ADCE.cpp revision 27c694bacb40be5f727c469095d7a44fe05c3334
1//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This file implements "aggressive" dead code elimination. ADCE is DCe where 11// values are assumed to be dead until proven otherwise. This is similar to 12// SCCP, except applied to the liveness of values. 13// 14//===----------------------------------------------------------------------===// 15 16#include "llvm/Transforms/Scalar.h" 17#include "llvm/Constant.h" 18#include "llvm/Instructions.h" 19#include "llvm/Type.h" 20#include "llvm/Analysis/AliasAnalysis.h" 21#include "llvm/Analysis/PostDominators.h" 22#include "llvm/Support/CFG.h" 23#include "llvm/Transforms/Utils/BasicBlockUtils.h" 24#include "llvm/Transforms/Utils/Local.h" 25#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h" 26#include "Support/Debug.h" 27#include "Support/DepthFirstIterator.h" 28#include "Support/Statistic.h" 29#include "Support/STLExtras.h" 30#include <algorithm> 31using namespace llvm; 32 33namespace { 34 Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed"); 35 Statistic<> NumInstRemoved ("adce", "Number of instructions removed"); 36 Statistic<> NumCallRemoved ("adce", "Number of calls and invokes removed"); 37 38//===----------------------------------------------------------------------===// 39// ADCE Class 40// 41// This class does all of the work of Aggressive Dead Code Elimination. 42// It's public interface consists of a constructor and a doADCE() method. 43// 44class ADCE : public FunctionPass { 45 Function *Func; // The function that we are working on 46 AliasAnalysis *AA; // Current AliasAnalysis object 47 std::vector<Instruction*> WorkList; // Instructions that just became live 48 std::set<Instruction*> LiveSet; // The set of live instructions 49 50 //===--------------------------------------------------------------------===// 51 // The public interface for this class 52 // 53public: 54 // Execute the Aggressive Dead Code Elimination Algorithm 55 // 56 virtual bool runOnFunction(Function &F) { 57 Func = &F; 58 AA = &getAnalysis<AliasAnalysis>(); 59 bool Changed = doADCE(); 60 assert(WorkList.empty()); 61 LiveSet.clear(); 62 return Changed; 63 } 64 // getAnalysisUsage - We require post dominance frontiers (aka Control 65 // Dependence Graph) 66 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 67 // We require that all function nodes are unified, because otherwise code 68 // can be marked live that wouldn't necessarily be otherwise. 69 AU.addRequired<UnifyFunctionExitNodes>(); 70 AU.addRequired<AliasAnalysis>(); 71 AU.addRequired<PostDominatorTree>(); 72 AU.addRequired<PostDominanceFrontier>(); 73 } 74 75 76 //===--------------------------------------------------------------------===// 77 // The implementation of this class 78 // 79private: 80 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning 81 // true if the function was modified. 82 // 83 bool doADCE(); 84 85 void markBlockAlive(BasicBlock *BB); 86 87 88 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the 89 // instructions in the specified basic block, dropping references on 90 // instructions that are dead according to LiveSet. 91 bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB); 92 93 TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI); 94 95 inline void markInstructionLive(Instruction *I) { 96 if (LiveSet.count(I)) return; 97 DEBUG(std::cerr << "Insn Live: " << I); 98 LiveSet.insert(I); 99 WorkList.push_back(I); 100 } 101 102 inline void markTerminatorLive(const BasicBlock *BB) { 103 DEBUG(std::cerr << "Terminator Live: " << BB->getTerminator()); 104 markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator())); 105 } 106}; 107 108 RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination"); 109} // End of anonymous namespace 110 111Pass *llvm::createAggressiveDCEPass() { return new ADCE(); } 112 113void ADCE::markBlockAlive(BasicBlock *BB) { 114 // Mark the basic block as being newly ALIVE... and mark all branches that 115 // this block is control dependent on as being alive also... 116 // 117 PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>(); 118 119 PostDominanceFrontier::const_iterator It = CDG.find(BB); 120 if (It != CDG.end()) { 121 // Get the blocks that this node is control dependent on... 122 const PostDominanceFrontier::DomSetType &CDB = It->second; 123 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live 124 bind_obj(this, &ADCE::markTerminatorLive)); 125 } 126 127 // If this basic block is live, and it ends in an unconditional branch, then 128 // the branch is alive as well... 129 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) 130 if (BI->isUnconditional()) 131 markTerminatorLive(BB); 132} 133 134// dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the 135// instructions in the specified basic block, dropping references on 136// instructions that are dead according to LiveSet. 137bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) { 138 bool Changed = false; 139 for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; ) 140 if (!LiveSet.count(I)) { // Is this instruction alive? 141 I->dropAllReferences(); // Nope, drop references... 142 if (PHINode *PN = dyn_cast<PHINode>(I)) { 143 // We don't want to leave PHI nodes in the program that have 144 // #arguments != #predecessors, so we remove them now. 145 // 146 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType())); 147 148 // Delete the instruction... 149 ++I; 150 BB->getInstList().erase(PN); 151 Changed = true; 152 ++NumInstRemoved; 153 } else { 154 ++I; 155 } 156 } else { 157 ++I; 158 } 159 return Changed; 160} 161 162 163/// convertToUnconditionalBranch - Transform this conditional terminator 164/// instruction into an unconditional branch because we don't care which of the 165/// successors it goes to. This eliminate a use of the condition as well. 166/// 167TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) { 168 BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI); 169 BasicBlock *BB = TI->getParent(); 170 171 // Remove entries from PHI nodes to avoid confusing ourself later... 172 for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i) 173 TI->getSuccessor(i)->removePredecessor(BB); 174 175 // Delete the old branch itself... 176 BB->getInstList().erase(TI); 177 return NB; 178} 179 180 181// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning 182// true if the function was modified. 183// 184bool ADCE::doADCE() { 185 bool MadeChanges = false; 186 187 // Iterate over all of the instructions in the function, eliminating trivially 188 // dead instructions, and marking instructions live that are known to be 189 // needed. Perform the walk in depth first order so that we avoid marking any 190 // instructions live in basic blocks that are unreachable. These blocks will 191 // be eliminated later, along with the instructions inside. 192 // 193 for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func); 194 BBI != BBE; ++BBI) { 195 BasicBlock *BB = *BBI; 196 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) { 197 Instruction *I = II++; 198 if (CallInst *CI = dyn_cast<CallInst>(I)) { 199 Function *F = CI->getCalledFunction(); 200 if (F && AA->onlyReadsMemory(F)) { 201 if (CI->use_empty()) { 202 BB->getInstList().erase(CI); 203 ++NumCallRemoved; 204 } 205 } else { 206 markInstructionLive(I); 207 } 208 } else if (InvokeInst *II = dyn_cast<InvokeInst>(I)) { 209 Function *F = II->getCalledFunction(); 210 if (F && AA->onlyReadsMemory(F)) { 211 // The function cannot unwind. Convert it to a call with a branch 212 // after it to the normal destination. 213 std::vector<Value*> Args(II->op_begin()+1, II->op_end()); 214 std::string Name = II->getName(); II->setName(""); 215 Instruction *NewCall = new CallInst(F, Args, Name, II); 216 II->replaceAllUsesWith(NewCall); 217 new BranchInst(II->getNormalDest(), II); 218 BB->getInstList().erase(II); 219 220 if (NewCall->use_empty()) { 221 BB->getInstList().erase(NewCall); 222 ++NumCallRemoved; 223 } 224 } else { 225 markInstructionLive(I); 226 } 227 } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) || 228 isa<UnwindInst>(I)) { 229 markInstructionLive(I); 230 } else if (isInstructionTriviallyDead(I)) { 231 // Remove the instruction from it's basic block... 232 BB->getInstList().erase(I); 233 ++NumInstRemoved; 234 } 235 } 236 } 237 238 // Check to ensure we have an exit node for this CFG. If we don't, we won't 239 // have any post-dominance information, thus we cannot perform our 240 // transformations safely. 241 // 242 PostDominatorTree &DT = getAnalysis<PostDominatorTree>(); 243 if (DT[&Func->getEntryBlock()] == 0) { 244 WorkList.clear(); 245 return MadeChanges; 246 } 247 248 // Scan the function marking blocks without post-dominance information as 249 // live. Blocks without post-dominance information occur when there is an 250 // infinite loop in the program. Because the infinite loop could contain a 251 // function which unwinds, exits or has side-effects, we don't want to delete 252 // the infinite loop or those blocks leading up to it. 253 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) 254 if (DT[I] == 0) 255 for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI) 256 markInstructionLive((*PI)->getTerminator()); 257 258 259 260 DEBUG(std::cerr << "Processing work list\n"); 261 262 // AliveBlocks - Set of basic blocks that we know have instructions that are 263 // alive in them... 264 // 265 std::set<BasicBlock*> AliveBlocks; 266 267 // Process the work list of instructions that just became live... if they 268 // became live, then that means that all of their operands are necessary as 269 // well... make them live as well. 270 // 271 while (!WorkList.empty()) { 272 Instruction *I = WorkList.back(); // Get an instruction that became live... 273 WorkList.pop_back(); 274 275 BasicBlock *BB = I->getParent(); 276 if (!AliveBlocks.count(BB)) { // Basic block not alive yet... 277 AliveBlocks.insert(BB); // Block is now ALIVE! 