker's input.
144 </p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]145</div>
146
147<!-- ======================================================================= -->
148<div class="doc_subsection">
149 <a name="alternative_approaches">Alternative Approaches</a>
150</div>
151
152<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]153 <dl>
154 <dt><b>Compiler driver invokes link time optimizer separately.</b></dt>
155 <dd>In this model the link time optimizer is not able to take advantage of
156 information collected during the linker's normal symbol resolution phase.
157 In the above example, the optimizer can not remove <tt>foo2()</tt> without
158 the linker's input because it is externally visible. This in turn prohibits
159 the optimizer from removing <tt>foo3()</tt>.</dd>
160 <dt><b>Use separate tool to collect symbol information from all object
161 files.</b></dt>
162 <dd>In this model, a new, separate, tool or library replicates the linker's
163 capability to collect information for link time optimization. Not only is
164 this code duplication difficult to justify, but it also has several other
165 disadvantages. For example, the linking semantics and the features
166 provided by the linker on various platform are not unique. This means,
167 this new tool needs to support all such features and platforms in one
168 super tool or a separate tool per platform is required. This increases
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]169 maintenance cost for link time optimizer significantly, which is not
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]170 necessary. This approach also requires staying synchronized with linker
171 developements on various platforms, which is not the main focus of the link
172 time optimizer. Finally, this approach increases end user's build time due
173 to the duplication of work done by this separate tool and the linker itself.
174 </dd>
175 </dl>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]176</div>
177
178<!-- *********************************************************************** -->
179<div class="doc_section">
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]180 <a name="multiphase">Multi-phase communication between libLTO and linker</a>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]181</div>
182
183<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]184 <p>The linker collects information about symbol defininitions and uses in
185 various link objects which is more accurate than any information collected
186 by other tools during typical build cycles. The linker collects this
187 information by looking at the definitions and uses of symbols in native .o
188 files and using symbol visibility information. The linker also uses
189 user-supplied information, such as a list of exported symbols. LLVM
190 optimizer collects control flow information, data flow information and knows
191 much more about program structure from the optimizer's point of view.
Benjamin Kramer8040cd32009-10-12 14:46:08 +0000[diff] [blame]192 Our goal is to take advantage of tight integration between the linker and
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]193 the optimizer by sharing this information during various linking phases.
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]194</p>
195</div>
196
197<!-- ======================================================================= -->
198<div class="doc_subsection">
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]199 <a name="phase1">Phase 1 : Read LLVM Bitcode Files</a>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]200</div>
201
202<div class="doc_text">
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]203 <p>The linker first reads all object files in natural order and collects
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]204 symbol information. This includes native object files as well as LLVM bitcode
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]205 files. To minimize the cost to the linker in the case that all .o files
206 are native object files, the linker only calls <tt>lto_module_create()</tt>
207 when a supplied object file is found to not be a native object file. If
208 <tt>lto_module_create()</tt> returns that the file is an LLVM bitcode file,
209 the linker
210 then iterates over the module using <tt>lto_module_get_symbol_name()</tt> and
211 <tt>lto_module_get_symbol_attribute()</tt> to get all symbols defined and
212 referenced.
213 This information is added to the linker's global symbol table.
214</p>
215 <p>The lto* functions are all implemented in a shared object libLTO. This
216 allows the LLVM LTO code to be updated independently of the linker tool.
217 On platforms that support it, the shared object is lazily loaded.
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]218</p>
219</div>
220
221<!-- ======================================================================= -->
222<div class="doc_subsection">
223 <a name="phase2">Phase 2 : Symbol Resolution</a>
224</div>
225
226<div class="doc_text">
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]227 <p>In this stage, the linker resolves symbols using global symbol table.
228 It may report undefined symbol errors, read archive members, replace
229 weak symbols, etc. The linker is able to do this seamlessly even though it
230 does not know the exact content of input LLVM bitcode files. If dead code
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]231 stripping is enabled then the linker collects the list of live symbols.
232 </p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]233</div>
234
235<!-- ======================================================================= -->
236<div class="doc_subsection">
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]237 <a name="phase3">Phase 3 : Optimize Bitcode Files</a>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]238</div>
239<div class="doc_text">
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]240 <p>After symbol resolution, the linker tells the LTO shared object which
241 symbols are needed by native object files. In the example above, the linker
242 reports that only <tt>foo1()</tt> is used by native object files using
243 <tt>lto_codegen_add_must_preserve_symbol()</tt>. Next the linker invokes
244 the LLVM optimizer and code generators using <tt>lto_codegen_compile()</tt>
245 which returns a native object file creating by merging the LLVM bitcode files
246 and applying various optimization passes.
