1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 6 <title>Writing an LLVM Pass</title> 7 <link rel="stylesheet" href="llvm.css" type="text/css"> 8 </head> 9 <body> 10 11 <h1> 12 Writing an LLVM Pass 13 </h1> 14 15 <ol> 16 <li><a href="#introduction">Introduction - What is a pass?</a></li> 17 <li><a href="#quickstart">Quick Start - Writing hello world</a> 18 <ul> 19 <li><a href="#makefile">Setting up the build environment</a></li> 20 <li><a href="#basiccode">Basic code required</a></li> 21 <li><a href="#running">Running a pass with <tt>opt</tt></a></li> 22 </ul></li> 23 <li><a href="#passtype">Pass classes and requirements</a> 24 <ul> 25 <li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a></li> 26 <li><a href="#ModulePass">The <tt>ModulePass</tt> class</a> 27 <ul> 28 <li><a href="#runOnModule">The <tt>runOnModule</tt> method</a></li> 29 </ul></li> 30 <li><a href="#CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a> 31 <ul> 32 <li><a href="#doInitialization_scc">The <tt>doInitialization(CallGraph 33 &)</tt> method</a></li> 34 <li><a href="#runOnSCC">The <tt>runOnSCC</tt> method</a></li> 35 <li><a href="#doFinalization_scc">The <tt>doFinalization(CallGraph 36 &)</tt> method</a></li> 37 </ul></li> 38 <li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a> 39 <ul> 40 <li><a href="#doInitialization_mod">The <tt>doInitialization(Module 41 &)</tt> method</a></li> 42 <li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a></li> 43 <li><a href="#doFinalization_mod">The <tt>doFinalization(Module 44 &)</tt> method</a></li> 45 </ul></li> 46 <li><a href="#LoopPass">The <tt>LoopPass</tt> class</a> 47 <ul> 48 <li><a href="#doInitialization_loop">The <tt>doInitialization(Loop *, 49 LPPassManager &)</tt> method</a></li> 50 <li><a href="#runOnLoop">The <tt>runOnLoop</tt> method</a></li> 51 <li><a href="#doFinalization_loop">The <tt>doFinalization() 52 </tt> method</a></li> 53 </ul></li> 54 <li><a href="#RegionPass">The <tt>RegionPass</tt> class</a> 55 <ul> 56 <li><a href="#doInitialization_region">The <tt>doInitialization(Region *, 57 RGPassManager &)</tt> method</a></li> 58 <li><a href="#runOnRegion">The <tt>runOnRegion</tt> method</a></li> 59 <li><a href="#doFinalization_region">The <tt>doFinalization() 60 </tt> method</a></li> 61 </ul></li> 62 <li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a> 63 <ul> 64 <li><a href="#doInitialization_fn">The <tt>doInitialization(Function 65 &)</tt> method</a></li> 66 <li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt> 67 method</a></li> 68 <li><a href="#doFinalization_fn">The <tt>doFinalization(Function 69 &)</tt> method</a></li> 70 </ul></li> 71 <li><a href="#MachineFunctionPass">The <tt>MachineFunctionPass</tt> 72 class</a> 73 <ul> 74 <li><a href="#runOnMachineFunction">The 75 <tt>runOnMachineFunction(MachineFunction &)</tt> method</a></li> 76 </ul></li> 77 </ul> 78 <li><a href="#registration">Pass Registration</a> 79 <ul> 80 <li><a href="#print">The <tt>print</tt> method</a></li> 81 </ul></li> 82 <li><a href="#interaction">Specifying interactions between passes</a> 83 <ul> 84 <li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt> 85 method</a></li> 86 <li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a></li> 87 <li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a></li> 88 <li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li> 89 <li><a href="#getAnalysis">The <tt>getAnalysis<></tt> and 90 <tt>getAnalysisIfAvailable<></tt> methods</a></li> 91 </ul></li> 92 <li><a href="#analysisgroup">Implementing Analysis Groups</a> 93 <ul> 94 <li><a href="#agconcepts">Analysis Group Concepts</a></li> 95 <li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a></li> 96 </ul></li> 97 <li><a href="#passStatistics">Pass Statistics</a> 98 <li><a href="#passmanager">What PassManager does</a> 99 <ul> 100 <li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a></li> 101 </ul></li> 102 <li><a href="#registering">Registering dynamically loaded passes</a> 103 <ul> 104 <li><a href="#registering_existing">Using existing registries</a></li> 105 <li><a href="#registering_new">Creating new registries</a></li> 106 </ul></li> 107 <li><a href="#debughints">Using GDB with dynamically loaded passes</a> 108 <ul> 109 <li><a href="#breakpoint">Setting a breakpoint in your pass</a></li> 110 <li><a href="#debugmisc">Miscellaneous Problems</a></li> 111 </ul></li> 112 <li><a href="#future">Future extensions planned</a> 113 <ul> 114 <li><a href="#SMP">Multithreaded LLVM</a></li> 115 </ul></li> 116 </ol> 117 118 <div class="doc_author"> 119 <p>Written by <a href="mailto:sabre (a] nondot.org">Chris Lattner</a> and 120 <a href="mailto:jlaskey (a] mac.com">Jim Laskey</a></p> 121 </div> 122 123 <!-- *********************************************************************** --> 124 <h2> 125 <a name="introduction">Introduction - What is a pass?</a> 126 </h2> 127 <!-- *********************************************************************** --> 128 129 <div> 130 131 <p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM 132 passes are where most of the interesting parts of the compiler exist. Passes 133 perform the transformations and optimizations that make up the compiler, they 134 build the analysis results that are used by these transformations, and they are, 135 above all, a structuring technique for compiler code.</p> 136 137 <p>All LLVM passes are subclasses of the <tt><a 138 href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt> 139 class, which implement functionality by overriding virtual methods inherited 140 from <tt>Pass</tt>. Depending on how your pass works, you should inherit from 141 the <tt><a href="#ModulePass">ModulePass</a></tt>, <tt><a 142 href="#CallGraphSCCPass">CallGraphSCCPass</a></tt>, <tt><a 143 href="#FunctionPass">FunctionPass</a></tt>, or <tt><a 144 href="#LoopPass">LoopPass</a></tt>, or <tt><a 145 href="#RegionPass">RegionPass</a></tt>, or <tt><a 146 href="#BasicBlockPass">BasicBlockPass</a></tt> classes, which gives the system 147 more information about what your pass does, and how it can be combined with 148 other passes. One of the main features of the LLVM Pass Framework is that it 149 schedules passes to run in an efficient way based on the constraints that your 150 pass meets (which are indicated by which class they derive from).</p> 151 152 <p>We start by showing you how to construct a pass, everything from setting up 153 the code, to compiling, loading, and executing it. After the basics are down, 154 more advanced features are discussed.</p> 155 156 </div> 157 158 <!-- *********************************************************************** --> 159 <h2> 160 <a name="quickstart">Quick Start - Writing hello world</a> 161 </h2> 162 <!-- *********************************************************************** --> 163 164 <div> 165 166 <p>Here we describe how to write the "hello world" of passes. The "Hello" pass 167 is designed to simply print out the name of non-external functions that exist in 168 the program being compiled. It does not modify the program at all, it just 169 inspects it. The source code and files for this pass are available in the LLVM 170 source tree in the <tt>lib/Transforms/Hello</tt> directory.</p> 171 172 <!-- ======================================================================= --> 173 <h3> 174 <a name="makefile">Setting up the build environment</a> 175 </h3> 176 177 <div> 178 179 <p>First, configure and build LLVM. This needs to be done directly inside the 180 LLVM source tree rather than in a separate objects directory. 181 Next, you need to create a new directory somewhere in the LLVM source 182 base. For this example, we'll assume that you made 183 <tt>lib/Transforms/Hello</tt>. Finally, you must set up a build script 184 (Makefile) that will compile the source code for the new pass. To do this, 185 copy the following into <tt>Makefile</tt>:</p> 186 <hr> 187 188 <div class="doc_code"><pre> 189 # Makefile for hello pass 190 191 # Path to top level of LLVM hierarchy 192 LEVEL = ../../.. 193 194 # Name of the library to build 195 LIBRARYNAME = Hello 196 197 # Make the shared library become a loadable module so the tools can 198 # dlopen/dlsym on the resulting library. 199 LOADABLE_MODULE = 1 200 201 # Include the makefile implementation stuff 202 include $(LEVEL)/Makefile.common 203 </pre></div> 204 205 <p>This makefile specifies that all of the <tt>.cpp</tt> files in the current 206 directory are to be compiled and linked together into a shared object 207 <tt>$(LEVEL)/Debug+Asserts/lib/Hello.so</tt> that can be dynamically loaded by 208 the <tt>opt</tt> or <tt>bugpoint</tt> tools via their <tt>-load</tt> options. 209 If your operating system uses a suffix other than .so (such as windows or 210 Mac OS/X), the appropriate extension will be used.</p> 211 212 <p>If you are used CMake to build LLVM, see 213 <a href="CMake.html#passdev">Developing an LLVM pass with CMake</a>.</p> 214 215 <p>Now that we have the build scripts set up, we just need to write the code for 216 the pass itself.</p> 217 218 </div> 219 220 <!-- ======================================================================= --> 221 <h3> 222 <a name="basiccode">Basic code required</a> 223 </h3> 224 225 <div> 226 227 <p>Now that we have a way to compile our new pass, we just have to write it. 228 Start out with:</p> 229 230 <div class="doc_code"> 231 <pre> 232 <b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>" 233 <b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>" 234 <b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>" 235 </pre> 236 </div> 237 238 <p>Which are needed because we are writing a <tt><a 239 href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>, 240 we are operating on <tt><a 241 href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s, 242 and we will be doing some printing.