Local.cpp revision 210299
1//===-- Local.cpp - Functions to perform local transformations ------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file is distributed under the University of Illinois Open Source 6// License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This family of functions perform various local transformations to the 11// program. 12// 13//===----------------------------------------------------------------------===// 14 15#include "llvm/Transforms/Utils/Local.h" 16#include "llvm/Constants.h" 17#include "llvm/GlobalAlias.h" 18#include "llvm/GlobalVariable.h" 19#include "llvm/DerivedTypes.h" 20#include "llvm/Instructions.h" 21#include "llvm/Intrinsics.h" 22#include "llvm/IntrinsicInst.h" 23#include "llvm/ADT/DenseMap.h" 24#include "llvm/ADT/SmallPtrSet.h" 25#include "llvm/Analysis/ConstantFolding.h" 26#include "llvm/Analysis/InstructionSimplify.h" 27#include "llvm/Analysis/ProfileInfo.h" 28#include "llvm/Target/TargetData.h" 29#include "llvm/Support/CFG.h" 30#include "llvm/Support/Debug.h" 31#include "llvm/Support/GetElementPtrTypeIterator.h" 32#include "llvm/Support/MathExtras.h" 33#include "llvm/Support/ValueHandle.h" 34#include "llvm/Support/raw_ostream.h" 35using namespace llvm; 36 37//===----------------------------------------------------------------------===// 38// Local constant propagation. 39// 40 41// ConstantFoldTerminator - If a terminator instruction is predicated on a 42// constant value, convert it into an unconditional branch to the constant 43// destination. 44// 45bool llvm::ConstantFoldTerminator(BasicBlock *BB) { 46 TerminatorInst *T = BB->getTerminator(); 47 48 // Branch - See if we are conditional jumping on constant 49 if (BranchInst *BI = dyn_cast<BranchInst>(T)) { 50 if (BI->isUnconditional()) return false; // Can't optimize uncond branch 51 BasicBlock *Dest1 = BI->getSuccessor(0); 52 BasicBlock *Dest2 = BI->getSuccessor(1); 53 54 if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) { 55 // Are we branching on constant? 56 // YES. Change to unconditional branch... 57 BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2; 58 BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1; 59 60 //cerr << "Function: " << T->getParent()->getParent() 61 // << "\nRemoving branch from " << T->getParent() 62 // << "\n\nTo: " << OldDest << endl; 63 64 // Let the basic block know that we are letting go of it. Based on this, 65 // it will adjust it's PHI nodes. 66 assert(BI->getParent() && "Terminator not inserted in block!"); 67 OldDest->removePredecessor(BI->getParent()); 68 69 // Set the unconditional destination, and change the insn to be an 70 // unconditional branch. 71 BI->setUnconditionalDest(Destination); 72 return true; 73 } 74 75 if (Dest2 == Dest1) { // Conditional branch to same location? 76 // This branch matches something like this: 77 // br bool %cond, label %Dest, label %Dest 78 // and changes it into: br label %Dest 79 80 // Let the basic block know that we are letting go of one copy of it. 81 assert(BI->getParent() && "Terminator not inserted in block!"); 82 Dest1->removePredecessor(BI->getParent()); 83 84 // Change a conditional branch to unconditional. 85 BI->setUnconditionalDest(Dest1); 86 return true; 87 } 88 return false; 89 } 90 91 if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) { 92 // If we are switching on a constant, we can convert the switch into a 93 // single branch instruction! 94 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition()); 95 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest 96 BasicBlock *DefaultDest = TheOnlyDest; 97 assert(TheOnlyDest == SI->getDefaultDest() && 98 "Default destination is not successor #0?"); 99 100 // Figure out which case it goes to. 101 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) { 102 // Found case matching a constant operand? 103 if (SI->getSuccessorValue(i) == CI) { 104 TheOnlyDest = SI->getSuccessor(i); 105 break; 106 } 107 108 // Check to see if this branch is going to the same place as the default 109 // dest. If so, eliminate it as an explicit compare. 