Local.cpp revision 1a4021a2be4a59e9f9010776cb6f72107241aeb5
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/Dominators.h" 26#include "llvm/Analysis/ConstantFolding.h" 27#include "llvm/Analysis/InstructionSimplify.h" 28#include "llvm/Analysis/ProfileInfo.h" 29#include "llvm/Analysis/ValueTracking.h" 30#include "llvm/Target/TargetData.h" 31#include "llvm/Support/CFG.h" 32#include "llvm/Support/Debug.h" 33#include "llvm/Support/GetElementPtrTypeIterator.h" 34#include "llvm/Support/MathExtras.h" 35#include "llvm/Support/ValueHandle.h" 36#include "llvm/Support/raw_ostream.h" 37using namespace llvm; 38 39//===----------------------------------------------------------------------===// 40// Local constant propagation. 41// 42 43// ConstantFoldTerminator - If a terminator instruction is predicated on a 44// constant value, convert it into an unconditional branch to the constant 45// destination. 46// 47bool llvm::ConstantFoldTerminator(BasicBlock *BB) { 48 TerminatorInst *T = BB->getTerminator(); 49 50 // Branch - See if we are conditional jumping on constant 51 if (BranchInst *BI = dyn_cast<BranchInst>(T)) { 52 if (BI->isUnconditional()) return false; // Can't optimize uncond branch 53 BasicBlock *Dest1 = BI->getSuccessor(0); 54 BasicBlock *Dest2 = BI->getSuccessor(1); 55 56 if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) { 57 // Are we branching on constant? 58 // YES. Change to unconditional branch... 59 BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2; 60 BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1; 61 62 //cerr << "Function: " << T->getParent()->getParent() 63 // << "\nRemoving branch from " << T->getParent() 64 // << "\n\nTo: " << OldDest << endl; 65 66 // Let the basic block know that we are letting go of it. Based on this, 67 // it will adjust it's PHI nodes. 68 assert(BI->getParent() && "Terminator not inserted in block!"); 69 OldDest->removePredecessor(BI->getParent()); 70 71 // Replace the conditional branch with an unconditional one. 72 BranchInst::Create(Destination, BI); 73 BI->eraseFromParent(); 74 return true; 75 } 76 77 if (Dest2 == Dest1) { // Conditional branch to same location? 78 // This branch matches something like this: 79 // br bool %cond, label %Dest, label %Dest 80 // and changes it into: br label %Dest 81 82 // Let the basic block know that we are letting go of one copy of it. 83 assert(BI->getParent() && "Terminator not inserted in block!"); 84 Dest1->removePredecessor(BI->getParent()); 85 86 // Replace the conditional branch with an unconditional one. 87 BranchInst::Create(Dest1, BI); 88 BI->eraseFromParent(); 89 return true; 90 } 91 return false; 92 } 93 94 if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) { 95 // If we are switching on a constant, we can convert the switch into a 96 // single branch instruction! 97 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition()); 98 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest 99 BasicBlock *DefaultDest = TheOnlyDest; 100 assert(TheOnlyDest == SI->getDefaultDest() && 101 "Default destination is not successor #0?"); 102 103 // Figure out which case it goes to. 104 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) { 105 // Found case matching a constant operand? 106 if (SI->getSuccessorValue(i) == CI) { 107 TheOnlyDest = SI->getSuccessor(i); 108 break; 109 } 110 111 // Check to see if this branch is going to the same place as the default 112 // dest. If so, eliminate it as an explicit compare. 113 if (SI->getSuccessor(i) == DefaultDest) { 114 // Remove this entry. 115 DefaultDest->removePredecessor(SI->getParent()); 116 SI->removeCase(i); 117 --i; --e; // Don't skip an entry... 118 continue; 119 } 120 121 // Otherwise, check to see if the switch only branches to one destination. 122 // We do this by reseting "TheOnlyDest" to null when we find two non-equal 123 // destinations. 124 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0; 125 } 126 127 if (CI && !TheOnlyDest) { 128 // Branching on a constant, but not any of the cases, go to the default 129 // successor. 130 TheOnlyDest = SI->getDefaultDest(); 131 } 132 133 // If we found a single destination that we can fold the switch into, do so 134 // now. 135 if (TheOnlyDest) { 136 // Insert the new branch. 137 BranchInst::Create(TheOnlyDest, SI); 138 BasicBlock *BB = SI->getParent(); 139 140 // Remove entries from PHI nodes which we no longer branch to... 141 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) { 142 // Found case matching a constant operand? 