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11 LLVM Link Time Optimization: Design and Implementation
15 <li><a href="#desc">Description</a></li>
16 <li><a href="#design">Design Philosophy</a>
18 <li><a href="#example1">Example of link time optimization</a></li>
19 <li><a href="#alternative_approaches">Alternative Approaches</a></li>
21 <li><a href="#multiphase">Multi-phase communication between LLVM and linker</a>
23 <li><a href="#phase1">Phase 1 : Read LLVM Bitcode Files</a></li>
24 <li><a href="#phase2">Phase 2 : Symbol Resolution</a></li>
25 <li><a href="#phase3">Phase 3 : Optimize Bitcode Files</a></li>
26 <li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
28 <li><a href="#lto">libLTO</a>
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>
35 <div class="doc_author">
36 <p>Written by Devang Patel and Nick Kledzik</p>
39 <!-- *********************************************************************** -->
41 <a name="desc">Description</a>
43 <!-- *********************************************************************** -->
47 LLVM features powerful intermodular optimizations which can be used at link
48 time. Link Time Optimization (LTO) is another name for intermodular optimization
49 when performed during the link stage. This document describes the interface
50 and design between the LTO optimizer and the linker.</p>
53 <!-- *********************************************************************** -->
55 <a name="design">Design Philosophy</a>
57 <!-- *********************************************************************** -->
61 The LLVM Link Time Optimizer provides complete transparency, while doing
62 intermodular optimization, in the compiler tool chain. Its main goal is to let
63 the developer take advantage of intermodular optimizations without making any
64 significant changes to the developer's makefiles or build system. This is
65 achieved through tight integration with the linker. In this model, the linker
66 treates LLVM bitcode files like native object files and allows mixing and
67 matching among them. The linker uses <a href="#lto">libLTO</a>, a shared
68 object, to handle LLVM bitcode files. This tight integration between
69 the linker and LLVM optimizer helps to do optimizations that are not possible
70 in other models. The linker input allows the optimizer to avoid relying on
71 conservative escape analysis.
74 <!-- ======================================================================= -->
76 <a name="example1">Example of link time optimization</a>
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,
83 clang transparently invokes system linker. </p>
85 <li> Input source file <tt>a.c</tt> is compiled into LLVM bitcode form.
86 <li> Input source file <tt>main.c</tt> is compiled into native object code.
88 <pre class="doc_code">
90 extern int foo1(void);
91 extern void foo2(void);
92 extern void foo4(void);
97 static signed int i = 0;
119 #include <stdio.h>
130 --- command lines ---
131 $ clang -emit-llvm -c a.c -o a.o # <-- a.o is LLVM bitcode file
132 $ clang -c main.c -o main.o # <-- main.o is native object file
133 $ clang a.o main.o -o main # <-- standard link command without any modifications
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 < 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>
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
154 <!-- ======================================================================= -->
156 <a name="alternative_approaches">Alternative Approaches</a>
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
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
176 maintenance cost for link time optimizer significantly, which is not
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.
187 <!-- *********************************************************************** -->
189 <a name="multiphase">Multi-phase communication between libLTO and linker</a>
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.
201 Our goal is to take advantage of tight integration between the linker and
202 the optimizer by sharing this information during various linking phases.
205 <!-- ======================================================================= -->
207 <a name="phase1">Phase 1 : Read LLVM Bitcode Files</a>
211 <p>The linker first reads all object files in natural order and collects
212 symbol information. This includes native object files as well as LLVM bitcode
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,
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
221 This information is added to the linker's global symbol table.
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.
229 <!-- ======================================================================= -->
231 <a name="phase2">Phase 2 : Symbol Resolution</a>
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
239 stripping is enabled then the linker collects the list of live symbols.
243 <!-- ======================================================================= -->
245 <a name="phase3">Phase 3 : Optimize Bitcode Files</a>
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.
258 <!-- ======================================================================= -->
260 <a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
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
267 LLVM bitcode files. In the example above, the linker notes that
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
277 <!-- *********************************************************************** -->
279 <a name="lto">libLTO</a>
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
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
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
292 <!-- ======================================================================= -->
294 <a name="lto_module_t">lto_module_t</a>
299 <p>A non-native object file is handled via an <tt>lto_module_t</tt>.
300 The following functions allow the linker to check if a file (on disk
301 or in a memory buffer) is a file which libLTO can process:</p>
303 <pre class="doc_code">
304 lto_module_is_object_file(const char*)
305 lto_module_is_object_file_for_target(const char*, const char*)
306 lto_module_is_object_file_in_memory(const void*, size_t)
307 lto_module_is_object_file_in_memory_for_target(const void*, size_t, const char*)
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>
313 <pre class="doc_code">
314 lto_module_create(const char*)
315 lto_module_create_from_memory(const void*, size_t)
318 <p>and when done, the handle is released via</p>
320 <pre class="doc_code">
321 lto_module_dispose(lto_module_t)
324 <p>The linker can introspect the non-native object file by getting the number of
325 symbols and getting the name and attributes of each symbol via:</p>
327 <pre class="doc_code">
328 lto_module_get_num_symbols(lto_module_t)
329 lto_module_get_symbol_name(lto_module_t, unsigned int)
330 lto_module_get_symbol_attribute(lto_module_t, unsigned int)
333 <p>The attributes of a symbol include the alignment, visibility, and kind.</p>
336 <!-- ======================================================================= -->
338 <a name="lto_code_gen_t">lto_code_gen_t</a>
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
345 generate a native object file. This is done in a couple of steps.
346 First, a code generator is created with:</p>
348 <pre class="doc_code">lto_codegen_create()</pre>
350 <p>Then, each non-native object file is added to the code generator with:</p>
352 <pre class="doc_code">
353 lto_codegen_add_module(lto_code_gen_t, lto_module_t)
356 <p>The linker then has the option of setting some codegen options. Whether or
357 not to generate DWARF debug info is set with:</p>
359 <pre class="doc_code">lto_codegen_set_debug_model(lto_code_gen_t)</pre>
361 <p>Which kind of position independence is set with:</p>
363 <pre class="doc_code">lto_codegen_set_pic_model(lto_code_gen_t) </pre>
365 <p>And each symbol that is referenced by a native object file or otherwise must
366 not be optimized away is set with:</p>
368 <pre class="doc_code">
369 lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*)
372 <p>After all these settings are done, the linker requests that a native object
373 file be created from the modules with the settings using:</p>
375 <pre class="doc_code">lto_codegen_compile(lto_code_gen_t, size*)</pre>
377 <p>which returns a pointer to a buffer containing the generated native
378 object file. The linker then parses that and links it with the rest
379 of the native object files.</p>
385 <!-- *********************************************************************** -->
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394 Devang Patel and Nick Kledzik<br>
395 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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