110 if (SI->getSuccessor(i) == DefaultDest) { 111 // Remove this entry. 112 DefaultDest->removePredecessor(SI->getParent()); 113 SI->removeCase(i); 114 --i; --e; // Don't skip an entry... 115 continue; 116 } 117 118 // Otherwise, check to see if the switch only branches to one destination. 119 // We do this by reseting "TheOnlyDest" to null when we find two non-equal 120 // destinations. 121 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0; 122 } 123 124 if (CI && !TheOnlyDest) { 125 // Branching on a constant, but not any of the cases, go to the default 126 // successor. 127 TheOnlyDest = SI->getDefaultDest(); 128 } 129 130 // If we found a single destination that we can fold the switch into, do so 131 // now. 132 if (TheOnlyDest) { 133 // Insert the new branch. 134 BranchInst::Create(TheOnlyDest, SI); 135 BasicBlock *BB = SI->getParent(); 136 137 // Remove entries from PHI nodes which we no longer branch to... 138 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) { 139 // Found case matching a constant operand? 140 BasicBlock *Succ = SI->getSuccessor(i); 141 if (Succ == TheOnlyDest) 142 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest 143 else 144 Succ->removePredecessor(BB); 145 } 146 147 // Delete the old switch. 148 BB->getInstList().erase(SI); 149 return true; 150 } 151 152 if (SI->getNumSuccessors() == 2) { 153 // Otherwise, we can fold this switch into a conditional branch 154 // instruction if it has only one non-default destination. 155 Value *Cond = new ICmpInst(SI, ICmpInst::ICMP_EQ, SI->getCondition(), 156 SI->getSuccessorValue(1), "cond"); 157 // Insert the new branch. 158 BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI); 159 160 // Delete the old switch. 161 SI->eraseFromParent(); 162 return true; 163 } 164 return false; 165 } 166 167 if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(T)) { 168 // indirectbr blockaddress(@F, @BB) -> br label @BB 169 if (BlockAddress *BA = 170 dyn_cast<BlockAddress>(IBI->getAddress()->stripPointerCasts())) { 171 BasicBlock *TheOnlyDest = BA->getBasicBlock(); 172 // Insert the new branch. 173 BranchInst::Create(TheOnlyDest, IBI); 174 175 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) { 176 if (IBI->getDestination(i) == TheOnlyDest) 177 TheOnlyDest = 0; 178 else 179 IBI->getDestination(i)->removePredecessor(IBI->getParent()); 180 } 181 IBI->eraseFromParent(); 182 183 // If we didn't find our destination in the IBI successor list, then we 184 // have undefined behavior. Replace the unconditional branch with an 185 // 'unreachable' instruction. 186 if (TheOnlyDest) { 187 BB->getTerminator()->eraseFromParent(); 188 new UnreachableInst(BB->getContext(), BB); 189 } 190 191 return true; 192 } 193 } 194 195 return false; 196} 197 198 199//===----------------------------------------------------------------------===// 200// Local dead code elimination. 201// 202 203/// isInstructionTriviallyDead - Return true if the result produced by the 204/// instruction is not used, and the instruction has no side effects. 205/// 206bool llvm::isInstructionTriviallyDead(Instruction *I) { 207 if (!I->use_empty() || isa<TerminatorInst>(I)) return false; 208 209 // We don't want debug info removed by anything this general. 210 if (isa<DbgInfoIntrinsic>(I)) return false; 211 212 // Likewise for memory use markers. 213 if (isa<MemoryUseIntrinsic>(I)) return false; 214 215 if (!I->mayHaveSideEffects()) return true; 216 217 // Special case intrinsics that "may have side effects" but can be deleted 218 // when dead. 219 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) 220 // Safe to delete llvm.stacksave if dead. 221 if (II->getIntrinsicID() == Intrinsic::stacksave) 222 return true; 223 return false; 224} 225 226/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a 227/// trivially dead instruction, delete it. If that makes any of its operands 228/// trivially dead, delete them too, recursively. Return true if any 229/// instructions were deleted. 