143 BasicBlock *Succ = SI->getSuccessor(i); 144 if (Succ == TheOnlyDest) 145 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest 146 else 147 Succ->removePredecessor(BB); 148 } 149 150 // Delete the old switch. 151 BB->getInstList().erase(SI); 152 return true; 153 } 154 155 if (SI->getNumSuccessors() == 2) { 156 // Otherwise, we can fold this switch into a conditional branch 157 // instruction if it has only one non-default destination. 158 Value *Cond = new ICmpInst(SI, ICmpInst::ICMP_EQ, SI->getCondition(), 159 SI->getSuccessorValue(1), "cond"); 160 // Insert the new branch. 161 BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI); 162 163 // Delete the old switch. 164 SI->eraseFromParent(); 165 return true; 166 } 167 return false; 168 } 169 170 if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(T)) { 171 // indirectbr blockaddress(@F, @BB) -> br label @BB 172 if (BlockAddress *BA = 173 dyn_cast<BlockAddress>(IBI->getAddress()->stripPointerCasts())) { 174 BasicBlock *TheOnlyDest = BA->getBasicBlock(); 175 // Insert the new branch. 176 BranchInst::Create(TheOnlyDest, IBI); 177 178 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) { 179 if (IBI->getDestination(i) == TheOnlyDest) 180 TheOnlyDest = 0; 181 else 182 IBI->getDestination(i)->removePredecessor(IBI->getParent()); 183 } 184 IBI->eraseFromParent(); 185 186 // If we didn't find our destination in the IBI successor list, then we 187 // have undefined behavior. Replace the unconditional branch with an 188 // 'unreachable' instruction. 189 if (TheOnlyDest) { 190 BB->getTerminator()->eraseFromParent(); 191 new UnreachableInst(BB->getContext(), BB); 192 } 193 194 return true; 195 } 196 } 197 198 return false; 199} 200 201 202//===----------------------------------------------------------------------===// 203// Local dead code elimination. 204// 205 206/// isInstructionTriviallyDead - Return true if the result produced by the 207/// instruction is not used, and the instruction has no side effects. 208/// 209bool llvm::isInstructionTriviallyDead(Instruction *I) { 210 if (!I->use_empty() || isa<TerminatorInst>(I)) return false; 211 212 // We don't want debug info removed by anything this general. 213 if (isa<DbgInfoIntrinsic>(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/// areAllUsesEqual - Check whether the uses of a value are all the same. 264/// This is similar to Instruction::hasOneUse() except this will also return 265/// true when there are multiple uses that all refer to the same value. 266static bool areAllUsesEqual(Instruction *I) { 267 Value::use_iterator UI = I->use_begin(); 268 Value::use_iterator UE = I->use_end(); 269 if (UI == UE) 270 return false; 271 272 User *TheUse = *UI; 273 for (++UI; UI != UE; ++UI) { 274 if (*UI != TheUse) 275 return false; 276 } 277 return true; 278} 279 280/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively 281/// dead PHI node, due to being a def-use chain of single-use nodes that 282/// either forms a cycle or is terminated by a trivially dead instruction, 283/// delete it. If that makes any of its operands trivially dead, delete them 284/// too, recursively. Return true if the PHI node is actually deleted. 285bool llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) { 286 // We can remove a PHI if it is on a cycle in the def-use graph 287 // where each node in the cycle has degree one, i.e. only one use, 288 // and is an instruction with no side effects. 289 if (!areAllUsesEqual(PN)) 290 return false; 291 292 bool Changed = false; 293 SmallPtrSet<PHINode *, 4> PHIs; 294 PHIs.insert(PN); 295 for (Instruction *J = cast<Instruction>(*PN->use_begin()); 296 areAllUsesEqual(J) && !J->mayHaveSideEffects(); 297 J = cast<Instruction>(*J->use_begin())) 298 // If we find a PHI more than once, we're on a cycle that 299 // won't prove fruitful. 300 if (PHINode *JP = dyn_cast<PHINode>(J)) 301 if (!PHIs.insert(JP)) { 302 // Break the cycle and delete the PHI and its operands. 303 JP->replaceAllUsesWith(UndefValue::get(JP->getType())); 304 (void)RecursivelyDeleteTriviallyDeadInstructions(JP); 305 Changed = true; 306 break; 307 } 308 return Changed; 309} 310 311/// SimplifyInstructionsInBlock - Scan the specified basic block and try to 312/// simplify any instructions in it and recursively delete dead instructions. 