docs/LinkTimeOptimization.html

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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
  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>
  20   <li><a href="#multiphase">Multi-phase communication between LLVM and linker</a>
  21   <ul>
  22     <li><a href="#phase1">Phase 1 : Read LLVM Bitcode Files</a></li>
  23     <li><a href="#phase2">Phase 2 : Symbol Resolution</a></li>
  24     <li><a href="#phase3">Phase 3 : Optimize Bitcode Files</a></li>
  25     <li><a href="#phase4">Phase 4 : Symbol Resolution after optimization</a></li>
  26   </ul></li>
  27   <li><a href="#lto">libLTO</a>
  28   <ul>
  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>
  32 </ul>
  33
  34 <div class="doc_author">
  35 <p>Written by Devang Patel and Nick Kledzik</p>
  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>
  46 LLVM features powerful intermodular optimizations which can be used at link
  47 time.  Link Time Optimization (LTO) is another name for intermodular optimization
  48 when performed during the link stage. This document describes the interface
  49 and design between the LTO optimizer and the linker.</p>
  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>
  60 The LLVM Link Time Optimizer provides complete transparency, while doing
  61 intermodular optimization, in the compiler tool chain. Its main goal is to let
  62 the developer take advantage of intermodular optimizations without making any
  63 significant changes to the developer's makefiles or build system. This is
  64 achieved through tight integration with the linker. In this model, the linker
  65 treates LLVM bitcode files like native object files and allows mixing and
  66 matching among them. The linker uses <a href="#lto">libLTO</a>, a shared
  67 object, to handle LLVM bitcode files. This tight integration between
  68 the linker and LLVM optimizer helps to do optimizations that are not possible
  69 in other models. The linker input allows the optimizer to avoid relying on
  70 conservative escape analysis.
  71 </p>
  72 </div>
  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">
  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   llvm-gcc transparently invokes system linker. </p>
  84   <ul>
  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.
  87   </ul>
  88 <pre class="doc_code">
  89 --- a.h ---
  90 extern int foo1(void);
  91 extern void foo2(void);
  92 extern void foo4(void);
  93 --- a.c ---
  94 #include "a.h"
  95
  96 static signed int i = 0;
  97
  98 void foo2(void) {
  99  i = -1;
 100 }
 101
 102 static int foo3() {
 103 foo4();
 104 return 10;
 105 }
 106
 107 int foo1(void) {
 108 int data = 0;
 109
 110 if (i &lt; 0) { data = foo3(); }
 111
 112 data = data + 42;
 113 return data;
 114 }
 115
 116 --- main.c ---
 117 #include &lt;stdio.h&gt;
 118 #include "a.h"
 119
 120 void foo4(void) {
 121  printf ("Hi\n");
 122 }
 123
 124 int main() {
 125  return foo1();
 126 }
 127
 128 --- command lines ---
 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
 132 </pre>
 133   <p>In this example, the linker recognizes that <tt>foo2()</tt> is an
 134   externally visible symbol defined in LLVM bitcode file. The linker completes
 135   its usual symbol resolution
 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>
 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">
 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
 169     maintenance cost for link time optimizer significantly, which is not
 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>
 176 </div>
 177
 178 <!-- *********************************************************************** -->
 179 <div class="doc_section">
 180   <a name="multiphase">Multi-phase communication between libLTO and linker</a>
 181 </div>
 182
 183 <div class="doc_text">
 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.
 192   Our goal is to take advantage of tight integration between the linker and
 193   the optimizer by sharing this information during various linking phases.
 194 </p>
 195 </div>
 196
 197 <!-- ======================================================================= -->
 198 <div class="doc_subsection">
 199   <a name="phase1">Phase 1 : Read LLVM Bitcode Files</a>
 200 </div>
 201
 202 <div class="doc_text">
 203   <p>The linker first reads all object files in natural order and collects
 204   symbol information. This includes native object files as well as LLVM bitcode
 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.
 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">
 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
 231   stripping is enabled then the linker collects the list of live symbols.
 232   </p>
 233 </div>
 234
 235 <!-- ======================================================================= -->
 236 <div class="doc_subsection">
 237   <a name="phase3">Phase 3 : Optimize Bitcode Files</a>
 238 </div>
 239 <div class="doc_text">
 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.
 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">
 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 bitcode files. In the example 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   bitcode files.</p>
 265 </div>
 266
 267 <!-- *********************************************************************** -->
 268 <div class="doc_section">
 269 <a name="lto">libLTO</a>
 270 </div>
 271
 272 <div class="doc_text">
 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
 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. 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>
 281 </div>
 282
 283 <!-- ======================================================================= -->
 284 <div class="doc_subsection">
 285   <a name="lto_module_t">lto_module_t</a>
 286 </div>
 287
 288 <div class="doc_text">
 289
 290 <p>A non-native object file is handled via an <tt>lto_module_t</tt>.
 291 The following functions allow the linker to check if a file (on disk
 292 or in a memory buffer) is a file which libLTO can process:</p>
 293
 294 <pre class="doc_code">
 295 lto_module_is_object_file(const char*)
 296 lto_module_is_object_file_for_target(const char*, const char*)
 297 lto_module_is_object_file_in_memory(const void*, size_t)
 298 lto_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">
 305 lto_module_create(const char*)
 306 lto_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">
 312 lto_module_dispose(lto_module_t)
 313 </pre>
 314
 315 <p>The linker can introspect the non-native object file by getting the number of
 316 symbols and getting the name and attributes of each symbol via:</p>
 317
 318 <pre class="doc_code">
 319 lto_module_get_num_symbols(lto_module_t)
 320 lto_module_get_symbol_name(lto_module_t, unsigned int)
 321 lto_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>
 325 </div>
 326
 327 <!-- ======================================================================= -->
 328 <div class="doc_subsection">
 329   <a name="lto_code_gen_t">lto_code_gen_t</a>
 330 </div>
 331
 332 <div class="doc_text">
 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
 336 generate a native object file.  This is done in a couple of steps.
 337 First, 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">
 344 lto_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
 348 not 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
 357 not be optimized away is set with:</p>
 358
 359 <pre class="doc_code">
 360 lto_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
 364 file 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
 369 object file.  The linker then parses that and links it with the rest
 370 of the native object files.</p>
 371
 372 </div>
 373
 374 <!-- *********************************************************************** -->
 375
 376 <hr>
 377 <address>
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 382
 383   Devang Patel and Nick Kledzik<br>
 384   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
 385   Last modified: $Date$
 386 </address>
 387
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