230bool llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) { 231 Instruction *I = dyn_cast<Instruction>(V); 232 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I)) 233 return false; 234 235 SmallVector<Instruction*, 16> DeadInsts; 236 DeadInsts.push_back(I); 237 238 do { 239 I = DeadInsts.pop_back_val(); 240 241 // Null out all of the instruction's operands to see if any operand becomes 242 // dead as we go. 243 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 244 Value *OpV = I->getOperand(i); 245 I->setOperand(i, 0); 246 247 if (!OpV->use_empty()) continue; 248 249 // If the operand is an instruction that became dead as we nulled out the 250 // operand, and if it is 'trivially' dead, delete it in a future loop 251 // iteration. 252 if (Instruction *OpI = dyn_cast<Instruction>(OpV)) 253 if (isInstructionTriviallyDead(OpI)) 254 DeadInsts.push_back(OpI); 255 } 256 257 I->eraseFromParent(); 258 } while (!DeadInsts.empty()); 259 260 return true; 261} 262 263/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively 264/// dead PHI node, due to being a def-use chain of single-use nodes that 265/// either forms a cycle or is terminated by a trivially dead instruction, 266/// delete it. If that makes any of its operands trivially dead, delete them 267/// too, recursively. Return true if the PHI node is actually deleted. 268bool 269llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) { 270 // We can remove a PHI if it is on a cycle in the def-use graph 271 // where each node in the cycle has degree one, i.e. only one use, 272 // and is an instruction with no side effects. 273 if (!PN->hasOneUse()) 274 return false; 275 276 bool Changed = false; 277 SmallPtrSet<PHINode *, 4> PHIs; 278 PHIs.insert(PN); 279 for (Instruction *J = cast<Instruction>(*PN->use_begin()); 280 J->hasOneUse() && !J->mayHaveSideEffects(); 281 J = cast<Instruction>(*J->use_begin())) 282 // If we find a PHI more than once, we're on a cycle that 283 // won't prove fruitful. 284 if (PHINode *JP = dyn_cast<PHINode>(J)) 285 if (!PHIs.insert(cast<PHINode>(JP))) { 286 // Break the cycle and delete the PHI and its operands. 287 JP->replaceAllUsesWith(UndefValue::get(JP->getType())); 288 (void)RecursivelyDeleteTriviallyDeadInstructions(JP); 289 Changed = true; 290 break; 291 } 292 return Changed; 293} 294 295/// SimplifyInstructionsInBlock - Scan the specified basic block and try to 296/// simplify any instructions in it and recursively delete dead instructions. 297/// 298/// This returns true if it changed the code, note that it can delete 299/// instructions in other blocks as well in this block. 300bool llvm::SimplifyInstructionsInBlock(BasicBlock *BB, const TargetData *TD) { 301 bool MadeChange = false; 302 for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) { 303 Instruction *Inst = BI++; 304 305 if (Value *V = SimplifyInstruction(Inst, TD)) { 306 WeakVH BIHandle(BI); 307 ReplaceAndSimplifyAllUses(Inst, V, TD); 308 MadeChange = true; 309 if (BIHandle != BI) 310 BI = BB->begin(); 311 continue; 312 } 313 314 MadeChange |= RecursivelyDeleteTriviallyDeadInstructions(Inst); 315 } 316 return MadeChange; 317} 318 319//===----------------------------------------------------------------------===// 320// Control Flow Graph Restructuring. 321// 322 323 324/// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this 325/// method is called when we're about to delete Pred as a predecessor of BB. If 326/// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred. 327/// 328/// Unlike the removePredecessor method, this attempts to simplify uses of PHI 329/// nodes that collapse into identity values. For example, if we have: 330/// x = phi(1, 0, 0, 0) 331/// y = and x, z 332/// 333/// .. and delete the predecessor corresponding to the '1', this will attempt to 334/// recursively fold the and to 0. 335void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred, 336 TargetData *TD) { 337 // This only adjusts blocks with PHI nodes. 338 if (!isa<PHINode>(BB->begin())) 339 return; 340 341 // Remove the entries for Pred from the PHI nodes in BB, but do not simplify 342 // them down. This will leave us with single entry phi nodes and other phis 343 // that can be removed. 