313/// 314/// This returns true if it changed the code, note that it can delete 315/// instructions in other blocks as well in this block. 316bool llvm::SimplifyInstructionsInBlock(BasicBlock *BB, const TargetData *TD) { 317 bool MadeChange = false; 318 for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) { 319 Instruction *Inst = BI++; 320 321 if (Value *V = SimplifyInstruction(Inst, TD)) { 322 WeakVH BIHandle(BI); 323 ReplaceAndSimplifyAllUses(Inst, V, TD); 324 MadeChange = true; 325 if (BIHandle != BI) 326 BI = BB->begin(); 327 continue; 328 } 329 330 MadeChange |= RecursivelyDeleteTriviallyDeadInstructions(Inst); 331 } 332 return MadeChange; 333} 334 335//===----------------------------------------------------------------------===// 336// Control Flow Graph Restructuring. 337// 338 339 340/// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this 341/// method is called when we're about to delete Pred as a predecessor of BB. If 342/// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred. 343/// 344/// Unlike the removePredecessor method, this attempts to simplify uses of PHI 345/// nodes that collapse into identity values. For example, if we have: 346/// x = phi(1, 0, 0, 0) 347/// y = and x, z 348/// 349/// .. and delete the predecessor corresponding to the '1', this will attempt to 350/// recursively fold the and to 0. 351void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred, 352 TargetData *TD) { 353 // This only adjusts blocks with PHI nodes. 354 if (!isa<PHINode>(BB->begin())) 355 return; 356 357 // Remove the entries for Pred from the PHI nodes in BB, but do not simplify 358 // them down. This will leave us with single entry phi nodes and other phis 359 // that can be removed. 360 BB->removePredecessor(Pred, true); 361 362 WeakVH PhiIt = &BB->front(); 363 while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) { 364 PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt)); 365 366 Value *PNV = SimplifyInstruction(PN, TD); 367 if (PNV == 0) continue; 368 369 // If we're able to simplify the phi to a single value, substitute the new 370 // value into all of its uses. 371 assert(PNV != PN && "SimplifyInstruction broken!"); 372 373 Value *OldPhiIt = PhiIt; 374 ReplaceAndSimplifyAllUses(PN, PNV, TD); 375 376 // If recursive simplification ended up deleting the next PHI node we would 377 // iterate to, then our iterator is invalid, restart scanning from the top 378 // of the block. 379 if (PhiIt != OldPhiIt) PhiIt = &BB->front(); 380 } 381} 382 383 384/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its 385/// predecessor is known to have one successor (DestBB!). Eliminate the edge 386/// between them, moving the instructions in the predecessor into DestBB and 387/// deleting the predecessor block. 388/// 389void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) { 390 // If BB has single-entry PHI nodes, fold them. 391 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { 392 Value *NewVal = PN->getIncomingValue(0); 393 // Replace self referencing PHI with undef, it must be dead. 394 if (NewVal == PN) NewVal = UndefValue::get(PN->getType()); 395 PN->replaceAllUsesWith(NewVal); 396 PN->eraseFromParent(); 397 } 398 399 BasicBlock *PredBB = DestBB->getSinglePredecessor(); 400 assert(PredBB && "Block doesn't have a single predecessor!"); 401 402 // Splice all the instructions from PredBB to DestBB. 403 PredBB->getTerminator()->eraseFromParent(); 404 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList()); 405 406 // Zap anything that took the address of DestBB. Not doing this will give the 407 // address an invalid value. 408 if (DestBB->hasAddressTaken()) { 409 BlockAddress *BA = BlockAddress::get(DestBB); 410 Constant *Replacement = 411 ConstantInt::get(llvm::Type::getInt32Ty(BA->getContext()), 1); 412 BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(Replacement, 413 BA->getType())); 414 BA->destroyConstant(); 415 } 416 417 // Anything that branched to PredBB now branches to DestBB. 