Local.cpp revision 6c146eefbf75875250af37a0f1ea70fc6b4716ee
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 analysis. 39// 40 41/// isSafeToLoadUnconditionally - Return true if we know that executing a load 42/// from this value cannot trap. If it is not obviously safe to load from the 43/// specified pointer, we do a quick local scan of the basic block containing 44/// ScanFrom, to determine if the address is already accessed. 45bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom) { 46 // If it is an alloca it is always safe to load from. 47 if (isa<AllocaInst>(V)) return true; 48 49 // If it is a global variable it is mostly safe to load from. 50 if (const GlobalValue *GV = dyn_cast<GlobalVariable>(V)) 51 // Don't try to evaluate aliases. External weak GV can be null. 52 return !isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage(); 53 54 // Otherwise, be a little bit agressive by scanning the local block where we 55 // want to check to see if the pointer is already being loaded or stored 56 // from/to. If so, the previous load or store would have already trapped, 57 // so there is no harm doing an extra load (also, CSE will later eliminate 58 // the load entirely). 59 BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin(); 60 61 while (BBI != E) { 62 --BBI; 63 64 // If we see a free or a call which may write to memory (i.e. which might do 65 // a free) the pointer could be marked invalid. 66 if (isa<CallInst>(BBI) && BBI->mayWriteToMemory() && 67 !isa<DbgInfoIntrinsic>(BBI)) 68 return false; 69 70 if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) { 71 if (LI->getOperand(0) == V) return true; 72 } else if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) { 73 if (SI->getOperand(1) == V) return true; 74 } 75 } 76 return false; 77} 78 79 80//===----------------------------------------------------------------------===// 81// Local constant propagation. 82// 83 84// ConstantFoldTerminator - If a terminator instruction is predicated on a 85// constant value, convert it into an unconditional branch to the constant 86// destination. 87// 88bool llvm::ConstantFoldTerminator(BasicBlock *BB) { 89 TerminatorInst *T = BB->getTerminator(); 90 91 // Branch - See if we are conditional jumping on constant 92 if (BranchInst *BI = dyn_cast<BranchInst>(T)) { 93 if (BI->isUnconditional()) return false; // Can't optimize uncond branch 94 BasicBlock *Dest1 = BI->getSuccessor(0); 95 BasicBlock *Dest2 = BI->getSuccessor(1); 96 97 if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) { 98 // Are we branching on constant? 99 // YES. Change to unconditional branch... 100 BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2; 101 BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1; 102 103 //cerr << "Function: " << T->getParent()->getParent() 104 // << "\nRemoving branch from " << T->getParent() 105 // << "\n\nTo: " << OldDest << endl; 106 107 // Let the basic block know that we are letting go of it. Based on this, 108 // it will adjust it's PHI nodes. 109 assert(BI->getParent() && "Terminator not inserted in block!"); 110 OldDest->removePredecessor(BI->getParent()); 111 112 // Set the unconditional destination, and change the insn to be an 113 // unconditional branch. 114 BI->setUnconditionalDest(Destination); 115 return true; 116 } 117 118 if (Dest2 == Dest1) { // Conditional branch to same location? 119 // This branch matches something like this: 120 // br bool %cond, label %Dest, label %Dest 121 // and changes it into: br label %Dest 122 123 // Let the basic block know that we are letting go of one copy of it. 124 assert(BI->getParent() && "Terminator not inserted in block!"); 125 Dest1->removePredecessor(BI->getParent()); 126 127 // Change a conditional branch to unconditional. 128 BI->setUnconditionalDest(Dest1); 129 return true; 130 } 131 return false; 132 } 133 134 if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) { 135 // If we are switching on a constant, we can convert the switch into a 136 // single branch instruction! 