344 BB->removePredecessor(Pred, true); 345 346 WeakVH PhiIt = &BB->front(); 347 while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) { 348 PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt)); 349 350 Value *PNV = PN->hasConstantValue(); 351 if (PNV == 0) continue; 352 353 // If we're able to simplify the phi to a single value, substitute the new 354 // value into all of its uses. 355 assert(PNV != PN && "hasConstantValue broken"); 356 357 Value *OldPhiIt = PhiIt; 358 ReplaceAndSimplifyAllUses(PN, PNV, TD); 359 360 // If recursive simplification ended up deleting the next PHI node we would 361 // iterate to, then our iterator is invalid, restart scanning from the top 362 // of the block. 363 if (PhiIt != OldPhiIt) PhiIt = &BB->front(); 364 } 365} 366 367 368/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its 369/// predecessor is known to have one successor (DestBB!). Eliminate the edge 370/// between them, moving the instructions in the predecessor into DestBB and 371/// deleting the predecessor block. 372/// 373void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) { 374 // If BB has single-entry PHI nodes, fold them. 375 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { 376 Value *NewVal = PN->getIncomingValue(0); 377 // Replace self referencing PHI with undef, it must be dead. 378 if (NewVal == PN) NewVal = UndefValue::get(PN->getType()); 379 PN->replaceAllUsesWith(NewVal); 380 PN->eraseFromParent(); 381 } 382 383 BasicBlock *PredBB = DestBB->getSinglePredecessor(); 384 assert(PredBB && "Block doesn't have a single predecessor!"); 385 386 // Splice all the instructions from PredBB to DestBB. 387 PredBB->getTerminator()->eraseFromParent(); 388 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList()); 389 390 // Zap anything that took the address of DestBB. Not doing this will give the 391 // address an invalid value. 392 if (DestBB->hasAddressTaken()) { 393 BlockAddress *BA = BlockAddress::get(DestBB); 394 Constant *Replacement = 395 ConstantInt::get(llvm::Type::getInt32Ty(BA->getContext()), 1); 396 BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(Replacement, 397 BA->getType())); 398 BA->destroyConstant(); 399 } 400 401 // Anything that branched to PredBB now branches to DestBB. 402 PredBB->replaceAllUsesWith(DestBB); 403 404 if (P) { 405 ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>(); 406 if (PI) { 407 PI->replaceAllUses(PredBB, DestBB); 408 PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB)); 409 } 410 } 411 // Nuke BB. 412 PredBB->eraseFromParent(); 413} 414 415/// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an 416/// almost-empty BB ending in an unconditional branch to Succ, into succ. 417/// 418/// Assumption: Succ is the single successor for BB. 419/// 420static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { 421 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!"); 422 423 DEBUG(dbgs() << "Looking to fold " << BB->getName() << " into " 424 << Succ->getName() << "\n"); 425 // Shortcut, if there is only a single predecessor it must be BB and merging 426 // is always safe 427 if (Succ->getSinglePredecessor()) return true; 428 429 // Make a list of the predecessors of BB 430 typedef SmallPtrSet<BasicBlock*, 16> BlockSet; 431 BlockSet BBPreds(pred_begin(BB), pred_end(BB)); 432 433 // Use that list to make another list of common predecessors of BB and Succ 434 BlockSet CommonPreds; 435 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); 436 PI != PE; ++PI) { 437 BasicBlock *P = *PI; 438 if (BBPreds.count(P)) 439 CommonPreds.insert(P); 440 } 441 442 // Shortcut, if there are no common predecessors, merging is always safe 443 if (CommonPreds.empty()) 444 return true; 445 446 // Look at all the phi nodes in Succ, to see if they present a conflict when 447 // merging these blocks 448 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 449 PHINode *PN = cast<PHINode>(I); 450 451 // If the