418 PredBB->replaceAllUsesWith(DestBB); 419 420 if (P) { 421 DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>(); 422 if (DT) { 423 BasicBlock *PredBBIDom = DT->getNode(PredBB)->getIDom()->getBlock(); 424 DT->changeImmediateDominator(DestBB, PredBBIDom); 425 DT->eraseNode(PredBB); 426 } 427 ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>(); 428 if (PI) { 429 PI->replaceAllUses(PredBB, DestBB); 430 PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB)); 431 } 432 } 433 // Nuke BB. 434 PredBB->eraseFromParent(); 435} 436 437/// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an 438/// almost-empty BB ending in an unconditional branch to Succ, into succ. 439/// 440/// Assumption: Succ is the single successor for BB. 441/// 442static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { 443 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!"); 444 445 DEBUG(dbgs() << "Looking to fold " << BB->getName() << " into " 446 << Succ->getName() << "\n"); 447 // Shortcut, if there is only a single predecessor it must be BB and merging 448 // is always safe 449 if (Succ->getSinglePredecessor()) return true; 450 451 // Make a list of the predecessors of BB 452 typedef SmallPtrSet<BasicBlock*, 16> BlockSet; 453 BlockSet BBPreds(pred_begin(BB), pred_end(BB)); 454 455 // Use that list to make another list of common predecessors of BB and Succ 456 BlockSet CommonPreds; 457 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); 458 PI != PE; ++PI) { 459 BasicBlock *P = *PI; 460 if (BBPreds.count(P)) 461 CommonPreds.insert(P); 462 } 463 464 // Shortcut, if there are no common predecessors, merging is always safe 465 if (CommonPreds.empty()) 466 return true; 467 468 // Look at all the phi nodes in Succ, to see if they present a conflict when 469 // merging these blocks 470 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 471 PHINode *PN = cast<PHINode>(I); 472 473 // If the incoming value from BB is again a PHINode in 474 // BB which has the same incoming value for *PI as PN does, we can 475 // merge the phi nodes and then the blocks can still be merged 476 PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB)); 477 if (BBPN && BBPN->getParent() == BB) { 478 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end(); 479 PI != PE; PI++) { 480 if (BBPN->getIncomingValueForBlock(*PI) 481 != PN->getIncomingValueForBlock(*PI)) { 482 DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in " 483 << Succ->getName() << " is conflicting with " 484 << BBPN->getName() << " with regard to common predecessor " 485 << (*PI)->getName() << "\n"); 486 return false; 487 } 488 } 489 } else { 490 Value* Val = PN->getIncomingValueForBlock(BB); 491 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end(); 492 PI != PE; PI++) { 493 // See if the incoming value for the common predecessor is equal to the 494 // one for BB, in which case this phi node will not prevent the merging 495 // of the block. 496 if (Val != PN->getIncomingValueForBlock(*PI)) { 497 DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in " 498 << Succ->getName() << " is conflicting with regard to common " 499 << "predecessor " << (*PI)->getName() << "\n"); 500 return false; 501 } 502 } 503 } 504 } 505 506 return true; 507} 508 509/// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an 510/// unconditional branch, and contains no instructions other than PHI nodes, 511/// potential debug intrinsics and the branch. If possible, eliminate BB by 512/// rewriting all the predecessors to branch to the successor block and return 513/// true. If we can't transform, return false. 514bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) { 515 assert(BB != &BB->getParent()->getEntryBlock() && 516 "TryToSimplifyUncondBranchFromEmptyBlock called on entry block!"); 517 518 // We can't eliminate infinite loops. 519 BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0); 520 if (BB == Succ) return false; 521 522 // Check to see if merging these blocks would cause conflicts for any of the 523 // phi nodes in BB or Succ. If not, we can safely merge. 524 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false; 525 526 // Check for cases where Succ has multiple predecessors and a PHI node in BB 527 // has uses which will not disappear when the PHI nodes are merged. It is 528 // possible to handle such cases, but difficult: it requires checking whether 529 // BB dominates Succ, which is non-trivial to calculate in the case where 530 // Succ has multiple predecessors. Also, it requires checking whether 531 // constructing the necessary self-referential PHI node doesn't intoduce any 532 // conflicts; this isn't too difficult, but the previous code for doing this 533 // was incorrect. 534 // 535 // Note that if this check finds a live use, BB dominates Succ, so BB is 536 // something like a loop pre-header (or rarely, a part of an irreducible CFG); 537 // folding the branch isn't profitable in that case anyway. 