docs/LinkTimeOptimization.html

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   5  <title>LLVM Link Time Optimization: Design and Implementation</title>
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   9 <h1>
  10   LLVM Link Time Optimization: Design and Implementation
  11 </h1>
  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 <h2>
  40 <a name="desc">Description</a>
  41 </h2>
  42 <!-- *********************************************************************** -->
  43
  44 <div>
  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 <h2>
  54 <a name="design">Design Philosophy</a>
  55 </h2>
  56 <!-- *********************************************************************** -->
  57
  58 <div>
  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
  73 <!-- ======================================================================= -->
  74 <h3>
  75   <a name="example1">Example of link time optimization</a>
  76 </h3>
  77
  78 <div>
  79   <p>The following example illustrates the advantages of LTO's integrated
  80   approach and clean interface. This example requires a system linker which
  81   supports LTO through the interface described in this document.  Here,
  82   llvm-gcc transparently invokes system linker. </p>
  83   <ul>
  84     <li> Input source file <tt>a.c</tt> is compiled into LLVM bitcode form.
  85     <li> Input source file <tt>main.c</tt> is compiled into native object code.
  86   </ul>
  87 <pre class="doc_code">
  88 --- a.h ---
  89 extern int foo1(void);
  90 extern void foo2(void);
  91 extern void foo4(void);
  92 --- a.c ---
  93 #include "a.h"
  94
  95 static signed int i = 0;
  96
  97 void foo2(void) {
  98  i = -1;
  99 }
 100
 101 static int foo3() {
 102 foo4();
 103 return 10;
 104 }
 105
 106 int foo1(void) {
 107 int data = 0;
 108
 109 if (i &lt; 0) { data = foo3(); }
 110
 111 data = data + 42;
 112 return data;
 113 }
 114
 115 --- main.c ---
 116 #include &lt;stdio.h&gt;
 117 #include "a.h"
 118
 119 void foo4(void) {
 120  printf ("Hi\n");
 121 }
 122
 123 int main() {
 124  return foo1();
 125 }
 126
 127 --- command lines ---
 128 $ llvm-gcc --emit-llvm -c a.c -o a.o  # &lt;-- a.o is LLVM bitcode file
 129 $ llvm-gcc -c main.c -o main.o # &lt;-- main.o is native object file
 130 $ llvm-gcc a.o main.o -o main # &lt;-- standard link command without any modifications
 131 </pre>
 132   <p>In this example, the linker recognizes that <tt>foo2()</tt> is an
 133   externally visible symbol defined in LLVM bitcode file. The linker completes
 134   its usual symbol resolution
 135   pass and finds that <tt>foo2()</tt> is not used anywhere. This information
 136   is used by the LLVM optimizer and it removes <tt>foo2()</tt>. As soon as
 137   <tt>foo2()</tt> is removed, the optimizer recognizes that condition
 138   <tt>i &lt; 0</tt> is always false, which means <tt>foo3()</tt> is never
 139   used. Hence, the optimizer removes <tt>foo3()</tt>, also.  And this in turn,
 140   enables linker to remove <tt>foo4()</tt>.  This example illustrates the
 141   advantage of tight integration with the linker. Here, the optimizer can not
 142   remove <tt>foo3()</tt> without the linker's input.
 143   </p>
 144 </div>
 145
 146 <!-- ======================================================================= -->
 147 <h3>
 148   <a name="alternative_approaches">Alternative Approaches</a>
 149 </h3>
 150
 151 <div>
 152   <dl>
 153     <dt><b>Compiler driver invokes link time optimizer separately.</b></dt>
 154     <dd>In this model the link time optimizer is not able to take advantage of
 155     information collected during the linker's normal symbol resolution phase.
 156     In the above example, the optimizer can not remove <tt>foo2()</tt> without
 157     the linker's input because it is externally visible. This in turn prohibits
 158     the optimizer from removing <tt>foo3()</tt>.</dd>
 159     <dt><b>Use separate tool to collect symbol information from all object
 160     files.</b></dt>
 161     <dd>In this model, a new, separate, tool or library replicates the linker's
 162     capability to collect information for link time optimization. Not only is
 163     this code duplication difficult to justify, but it also has several other
 164     disadvantages.  For example, the linking semantics and the features
 165     provided by the linker on various platform are not unique. This means,
 166     this new tool needs to support all such features and platforms in one
 167     super tool or a separate tool per platform is required. This increases
 168     maintenance cost for link time optimizer significantly, which is not
 169     necessary. This approach also requires staying synchronized with linker
 170     developements on various platforms, which is not the main focus of the link
 171     time optimizer. Finally, this approach increases end user's build time due
 172     to the duplication of work done by this separate tool and the linker itself.
 173     </dd>
 174   </dl>
 175 </div>
 176
 177 </div>
 178
 179 <!-- *********************************************************************** -->
 180 <h2>
 181   <a name="multiphase">Multi-phase communication between libLTO and linker</a>
 182 </h2>
 183
 184 <div>
 185   <p>The linker collects information about symbol defininitions and uses in
 186   various link objects which is more accurate than any information collected
 187   by other tools during typical build cycles.  The linker collects this
 188   information by looking at the definitions and uses of symbols in native .o
 189   files and using symbol visibility information. The linker also uses
 190   user-supplied information, such as a list of exported symbols. LLVM
 191   optimizer collects control flow information, data flow information and knows
 192   much more about program structure from the optimizer's point of view.