incoming value from BB is again a PHINode in 452 // BB which has the same incoming value for *PI as PN does, we can 453 // merge the phi nodes and then the blocks can still be merged 454 PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB)); 455 if (BBPN && BBPN->getParent() == BB) { 456 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end(); 457 PI != PE; PI++) { 458 if (BBPN->getIncomingValueForBlock(*PI) 459 != PN->getIncomingValueForBlock(*PI)) { 460 DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in " 461 << Succ->getName() << " is conflicting with " 462 << BBPN->getName() << " with regard to common predecessor " 463 << (*PI)->getName() << "\n"); 464 return false; 465 } 466 } 467 } else { 468 Value* Val = PN->getIncomingValueForBlock(BB); 469 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end(); 470 PI != PE; PI++) { 471 // See if the incoming value for the common predecessor is equal to the 472 // one for BB, in which case this phi node will not prevent the merging 473 // of the block. 474 if (Val != PN->getIncomingValueForBlock(*PI)) { 475 DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in " 476 << Succ->getName() << " is conflicting with regard to common " 477 << "predecessor " << (*PI)->getName() << "\n"); 478 return false; 479 } 480 } 481 } 482 } 483 484 return true; 485} 486 487/// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an 488/// unconditional branch, and contains no instructions other than PHI nodes, 489/// potential debug intrinsics and the branch. If possible, eliminate BB by 490/// rewriting all the predecessors to branch to the successor block and return 491/// true. If we can't transform, return false. 492bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) { 493 // We can't eliminate infinite loops. 494 BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0); 495 if (BB == Succ) return false; 496 497 // Check to see if merging these blocks would cause conflicts for any of the 498 // phi nodes in BB or Succ. If not, we can safely merge. 499 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false; 500 501 // Check for cases where Succ has multiple predecessors and a PHI node in BB 502 // has uses which will not disappear when the PHI nodes are merged. It is 503 // possible to handle such cases, but difficult: it requires checking whether 504 // BB dominates Succ, which is non-trivial to calculate in the case where 505 // Succ has multiple predecessors. Also, it requires checking whether 506 // constructing the necessary self-referential PHI node doesn't intoduce any 507 // conflicts; this isn't too difficult, but the previous code for doing this 508 // was incorrect. 509 // 510 // Note that if this check finds a live use, BB dominates Succ, so BB is 511 // something like a loop pre-header (or rarely, a part of an irreducible CFG); 512 // folding the branch isn't profitable in that case anyway. 513 if (!Succ->getSinglePredecessor()) { 514 BasicBlock::iterator BBI = BB->begin(); 515 while (isa<PHINode>(*BBI)) { 516 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end(); 517 UI != E; ++UI) { 518 if (PHINode* PN = dyn_cast<PHINode>(*UI)) { 519 if (PN->getIncomingBlock(UI) != BB) 520 return false; 521 } else { 522 return false; 523 } 524 } 525 ++BBI; 526 } 527 } 528 529 DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB); 530 531 if (isa<PHINode>(Succ->begin())) { 532 // If there is more than one pred of succ, and there are PHI nodes in 533 // the successor, then we need to add incoming edges for the PHI nodes 534 // 535 const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB)); 536 537 // Loop over all of the PHI nodes in the successor of BB. 538 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 539 PHINode *PN = cast<PHINode>(I); 540 Value *OldVal = PN->removeIncomingValue(BB, false); 541 assert(OldVal && "No entry in PHI for Pred BB!"); 542 543 // If this incoming value is one of the PHI nodes in BB, the new entries 544 // in the PHI node are the entries from the old PHI. 545 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) { 546 PHINode *OldValPN = cast<PHINode>(OldVal); 547 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) 548 // Note that, since we are merging phi nodes and BB and Succ might 549 // have common predecessors, we could end up with a phi node with 550 // identical incoming branches. This will be cleaned up later (and 551 // will trigger asserts if we try to clean it up now, without also 552 // simplifying the corresponding conditional branch). 