</p> 243 244 <p>Next we have:</p> 245 246 <div class="doc_code"> 247 <pre> 248 <b>using namespace llvm;</b> 249 </pre> 250 </div> 251 252 <p>... which is required because the functions from the include files 253 live in the llvm namespace.</p> 254 255 <p>Next we have:</p> 256 257 <div class="doc_code"> 258 <pre> 259 <b>namespace</b> { 260 </pre> 261 </div> 262 263 <p>... which starts out an anonymous namespace. Anonymous namespaces are to C++ 264 what the "<tt>static</tt>" keyword is to C (at global scope). It makes the 265 things declared inside of the anonymous namespace visible only to the current 266 file. If you're not familiar with them, consult a decent C++ book for more 267 information.</p> 268 269 <p>Next, we declare our pass itself:</p> 270 271 <div class="doc_code"> 272 <pre> 273 <b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> { 274 </pre> 275 </div> 276 277 <p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a 278 href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass</a></tt>. 279 The different builtin pass subclasses are described in detail <a 280 href="#passtype">later</a>, but for now, know that <a 281 href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate on a function at a 282 time.</p> 283 284 <div class="doc_code"> 285 <pre> 286 static char ID; 287 Hello() : FunctionPass(ID) {} 288 </pre> 289 </div> 290 291 <p>This declares pass identifier used by LLVM to identify pass. This allows LLVM 292 to avoid using expensive C++ runtime information.</p> 293 294 <div class="doc_code"> 295 <pre> 296 <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) { 297 errs() << "<i>Hello: </i>"; 298 errs().write_escaped(F.getName()) << "\n"; 299 <b>return false</b>; 300 } 301 }; <i>// end of struct Hello</i> 302 } <i>// end of anonymous namespace</i> 303 </pre> 304 </div> 305 306 <p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method, 307 which overloads an abstract virtual method inherited from <a 308 href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed 309 to do our thing, so we just print out our message with the name of each 310 function.</p> 311 312 <div class="doc_code"> 313 <pre> 314 char Hello::ID = 0; 315 </pre> 316 </div> 317 318 <p>We initialize pass ID here. LLVM uses ID's address to identify a pass, so 319 initialization value is not important.</p> 320 321 <div class="doc_code"> 322 <pre> 323 static RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>", 324 false /* Only looks at CFG */, 325 false /* Analysis Pass */); 326 </pre> 327 </div> 328 329 <p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>, 330 giving it a command line argument "<tt>hello</tt>", and a name "<tt>Hello World 331 Pass</tt>". The last two arguments describe its behavior: if a pass walks CFG 332 without modifying it then the third argument is set to <tt>true</tt>; if a pass 333 is an analysis pass, for example dominator tree pass, then <tt>true</tt> is 334 supplied as the fourth argument.</p> 335 336 <p>As a whole, the <tt>.cpp</tt> file looks like:</p> 337 338 <div class="doc_code"> 339 <pre> 340 <b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>" 341 <b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>" 342 <b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>" 343 344 <b>using namespace llvm;</b> 345 346 <b>namespace</b> { 347 <b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> { 348 349 static char ID; 350 Hello() : FunctionPass(ID) {} 351 352 <b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) { 353 errs() << "<i>Hello: </i>"; 354 errs().write_escaped(F.getName()) << '\n'; 355 <b>return false</b>; 356 } 357 358 }; 359 } 360 361 char Hello::ID = 0; 362 static RegisterPass<Hello> X("hello", "Hello World Pass", false, false); 363 </pre> 364 </div> 365 366 <p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>" 367 command in the local directory and you should get a new file 368 "<tt>Debug+Asserts/lib/Hello.so</tt>" under the top level directory of the LLVM 369 source tree (not in the local directory). Note that everything in this file is 370 contained in an anonymous namespace — this reflects the fact that passes 371 are self contained units that do not need external interfaces (although they can 372 have them) to be useful.</p> 373 374 </div> 375 376 <!-- ======================================================================= --> 377 <h3> 378 <a name="running">Running a pass with <tt>opt</tt></a> 379 </h3> 380 381 <div> 382 383 <p>Now that you have a brand new shiny shared object file, we can use the 384 <tt>opt</tt> command to run an LLVM program through your pass. Because you 385 registered your pass with <tt>RegisterPass</tt>, you will be able to 386 use the <tt>opt</tt> tool to access it, once loaded.</p> 387 388 <p>To test it, follow the example at the end of the <a 389 href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to 390 LLVM. We can now run the bitcode file (<tt>hello.bc</tt>) for the program 391 through our transformation like this (or course, any bitcode file will 392 work):</p> 393 394 <div class="doc_code"><pre> 395 $ opt -load ../../../Debug+Asserts/lib/Hello.so -hello < hello.bc > /dev/null 396 Hello: __main 397 Hello: puts 398 Hello: main 399 </pre></div> 400 401 <p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your 402 pass as a shared object, which makes '<tt>-hello</tt>' a valid command line 403 argument (which is one reason you need to <a href="#registration">register your 404 pass</a>). Because the hello pass does not modify the program in any 405 interesting way, we just throw away the result of <tt>opt</tt> (sending it to 406 <tt>/dev/null</tt>).</p> 407 408 <p>To see what happened to the other string you registered, try running 409 <tt>opt</tt> with the <tt>-help</tt> option:</p> 410 411 <div class="doc_code"><pre> 412 $ opt -load ../../../Debug+Asserts/lib/Hello.so -help 413 OVERVIEW: llvm .bc -> .bc modular optimizer 414 415 USAGE: opt [options] <input bitcode> 416 417 OPTIONS: 418 Optimizations available: 419 ... 420 -funcresolve - Resolve Functions 421 -gcse - Global Common Subexpression Elimination 422 -globaldce - Dead Global Elimination 423 <b>-hello - Hello World Pass</b> 424 -indvars - Canonicalize Induction Variables 425 -inline - Function Integration/Inlining 426 -instcombine - Combine redundant instructions 427 ... 428 </pre></div> 429 430 <p>The pass name get added as the information string for your pass, giving some 431 documentation to users of <tt>opt</tt>. Now that you have a working pass, you 432 would go ahead and make it do the cool transformations you want. Once you get 433 it all working and tested, it may become useful to find out how fast your pass 434 is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command 435 line option (<tt>--time-passes</tt>) that allows you to get information about 436 the execution time of your pass along with the other passes you queue up. For 437 example:</p> 438 439 <div class="doc_code"><pre> 440 $ opt -load ../../../Debug+Asserts/lib/Hello.so -hello -time-passes < hello.bc > /dev/null 441 Hello: __main 442 Hello: puts 443 Hello: main 444 =============================================================================== 445 ... Pass execution timing report ... 446 =============================================================================== 447 Total Execution Time: 0.02 seconds (0.0479059 wall clock) 448 449 ---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name --- 450 0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer 451 0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction 452 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier 453 <b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b> 454 0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL 455 </pre></div> 456 457 <p>As you can see, our implementation above is pretty fast :). The additional 458 passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify 459 that the LLVM emitted by your pass is still valid and well formed LLVM, which 460 hasn't been broken somehow.</p> 461 462 <p>Now that you have seen the basics of the mechanics behind passes, we can talk 463 about some more details of how they work and how to use them.</p> 464 465 </div> 466 467 </div> 468 469 <!-- *********************************************************************** --> 470 <h2> 471 <a name="passtype">Pass classes and requirements</a> 472 </h2> 473 <!-- *********************************************************************** --> 474 475 <div> 476 477 <p>One of the first things that you should do when designing a new pass is to 478 decide what class you should subclass for your pass. The <a 479 href="#basiccode">Hello World</a> example uses the <tt><a 480 href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we 481 did not discuss why or when this should occur. Here we talk about the classes 482 available, from the most general to the most specific.</p> 483 484 <p>When choosing a superclass for your Pass, you should choose the <b>most 485 specific</b> class possible, while still being able to meet the requirements 486 listed. This gives the LLVM Pass Infrastructure information necessary to 487 optimize how passes are run, so that the resultant compiler isn't unnecessarily 488 slow.</p> 489 490 <!-- ======================================================================= --> 491 <h3> 492 <a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a> 493 </h3> 494 495 <div> 496 497 <p>The most plain and boring type of pass is the "<tt><a 498 href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>" 499 class. This pass type is used for passes that do not have to be run, do not 500 change state, and never need to be updated. This is not a normal type of 501 transformation or analysis, but can provide information about the current 502 compiler configuration.