538 if (!Succ->getSinglePredecessor()) { 539 BasicBlock::iterator BBI = BB->begin(); 540 while (isa<PHINode>(*BBI)) { 541 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end(); 542 UI != E; ++UI) { 543 if (PHINode* PN = dyn_cast<PHINode>(*UI)) { 544 if (PN->getIncomingBlock(UI) != BB) 545 return false; 546 } else { 547 return false; 548 } 549 } 550 ++BBI; 551 } 552 } 553 554 DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB); 555 556 if (isa<PHINode>(Succ->begin())) { 557 // If there is more than one pred of succ, and there are PHI nodes in 558 // the successor, then we need to add incoming edges for the PHI nodes 559 // 560 const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB)); 561 562 // Loop over all of the PHI nodes in the successor of BB. 563 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 564 PHINode *PN = cast<PHINode>(I); 565 Value *OldVal = PN->removeIncomingValue(BB, false); 566 assert(OldVal && "No entry in PHI for Pred BB!"); 567 568 // If this incoming value is one of the PHI nodes in BB, the new entries 569 // in the PHI node are the entries from the old PHI. 570 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) { 571 PHINode *OldValPN = cast<PHINode>(OldVal); 572 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) 573 // Note that, since we are merging phi nodes and BB and Succ might 574 // have common predecessors, we could end up with a phi node with 575 // identical incoming branches. This will be cleaned up later (and 576 // will trigger asserts if we try to clean it up now, without also 577 // simplifying the corresponding conditional branch). 578 PN->addIncoming(OldValPN->getIncomingValue(i), 579 OldValPN->getIncomingBlock(i)); 580 } else { 581 // Add an incoming value for each of the new incoming values. 582 for (unsigned i = 0, e = BBPreds.size(); i != e; ++i) 583 PN->addIncoming(OldVal, BBPreds[i]); 584 } 585 } 586 } 587 588 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 589 if (Succ->getSinglePredecessor()) { 590 // BB is the only predecessor of Succ, so Succ will end up with exactly 591 // the same predecessors BB had. 592 Succ->getInstList().splice(Succ->begin(), 593 BB->getInstList(), BB->begin()); 594 } else { 595 // We explicitly check for such uses in CanPropagatePredecessorsForPHIs. 596 assert(PN->use_empty() && "There shouldn't be any uses here!"); 597 PN->eraseFromParent(); 598 } 599 } 600 601 // Everything that jumped to BB now goes to Succ. 602 BB->replaceAllUsesWith(Succ); 603 if (!Succ->hasName()) Succ->takeName(BB); 604 BB->eraseFromParent(); // Delete the old basic block. 605 return true; 606} 607 608/// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI 609/// nodes in this block. This doesn't try to be clever about PHI nodes 610/// which differ only in the order of the incoming values, but instcombine 611/// orders them so it usually won't matter. 612/// 613bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) { 614 bool Changed = false; 615 616 // This implementation doesn't currently consider undef operands 617 // specially. Theroetically, two phis which are identical except for 618 // one having an undef where the other doesn't could be collapsed. 619 620 // Map from PHI hash values to PHI nodes. If multiple PHIs have 621 // the same hash value, the element is the first PHI in the 622 // linked list in CollisionMap. 623 DenseMap<uintptr_t, PHINode *> HashMap; 624 625 // Maintain linked lists of PHI nodes with common hash values. 626 DenseMap<PHINode *, PHINode *> CollisionMap; 627 628 // Examine each PHI. 629 for (BasicBlock::iterator I = BB->begin(); 630 PHINode *PN = dyn_cast<PHINode>(I++); ) { 631 // Compute a hash value on the operands. Instcombine will likely have sorted 632 // them, which helps expose duplicates, but we have to check all the 633 // operands to be safe in case instcombine hasn't run. 634 uintptr_t Hash = 0; 635 for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) { 636 // This hash algorithm is quite weak as hash functions go, but it seems 637 // to do a good enough job for this particular purpose, and is very quick. 638 Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I)); 639 Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7)); 640 } 641 // If we've never seen this hash value before, it's a unique PHI. 642 std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair = 643 HashMap.insert(std::make_pair(Hash, PN)); 644 if (Pair.second) continue; 645 // Otherwise it's either a duplicate or a hash collision. 