Local.cpp revision 43a8241b65b70ded3a87fb26852719633908a1e4
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/LLVMContext.h" 24#include "llvm/ADT/SmallPtrSet.h" 25#include "llvm/Analysis/ConstantFolding.h" 26#include "llvm/Analysis/DebugInfo.h" 27#include "llvm/Analysis/InstructionSimplify.h" 28#include "llvm/Analysis/ProfileInfo.h" 29#include "llvm/Target/TargetData.h" 30#include "llvm/Support/CFG.h" 31#include "llvm/Support/Debug.h" 32#include "llvm/Support/GetElementPtrTypeIterator.h" 33#include "llvm/Support/MathExtras.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. 272void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) { 273 Instruction *I = dyn_cast<Instruction>(V); 274 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I)) 275 return; 276 277 SmallVector<Instruction*, 16> DeadInsts; 278 DeadInsts.push_back(I); 279 280 while (!DeadInsts.empty()) { 281 I = DeadInsts.pop_back_val(); 282 283 // Null out all of the instruction's operands to see if any operand becomes 284 // dead as we go. 285 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 286 Value *OpV = I->getOperand(i); 287 I->setOperand(i, 0); 288 289 if (!OpV->use_empty()) continue; 290 291 // If the operand is an instruction that became dead as we nulled out the 292 // operand, and if it is 'trivially' dead, delete it in a future loop 293 // iteration. 294 if (Instruction *OpI = dyn_cast<Instruction>(OpV)) 295 if (isInstructionTriviallyDead(OpI)) 296 DeadInsts.push_back(OpI); 297 } 298 299 I->eraseFromParent(); 300 } 301} 302 303/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively 304/// dead PHI node, due to being a def-use chain of single-use nodes that 305/// either forms a cycle or is terminated by a trivially dead instruction, 306/// delete it. If that makes any of its operands trivially dead, delete them 307/// too, recursively. 308void 309llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) { 310 // We can remove a PHI if it is on a cycle in the def-use graph 311 // where each node in the cycle has degree one, i.e. only one use, 312 // and is an instruction with no side effects. 313 if (!PN->hasOneUse()) 314 return; 315 316 SmallPtrSet<PHINode *, 4> PHIs; 317 PHIs.insert(PN); 318 for (Instruction *J = cast<Instruction>(*PN->use_begin()); 319 J->hasOneUse() && !J->mayHaveSideEffects(); 320 J = cast<Instruction>(*J->use_begin())) 321 // If we find a PHI more than once, we're on a cycle that 322 // won't prove fruitful. 323 if (PHINode *JP = dyn_cast<PHINode>(J)) 324 if (!PHIs.insert(cast<PHINode>(JP))) { 325 // Break the cycle and delete the PHI and its operands. 326 JP->replaceAllUsesWith(UndefValue::get(JP->getType())); 327 RecursivelyDeleteTriviallyDeadInstructions(JP); 328 break; 329 } 330} 331 332//===----------------------------------------------------------------------===// 333// Control Flow Graph Restructuring. 334// 335 336 337/// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this 338/// method is called when we're about to delete Pred as a predecessor of BB. If 339/// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred. 340/// 341/// Unlike the removePredecessor method, this attempts to simplify uses of PHI 342/// nodes that collapse into identity values. For example, if we have: 343/// x = phi(1, 0, 0, 0) 344/// y = and x, z 345/// 346/// .. and delete the predecessor corresponding to the '1', this will attempt to 347/// recursively fold the and to 0. 348void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred, 349 TargetData *TD) { 350 // This only adjusts blocks with PHI nodes. 351 if (!isa<PHINode>(BB->begin())) 352 return; 353 354 // Remove the entries for Pred from the PHI nodes in BB, but do not simplify 355 // them down. This will leave us with single entry phi nodes and other phis 356 // that can be removed. 