553 PN->addIncoming(OldValPN->getIncomingValue(i), 554 OldValPN->getIncomingBlock(i)); 555 } else { 556 // Add an incoming value for each of the new incoming values. 557 for (unsigned i = 0, e = BBPreds.size(); i != e; ++i) 558 PN->addIncoming(OldVal, BBPreds[i]); 559 } 560 } 561 } 562 563 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 564 if (Succ->getSinglePredecessor()) { 565 // BB is the only predecessor of Succ, so Succ will end up with exactly 566 // the same predecessors BB had. 567 Succ->getInstList().splice(Succ->begin(), 568 BB->getInstList(), BB->begin()); 569 } else { 570 // We explicitly check for such uses in CanPropagatePredecessorsForPHIs. 571 assert(PN->use_empty() && "There shouldn't be any uses here!"); 572 PN->eraseFromParent(); 573 } 574 } 575 576 // Everything that jumped to BB now goes to Succ. 577 BB->replaceAllUsesWith(Succ); 578 if (!Succ->hasName()) Succ->takeName(BB); 579 BB->eraseFromParent(); // Delete the old basic block. 580 return true; 581} 582 583/// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI 584/// nodes in this block. This doesn't try to be clever about PHI nodes 585/// which differ only in the order of the incoming values, but instcombine 586/// orders them so it usually won't matter. 587/// 588bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) { 589 bool Changed = false; 590 591 // This implementation doesn't currently consider undef operands 592 // specially. Theroetically, two phis which are identical except for 593 // one having an undef where the other doesn't could be collapsed. 594 595 // Map from PHI hash values to PHI nodes. If multiple PHIs have 596 // the same hash value, the element is the first PHI in the 597 // linked list in CollisionMap. 598 DenseMap<uintptr_t, PHINode *> HashMap; 599 600 // Maintain linked lists of PHI nodes with common hash values. 601 DenseMap<PHINode *, PHINode *> CollisionMap; 602 603 // Examine each PHI. 604 for (BasicBlock::iterator I = BB->begin(); 605 PHINode *PN = dyn_cast<PHINode>(I++); ) { 606 // Compute a hash value on the operands. Instcombine will likely have sorted 607 // them, which helps expose duplicates, but we have to check all the 608 // operands to be safe in case instcombine hasn't run. 609 uintptr_t Hash = 0; 610 for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) { 611 // This hash algorithm is quite weak as hash functions go, but it seems 612 // to do a good enough job for this particular purpose, and is very quick. 613 Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I)); 614 Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7)); 615 } 616 // If we've never seen this hash value before, it's a unique PHI. 617 std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair = 618 HashMap.insert(std::make_pair(Hash, PN)); 619 if (Pair.second) continue; 620 // Otherwise it's either a duplicate or a hash collision. 621 for (PHINode *OtherPN = Pair.first->second; ; ) { 622 if (OtherPN->isIdenticalTo(PN)) { 623 // A duplicate. Replace this PHI with its duplicate. 624 PN->replaceAllUsesWith(OtherPN); 625 PN->eraseFromParent(); 626 Changed = true; 627 break; 628 } 629 // A non-duplicate hash collision. 630 DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN); 631 if (I == CollisionMap.end()) { 632 // Set this PHI to be the head of the linked list of colliding PHIs. 633 PHINode *Old = Pair.first->second; 634 Pair.first->second = PN; 635 CollisionMap[PN] = Old; 636 break; 637 } 638 // Procede to the next PHI in the list. 639 OtherPN = I->second; 640 } 641 } 642 643 return Changed; 644} 645