137 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition()); 138 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest 139 BasicBlock *DefaultDest = TheOnlyDest; 140 assert(TheOnlyDest == SI->getDefaultDest() && 141 "Default destination is not successor #0?"); 142 143 // Figure out which case it goes to. 144 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) { 145 // Found case matching a constant operand? 146 if (SI->getSuccessorValue(i) == CI) { 147 TheOnlyDest = SI->getSuccessor(i); 148 break; 149 } 150 151 // Check to see if this branch is going to the same place as the default 152 // dest. If so, eliminate it as an explicit compare. 153 if (SI->getSuccessor(i) == DefaultDest) { 154 // Remove this entry. 155 DefaultDest->removePredecessor(SI->getParent()); 156 SI->removeCase(i); 157 --i; --e; // Don't skip an entry... 158 continue; 159 } 160 161 // Otherwise, check to see if the switch only branches to one destination. 162 // We do this by reseting "TheOnlyDest" to null when we find two non-equal 163 // destinations. 164 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0; 165 } 166 167 if (CI && !TheOnlyDest) { 168 // Branching on a constant, but not any of the cases, go to the default 169 // successor. 170 TheOnlyDest = SI->getDefaultDest(); 171 } 172 173 // If we found a single destination that we can fold the switch into, do so 174 // now. 175 if (TheOnlyDest) { 176 // Insert the new branch. 177 BranchInst::Create(TheOnlyDest, SI); 178 BasicBlock *BB = SI->getParent(); 179 180 // Remove entries from PHI nodes which we no longer branch to... 181 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) { 182 // Found case matching a constant operand? 183 BasicBlock *Succ = SI->getSuccessor(i); 184 if (Succ == TheOnlyDest) 185 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest 186 else 187 Succ->removePredecessor(BB); 188 } 189 190 // Delete the old switch. 191 BB->getInstList().erase(SI); 192 return true; 193 } 194 195 if (SI->getNumSuccessors() == 2) { 196 // Otherwise, we can fold this switch into a conditional branch 197 // instruction if it has only one non-default destination. 198 Value *Cond = new ICmpInst(SI, ICmpInst::ICMP_EQ, SI->getCondition(), 199 SI->getSuccessorValue(1), "cond"); 200 // Insert the new branch. 201 BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI); 202 203 // Delete the old switch. 204 SI->eraseFromParent(); 205 return true; 206 } 207 return false; 208 } 209 210 if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(T)) { 211 // indirectbr blockaddress(@F, @BB) -> br label @BB 212 if (BlockAddress *BA = 213 dyn_cast<BlockAddress>(IBI->getAddress()->stripPointerCasts())) { 214 BasicBlock *TheOnlyDest = BA->getBasicBlock(); 215 // Insert the new branch. 216 BranchInst::Create(TheOnlyDest, IBI); 217 218 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) { 219 if (IBI->getDestination(i) == TheOnlyDest) 220 TheOnlyDest = 0; 221 else 222 IBI->getDestination(i)->removePredecessor(IBI->getParent()); 223 } 224 IBI->eraseFromParent(); 225 226 // If we didn't find our destination in the IBI successor list, then we 227 // have undefined behavior. Replace the unconditional branch with an 228 // 'unreachable' instruction. 229 if (TheOnlyDest) { 230 BB->getTerminator()->eraseFromParent(); 231 new UnreachableInst(BB->getContext(), BB); 232 } 233 234 return true; 235 } 236 } 237 238 return false; 239} 240 241 242//===----------------------------------------------------------------------===// 243// Local dead code elimination. 244// 245 246/// isInstructionTriviallyDead - Return true if the result produced by the 247/// instruction is not used, and the instruction has no side effects. 248/// 249bool llvm::isInstructionTriviallyDead(Instruction *I) { 250 if (!I->use_empty() || isa<TerminatorInst>(I)) return false; 251 252 // We don't want debug info removed by anything this general. 253 if (isa<DbgInfoIntrinsic>(I)) return false; 254 255 // Likewise for memory use markers. 