278 markBlockAlive(BB); // Make it so now! 279 } 280 281 // PHI nodes are a special case, because the incoming values are actually 282 // defined in the predecessor nodes of this block, meaning that the PHI 283 // makes the predecessors alive. 284 // 285 if (PHINode *PN = dyn_cast<PHINode>(I)) 286 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI) 287 if (!AliveBlocks.count(*PI)) { 288 AliveBlocks.insert(BB); // Block is now ALIVE! 289 markBlockAlive(*PI); 290 } 291 292 // Loop over all of the operands of the live instruction, making sure that 293 // they are known to be alive as well... 294 // 295 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op) 296 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op))) 297 markInstructionLive(Operand); 298 } 299 300 DEBUG( 301 std::cerr << "Current Function: X = Live\n"; 302 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){ 303 std::cerr << I->getName() << ":\t" 304 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n"); 305 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){ 306 if (LiveSet.count(BI)) std::cerr << "X "; 307 std::cerr << *BI; 308 } 309 }); 310 311 // Find the first postdominator of the entry node that is alive. Make it the 312 // new entry node... 313 // 314 if (AliveBlocks.size() == Func->size()) { // No dead blocks? 315 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) { 316 // Loop over all of the instructions in the function, telling dead 317 // instructions to drop their references. This is so that the next sweep 318 // over the program can safely delete dead instructions without other dead 319 // instructions still referring to them. 320 // 321 dropReferencesOfDeadInstructionsInLiveBlock(I); 322 323 // Check to make sure the terminator instruction is live. If it isn't, 324 // this means that the condition that it branches on (we know it is not an 325 // unconditional branch), is not needed to make the decision of where to 326 // go to, because all outgoing edges go to the same place. We must remove 327 // the use of the condition (because it's probably dead), so we convert 328 // the terminator to a conditional branch. 329 // 330 TerminatorInst *TI = I->getTerminator(); 331 if (!LiveSet.count(TI)) 332 convertToUnconditionalBranch(TI); 333 } 334 335 } else { // If there are some blocks dead... 336 // If the entry node is dead, insert a new entry node to eliminate the entry 337 // node as a special case. 338 // 339 if (!AliveBlocks.count(&Func->front())) { 340 BasicBlock *NewEntry = new BasicBlock(); 341 new BranchInst(&Func->front(), NewEntry); 342 Func->getBasicBlockList().push_front(NewEntry); 343 AliveBlocks.insert(NewEntry); // This block is always alive! 344 LiveSet.insert(NewEntry->getTerminator()); // The branch is live 345 } 346 347 // Loop over all of the alive blocks in the function. If any successor 348 // blocks are not alive, we adjust the outgoing branches to branch to the 349 // first live postdominator of the live block, adjusting any PHI nodes in 350 // the block to reflect this. 351 // 352 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) 353 if (AliveBlocks.count(I)) { 354 BasicBlock *BB = I; 355 TerminatorInst *TI = BB->getTerminator(); 356 357 // If the terminator instruction is alive, but the block it is contained 358 // in IS alive, this means that this terminator is a conditional branch 359 // on a condition that doesn't matter. Make it an unconditional branch 360 // to ONE of the successors. This has the side effect of dropping a use 361 // of the conditional value, which may also be dead. 362 if (!LiveSet.count(TI)) 363 TI = convertToUnconditionalBranch(TI); 364 365 // Loop over all of the successors, looking for ones that are not alive. 366 // We cannot save the number of successors in the terminator instruction 367 // here because we may remove them if we don't have a postdominator... 368 // 369 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i) 370 if (!AliveBlocks.count(TI->getSuccessor(i))) { 371 // Scan up the postdominator tree, looking for the first 372 // postdominator that is alive, and the last postdominator that is 373 // dead... 374 // 375 PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)]; 376 377 // There is a special case here... if there IS no post-dominator for 378 // the block we have no owhere to point our branch to. Instead, 379 // convert it to a return. This can only happen if the code 380 // branched into an infinite loop. Note that this may not be 381 // desirable, because we _are_ altering the behavior of the code. 382 // This is a well known drawback of ADCE, so in the future if we 383 // choose to revisit the decision, this is where it should be. 384 // 385 if (LastNode == 0) { // No postdominator! 