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5 <title>LLVM Link Time Optimization: Design and Implementation</title>
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9 <div class="doc_title">
10 LLVM Link Time Optimization: Design and Implementation
14 <li><a href="#desc">Description</a></li>
15 <li><a href="#design">Design Philosophy</a>
17 <li><a href="#example1">Example of link time optimization</a></li>
18 <li><a href="#alternative_approaches">Alternative Approaches</a></li>
20 <li><a href="#multiphase">Multi-phase communication between LLVM and linker</a>
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>
27 <li><a href="#lto">LLVMlto</a>
29 <li><a href="#llvmsymbol">LLVMSymbol</a></li>
30 <li><a href="#readllvmobjectfile">readLLVMObjectFile()</a></li>
31 <li><a href="#optimizemodules">optimizeModules()</a></li>
32 <li><a href="#gettargettriple">getTargetTriple()</a></li>
34 <li><a href="#debug">Debugging Information</a></li>
37 <div class="doc_author">
38 <p>Written by Devang Patel</p>
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42 <div class="doc_section">
43 <a name="desc">Description</a>
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47 <div class="doc_text">
49 LLVM features powerful intermodular optimizations which can be used at link
50 time. Link Time Optimization is another name for intermodular optimization
51 when performed during the link stage. This document describes the interface
52 and design between the LLVM intermodular optimizer and the linker.</p>
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56 <div class="doc_section">
57 <a name="design">Design Philosophy</a>
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61 <div class="doc_text">
63 The LLVM Link Time Optimizer provides complete transparency, while doing
64 intermodular optimization, in the compiler tool chain. Its main goal is to let
65 the developer take advantage of intermodular optimizations without making any
66 significant changes to the developer's makefiles or build system. This is
67 achieved through tight integration with the linker. In this model, the linker
68 treates LLVM bytecode files like native object files and allows mixing and
69 matching among them. The linker uses <a href="#lto">LLVMlto</a>, a dynamically
70 loaded library, to handle LLVM bytecode files. This tight integration between
71 the linker and LLVM optimizer helps to do optimizations that are not possible
72 in other models. The linker input allows the optimizer to avoid relying on
73 conservative escape analysis.
77 <!-- ======================================================================= -->
78 <div class="doc_subsection">
79 <a name="example1">Example of link time optimization</a>
82 <div class="doc_text">
83 <p>The following example illustrates the advantages of LTO's integrated
84 approach and clean interface.</p>
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.
91 extern int foo1(void);
92 extern void foo2(void);
93 extern void foo4(void);
97 static signed int i = 0;
111 if (i < 0) { data = foo3(); }
118 #include <stdio.h>
129 --- command lines ---
130 $ llvm-gcc4 --emit-llvm -c a.c -o a.o # <-- a.o is LLVM bytecode file
131 $ llvm-gcc4 -c main.c -o main.o # <-- main.o is native object file
132 $ llvm-gcc4 a.o main.o -o main # <-- standard link command without any modifications
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 < 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.
149 <!-- ======================================================================= -->
150 <div class="doc_subsection">
151 <a name="alternative_approaches">Alternative Approaches</a>
154 <div class="doc_text">
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
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.
180 <!-- *********************************************************************** -->
181 <div class="doc_section">
182 <a name="multiphase">Multi-phase communication between LLVM and linker</a>
185 <div class="doc_text">
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.
199 <!-- ======================================================================= -->
200 <div class="doc_subsection">
201 <a name="phase1">Phase 1 : Read LLVM Bytecode Files</a>
204 <div class="doc_text">
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.
218 <!-- ======================================================================= -->
219 <div class="doc_subsection">
220 <a name="phase2">Phase 2 : Symbol Resolution</a>
223 <div class="doc_text">
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.
234 <!-- ======================================================================= -->
235 <div class="doc_subsection">
236 <a name="phase3">Phase 3 : Optimize Bytecode Files</a>
238 <div class="doc_text">
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
250 <!-- ======================================================================= -->
251 <div class="doc_subsection">
252 <a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
255 <div class="doc_text">
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
267 <!-- *********************************************************************** -->
268 <div class="doc_section">
269 <a name="lto">LLVMlto</a>
272 <div class="doc_text">
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>
280 <!-- ======================================================================= -->
281 <div class="doc_subsection">
282 <a name="llvmsymbol">LLVMSymbol</a>
285 <div class="doc_text">
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>
302 <!-- ======================================================================= -->
303 <div class="doc_subsection">
304 <a name="readllvmobjectfile">readLLVMObjectFile()</a>
307 <div class="doc_text">
308 <p>The <tt>readLLVMObjectFile()</tt> function is used by the linker to read
309 LLVM bytecode files and collect LLVMSymbol information. This routine also
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
317 <!-- ======================================================================= -->
318 <div class="doc_subsection">
319 <a name="optimizemodules">optimizeModules()</a>
322 <div class="doc_text">
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
330 <!-- ======================================================================= -->
331 <div class="doc_subsection">
332 <a name="gettargettriple">getTargetTriple()</a>
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>
340 <!-- *********************************************************************** -->
341 <div class="doc_section">
342 <a name="debug">Debugging Information</a>
344 <!-- *********************************************************************** -->
346 <div class="doc_text">
348 <p><tt> ... To be completed ... </tt></p>
352 <!-- *********************************************************************** -->
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