</p> 503 504 <p>Although this pass class is very infrequently used, it is important for 505 providing information about the current target machine being compiled for, and 506 other static information that can affect the various transformations.</p> 507 508 <p><tt>ImmutablePass</tt>es never invalidate other transformations, are never 509 invalidated, and are never "run".</p> 510 511 </div> 512 513 <!-- ======================================================================= --> 514 <h3> 515 <a name="ModulePass">The <tt>ModulePass</tt> class</a> 516 </h3> 517 518 <div> 519 520 <p>The "<tt><a 521 href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass</a></tt>" 522 class is the most general of all superclasses that you can use. Deriving from 523 <tt>ModulePass</tt> indicates that your pass uses the entire program as a unit, 524 referring to function bodies in no predictable order, or adding and removing 525 functions. Because nothing is known about the behavior of <tt>ModulePass</tt> 526 subclasses, no optimization can be done for their execution.</p> 527 528 <p>A module pass can use function level passes (e.g. dominators) using 529 the getAnalysis interface 530 <tt>getAnalysis<DominatorTree>(llvm::Function *)</tt> to provide the 531 function to retrieve analysis result for, if the function pass does not require 532 any module or immutable passes. Note that this can only be done for functions for which the 533 analysis ran, e.g. in the case of dominators you should only ask for the 534 DominatorTree for function definitions, not declarations.</p> 535 536 <p>To write a correct <tt>ModulePass</tt> subclass, derive from 537 <tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the 538 following signature:</p> 539 540 <!-- _______________________________________________________________________ --> 541 <h4> 542 <a name="runOnModule">The <tt>runOnModule</tt> method</a> 543 </h4> 544 545 <div> 546 547 <div class="doc_code"><pre> 548 <b>virtual bool</b> runOnModule(Module &M) = 0; 549 </pre></div> 550 551 <p>The <tt>runOnModule</tt> method performs the interesting work of the pass. 552 It should return true if the module was modified by the transformation and 553 false otherwise.</p> 554 555 </div> 556 557 </div> 558 559 <!-- ======================================================================= --> 560 <h3> 561 <a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a> 562 </h3> 563 564 <div> 565 566 <p>The "<tt><a 567 href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>" 568 is used by passes that need to traverse the program bottom-up on the call graph 569 (callees before callers). Deriving from CallGraphSCCPass provides some 570 mechanics for building and traversing the CallGraph, but also allows the system 571 to optimize execution of CallGraphSCCPass's. If your pass meets the 572 requirements outlined below, and doesn't meet the requirements of a <tt><a 573 href="#FunctionPass">FunctionPass</a></tt> or <tt><a 574 href="#BasicBlockPass">BasicBlockPass</a></tt>, you should derive from 575 <tt>CallGraphSCCPass</tt>.</p> 576 577 <p><b>TODO</b>: explain briefly what SCC, Tarjan's algo, and B-U mean.</p> 578 579 <p>To be explicit, <tt>CallGraphSCCPass</tt> subclasses are:</p> 580 581 <ol> 582 583 <li>... <em>not allowed</em> to inspect or modify any <tt>Function</tt>s other 584 than those in the current SCC and the direct callers and direct callees of the 585 SCC.</li> 586 587 <li>... <em>required</em> to preserve the current CallGraph object, updating it 588 to reflect any changes made to the program.</li> 589 590 <li>... <em>not allowed</em> to add or remove SCC's from the current Module, 591 though they may change the contents of an SCC.</li> 592 593 <li>... <em>allowed</em> to add or remove global variables from the current 594 Module.</li> 595 596 <li>... <em>allowed</em> to maintain state across invocations of 597 <a href="#runOnSCC"><tt>runOnSCC</tt></a> (including global data).</li> 598 </ol> 599 600 <p>Implementing a <tt>CallGraphSCCPass</tt> is slightly tricky in some cases 601 because it has to handle SCCs with more than one node in it. All of the virtual 602 methods described below should return true if they modified the program, or 603 false if they didn't.</p> 604 605 <!-- _______________________________________________________________________ --> 606 <h4> 607 <a name="doInitialization_scc"> 608 The <tt>doInitialization(CallGraph &)</tt> method 609 </a> 610 </h4> 611 612 <div> 613 614 <div class="doc_code"><pre> 615 <b>virtual bool</b> doInitialization(CallGraph &CG); 616 </pre></div> 617 618 <p>The <tt>doIninitialize</tt> method is allowed to do most of the things that 619 <tt>CallGraphSCCPass</tt>'s are not allowed to do. They can add and remove 620 functions, get pointers to functions, etc. The <tt>doInitialization</tt> method 621 is designed to do simple initialization type of stuff that does not depend on 622 the SCCs being processed. The <tt>doInitialization</tt> method call is not 623 scheduled to overlap with any other pass executions (thus it should be very 624 fast).</p> 625 626 </div> 627 628 <!-- _______________________________________________________________________ --> 629 <h4> 630 <a name="runOnSCC">The <tt>runOnSCC</tt> method</a> 631 </h4> 632 633 <div> 634 635 <div class="doc_code"><pre> 636 <b>virtual bool</b> runOnSCC(CallGraphSCC &SCC) = 0; 637 </pre></div> 638 639 <p>The <tt>runOnSCC</tt> method performs the interesting work of the pass, and 640 should return true if the module was modified by the transformation, false 641 otherwise.</p> 642 643 </div> 644 645 <!-- _______________________________________________________________________ --> 646 <h4> 647 <a name="doFinalization_scc"> 648 The <tt>doFinalization(CallGraph &)</tt> method 649 </a> 650 </h4> 651 652 <div> 653 654 <div class="doc_code"><pre> 655 <b>virtual bool</b> doFinalization(CallGraph &CG); 656 </pre></div> 657 658 <p>The <tt>doFinalization</tt> method is an infrequently used method that is 659 called when the pass framework has finished calling <a 660 href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the 661 program being compiled.</p> 662 663 </div> 664 665 </div> 666 667 <!-- ======================================================================= --> 668 <h3> 669 <a name="FunctionPass">The <tt>FunctionPass</tt> class</a> 670 </h3> 671 672 <div> 673 674 <p>In contrast to <tt>ModulePass</tt> subclasses, <tt><a 675 href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt> 676 subclasses do have a predictable, local behavior that can be expected by the 677 system. All <tt>FunctionPass</tt> execute on each function in the program 678 independent of all of the other functions in the program. 679 <tt>FunctionPass</tt>'s do not require that they are executed in a particular 680 order, and <tt>FunctionPass</tt>'s do not modify external functions.</p> 681 682 <p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p> 683 684 <ol> 685 <li>Modify a Function other than the one currently being processed.</li> 686 <li>Add or remove Function's from the current Module.</li> 687 <li>Add or remove global variables from the current Module.</li> 688 <li>Maintain state across invocations of 689 <a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li> 690 </ol> 691 692 <p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a 693 href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s 694 may overload three virtual methods to do their work. All of these methods 695 should return true if they modified the program, or false if they didn't.</p> 696 697 <!-- _______________________________________________________________________ --> 698 <h4> 699 <a name="doInitialization_mod"> 700 The <tt>doInitialization(Module &)</tt> method 701 </a> 702 </h4> 703 704 <div> 705 706 <div class="doc_code"><pre> 707 <b>virtual bool</b> doInitialization(Module &M); 708 </pre></div> 709 710 <p>The <tt>doIninitialize</tt> method is allowed to do most of the things that 711 <tt>FunctionPass</tt>'s are not allowed to do. They can add and remove 712 functions, get pointers to functions, etc. The <tt>doInitialization</tt> method 713 is designed to do simple initialization type of stuff that does not depend on 714 the functions being processed. The <tt>doInitialization</tt> method call is not 715 scheduled to overlap with any other pass executions (thus it should be very 716 fast).</p> 717 718 <p>A good example of how this method should be used is the <a 719 href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a> 720 pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into 721 platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls. It 722 uses the <tt>doInitialization</tt> method to get a reference to the malloc and 723 free functions that it needs, adding prototypes to the module if necessary.</p> 724 725 </div> 726 727 <!-- _______________________________________________________________________ --> 728 <h4> 729 <a name="runOnFunction">The <tt>runOnFunction</tt> method</a> 730 </h4> 731 732 <div> 733 734 <div class="doc_code"><pre> 735 <b>virtual bool</b> runOnFunction(Function &F) = 0; 736 </pre></div><p> 737 738 <p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do 739 the transformation or analysis work of your pass. As usual, a true value should 740 be returned if the function is modified.</p> 741 742 </div> 743 744 <!-- _______________________________________________________________________ --> 745 <h4> 746 <a name="doFinalization_mod"> 747 The <tt>doFinalization(Module &)</tt> method 748 </a> 749 </h4> 750 751 <div> 752 753 <div class="doc_code"><pre> 754 <b>virtual bool</b> doFinalization(Module &M); 755 </pre></div> 756 757 <p>The <tt>doFinalization</tt> method is an infrequently used method that is 758 called when the pass framework has finished calling <a 759 href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the 760 program being compiled.</p> 761 762 </div> 763 764 </div> 765 766 <!