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