386 // Call RemoveSuccessor to transmogrify the terminator instruction 387 // to not contain the outgoing branch, or to create a new 388 // terminator if the form fundamentally changes (i.e., 389 // unconditional branch to return). Note that this will change a 390 // branch into an infinite loop into a return instruction! 391 // 392 RemoveSuccessor(TI, i); 393 394 // RemoveSuccessor may replace TI... make sure we have a fresh 395 // pointer... and e variable. 396 // 397 TI = BB->getTerminator(); 398 399 // Rescan this successor... 400 --i; 401 } else { 402 PostDominatorTree::Node *NextNode = LastNode->getIDom(); 403 404 while (!AliveBlocks.count(NextNode->getBlock())) { 405 LastNode = NextNode; 406 NextNode = NextNode->getIDom(); 407 } 408 409 // Get the basic blocks that we need... 410 BasicBlock *LastDead = LastNode->getBlock(); 411 BasicBlock *NextAlive = NextNode->getBlock(); 412 413 // Make the conditional branch now go to the next alive block... 414 TI->getSuccessor(i)->removePredecessor(BB); 415 TI->setSuccessor(i, NextAlive); 416 417 // If there are PHI nodes in NextAlive, we need to add entries to 418 // the PHI nodes for the new incoming edge. The incoming values 419 // should be identical to the incoming values for LastDead. 420 // 421 for (BasicBlock::iterator II = NextAlive->begin(); 422 PHINode *PN = dyn_cast<PHINode>(II); ++II) 423 if (LiveSet.count(PN)) { // Only modify live phi nodes 424 // Get the incoming value for LastDead... 425 int OldIdx = PN->getBasicBlockIndex(LastDead); 426 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!"); 427 Value *InVal = PN->getIncomingValue(OldIdx); 428 429 // Add an incoming value for BB now... 430 PN->addIncoming(InVal, BB); 431 } 432 } 433 } 434 435 // Now loop over all of the instructions in the basic block, telling 436 // dead instructions to drop their references. This is so that the next 437 // sweep over the program can safely delete dead instructions without 438 // other dead instructions still referring to them. 439 // 440 dropReferencesOfDeadInstructionsInLiveBlock(BB); 441 } 442 } 443 444 // We make changes if there are any dead blocks in the function... 445 if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) { 446 MadeChanges = true; 447 NumBlockRemoved += NumDeadBlocks; 448 } 449 450 // Loop over all of the basic blocks in the function, removing control flow 451 // edges to live blocks (also eliminating any entries in PHI functions in 452 // referenced blocks). 453 // 454 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) 455 if (!AliveBlocks.count(BB)) { 456 // Remove all outgoing edges from this basic block and convert the 457 // terminator into a return instruction. 458 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB)); 459 460 if (!Succs.empty()) { 461 // Loop over all of the successors, removing this block from PHI node 462 // entries that might be in the block... 463 while (!Succs.empty()) { 464 Succs.back()->removePredecessor(BB); 465 Succs.pop_back(); 466 } 467 468 // Delete the old terminator instruction... 469 const Type *TermTy = BB->getTerminator()->getType(); 470 if (TermTy != Type::VoidTy) 471 BB->getTerminator()->replaceAllUsesWith( 472 Constant::getNullValue(TermTy)); 473 BB->getInstList().pop_back(); 474 const Type *RetTy = Func->getReturnType(); 475 new ReturnInst(RetTy != Type::VoidTy ? 476 Constant::getNullValue(RetTy) : 0, BB); 477 } 478 } 479 480 481 // Loop over all of the basic blocks in the function, dropping references of 482 // the dead basic blocks. We must do this after the previous step to avoid 483 // dropping references to PHIs which still have entries... 484 // 485 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) 486 if (!AliveBlocks.count(BB)) 487 BB->dropAllReferences(); 488 489 // Now loop through all of the blocks and delete the dead ones. We can safely 490 // do this now because we know that there are no references to dead blocks 491 // (because they have dropped all of their references... we also remove dead 492 // instructions from alive blocks. 493 // 494 for (Function::iterator BI = Func->begin(); BI != Func->end(); ) 495 if (!AliveBlocks.count(BI)) { // Delete dead blocks... 496 BI = Func->getBasicBlockList().erase(BI); 497 } else { // Scan alive blocks... 498 for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); ) 499 if (!LiveSet.count(II)) { // Is this instruction alive? 500 // Nope... remove the instruction from it's basic block... 501 if (isa<CallInst>(II)) 502 ++NumCallRemoved; 503 else 504 ++NumInstRemoved; 505 II = BI->getInstList().erase(II); 506 MadeChanges = true; 507 } else { 508 ++II; 509 } 510 511 ++BI; // Increment iterator... 512 } 513 514 return MadeChanges; 515} 516