-- ======================================================================= --> 767 <h3> 768 <a name="LoopPass">The <tt>LoopPass</tt> class </a> 769 </h3> 770 771 <div> 772 773 <p> All <tt>LoopPass</tt> execute on each loop in the function independent of 774 all of the other loops in the function. <tt>LoopPass</tt> processes loops in 775 loop nest order such that outer most loop is processed last. </p> 776 777 <p> <tt>LoopPass</tt> subclasses are allowed to update loop nest using 778 <tt>LPPassManager</tt> interface. Implementing a loop pass is usually 779 straightforward. <tt>LoopPass</tt>'s may overload three virtual methods to 780 do their work. All these methods should return true if they modified the 781 program, or false if they didn't. </p> 782 783 <!-- _______________________________________________________________________ --> 784 <h4> 785 <a name="doInitialization_loop"> 786 The <tt>doInitialization(Loop *,LPPassManager &)</tt> method 787 </a> 788 </h4> 789 790 <div> 791 792 <div class="doc_code"><pre> 793 <b>virtual bool</b> doInitialization(Loop *, LPPassManager &LPM); 794 </pre></div> 795 796 <p>The <tt>doInitialization</tt> method is designed to do simple initialization 797 type of stuff that does not depend on the functions being processed. The 798 <tt>doInitialization</tt> method call is not scheduled to overlap with any 799 other pass executions (thus it should be very fast). LPPassManager 800 interface should be used to access Function or Module level analysis 801 information.</p> 802 803 </div> 804 805 806 <!-- _______________________________________________________________________ --> 807 <h4> 808 <a name="runOnLoop">The <tt>runOnLoop</tt> method</a> 809 </h4> 810 811 <div> 812 813 <div class="doc_code"><pre> 814 <b>virtual bool</b> runOnLoop(Loop *, LPPassManager &LPM) = 0; 815 </pre></div><p> 816 817 <p>The <tt>runOnLoop</tt> method must be implemented by your subclass to do 818 the transformation or analysis work of your pass. As usual, a true value should 819 be returned if the function is modified. <tt>LPPassManager</tt> interface 820 should be used to update loop nest.</p> 821 822 </div> 823 824 <!-- _______________________________________________________________________ --> 825 <h4> 826 <a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a> 827 </h4> 828 829 <div> 830 831 <div class="doc_code"><pre> 832 <b>virtual bool</b> doFinalization(); 833 </pre></div> 834 835 <p>The <tt>doFinalization</tt> method is an infrequently used method that is 836 called when the pass framework has finished calling <a 837 href="#runOnLoop"><tt>runOnLoop</tt></a> for every loop in the 838 program being compiled. </p> 839 840 </div> 841 842 </div> 843 844 <!-- ======================================================================= --> 845 <h3> 846 <a name="RegionPass">The <tt>RegionPass</tt> class </a> 847 </h3> 848 849 <div> 850 851 <p> <tt>RegionPass</tt> is similar to <a href="#LoopPass"><tt>LoopPass</tt></a>, 852 but executes on each single entry single exit region in the function. 853 <tt>RegionPass</tt> processes regions in nested order such that the outer most 854 region is processed last. </p> 855 856 <p> <tt>RegionPass</tt> subclasses are allowed to update the region tree by using 857 the <tt>RGPassManager</tt> interface. You may overload three virtual methods of 858 <tt>RegionPass</tt> to implement your own region pass. All these 859 methods should return true if they modified the program, or false if they didn not. 860 </p> 861 862 <!-- _______________________________________________________________________ --> 863 <h4> 864 <a name="doInitialization_region"> 865 The <tt>doInitialization(Region *, RGPassManager &)</tt> method 866 </a> 867 </h4> 868 869 <div> 870 871 <div class="doc_code"><pre> 872 <b>virtual bool</b> doInitialization(Region *, RGPassManager &RGM); 873 </pre></div> 874 875 <p>The <tt>doInitialization</tt> method is designed to do simple initialization 876 type of stuff that does not depend on the functions being processed. The 877 <tt>doInitialization</tt> method call is not scheduled to overlap with any 878 other pass executions (thus it should be very fast). RPPassManager 879 interface should be used to access Function or Module level analysis 880 information.</p> 881 882 </div> 883 884 885 <!-- _______________________________________________________________________ --> 886 <h4> 887 <a name="runOnRegion">The <tt>runOnRegion</tt> method</a> 888 </h4> 889 890 <div> 891 892 <div class="doc_code"><pre> 893 <b>virtual bool</b> runOnRegion(Region *, RGPassManager &RGM) = 0; 894 </pre></div><p> 895 896 <p>The <tt>runOnRegion</tt> method must be implemented by your subclass to do 897 the transformation or analysis work of your pass. As usual, a true value should 898 be returned if the region is modified. <tt>RGPassManager</tt> interface 899 should be used to update region tree.</p> 900 901 </div> 902 903 <!-- _______________________________________________________________________ --> 904 <h4> 905 <a name="doFinalization_region">The <tt>doFinalization()</tt> method</a> 906 </h4> 907 908 <div> 909 910 <div class="doc_code"><pre> 911 <b>virtual bool</b> doFinalization(); 912 </pre></div> 913 914 <p>The <tt>doFinalization</tt> method is an infrequently used method that is 915 called when the pass framework has finished calling <a 916 href="#runOnRegion"><tt>runOnRegion</tt></a> for every region in the 917 program being compiled. </p> 918 919 </div> 920 921 </div> 922 923 <!-- ======================================================================= --> 924 <h3> 925 <a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a> 926 </h3> 927 928 <div> 929 930 <p><tt>BasicBlockPass</tt>'s are just like <a 931 href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit 932 their scope of inspection and modification to a single basic block at a time. 933 As such, they are <b>not</b> allowed to do any of the following:</p> 934 935 <ol> 936 <li>Modify or inspect any basic blocks outside of the current one</li> 937 <li>Maintain state across invocations of 938 <a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li> 939 <li>Modify the control flow graph (by altering terminator instructions)</li> 940 <li>Any of the things forbidden for 941 <a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li> 942 </ol> 943 944 <p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole" 945 optimizations. They may override the same <a 946 href="#doInitialization_mod"><tt>doInitialization(Module &)</tt></a> and <a 947 href="#doFinalization_mod"><tt>doFinalization(Module &)</tt></a> methods that <a 948 href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p> 949 950 <!-- _______________________________________________________________________ --> 951 <h4> 952 <a name="doInitialization_fn"> 953 The <tt>doInitialization(Function &)</tt> method 954 </a> 955 </h4> 956 957 <div> 958 959 <div class="doc_code"><pre> 960 <b>virtual bool</b> doInitialization(Function &F); 961 </pre></div> 962 963 <p>The <tt>doIninitialize</tt> method is allowed to do most of the things that 964 <tt>BasicBlockPass</tt>'s are not allowed to do, but that 965 <tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed 966 to do simple initialization that does not depend on the 967 BasicBlocks being processed. The <tt>doInitialization</tt> method call is not 968 scheduled to overlap with any other pass executions (thus it should be very 969 fast).</p> 970 971 </div> 972 973 <!-- _______________________________________________________________________ --> 974 <h4> 975 <a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a> 976 </h4> 977 978 <div> 979 980 <div class="doc_code"><pre> 981 <b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0; 982 </pre></div> 983 984 <p>Override this function to do the work of the <tt>BasicBlockPass</tt>. This 985 function is not allowed to inspect or modify basic blocks other than the 986 parameter, and are not allowed to modify the CFG. A true value must be returned 987 if the basic block is modified.</p> 988 989 </div> 990 991 <!-- _______________________________________________________________________ --> 992 <h4> 993 <a name="doFinalization_fn"> 994 The <tt>doFinalization(Function &)</tt> method 995 </a> 996 </h4> 997 998 <div> 999 1000 <div class="doc_code"><pre> 1001 <b>virtual bool</b> doFinalization(Function &F); 1002 </pre></div> 1003 1004 <p>The <tt>doFinalization</tt> method is an infrequently used method that is 1005 called when the pass framework has finished calling <a 1006 href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the 1007 program being compiled. This can be used to perform per-function 1008 finalization.</p> 1009 1010 </div> 1011 1012 </div> 1013 1014 <!-- ======================================================================= --> 1015 <h3> 1016 <a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a> 1017 </h3> 1018 1019 <div> 1020 1021 <p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that 1022 executes on the machine-dependent representation of each LLVM function in the 1023 program.</p> 1024 1025 <p>Code generator passes are registered and initialized specially by 1026 <tt>TargetMachine::addPassesToEmitFile</tt> and similar routines, so they 1027 cannot generally be run from the <tt>opt</tt> or <tt>bugpoint</tt> 1028 commands.</p> 1029 1030 <p>A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all 1031 the restrictions that apply to a <tt>FunctionPass</tt> also apply to it. 1032 <tt>MachineFunctionPass</tt>es also have additional restrictions. In particular, 1033 <tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p> 1034 1035 <ol> 1036 <li>Modify or create any LLVM IR Instructions, BasicBlocks, Arguments, 1037 Functions, GlobalVariables, GlobalAliases, or Modules.</li> 1038 <li>Modify a MachineFunction other than the one currently being processed.