357 BB->removePredecessor(Pred, true); 358 359 WeakVH PhiIt = &BB->front(); 360 while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) { 361 PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt)); 362 363 Value *PNV = PN->hasConstantValue(); 364 if (PNV == 0) continue; 365 366 // If we're able to simplify the phi to a single value, substitute the new 367 // value into all of its uses. 368 assert(PNV != PN && "hasConstantValue broken"); 369 370 ReplaceAndSimplifyAllUses(PN, PNV, TD); 371 372 // If recursive simplification ended up deleting the next PHI node we would 373 // iterate to, then our iterator is invalid, restart scanning from the top 374 // of the block. 375 if (PhiIt == 0) PhiIt = &BB->front(); 376 } 377} 378 379 380/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its 381/// predecessor is known to have one successor (DestBB!). Eliminate the edge 382/// between them, moving the instructions in the predecessor into DestBB and 383/// deleting the predecessor block. 384/// 385void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) { 386 // If BB has single-entry PHI nodes, fold them. 387 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { 388 Value *NewVal = PN->getIncomingValue(0); 389 // Replace self referencing PHI with undef, it must be dead. 390 if (NewVal == PN) NewVal = UndefValue::get(PN->getType()); 391 PN->replaceAllUsesWith(NewVal); 392 PN->eraseFromParent(); 393 } 394 395 BasicBlock *PredBB = DestBB->getSinglePredecessor(); 396 assert(PredBB && "Block doesn't have a single predecessor!"); 397 398 // Splice all the instructions from PredBB to DestBB. 399 PredBB->getTerminator()->eraseFromParent(); 400 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList()); 401 402 // Anything that branched to PredBB now branches to DestBB. 403 PredBB->replaceAllUsesWith(DestBB); 404 405 if (P) { 406 ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>(); 407 if (PI) { 408 PI->replaceAllUses(PredBB, DestBB); 409 PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB)); 410 } 411 } 412 // Nuke BB. 413 PredBB->eraseFromParent(); 414} 415 416/// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an 417/// almost-empty BB ending in an unconditional branch to Succ, into succ. 418/// 419/// Assumption: Succ is the single successor for BB. 420/// 421static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { 422 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!"); 423 424 DEBUG(errs() << "Looking to fold " << BB->getName() << " into " 425 << Succ->getName() << "\n"); 426 // Shortcut, if there is only a single predecessor it must be BB and merging 427 // is always safe 428 if (Succ->getSinglePredecessor()) return true; 429 430 // Make a list of the predecessors of BB 431 typedef SmallPtrSet<BasicBlock*, 16> BlockSet; 432 BlockSet BBPreds(pred_begin(BB), pred_end(BB)); 433 434 // Use that list to make another list of common predecessors of BB and Succ 435 BlockSet CommonPreds; 436 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); 437 PI != PE; ++PI) 438 if (BBPreds.count(*PI)) 439 CommonPreds.insert(*PI); 440 441 // Shortcut, if there are no common predecessors, merging is always safe 442 if (CommonPreds.empty()) 443 return true; 444 445 // Look at all the phi nodes in Succ, to see if they present a conflict when 446 // merging these blocks 447 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 448 PHINode *PN = cast<PHINode>(I); 449 450 // If the incoming value from BB is again a PHINode in 451 // BB which has the same incoming value for *PI as PN does, we can 452 // merge the phi nodes and then the blocks can still be merged 453 PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB)); 454 if (BBPN && BBPN->getParent() == BB) { 455 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end(); 456 PI != PE; PI++) { 457 if (BBPN->getIncomingValueForBlock(*PI) 458 != PN->getIncomingValueForBlock(*PI)) { 459 DEBUG(errs() << "Can't fold, phi node " << PN->getName() << " in " 460 << Succ->getName() << " is conflicting with " 461 << BBPN->getName() << " with regard to common predecessor " 462 << (*PI)->getName() << "\n"); 463 return false; 464 } 465 } 466 } else { 467 Value* Val = PN->getIncomingValueForBlock(BB); 468 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end(); 469 PI != PE; PI++) { 470 // See if the incoming value for the common predecessor is equal to the 471 // one for BB, in which case this phi node will not prevent the merging 472 // of the block. 