256 if (isa<MemoryUseIntrinsic>(I)) return false; 257 258 if (!I->mayHaveSideEffects()) return true; 259 260 // Special case intrinsics that "may have side effects" but can be deleted 261 // when dead. 262 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) 263 // Safe to delete llvm.stacksave if dead. 264 if (II->getIntrinsicID() == Intrinsic::stacksave) 265 return true; 266 return false; 267} 268 269/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a 270/// trivially dead instruction, delete it. If that makes any of its operands 271/// trivially dead, delete them too, recursively. Return true if any 272/// instructions were deleted. 273bool llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) { 274 Instruction *I = dyn_cast<Instruction>(V); 275 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I)) 276 return false; 277 278 SmallVector<Instruction*, 16> DeadInsts; 279 DeadInsts.push_back(I); 280 281 do { 282 I = DeadInsts.pop_back_val(); 283 284 // Null out all of the instruction's operands to see if any operand becomes 285 // dead as we go. 286 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 287 Value *OpV = I->getOperand(i); 288 I->setOperand(i, 0); 289 290 if (!OpV->use_empty()) continue; 291 292 // If the operand is an instruction that became dead as we nulled out the 293 // operand, and if it is 'trivially' dead, delete it in a future loop 294 // iteration. 295 if (Instruction *OpI = dyn_cast<Instruction>(OpV)) 296 if (isInstructionTriviallyDead(OpI)) 297 DeadInsts.push_back(OpI); 298 } 299 300 I->eraseFromParent(); 301 } while (!DeadInsts.empty()); 302 303 return true; 304} 305 306/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively 307/// dead PHI node, due to being a def-use chain of single-use nodes that 308/// either forms a cycle or is terminated by a trivially dead instruction, 309/// delete it. If that makes any of its operands trivially dead, delete them 310/// too, recursively. Return true if the PHI node is actually deleted. 311bool 312llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) { 313 // We can remove a PHI if it is on a cycle in the def-use graph 314 // where each node in the cycle has degree one, i.e. only one use, 315 // and is an instruction with no side effects. 316 if (!PN->hasOneUse()) 317 return false; 318 319 bool Changed = false; 320 SmallPtrSet<PHINode *, 4> PHIs; 321 PHIs.insert(PN); 322 for (Instruction *J = cast<Instruction>(*PN->use_begin()); 323 J->hasOneUse() && !J->mayHaveSideEffects(); 324 J = cast<Instruction>(*J->use_begin())) 325 // If we find a PHI more than once, we're on a cycle that 326 // won't prove fruitful. 327 if (PHINode *JP = dyn_cast<PHINode>(J)) 328 if (!PHIs.insert(cast<PHINode>(JP))) { 329 // Break the cycle and delete the PHI and its operands. 330 JP->replaceAllUsesWith(UndefValue::get(JP->getType())); 331 (void)RecursivelyDeleteTriviallyDeadInstructions(JP); 332 Changed = true; 333 break; 334 } 335 return Changed; 336} 337 338/// SimplifyInstructionsInBlock - Scan the specified basic block and try to 339/// simplify any instructions in it and recursively delete dead instructions. 340/// 341/// This returns true if it changed the code, note that it can delete 342/// instructions in other blocks as well in this block. 343bool llvm::SimplifyInstructionsInBlock(BasicBlock *BB, const TargetData *TD) { 344 bool MadeChange = false; 345 for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) { 346 Instruction *Inst = BI++; 347 348 if (Value *V = SimplifyInstruction(Inst, TD)) { 349 WeakVH BIHandle(BI); 350 ReplaceAndSimplifyAllUses(Inst, V, TD); 351 MadeChange = true; 352 if (BIHandle == 0) 353 BI = BB->begin(); 354 continue; 355 } 356 357 MadeChange |= RecursivelyDeleteTriviallyDeadInstructions(Inst); 358 } 359 return MadeChange; 360} 361 362//===----------------------------------------------------------------------===// 363// Control Flow Graph Restructuring. 