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
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]259 LLVM bitcode files. In the examle above, the linker notes that
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]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
Gabor Greif04367bf2007-07-06 22:07:22 +0000[diff] [blame]264 bitcode files.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]265</div>
266
267<!-- *********************************************************************** -->
268<div class="doc_section">
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]269<a name="lto">libLTO</a>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]270</div>
271
272<div class="doc_text">
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]273 <p><tt>libLTO</tt> is a shared object that is part of the LLVM tools, and
274 is intended for use by a linker. <tt>libLTO</tt> provides an abstract C
Reid Spencerb5fc9f52006-08-14 19:19:55 +0000[diff] [blame]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
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]277 possible even when the LLVM optimizer continues to evolve. It should even
278 be possible for a completely different compilation technology to provide
279 a different libLTO that works with their object files and the standard
280 linker tool.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]281</div>
282
283<!-- ======================================================================= -->
284<div class="doc_subsection">
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]285 <a name="lto_module_t">lto_module_t</a>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]286</div>
287
288<div class="doc_text">
Misha Brukman64722e52008-12-16 03:07:49 +0000[diff] [blame]289
290<p>A non-native object file is handled via an <tt>lto_module_t</tt>.
291The following functions allow the linker to check if a file (on disk
292or in a memory buffer) is a file which libLTO can process:</p>
293
294<pre class="doc_code">
295lto_module_is_object_file(const char*)
296lto_module_is_object_file_for_target(const char*, const char*)
297lto_module_is_object_file_in_memory(const void*, size_t)
298lto_module_is_object_file_in_memory_for_target(const void*, size_t, const char*)
299</pre>
300
301<p>If the object file can be processed by libLTO, the linker creates a
302<tt>lto_module_t</tt> by using one of</p>
303
304<pre class="doc_code">
305lto_module_create(const char*)
306lto_module_create_from_memory(const void*, size_t)
307</pre>
308
309<p>and when done, the handle is released via</p>
310
311<pre class="doc_code">
312lto_module_dispose(lto_module_t)
313</pre>
314
315<p>The linker can introspect the non-native object file by getting the number of
316symbols and getting the name and attributes of each symbol via:</p>
317
318<pre class="doc_code">
319lto_module_get_num_symbols(lto_module_t)
320lto_module_get_symbol_name(lto_module_t, unsigned int)
321lto_module_get_symbol_attribute(lto_module_t, unsigned int)
322</pre>
323
324<p>The attributes of a symbol include the alignment, visibility, and kind.</p>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]325</div>
326
327<!-- ======================================================================= -->
328<div class="doc_subsection">
Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]329 <a name="lto_code_gen_t">lto_code_gen_t</a>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]330</div>
331
332<div class="doc_text">
Misha Brukman64722e52008-12-16 03:07:49 +0000[diff] [blame]333
334<p>Once the linker has loaded each non-native object files into an
335<tt>lto_module_t</tt>, it can request libLTO to process them all and
336generate a native object file. This is done in a couple of steps.
337First, a code generator is created with:</p>
338
339<pre class="doc_code">lto_codegen_create()</pre>
340
341<p>Then, each non-native object file is added to the code generator with:</p>
342
343<pre class="doc_code">
344lto_codegen_add_module(lto_code_gen_t, lto_module_t)
345</pre>
346
347<p>The linker then has the option of setting some codegen options. Whether or
348not to generate DWARF debug info is set with:</p>
349
350<pre class="doc_code">lto_codegen_set_debug_model(lto_code_gen_t)</pre>
351
352<p>Which kind of position independence is set with:</p>
353
354<pre class="doc_code">lto_codegen_set_pic_model(lto_code_gen_t) </pre>
355
356<p>And each symbol that is referenced by a native object file or otherwise must
357not be optimized away is set with:</p>
358
359<pre class="doc_code">
360lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*)
361</pre>
362
363<p>After all these settings are done, the linker requests that a native object
364file be created from the modules with the settings using:</p>
365
366<pre class="doc_code">lto_codegen_compile(lto_code_gen_t, size*)</pre>
367
368<p>which returns a pointer to a buffer containing the generated native
369object file. The linker then parses that and links it with the rest
370of the native object files.</p>
371
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]372</div>
373
374<!-- *********************************************************************** -->
375
376<hr>
377<address>
378 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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Nick Kledzik84e5f772008-02-29 19:34:52 +0000[diff] [blame]383 Devang Patel and Nick Kledzik<br>
Devang Patel93449f12006-08-14 18:03:40 +0000[diff] [blame]384 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
385 Last modified: $Date$
386</address>
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