 193   Our goal is to take advantage of tight integration between the linker and
 194   the optimizer by sharing this information during various linking phases.
 195 </p>
 196
 197 <!-- ======================================================================= -->
 198 <h3>
 199   <a name="phase1">Phase 1 : Read LLVM Bitcode Files</a>
 200 </h3>
 201
 202 <div>
 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 <h3>
 223   <a name="phase2">Phase 2 : Symbol Resolution</a>
 224 </h3>
 225
 226 <div>
 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 <h3>
 237   <a name="phase3">Phase 3 : Optimize Bitcode Files</a>
 238 </h3>
 239 <div>
 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 <h3>
 252   <a name="phase4">Phase 4 : Symbol Resolution after optimization</a>
 253 </h3>
 254
 255 <div>
 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 </div>
 268
 269 <!-- *********************************************************************** -->
 270 <h2>
 271 <a name="lto">libLTO</a>
 272 </h2>
 273
 274 <div>
 275   <p><tt>libLTO</tt> is a shared object that is part of the LLVM tools, and
 276   is intended for use by a linker. <tt>libLTO</tt> provides an abstract C
 277   interface to use the LLVM interprocedural optimizer without exposing details
 278   of LLVM's internals. The intention is to keep the interface as stable as
 279   possible even when the LLVM optimizer continues to evolve. It should even
 280   be possible for a completely different compilation technology to provide
 281   a different libLTO that works with their object files and the standard
 282   linker tool.</p>
 283
 284 <!-- ======================================================================= -->
 285 <h3>
 286   <a name="lto_module_t">lto_module_t</a>
 287 </h3>
 288
 289 <div>
 290
 291 <p>A non-native object file is handled via an <tt>lto_module_t</tt>.
 292 The following functions allow the linker to check if a file (on disk
 293 or in a memory buffer) is a file which libLTO can process:</p>
 294
 295 <pre class="doc_code">
 296 lto_module_is_object_file(const char*)
 297 lto_module_is_object_file_for_target(const char*, const char*)
 298 lto_module_is_object_file_in_memory(const void*, size_t)
 299 lto_module_is_object_file_in_memory_for_target(const void*, size_t, const char*)
 300 </pre>
 301
 302 <p>If the object file can be processed by libLTO, the linker creates a
 303 <tt>lto_module_t</tt> by using one of</p>
 304
 305 <pre class="doc_code">
 306 lto_module_create(const char*)
 307 lto_module_create_from_memory(const void*, size_t)
 308 </pre>
 309
 310 <p>and when done, the handle is released via</p>
 311
 312 <pre class="doc_code">
 313 lto_module_dispose(lto_module_t)
 314 </pre>
 315
 316 <p>The linker can introspect the non-native object file by getting the number of
 317 symbols and getting the name and attributes of each symbol via:</p>
 318
 319 <pre class="doc_code">
 320 lto_module_get_num_symbols(lto_module_t)
 321 lto_module_get_symbol_name(lto_module_t, unsigned int)
 322 lto_module_get_symbol_attribute(lto_module_t, unsigned int)
 323 </pre>
 324
 325 <p>The attributes of a symbol include the alignment, visibility, and kind.</p>
 326 </div>
 327
 328 <!-- ======================================================================= -->
 329 <h3>
 330   <a name="lto_code_gen_t">lto_code_gen_t</a>
 331 </h3>
 332
 333 <div>
 334
 335 <p>Once the linker has loaded each non-native object files into an
 336 <tt>lto_module_t</tt>, it can request libLTO to process them all and
 337 generate a native object file.  This is done in a couple of steps.
 338 First, a code generator is created with:</p>
 339
 340 <pre class="doc_code">lto_codegen_create()</pre>
 341
 342 <p>Then, each non-native object file is added to the code generator with:</p>
 343
 344 <pre class="doc_code">
 345 lto_codegen_add_module(lto_code_gen_t, lto_module_t)
 346 </pre>
 347
 348 <p>The linker then has the option of setting some codegen options.  Whether or
 349 not to generate DWARF debug info is set with:</p>
 350
 351 <pre class="doc_code">lto_codegen_set_debug_model(lto_code_gen_t)</pre>
 352
 353 <p>Which kind of position independence is set with:</p>
 354
 355 <pre class="doc_code">lto_codegen_set_pic_model(lto_code_gen_t) </pre>
 356
 357 <p>And each symbol that is referenced by a native object file or otherwise must
 358 not be optimized away is set with:</p>
 359
 360 <pre class="doc_code">
 361 lto_codegen_add_must_preserve_symbol(lto_code_gen_t, const char*)
 362 </pre>
 363
 364 <p>After all these settings are done, the linker requests that a native object
 365 file be created from the modules with the settings using:</p>
 366
 367 <pre class="doc_code">lto_codegen_compile(lto_code_gen_t, size*)</pre>
 368
 369 <p>which returns a pointer to a buffer containing the generated native
 370 object file.  The linker then parses that and links it with the rest
 371 of the native object files.</p>
 372
 373 </div>
 374
 375 </div>
 376
 377 <!-- *********************************************************************** -->
 378
 379 <hr>
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 385
 386   Devang Patel and Nick Kledzik<br>
 387   <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
 388   Last modified: $Date$
 389 </address>
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