364// 365 366 367/// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this 368/// method is called when we're about to delete Pred as a predecessor of BB. If 369/// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred. 370/// 371/// Unlike the removePredecessor method, this attempts to simplify uses of PHI 372/// nodes that collapse into identity values. For example, if we have: 373/// x = phi(1, 0, 0, 0) 374/// y = and x, z 375/// 376/// .. and delete the predecessor corresponding to the '1', this will attempt to 377/// recursively fold the and to 0. 378void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred, 379 TargetData *TD) { 380 // This only adjusts blocks with PHI nodes. 381 if (!isa<PHINode>(BB->begin())) 382 return; 383 384 // Remove the entries for Pred from the PHI nodes in BB, but do not simplify 385 // them down. This will leave us with single entry phi nodes and other phis 386 // that can be removed. 387 BB->removePredecessor(Pred, true); 388 389 WeakVH PhiIt = &BB->front(); 390 while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) { 391 PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt)); 392 393 Value *PNV = PN->hasConstantValue(); 394 if (PNV == 0) continue; 395 396 // If we're able to simplify the phi to a single value, substitute the new 397 // value into all of its uses. 398 assert(PNV != PN && "hasConstantValue broken"); 399 400 ReplaceAndSimplifyAllUses(PN, PNV, TD); 401 402 // If recursive simplification ended up deleting the next PHI node we would 403 // iterate to, then our iterator is invalid, restart scanning from the top 404 // of the block. 405 if (PhiIt == 0) PhiIt = &BB->front(); 406 } 407} 408 409 410/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its 411/// predecessor is known to have one successor (DestBB!). Eliminate the edge 412/// between them, moving the instructions in the predecessor into DestBB and 413/// deleting the predecessor block. 414/// 415void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) { 416 // If BB has single-entry PHI nodes, fold them. 417 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { 418 Value *NewVal = PN->getIncomingValue(0); 419 // Replace self referencing PHI with undef, it must be dead. 420 if (NewVal == PN) NewVal = UndefValue::get(PN->getType()); 421 PN->replaceAllUsesWith(NewVal); 422 PN->eraseFromParent(); 423 } 424 425 BasicBlock *PredBB = DestBB->getSinglePredecessor(); 426 assert(PredBB && "Block doesn't have a single predecessor!"); 427 428 // Splice all the instructions from PredBB to DestBB. 429 PredBB->getTerminator()->eraseFromParent(); 430 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList()); 431 432 // Anything that branched to PredBB now branches to DestBB. 433 PredBB->replaceAllUsesWith(DestBB); 434 435 if (P) { 436 ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>(); 437 if (PI) { 438 PI->replaceAllUses(PredBB, DestBB); 439 PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB)); 440 } 441 } 442 // Nuke BB. 443 PredBB->eraseFromParent(); 444} 445 446/// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an 447/// almost-empty BB ending in an unconditional branch to Succ, into succ. 448/// 449/// Assumption: Succ is the single successor for BB. 450/// 451static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { 452 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!"); 453 454 DEBUG(dbgs() << "Looking to fold " << BB->getName() << " into " 455 << Succ->getName() << "\n"); 456 // Shortcut, if there is only a single predecessor it must be BB and merging 457 // is always safe 458 if (Succ->getSinglePredecessor()) return true; 459 460 // Make a list of the predecessors of BB 461 typedef SmallPtrSet<BasicBlock*, 16> BlockSet; 462 BlockSet BBPreds(pred_begin(BB), pred_end(BB)); 463 464 // Use that list to make another list of common predecessors of BB and Succ 465 BlockSet CommonPreds; 466 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); 467 PI != PE; ++PI) 468 if (BBPreds.count(*PI)) 469 CommonPreds.insert(*PI); 470 471 // Shortcut, if there are no common predecessors, merging is always safe 472 if (CommonPreds.empty()) 473 return true; 474 475 // Look at all the phi nodes in Succ, to see if they present a conflict when 476 // merging these blocks 477 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 478 PHINode *PN = cast<PHINode>(I); 479 480 // If the incoming value from BB is again a PHINode in 481 // BB which has the same incoming value for *PI as PN does, we can 482 // merge the phi nodes and then the blocks can still be merged 483 PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB)); 484 if (BBPN && BBPN->getParent() == BB) { 485 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end(); 486 PI != PE; PI++) { 487 if (BBPN->getIncomingValueForBlock(*PI) 488 != PN->getIncomingValueForBlock(*PI)) { 489 DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in " 490 << Succ->getName() << " is conflicting with " 491 << BBPN->getName() << " with regard to common predecessor " 492 << (*PI)->getName() << "\n"); 493 return false; 494 } 495 } 496 } else { 497 Value* Val = PN->getIncomingValueForBlock(BB); 498 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end(); 499 PI != PE; PI++) { 500 // See if the incoming value for the common predecessor is equal to the 501 // one for BB, in which case this phi node will not prevent the merging 502 // of the block. 503 if (Val != PN->getIncomingValueForBlock(*PI)) { 504 DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in " 505 << Succ->getName() << " is conflicting with regard to common " 506 << "predecessor " << (*PI)->getName() << "\n"); 507 return false; 508 } 509 } 510 } 511 } 512 513 return true; 514} 515 516/// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an 517/// unconditional branch, and contains no instructions other than PHI nodes, 518/// potential debug intrinsics and the branch. If possible, eliminate BB by 519/// rewriting all the predecessors to branch to the successor block and return 520/// true. If we can't transform, return false. 521bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) { 522 // We can't eliminate infinite loops. 523 BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0); 524 if (BB == Succ) return false; 525 526 // Check to see if merging these blocks would cause conflicts for any of the 527 // phi nodes in BB or Succ. If not, we can safely merge. 528 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false; 529 530 // Check for cases where Succ has multiple predecessors and a PHI node in BB 531 // has uses which will not disappear when the PHI nodes are merged. It is 532 // possible to handle such cases, but difficult: it requires checking whether 533 // BB dominates Succ, which is non-trivial to calculate in the case where 534 // Succ has multiple predecessors. Also, it requires checking whether 535 // constructing the necessary self-referential PHI node doesn't intoduce any 536 // conflicts; this isn't too difficult, but the previous code for doing this 537 // was incorrect. 538 // 539 // Note that if this check finds a live use, BB dominates Succ, so BB is 540 // something like a loop pre-header (or rarely, a part of an irreducible CFG); 541 // folding the branch isn't profitable in that case anyway. 542 if (!Succ->getSinglePredecessor()) { 543 BasicBlock::iterator BBI = BB->begin(); 544 while (isa<PHINode>(*BBI)) { 545 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end(); 546 UI != E; ++UI) { 547 if (PHINode* PN = dyn_cast<PHINode>(*UI)) { 548 if (PN->getIncomingBlock(UI) != BB) 549 return false; 550 } else { 551 return false; 552 } 553 } 554 ++BBI; 555 } 556 } 557 558 DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB); 559 560 if (isa<PHINode>(Succ->begin())) { 561 // If there is more than one pred of succ, and there are PHI nodes in 562 // the successor, then we need to add incoming edges for the PHI nodes 563 // 564 const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB)); 565 566 // Loop over all of the PHI nodes in the successor of BB. 567 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 568 PHINode *PN = cast<PHINode>(I); 569 Value *OldVal = PN->removeIncomingValue(BB, false); 570 assert(OldVal && "No entry in PHI for Pred BB!"); 571 572 // If this incoming value is one of the PHI nodes in BB, the new entries 573 // in the PHI node are the entries from the old PHI. 574 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) { 575 PHINode *OldValPN = cast<PHINode>(OldVal); 576 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) 577 // Note that, since we are merging phi nodes and BB and Succ might 578 // have common predecessors, we could end up with a phi node with 579 // identical incoming branches. This will be cleaned up later (and 580 // will trigger asserts if we try to clean it up now, without also 581 // simplifying the corresponding conditional branch). 