</li> 1039 <li>Maintain state across invocations of <a 1040 href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global 1041 data)</li> 1042 </ol> 1043 1044 <!-- _______________________________________________________________________ --> 1045 <h4> 1046 <a name="runOnMachineFunction"> 1047 The <tt>runOnMachineFunction(MachineFunction &MF)</tt> method 1048 </a> 1049 </h4> 1050 1051 <div> 1052 1053 <div class="doc_code"><pre> 1054 <b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0; 1055 </pre></div> 1056 1057 <p><tt>runOnMachineFunction</tt> can be considered the main entry point of a 1058 <tt>MachineFunctionPass</tt>; that is, you should override this method to do the 1059 work of your <tt>MachineFunctionPass</tt>.</p> 1060 1061 <p>The <tt>runOnMachineFunction</tt> method is called on every 1062 <tt>MachineFunction</tt> in a <tt>Module</tt>, so that the 1063 <tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent 1064 representation of the function. If you want to get at the LLVM <tt>Function</tt> 1065 for the <tt>MachineFunction</tt> you're working on, use 1066 <tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but 1067 remember, you may not modify the LLVM <tt>Function</tt> or its contents from a 1068 <tt>MachineFunctionPass</tt>.</p> 1069 1070 </div> 1071 1072 </div> 1073 1074 </div> 1075 1076 <!-- *********************************************************************** --> 1077 <h2> 1078 <a name="registration">Pass registration</a> 1079 </h2> 1080 <!-- *********************************************************************** --> 1081 1082 <div> 1083 1084 <p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how 1085 pass registration works, and discussed some of the reasons that it is used and 1086 what it does. Here we discuss how and why passes are registered.</p> 1087 1088 <p>As we saw above, passes are registered with the <b><tt>RegisterPass</tt></b> 1089 template. The template parameter is the name of the pass that is to be used on 1090 the command line to specify that the pass should be added to a program (for 1091 example, with <tt>opt</tt> or <tt>bugpoint</tt>). The first argument is the 1092 name of the pass, which is to be used for the <tt>-help</tt> output of 1093 programs, as 1094 well as for debug output generated by the <tt>--debug-pass</tt> option.</p> 1095 1096 <p>If you want your pass to be easily dumpable, you should 1097 implement the virtual <tt>print</tt> method:</p> 1098 1099 <!-- _______________________________________________________________________ --> 1100 <h4> 1101 <a name="print">The <tt>print</tt> method</a> 1102 </h4> 1103 1104 <div> 1105 1106 <div class="doc_code"><pre> 1107 <b>virtual void</b> print(std::ostream &O, <b>const</b> Module *M) <b>const</b>; 1108 </pre></div> 1109 1110 <p>The <tt>print</tt> method must be implemented by "analyses" in order to print 1111 a human readable version of the analysis results. This is useful for debugging 1112 an analysis itself, as well as for other people to figure out how an analysis 1113 works. Use the <tt>opt -analyze</tt> argument to invoke this method.</p> 1114 1115 <p>The <tt>llvm::OStream</tt> parameter specifies the stream to write the results on, 1116 and the <tt>Module</tt> parameter gives a pointer to the top level module of the 1117 program that has been analyzed. Note however that this pointer may be null in 1118 certain circumstances (such as calling the <tt>Pass::dump()</tt> from a 1119 debugger), so it should only be used to enhance debug output, it should not be 1120 depended on.</p> 1121 1122 </div> 1123 1124 </div> 1125 1126 <!-- *********************************************************************** --> 1127 <h2> 1128 <a name="interaction">Specifying interactions between passes</a> 1129 </h2> 1130 <!-- *********************************************************************** --> 1131 1132 <div> 1133 1134 <p>One of the main responsibilities of the <tt>PassManager</tt> is to make sure 1135 that passes interact with each other correctly. Because <tt>PassManager</tt> 1136 tries to <a href="#passmanager">optimize the execution of passes</a> it must 1137 know how the passes interact with each other and what dependencies exist between 1138 the various passes. To track this, each pass can declare the set of passes that 1139 are required to be executed before the current pass, and the passes which are 1140 invalidated by the current pass.</p> 1141 1142 <p>Typically this functionality is used to require that analysis results are 1143 computed before your pass is run. Running arbitrary transformation passes can 1144 invalidate the computed analysis results, which is what the invalidation set 1145 specifies. If a pass does not implement the <tt><a 1146 href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not 1147 having any prerequisite passes, and invalidating <b>all</b> other passes.</p> 1148 1149 <!-- _______________________________________________________________________ --> 1150 <h4> 1151 <a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a> 1152 </h4> 1153 1154 <div> 1155 1156 <div class="doc_code"><pre> 1157 <b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>; 1158 </pre></div> 1159 1160 <p>By implementing the <tt>getAnalysisUsage</tt> method, the required and 1161 invalidated sets may be specified for your transformation. The implementation 1162 should fill in the <tt><a 1163 href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt> 1164 object with information about which passes are required and not invalidated. To 1165 do this, a pass may call any of the following methods on the AnalysisUsage 1166 object:</p> 1167 </div> 1168 1169 <!-- _______________________________________________________________________ --> 1170 <h4> 1171 <a name="AU::addRequired"> 1172 The <tt>AnalysisUsage::addRequired<></tt> 1173 and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods 1174 </a> 1175 </h4> 1176 1177 <div> 1178 <p> 1179 If your pass requires a previous pass to be executed (an analysis for example), 1180 it can use one of these methods to arrange for it to be run before your pass. 1181 LLVM has many different types of analyses and passes that can be required, 1182 spanning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>. 1183 Requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will 1184 be no critical edges in the CFG when your pass has been run. 1185 </p> 1186 1187 <p> 1188 Some analyses chain to other analyses to do their job. For example, an <a 1189 href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a 1190 href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In 1191 cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be 1192 used instead of the <tt>addRequired</tt> method. This informs the PassManager 1193 that the transitively required pass should be alive as long as the requiring 1194 pass is. 1195 </p> 1196 </div> 1197 1198 <!-- _______________________________________________________________________ --> 1199 <h4> 1200 <a name="AU::addPreserved"> 1201 The <tt>AnalysisUsage::addPreserved<></tt> method 1202 </a> 1203 </h4> 1204 1205 <div> 1206 <p> 1207 One of the jobs of the PassManager is to optimize how and when analyses are run. 1208 In particular, it attempts to avoid recomputing data unless it needs to. For 1209 this reason, passes are allowed to declare that they preserve (i.e., they don't 1210 invalidate) an existing analysis if it's available. For example, a simple 1211 constant folding pass would not modify the CFG, so it can't possibly affect the 1212 results of dominator analysis. By default, all passes are assumed to invalidate 1213 all others. 1214 </p> 1215 1216 <p> 1217 The <tt>AnalysisUsage</tt> class provides several methods which are useful in 1218 certain circumstances that are related to <tt>addPreserved</tt>. In particular, 1219 the <tt>setPreservesAll</tt> method can be called to indicate that the pass does 1220 not modify the LLVM program at all (which is true for analyses), and the 1221 <tt>setPreservesCFG</tt> method can be used by transformations that change 1222 instructions in the program but do not modify the CFG or terminator instructions 1223 (note that this property is implicitly set for <a 1224 href="#BasicBlockPass">BasicBlockPass</a>'s). 1225 </p> 1226 1227 <p> 1228 <tt>addPreserved</tt> is particularly useful for transformations like 1229 <tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop 1230 and dominator related analyses if they exist, so it can preserve them, despite 1231 the fact that it hacks on the CFG. 1232 </p> 1233 </div> 1234 1235 <!-- _______________________________________________________________________ --> 1236 <h4> 1237 <a name="AU::examples"> 1238 Example implementations of <tt>getAnalysisUsage</tt> 1239 </a> 1240 </h4> 1241 1242 <div> 1243 1244 <div class="doc_code"><pre> 1245 <i>// This example modifies the program, but does not modify the CFG</i> 1246 <b>void</b> <a href="http://llvm.org/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> { 1247 AU.setPreservesCFG(); 1248 AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>>(); 1249 } 1250 </pre></div> 1251 1252 </div> 1253 1254 <!-- _______________________________________________________________________ --> 1255 <h4> 1256 <a name="getAnalysis"> 1257 The <tt>getAnalysis<></tt> and 1258 <tt>getAnalysisIfAvailable<></tt> methods 1259 </a> 1260 </h4> 1261 1262 <div> 1263 1264 <p>The <tt>Pass::getAnalysis<></tt> method is automatically inherited by 1265 your class, providing you with access to the passes that you declared that you 1266 required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> 1267 method. It takes a single template argument that specifies which pass class you 1268 want, and returns a reference to that pass. For example:</p> 1269 1270 <div class="doc_code"><pre> 1271 bool LICM::runOnFunction(Function &F) { 1272 LoopInfo &LI = getAnalysis<LoopInfo>(); 1273 ... 