646 for (PHINode *OtherPN = Pair.first->second; ; ) { 647 if (OtherPN->isIdenticalTo(PN)) { 648 // A duplicate. Replace this PHI with its duplicate. 649 PN->replaceAllUsesWith(OtherPN); 650 PN->eraseFromParent(); 651 Changed = true; 652 break; 653 } 654 // A non-duplicate hash collision. 655 DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN); 656 if (I == CollisionMap.end()) { 657 // Set this PHI to be the head of the linked list of colliding PHIs. 658 PHINode *Old = Pair.first->second; 659 Pair.first->second = PN; 660 CollisionMap[PN] = Old; 661 break; 662 } 663 // Procede to the next PHI in the list. 664 OtherPN = I->second; 665 } 666 } 667 668 return Changed; 669} 670 671/// enforceKnownAlignment - If the specified pointer points to an object that 672/// we control, modify the object's alignment to PrefAlign. This isn't 673/// often possible though. If alignment is important, a more reliable approach 674/// is to simply align all global variables and allocation instructions to 675/// their preferred alignment from the beginning. 676/// 677static unsigned enforceKnownAlignment(Value *V, unsigned Align, 678 unsigned PrefAlign) { 679 680 User *U = dyn_cast<User>(V); 681 if (!U) return Align; 682 683 switch (Operator::getOpcode(U)) { 684 default: break; 685 case Instruction::BitCast: 686 return enforceKnownAlignment(U->getOperand(0), Align, PrefAlign); 687 case Instruction::GetElementPtr: { 688 // If all indexes are zero, it is just the alignment of the base pointer. 689 bool AllZeroOperands = true; 690 for (User::op_iterator i = U->op_begin() + 1, e = U->op_end(); i != e; ++i) 691 if (!isa<Constant>(*i) || 692 !cast<Constant>(*i)->isNullValue()) { 693 AllZeroOperands = false; 694 break; 695 } 696 697 if (AllZeroOperands) { 698 // Treat this like a bitcast. 699 return enforceKnownAlignment(U->getOperand(0), Align, PrefAlign); 700 } 701 return Align; 702 } 703 case Instruction::Alloca: { 704 AllocaInst *AI = cast<AllocaInst>(V); 705 // If there is a requested alignment and if this is an alloca, round up. 706 if (AI->getAlignment() >= PrefAlign) 707 return AI->getAlignment(); 708 AI->setAlignment(PrefAlign); 709 return PrefAlign; 710 } 711 } 712 713 if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) { 714 // If there is a large requested alignment and we can, bump up the alignment 715 // of the global. 716 if (GV->isDeclaration()) return Align; 717 718 if (GV->getAlignment() >= PrefAlign) 719 return GV->getAlignment(); 720 // We can only increase the alignment of the global if it has no alignment 721 // specified or if it is not assigned a section. If it is assigned a 722 // section, the global could be densely packed with other objects in the 723 // section, increasing the alignment could cause padding issues. 724 if (!GV->hasSection() || GV->getAlignment() == 0) 725 GV->setAlignment(PrefAlign); 726 return GV->getAlignment(); 727 } 728 729 return Align; 730} 731 732/// getOrEnforceKnownAlignment - If the specified pointer has an alignment that 733/// we can determine, return it, otherwise return 0. If PrefAlign is specified, 734/// and it is more than the alignment of the ultimate object, see if we can 735/// increase the alignment of the ultimate object, making this check succeed. 736unsigned llvm::getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign, 737 const TargetData *TD) { 738 assert(V->getType()->isPointerTy() && 739 "getOrEnforceKnownAlignment expects a pointer!"); 740 unsigned BitWidth = TD ? TD->getPointerSizeInBits() : 64; 741 APInt Mask = APInt::getAllOnesValue(BitWidth); 742 APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0); 743 ComputeMaskedBits(V, Mask, KnownZero, KnownOne, TD); 744 unsigned TrailZ = KnownZero.countTrailingOnes(); 745 746 // Avoid trouble with rediculously large TrailZ values, such as 747 // those computed from a null pointer. 748 TrailZ = std::min(TrailZ, unsigned(sizeof(unsigned) * CHAR_BIT - 1)); 749 750 unsigned Align = 1u << std::min(BitWidth - 1, TrailZ); 751 752 // LLVM doesn't support alignments larger than this currently. 753 Align = std::min(Align, +Value::MaximumAlignment); 754 755 if (PrefAlign > Align) 756 Align = enforceKnownAlignment(V, Align, PrefAlign); 757 758 // We don't need to make any adjustment. 759 return Align; 760} 761 762