582 PN->addIncoming(OldValPN->getIncomingValue(i), 583 OldValPN->getIncomingBlock(i)); 584 } else { 585 // Add an incoming value for each of the new incoming values. 586 for (unsigned i = 0, e = BBPreds.size(); i != e; ++i) 587 PN->addIncoming(OldVal, BBPreds[i]); 588 } 589 } 590 } 591 592 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 593 if (Succ->getSinglePredecessor()) { 594 // BB is the only predecessor of Succ, so Succ will end up with exactly 595 // the same predecessors BB had. 596 Succ->getInstList().splice(Succ->begin(), 597 BB->getInstList(), BB->begin()); 598 } else { 599 // We explicitly check for such uses in CanPropagatePredecessorsForPHIs. 600 assert(PN->use_empty() && "There shouldn't be any uses here!"); 601 PN->eraseFromParent(); 602 } 603 } 604 605 // Everything that jumped to BB now goes to Succ. 606 BB->replaceAllUsesWith(Succ); 607 if (!Succ->hasName()) Succ->takeName(BB); 608 BB->eraseFromParent(); // Delete the old basic block. 609 return true; 610} 611 612/// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI 613/// nodes in this block. This doesn't try to be clever about PHI nodes 614/// which differ only in the order of the incoming values, but instcombine 615/// orders them so it usually won't matter. 616/// 617bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) { 618 bool Changed = false; 619 620 // This implementation doesn't currently consider undef operands 621 // specially. Theroetically, two phis which are identical except for 622 // one having an undef where the other doesn't could be collapsed. 623 624 // Map from PHI hash values to PHI nodes. If multiple PHIs have 625 // the same hash value, the element is the first PHI in the 626 // linked list in CollisionMap. 627 DenseMap<uintptr_t, PHINode *> HashMap; 628 629 // Maintain linked lists of PHI nodes with common hash values. 630 DenseMap<PHINode *, PHINode *> CollisionMap; 631 632 // Examine each PHI. 633 for (BasicBlock::iterator I = BB->begin(); 634 PHINode *PN = dyn_cast<PHINode>(I++); ) { 635 // Compute a hash value on the operands. Instcombine will likely have sorted 636 // them, which helps expose duplicates, but we have to check all the 637 // operands to be safe in case instcombine hasn't run. 638 uintptr_t Hash = 0; 639 for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) { 640 // This hash algorithm is quite weak as hash functions go, but it seems 641 // to do a good enough job for this particular purpose, and is very quick. 642 Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I)); 643 Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7)); 644 } 645 // If we've never seen this hash value before, it's a unique PHI. 646 std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair = 647 HashMap.insert(std::make_pair(Hash, PN)); 648 if (Pair.second) continue; 649 // Otherwise it's either a duplicate or a hash collision. 650 for (PHINode *OtherPN = Pair.first->second; ; ) { 651 if (OtherPN->isIdenticalTo(PN)) { 652 // A duplicate. Replace this PHI with its duplicate. 653 PN->replaceAllUsesWith(OtherPN); 654 PN->eraseFromParent(); 655 Changed = true; 656 break; 657 } 658 // A non-duplicate hash collision. 659 DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN); 660 if (I == CollisionMap.end()) { 661 // Set this PHI to be the head of the linked list of colliding PHIs. 662 PHINode *Old = Pair.first->second; 663 Pair.first->second = PN; 664 CollisionMap[PN] = Old; 665 break; 666 } 667 // Procede to the next PHI in the list. 668 OtherPN = I->second; 669 } 670 } 671 672 return Changed; 673} 674