473 if (Val != PN->getIncomingValueForBlock(*PI)) { 474 DEBUG(errs() << "Can't fold, phi node " << PN->getName() << " in " 475 << Succ->getName() << " is conflicting with regard to common " 476 << "predecessor " << (*PI)->getName() << "\n"); 477 return false; 478 } 479 } 480 } 481 } 482 483 return true; 484} 485 486/// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an 487/// unconditional branch, and contains no instructions other than PHI nodes, 488/// potential debug intrinsics and the branch. If possible, eliminate BB by 489/// rewriting all the predecessors to branch to the successor block and return 490/// true. If we can't transform, return false. 491bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) { 492 // We can't eliminate infinite loops. 493 BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0); 494 if (BB == Succ) return false; 495 496 // Check to see if merging these blocks would cause conflicts for any of the 497 // phi nodes in BB or Succ. If not, we can safely merge. 498 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false; 499 500 // Check for cases where Succ has multiple predecessors and a PHI node in BB 501 // has uses which will not disappear when the PHI nodes are merged. It is 502 // possible to handle such cases, but difficult: it requires checking whether 503 // BB dominates Succ, which is non-trivial to calculate in the case where 504 // Succ has multiple predecessors. Also, it requires checking whether 505 // constructing the necessary self-referential PHI node doesn't intoduce any 506 // conflicts; this isn't too difficult, but the previous code for doing this 507 // was incorrect. 508 // 509 // Note that if this check finds a live use, BB dominates Succ, so BB is 510 // something like a loop pre-header (or rarely, a part of an irreducible CFG); 511 // folding the branch isn't profitable in that case anyway. 512 if (!Succ->getSinglePredecessor()) { 513 BasicBlock::iterator BBI = BB->begin(); 514 while (isa<PHINode>(*BBI)) { 515 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end(); 516 UI != E; ++UI) { 517 if (PHINode* PN = dyn_cast<PHINode>(*UI)) { 518 if (PN->getIncomingBlock(UI) != BB) 519 return false; 520 } else { 521 return false; 522 } 523 } 524 ++BBI; 525 } 526 } 527 528 DEBUG(errs() << "Killing Trivial BB: \n" << *BB); 529 530 if (isa<PHINode>(Succ->begin())) { 531 // If there is more than one pred of succ, and there are PHI nodes in 532 // the successor, then we need to add incoming edges for the PHI nodes 533 // 534 const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB)); 535 536 // Loop over all of the PHI nodes in the successor of BB. 537 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 538 PHINode *PN = cast<PHINode>(I); 539 Value *OldVal = PN->removeIncomingValue(BB, false); 540 assert(OldVal && "No entry in PHI for Pred BB!"); 541 542 // If this incoming value is one of the PHI nodes in BB, the new entries 543 // in the PHI node are the entries from the old PHI. 544 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) { 545 PHINode *OldValPN = cast<PHINode>(OldVal); 546 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) 547 // Note that, since we are merging phi nodes and BB and Succ might 548 // have common predecessors, we could end up with a phi node with 549 // identical incoming branches. This will be cleaned up later (and 550 // will trigger asserts if we try to clean it up now, without also 551 // simplifying the corresponding conditional branch). 552 PN->addIncoming(OldValPN->getIncomingValue(i), 553 OldValPN->getIncomingBlock(i)); 554 } else { 555 // Add an incoming value for each of the new incoming values. 