1274 } 1275 </pre></div> 1276 1277 <p>This method call returns a reference to the pass desired. You may get a 1278 runtime assertion failure if you attempt to get an analysis that you did not 1279 declare as required in your <a 1280 href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This 1281 method can be called by your <tt>run*</tt> method implementation, or by any 1282 other local method invoked by your <tt>run*</tt> method. 1283 1284 A module level pass can use function level analysis info using this interface. 1285 For example:</p> 1286 1287 <div class="doc_code"><pre> 1288 bool ModuleLevelPass::runOnModule(Module &M) { 1289 ... 1290 DominatorTree &DT = getAnalysis<DominatorTree>(Func); 1291 ... 1292 } 1293 </pre></div> 1294 1295 <p>In above example, runOnFunction for DominatorTree is called by pass manager 1296 before returning a reference to the desired pass.</p> 1297 1298 <p> 1299 If your pass is capable of updating analyses if they exist (e.g., 1300 <tt>BreakCriticalEdges</tt>, as described above), you can use the 1301 <tt>getAnalysisIfAvailable</tt> method, which returns a pointer to the analysis 1302 if it is active. For example:</p> 1303 1304 <div class="doc_code"><pre> 1305 ... 1306 if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) { 1307 <i>// A DominatorSet is active. This code will update it.</i> 1308 } 1309 ... 1310 </pre></div> 1311 1312 </div> 1313 1314 </div> 1315 1316 <!-- *********************************************************************** --> 1317 <h2> 1318 <a name="analysisgroup">Implementing Analysis Groups</a> 1319 </h2> 1320 <!-- *********************************************************************** --> 1321 1322 <div> 1323 1324 <p>Now that we understand the basics of how passes are defined, how they are 1325 used, and how they are required from other passes, it's time to get a little bit 1326 fancier. All of the pass relationships that we have seen so far are very 1327 simple: one pass depends on one other specific pass to be run before it can run. 1328 For many applications, this is great, for others, more flexibility is 1329 required.</p> 1330 1331 <p>In particular, some analyses are defined such that there is a single simple 1332 interface to the analysis results, but multiple ways of calculating them. 1333 Consider alias analysis for example. The most trivial alias analysis returns 1334 "may alias" for any alias query. The most sophisticated analysis a 1335 flow-sensitive, context-sensitive interprocedural analysis that can take a 1336 significant amount of time to execute (and obviously, there is a lot of room 1337 between these two extremes for other implementations). To cleanly support 1338 situations like this, the LLVM Pass Infrastructure supports the notion of 1339 Analysis Groups.</p> 1340 1341 <!-- _______________________________________________________________________ --> 1342 <h4> 1343 <a name="agconcepts">Analysis Group Concepts</a> 1344 </h4> 1345 1346 <div> 1347 1348 <p>An Analysis Group is a single simple interface that may be implemented by 1349 multiple different passes. Analysis Groups can be given human readable names 1350 just like passes, but unlike passes, they need not derive from the <tt>Pass</tt> 1351 class. An analysis group may have one or more implementations, one of which is 1352 the "default" implementation.</p> 1353 1354 <p>Analysis groups are used by client passes just like other passes are: the 1355 <tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods. 1356 In order to resolve this requirement, the <a href="#passmanager">PassManager</a> 1357 scans the available passes to see if any implementations of the analysis group 1358 are available. If none is available, the default implementation is created for 1359 the pass to use. All standard rules for <A href="#interaction">interaction 1360 between passes</a> still apply.</p> 1361 1362 <p>Although <a href="#registration">Pass Registration</a> is optional for normal 1363 passes, all analysis group implementations must be registered, and must use the 1364 <A href="#registerag"><tt>INITIALIZE_AG_PASS</tt></a> template to join the 1365 implementation pool. Also, a default implementation of the interface 1366 <b>must</b> be registered with <A 1367 href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p> 1368 1369 <p>As a concrete example of an Analysis Group in action, consider the <a 1370 href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a> 1371 analysis group. The default implementation of the alias analysis interface (the 1372 <tt><a 1373 href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt> 1374 pass) just does a few simple checks that don't require significant analysis to 1375 compute (such as: two different globals can never alias each other, etc). 1376 Passes that use the <tt><a 1377 href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt> 1378 interface (for example the <tt><a 1379 href="http://llvm.org/doxygen/structGCSE.html">gcse</a></tt> pass), do 1380 not care which implementation of alias analysis is actually provided, they just 1381 use the designated interface.</p> 1382 1383 <p>From the user's perspective, commands work just like normal. Issuing the 1384 command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be 1385 instantiated and added to the pass sequence. Issuing the command '<tt>opt 1386 -somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the 1387 <tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a 1388 hypothetical example) instead.</p> 1389 1390 </div> 1391 1392 <!-- _______________________________________________________________________ --> 1393 <h4> 1394 <a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a> 1395 </h4> 1396 1397 <div> 1398 1399 <p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis 1400 group itself, while the <tt>INITIALIZE_AG_PASS</tt> is used to add pass 1401 implementations to the analysis group. First, 1402 an analysis group should be registered, with a human readable name 1403 provided for it. 1404 Unlike registration of passes, there is no command line argument to be specified 1405 for the Analysis Group Interface itself, because it is "abstract":</p> 1406 1407 <div class="doc_code"><pre> 1408 <b>static</b> RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> A("<i>Alias Analysis</i>"); 1409 </pre></div> 1410 1411 <p>Once the analysis is registered, passes can declare that they are valid 1412 implementations of the interface by using the following code:</p> 1413 1414 <div class="doc_code"><pre> 1415 <b>namespace</b> { 1416 //<i> Declare that we implement the AliasAnalysis interface</i> 1417 INITIALIZE_AG_PASS(FancyAA, <a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, "<i>somefancyaa</i>", 1418 "<i>A more complex alias analysis implementation</i>", 1419 false, // <i>Is CFG Only?</i> 1420 true, // <i>Is Analysis?</i> 1421 false, // <i>Is default Analysis Group implementation?</i> 1422 ); 1423 } 1424 </pre></div> 1425 1426 <p>This just shows a class <tt>FancyAA</tt> that 1427 uses the <tt>INITIALIZE_AG_PASS</tt> macro both to register and 1428 to "join" the <tt><a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt> 1429 analysis group. Every implementation of an analysis group should join using 1430 this macro.</p> 1431 1432 <div class="doc_code"><pre> 1433 <b>namespace</b> { 1434 //<i> Declare that we implement the AliasAnalysis interface</i> 1435 INITIALIZE_AG_PASS(BasicAA, <a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, "<i>basicaa</i>", 1436 "<i>Basic Alias Analysis (default AA impl)</i>", 1437 false, // <i>Is CFG Only?</i> 1438 true, // <i>Is Analysis?</i> 1439 true, // <i>Is default Analysis Group implementation?</i> 1440 ); 1441 } 1442 </pre></div> 1443 1444 <p>Here we show how the default implementation is specified (using the final 1445 argument to the <tt>INITIALIZE_AG_PASS</tt> template). There must be exactly 1446 one default implementation available at all times for an Analysis Group to be 1447 used. Only default implementation can derive from <tt>ImmutablePass</tt>. 1448 Here we declare that the 1449 <tt><a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt> 1450 pass is the default implementation for the interface.</p> 1451 1452 </div> 1453 1454 </div> 1455 1456 <!-- *********************************************************************** --> 1457 <h2> 1458 <a name="passStatistics">Pass Statistics</a> 1459 </h2> 1460 <!-- *********************************************************************** --> 1461 1462 <div> 1463 <p>The <a 1464 href="http://llvm.org/doxygen/Statistic_8h-source.html"><tt>Statistic</tt></a> 1465 class is designed to be an easy way to expose various success 1466 metrics from passes. These statistics are printed at the end of a 1467 run, when the -stats command line option is enabled on the command 1468 line. See the <a href="http://llvm.org/docs/ProgrammersManual.html#Statistic">Statistics section</a> in the Programmer's Manual for details. 1469 1470 </div> 1471 1472 1473 <!-- *********************************************************************** --> 1474 <h2> 1475 <a name="passmanager">What PassManager does</a> 1476 </h2> 1477 <!-- *********************************************************************** --> 1478 1479 <div> 1480 1481 <p>The <a 1482 href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a> 1483 <a 1484 href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class</a> 1485 takes a list of passes, ensures their <a href="#interaction">prerequisites</a> 1486 are set up correctly, and then schedules passes to run efficiently. All of the 1487 LLVM tools that run passes use the <tt>PassManager</tt> for execution of these 1488 passes.