556 for (unsigned i = 0, e = BBPreds.size(); i != e; ++i) 557 PN->addIncoming(OldVal, BBPreds[i]); 558 } 559 } 560 } 561 562 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 563 if (Succ->getSinglePredecessor()) { 564 // BB is the only predecessor of Succ, so Succ will end up with exactly 565 // the same predecessors BB had. 566 Succ->getInstList().splice(Succ->begin(), 567 BB->getInstList(), BB->begin()); 568 } else { 569 // We explicitly check for such uses in CanPropagatePredecessorsForPHIs. 570 assert(PN->use_empty() && "There shouldn't be any uses here!"); 571 PN->eraseFromParent(); 572 } 573 } 574 575 // Everything that jumped to BB now goes to Succ. 576 BB->replaceAllUsesWith(Succ); 577 if (!Succ->hasName()) Succ->takeName(BB); 578 BB->eraseFromParent(); // Delete the old basic block. 579 return true; 580} 581 582 583 584/// OnlyUsedByDbgIntrinsics - Return true if the instruction I is only used 585/// by DbgIntrinsics. If DbgInUses is specified then the vector is filled 586/// with the DbgInfoIntrinsic that use the instruction I. 587bool llvm::OnlyUsedByDbgInfoIntrinsics(Instruction *I, 588 SmallVectorImpl<DbgInfoIntrinsic *> *DbgInUses) { 589 if (DbgInUses) 590 DbgInUses->clear(); 591 592 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); UI != UE; 593 ++UI) { 594 if (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(*UI)) { 595 if (DbgInUses) 596 DbgInUses->push_back(DI); 597 } else { 598 if (DbgInUses) 599 DbgInUses->clear(); 600 return false; 601 } 602 } 603 return true; 604} 605 606/// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI 607/// nodes in this block. This doesn't try to be clever about PHI nodes 608/// which differ only in the order of the incoming values, but instcombine 609/// orders them so it usually won't matter. 610/// 611bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) { 612 bool Changed = false; 613 614 // This implementation doesn't currently consider undef operands 615 // specially. Theroetically, two phis which are identical except for 616 // one having an undef where the other doesn't could be collapsed. 617 618 // Map from PHI hash values to PHI nodes. If multiple PHIs have 619 // the same hash value, the element is the first PHI in the 620 // linked list in CollisionMap. 621 DenseMap<uintptr_t, PHINode *> HashMap; 622 623 // Maintain linked lists of PHI nodes with common hash values. 624 DenseMap<PHINode *, PHINode *> CollisionMap; 625 626 // Examine each PHI. 627 for (BasicBlock::iterator I = BB->begin(); 628 PHINode *PN = dyn_cast<PHINode>(I++); ) { 629 // Compute a hash value on the operands. Instcombine will likely have sorted 630 // them, which helps expose duplicates, but we have to check all the 631 // operands to be safe in case instcombine hasn't run. 632 uintptr_t Hash = 0; 633 for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) { 634 // This hash algorithm is quite weak as hash functions go, but it seems 635 // to do a good enough job for this particular purpose, and is very quick. 636 Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I)); 637 Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7)); 638 } 639 // If we've never seen this hash value before, it's a unique PHI. 640 std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair = 641 HashMap.insert(std::make_pair(Hash, PN)); 642 if (Pair.second) continue; 643 // Otherwise it's either a duplicate or a hash collision. 644 for (PHINode *OtherPN = Pair.first->second; ; ) { 645 if (OtherPN->isIdenticalTo(PN)) { 646 // A duplicate. Replace this PHI with its duplicate. 647 PN->replaceAllUsesWith(OtherPN); 648 PN->eraseFromParent(); 649 Changed = true; 650 break; 651 } 652 // A non-duplicate hash collision. 653 DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN); 654 if (I == CollisionMap.end()) { 655 // Set this PHI to be the head of the linked list of colliding PHIs. 656 PHINode *Old = Pair.first->second; 657 Pair.first->second = PN; 658 CollisionMap[PN] = Old; 659 break; 660 } 661 // Procede to the next PHI in the list. 662 OtherPN = I->second; 663 } 664 } 665 666 return Changed; 667} 668