</p> 1489 1490 <p>The <tt>PassManager</tt> does two main things to try to reduce the execution 1491 time of a series of passes:</p> 1492 1493 <ol> 1494 <li><b>Share analysis results</b> - The PassManager attempts to avoid 1495 recomputing analysis results as much as possible. This means keeping track of 1496 which analyses are available already, which analyses get invalidated, and which 1497 analyses are needed to be run for a pass. An important part of work is that the 1498 <tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing 1499 it to <a href="#releaseMemory">free memory</a> allocated to holding analysis 1500 results as soon as they are no longer needed.</li> 1501 1502 <li><b>Pipeline the execution of passes on the program</b> - The 1503 <tt>PassManager</tt> attempts to get better cache and memory usage behavior out 1504 of a series of passes by pipelining the passes together. This means that, given 1505 a series of consecutive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it 1506 will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on 1507 the first function, then all of the <a 1508 href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function, 1509 etc... until the entire program has been run through the passes. 1510 1511 <p>This improves the cache behavior of the compiler, because it is only touching 1512 the LLVM program representation for a single function at a time, instead of 1513 traversing the entire program. It reduces the memory consumption of compiler, 1514 because, for example, only one <a 1515 href="http://llvm.org/doxygen/classllvm_1_1DominatorSet.html"><tt>DominatorSet</tt></a> 1516 needs to be calculated at a time. This also makes it possible to implement 1517 some <a 1518 href="#SMP">interesting enhancements</a> in the future.</p></li> 1519 1520 </ol> 1521 1522 <p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how 1523 much information it has about the behaviors of the passes it is scheduling. For 1524 example, the "preserved" set is intentionally conservative in the face of an 1525 unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method. 1526 Not implementing when it should be implemented will have the effect of not 1527 allowing any analysis results to live across the execution of your pass.</p> 1528 1529 <p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line 1530 options that is useful for debugging pass execution, seeing how things work, and 1531 diagnosing when you should be preserving more analyses than you currently are 1532 (To get information about all of the variants of the <tt>--debug-pass</tt> 1533 option, just type '<tt>opt -help-hidden</tt>').</p> 1534 1535 <p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see 1536 how our <a href="#basiccode">Hello World</a> pass interacts with other passes. 1537 Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p> 1538 1539 <div class="doc_code"><pre> 1540 $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null 1541 Module Pass Manager 1542 Function Pass Manager 1543 Dominator Set Construction 1544 Immediate Dominators Construction 1545 Global Common Subexpression Elimination 1546 -- Immediate Dominators Construction 1547 -- Global Common Subexpression Elimination 1548 Natural Loop Construction 1549 Loop Invariant Code Motion 1550 -- Natural Loop Construction 1551 -- Loop Invariant Code Motion 1552 Module Verifier 1553 -- Dominator Set Construction 1554 -- Module Verifier 1555 Bitcode Writer 1556 --Bitcode Writer 1557 </pre></div> 1558 1559 <p>This output shows us when passes are constructed and when the analysis 1560 results are known to be dead (prefixed with '<tt>--</tt>'). Here we see that 1561 GCSE uses dominator and immediate dominator information to do its job. The LICM 1562 pass uses natural loop information, which uses dominator sets, but not immediate 1563 dominators. Because immediate dominators are no longer useful after the GCSE 1564 pass, it is immediately destroyed. The dominator sets are then reused to 1565 compute natural loop information, which is then used by the LICM pass.</p> 1566 1567 <p>After the LICM pass, the module verifier runs (which is automatically added 1568 by the '<tt>opt</tt>' tool), which uses the dominator set to check that the 1569 resultant LLVM code is well formed. After it finishes, the dominator set 1570 information is destroyed, after being computed once, and shared by three 1571 passes.</p> 1572 1573 <p>Lets see how this changes when we run the <a href="#basiccode">Hello 1574 World</a> pass in between the two passes:</p> 1575 1576 <div class="doc_code"><pre> 1577 $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null 1578 Module Pass Manager 1579 Function Pass Manager 1580 Dominator Set Construction 1581 Immediate Dominators Construction 1582 Global Common Subexpression Elimination 1583 <b>-- Dominator Set Construction</b> 1584 -- Immediate Dominators Construction 1585 -- Global Common Subexpression Elimination 1586 <b> Hello World Pass 1587 -- Hello World Pass 1588 Dominator Set Construction</b> 1589 Natural Loop Construction 1590 Loop Invariant Code Motion 1591 -- Natural Loop Construction 1592 -- Loop Invariant Code Motion 1593 Module Verifier 1594 -- Dominator Set Construction 1595 -- Module Verifier 1596 Bitcode Writer 1597 --Bitcode Writer 1598 Hello: __main 1599 Hello: puts 1600 Hello: main 1601 </pre></div> 1602 1603 <p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the 1604 Dominator Set pass, even though it doesn't modify the code at all! To fix this, 1605 we need to add the following <a 1606 href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p> 1607 1608 <div class="doc_code"><pre> 1609 <i>// We don't modify the program, so we preserve all analyses</i> 1610 <b>virtual void</b> getAnalysisUsage(AnalysisUsage &AU) <b>const</b> { 1611 AU.setPreservesAll(); 1612 } 1613 </pre></div> 1614 1615 <p>Now when we run our pass, we get this output:</p> 1616 1617 <div class="doc_code"><pre> 1618 $ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null 1619 Pass Arguments: -gcse -hello -licm 1620 Module Pass Manager 1621 Function Pass Manager 1622 Dominator Set Construction 1623 Immediate Dominators Construction 1624 Global Common Subexpression Elimination 1625 -- Immediate Dominators Construction 1626 -- Global Common Subexpression Elimination 1627 Hello World Pass 1628 -- Hello World Pass 1629 Natural Loop Construction 1630 Loop Invariant Code Motion 1631 -- Loop Invariant Code Motion 1632 -- Natural Loop Construction 1633 Module Verifier 1634 -- Dominator Set Construction 1635 -- Module Verifier 1636 Bitcode Writer 1637 --Bitcode Writer 1638 Hello: __main 1639 Hello: puts 1640 Hello: main 1641 </pre></div> 1642 1643 <p>Which shows that we don't accidentally invalidate dominator information 1644 anymore, and therefore do not have to compute it twice.</p> 1645 1646 <!-- _______________________________________________________________________ --> 1647 <h4> 1648 <a name="releaseMemory">The <tt>releaseMemory</tt> method</a> 1649 </h4> 1650 1651 <div> 1652 1653 <div class="doc_code"><pre> 1654 <b>virtual void</b> releaseMemory(); 1655 </pre></div> 1656 1657 <p>The <tt>PassManager</tt> automatically determines when to compute analysis 1658 results, and how long to keep them around for. Because the lifetime of the pass 1659 object itself is effectively the entire duration of the compilation process, we 1660 need some way to free analysis results when they are no longer useful. The 1661 <tt>releaseMemory</tt> virtual method is the way to do this.</p> 1662 1663 <p>If you are writing an analysis or any other pass that retains a significant 1664 amount of state (for use by another pass which "requires" your pass and uses the 1665 <a href="#getAnalysis">getAnalysis</a> method) you should implement 1666 <tt>releaseMemory</tt> to, well, release the memory allocated to maintain this 1667 internal state. This method is called after the <tt>run*</tt> method for the 1668 class, before the next call of <tt>run*</tt> in your pass.</p> 1669 1670 </div> 1671 1672 </div> 1673 1674 <!-- *********************************************************************** --> 1675 <h2> 1676 <a name="registering">Registering dynamically loaded passes</a> 1677 </h2> 1678 <!-- *********************************************************************** --> 1679 1680 <div> 1681 1682 <p><i>Size matters</i> when constructing production quality tools using llvm, 1683 both for the purposes of distribution, and for regulating the resident code size 1684 when running on the target system. Therefore, it becomes desirable to 1685 selectively use some passes, while omitting others and maintain the flexibility 1686 to change configurations later on. You want to be able to do all this, and, 1687 provide feedback to the user. This is where pass registration comes into 1688 play.</p> 1689 1690 <p>The fundamental mechanisms for pass registration are the 1691 <tt>MachinePassRegistry</tt> class and subclasses of 1692 <tt>MachinePassRegistryNode</tt>.</p> 1693 1694 <p>An instance of <tt>MachinePassRegistry</tt> is used to maintain a list of 1695 <tt>MachinePassRegistryNode</tt> objects. This instance maintains the list and 1696 communicates additions and deletions to the command line interface.</p> 1697 1698 <p>An instance of <tt>MachinePassRegistryNode</tt> subclass is used to maintain 1699 information provided about a particular pass. This information includes the 1700 command line name, the command help string and the address of the function used 1701 to create an instance of the pass. A global static constructor of one of these 1702 instances <i>registers</i> with a corresponding <tt>MachinePassRegistry</tt>, 1703 the static destructor <i>unregisters</i>. Thus a pass that is statically linked 1704 in the tool will be registered at start up. A dynamically loaded pass will 1705 register on load and unregister at unload.</p> 1706 1707 <!-- _______________________________________________________________________ --> 1708 <h3> 1709 <a name="registering_existing">Using existing registries</a> 1710 </h3> 1711 1712 <div> 1713 1714 <p>There are predefined registries to track instruction scheduling 1715 (<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>) 1716 machine passes. Here we will describe how to <i>register</i> a register 1717 allocator machine pass.</p> 1718 1719 <p>Implement your register allocator machine pass. In your register allocator 1720 .cpp file add the following include;</p> 1721 1722 <div class="doc_code"><pre> 1723 #include "llvm/CodeGen/RegAllocRegistry.h" 1724 </pre></div> 1725 1726 <p>Also in your register allocator .cpp file, define a creator function in the 1727 form; </p> 1728 1729 <div class="doc_code"><pre> 1730 FunctionPass *createMyRegisterAllocator() { 1731 return new MyRegisterAllocator(); 1732 } 1733 </pre></div> 1734 1735 <p>Note that the signature of this function should match the type of 1736 <tt>RegisterRegAlloc::FunctionPassCtor</tt>. In the same file add the 1737 "installing" declaration, in the form;</p> 1738 1739 <div class="doc_code"><pre> 1740 static RegisterRegAlloc myRegAlloc("myregalloc", 1741 " my register allocator help string", 1742 createMyRegisterAllocator); 1743 </pre></div> 1744 1745 <p>Note the two spaces prior to the help string produces a tidy result on the 1746 -help query.</p> 1747 1748 <div class="doc_code"><pre> 1749 $ llc -help 1750 ... 1751 -regalloc - Register allocator to use (default=linearscan) 1752 =linearscan - linear scan register allocator 1753 =local - local register allocator 1754 =simple - simple register allocator 1755 =myregalloc - my register allocator help string 1756 ... 1757 </pre></div> 1758 1759 <p>And that's it. The user is now free to use <tt>-regalloc=myregalloc</tt> as 1760 an option. Registering instruction schedulers is similar except use the 1761 <tt>RegisterScheduler</tt> class. Note that the 1762 <tt>RegisterScheduler::FunctionPassCtor</tt> is significantly different from 1763 <tt>RegisterRegAlloc::FunctionPassCtor</tt>.</p> 1764 1765 <p>To force the load/linking of your register allocator into the llc/lli tools, 1766 add your creator function's global declaration to "Passes.h" and add a "pseudo" 1767 call line to <tt>llvm/Codegen/LinkAllCodegenComponents.h</tt>.</p> 1768 1769 </div> 1770 1771 1772 <!-- _______________________________________________________________________ --> 1773 <h3> 1774 <a name="registering_new">Creating new registries</a> 1775 </h3> 1776 1777 <div> 1778 1779 <p>The easiest way to get started is to clone one of the existing registries; we 1780 recommend <tt>llvm/CodeGen/RegAllocRegistry.h</tt>. The key things to modify 1781 are the class name and the <tt>FunctionPassCtor</tt> type.</p> 1782 1783 <p>Then you need to declare the registry. Example: if your pass registry is 1784 <tt>RegisterMyPasses</tt> then define;</p> 1785 1786 <div class="doc_code"><pre> 1787 MachinePassRegistry RegisterMyPasses::Registry; 1788 </pre></div> 1789 1790 <p>And finally, declare the command line option for your passes. Example:</p> 1791 1792 <div class="doc_code"><pre> 1793 cl::opt<RegisterMyPasses::FunctionPassCtor, false, 1794 RegisterPassParser<RegisterMyPasses> > 1795 MyPassOpt("mypass", 1796 cl::init(&createDefaultMyPass), 1797 cl::desc("my pass option help")); 1798 </pre></div> 1799 1800 <p>Here the command option is "mypass", with createDefaultMyPass as the default 1801 creator.</p> 1802 1803 </div> 1804 1805 </div> 1806 1807 <!-- *********************************************************************** --> 1808 <h2> 1809 <a name="debughints">Using GDB with dynamically loaded passes</a> 1810 </h2> 1811 <!-- *********************************************************************** --> 1812 1813 <div> 1814 1815 <p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it 1816 should be. First of all, you can't set a breakpoint in a shared object that has 1817 not been loaded yet, and second of all there are problems with inlined functions 1818 in shared objects. Here are some suggestions to debugging your pass with 1819 GDB.</p> 1820 1821 <p>For sake of discussion, I'm going to assume that you are debugging a 1822 transformation invoked by <tt>opt</tt>, although nothing described here depends 1823 on that.</p> 1824 1825 <!-- _______________________________________________________________________ --> 1826 <h4> 1827 <a name="breakpoint">Setting a breakpoint in your pass</a> 1828 </h4> 1829 1830 <div> 1831 1832 <p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p> 1833 1834 <div class="doc_code"><pre> 1835 $ <b>gdb opt</b> 1836 GNU gdb 5.0 1837 Copyright 2000 Free Software Foundation, Inc. 1838 GDB is free software, covered by the GNU General Public License, and you are 1839 welcome to change it and/or distribute copies of it under certain conditions. 1840 Type "show copying" to see the conditions. 1841 There is absolutely no warranty for GDB. Type "show warranty" for details. 1842 This GDB was configured as "sparc-sun-solaris2.6"... 1843 (gdb) 1844 </pre></div> 1845 1846 <p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes 1847 time to load. Be patient. Since we cannot set a breakpoint in our pass yet 1848 (the shared object isn't loaded until runtime), we must execute the process, and 1849 have it stop before it invokes our pass, but after it has loaded the shared 1850 object. The most foolproof way of doing this is to set a breakpoint in 1851 <tt>PassManager::run</tt> and then run the process with the arguments you 1852 want:</p> 1853 1854 <div class="doc_code"><pre> 1855 (gdb) <b>break llvm::PassManager::run</b> 1856 Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70. 1857 (gdb) <b>run test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]</b> 1858 Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption] 1859 Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70 1860 70 bool PassManager::run(Module &M) { return PM->run(M); } 1861 (gdb) 1862 </pre></div> 1863 1864 <p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are 1865 now free to set breakpoints in your pass so that you can trace through execution 1866 or do other standard debugging stuff.</p> 1867 1868 </div> 1869 1870 <!-- _______________________________________________________________________ --> 1871 <h4> 1872 <a name="debugmisc">Miscellaneous Problems</a> 1873 </h4> 1874 1875 <div> 1876 1877 <p>Once you have the basics down, there are a couple of problems that GDB has, 1878 some with solutions, some without.</p> 1879 1880 <ul> 1881 <li>Inline functions have bogus stack information. In general, GDB does a 1882 pretty good job getting stack traces and stepping through inline functions. 1883 When a pass is dynamically loaded however, it somehow completely loses this 1884 capability. The only solution I know of is to de-inline a function (move it 1885 from the body of a class to a .cpp file).</li> 1886 1887 <li>Restarting the program breaks breakpoints. After following the information 1888 above, you have succeeded in getting some breakpoints planted in your pass. Nex 1889 thing you know, you restart the program (i.e., you type '<tt>run</tt>' again), 1890 and you start getting errors about breakpoints being unsettable. The only way I 1891 have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are 1892 already set in your pass, run the program, and re-set the breakpoints once 1893 execution stops in <tt>PassManager::run</tt>.</li> 1894 1895 </ul> 1896 1897 <p>Hopefully these tips will help with common case debugging situations. If 1898 you'd like to contribute some tips of your own, just contact <a 1899 href="mailto:sabre (a] nondot.org">Chris</a>.</p> 1900 1901 </div> 1902 1903 </div> 1904 1905 <!-- *********************************************************************** --> 1906 <h2> 1907 <a name="future">Future extensions planned</a> 1908 </h2> 1909 <!-- *********************************************************************** --> 1910 1911 <div> 1912 1913 <p>Although the LLVM Pass Infrastructure is very capable as it stands, and does 1914 some nifty stuff, there are things we'd like to add in the future. Here is 1915 where we are going:</p> 1916 1917 <!-- _______________________________________________________________________ --> 1918 <h4> 1919 <a name="SMP">Multithreaded LLVM</a> 1920 </h4> 1921 1922 <div> 1923 1924 <p>Multiple CPU machines are becoming more common and compilation can never be 1925 fast enough: obviously we should allow for a multithreaded compiler. Because of 1926 the semantics defined for passes above (specifically they cannot maintain state 1927 across invocations of their <tt>run*</tt> methods), a nice clean way to 1928 implement a multithreaded compiler would be for the <tt>PassManager</tt> class 1929 to create multiple instances of each pass object, and allow the separate 1930 instances to be hacking on different parts of the program at the same time.</p> 1931 1932 <p>This implementation would prevent each of the passes from having to implement 1933 multithreaded constructs, requiring only the LLVM core to have locking in a few 1934 places (for global resources). Although this is a simple extension, we simply 1935 haven't had time (or multiprocessor machines, thus a reason) to implement this. 1936 Despite that, we have kept the LLVM passes SMP ready, and you should too.</p> 1937 1938 </div> 1939 1940 </div> 1941 1942 <!-- *********************************************************************** --> 1943 <hr> 1944 <address> 1945 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 1946 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 1947 <a href="http://validator.w3.org/check/referer"><img 1948 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 1949 1950 <a href="mailto:sabre (a] nondot.org">Chris Lattner</a><br> 1951 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br> 1952 Last modified: $Date$ 1953 </address> 1954 1955 </body> 1956 </html> 1957
