Local.cpp revision ae47be1ea023e4b1e6bbbdc4687333eea54c84c8
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/Analysis/ValueTracking.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/ValueHandle.h" 35#include "llvm/Support/raw_ostream.h" 36using namespace llvm; 37 38//===----------------------------------------------------------------------===// 39// Local constant propagation. 40// 41 42// ConstantFoldTerminator - If a terminator instruction is predicated on a 43// constant value, convert it into an unconditional branch to the constant 44// destination. 45// 46bool llvm::ConstantFoldTerminator(BasicBlock *BB) { 47 TerminatorInst *T = BB->getTerminator(); 48 49 // Branch - See if we are conditional jumping on constant 50 if (BranchInst *BI = dyn_cast<BranchInst>(T)) { 51 if (BI->isUnconditional()) return false; // Can't optimize uncond branch 52 BasicBlock *Dest1 = BI->getSuccessor(0); 53 BasicBlock *Dest2 = BI->getSuccessor(1); 54 55 if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) { 56 // Are we branching on constant? 57 // YES. Change to unconditional branch... 58 BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2; 59 BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1; 60 61 //cerr << "Function: " << T->getParent()->getParent() 62 // << "\nRemoving branch from " << T->getParent() 63 // << "\n\nTo: " << OldDest << endl; 64 65 // Let the basic block know that we are letting go of it. Based on this, 66 // it will adjust it's PHI nodes. 67 assert(BI->getParent() && "Terminator not inserted in block!"); 68 OldDest->removePredecessor(BI->getParent()); 69 70 // Set the unconditional destination, and change the insn to be an 71 // unconditional branch. 72 BI->setUnconditionalDest(Destination); 73 return true; 74 } 75 76 if (Dest2 == Dest1) { // Conditional branch to same location? 77 // This branch matches something like this: 78 // br bool %cond, label %Dest, label %Dest 79 // and changes it into: br label %Dest 80 81 // Let the basic block know that we are letting go of one copy of it. 82 assert(BI->getParent() && "Terminator not inserted in block!"); 83 Dest1->removePredecessor(BI->getParent()); 84 85 // Change a conditional branch to unconditional. 86 BI->setUnconditionalDest(Dest1); 87 return true; 88 } 89 return false; 90 } 91 92 if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) { 93 // If we are switching on a constant, we can convert the switch into a 94 // single branch instruction! 95 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition()); 96 BasicBlock *TheOnlyDest = SI->getSuccessor(0); // The default dest 97 BasicBlock *DefaultDest = TheOnlyDest; 98 assert(TheOnlyDest == SI->getDefaultDest() && 99 "Default destination is not successor #0?"); 100 101 // Figure out which case it goes to. 102 for (unsigned i = 1, e = SI->getNumSuccessors(); i != e; ++i) { 103 // Found case matching a constant operand? 104 if (SI->getSuccessorValue(i) == CI) { 105 TheOnlyDest = SI->getSuccessor(i); 106 break; 107 } 108 109 // Check to see if this branch is going to the same place as the default 110 // dest. If so, eliminate it as an explicit compare. 111 if (SI->getSuccessor(i) == DefaultDest) { 112 // Remove this entry. 113 DefaultDest->removePredecessor(SI->getParent()); 114 SI->removeCase(i); 115 --i; --e; // Don't skip an entry... 116 continue; 117 } 118 119 // Otherwise, check to see if the switch only branches to one destination. 120 // We do this by reseting "TheOnlyDest" to null when we find two non-equal 121 // destinations. 122 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0; 123 } 124 125 if (CI && !TheOnlyDest) { 126 // Branching on a constant, but not any of the cases, go to the default 127 // successor. 128 TheOnlyDest = SI->getDefaultDest(); 129 } 130 131 // If we found a single destination that we can fold the switch into, do so 132 // now. 133 if (TheOnlyDest) { 134 // Insert the new branch. 135 BranchInst::Create(TheOnlyDest, SI); 136 BasicBlock *BB = SI->getParent(); 137 138 // Remove entries from PHI nodes which we no longer branch to... 139 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) { 140 // Found case matching a constant operand? 141 BasicBlock *Succ = SI->getSuccessor(i); 142 if (Succ == TheOnlyDest) 143 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest 144 else 145 Succ->removePredecessor(BB); 146 } 147 148 // Delete the old switch. 149 BB->getInstList().erase(SI); 150 return true; 151 } 152 153 if (SI->getNumSuccessors() == 2) { 154 // Otherwise, we can fold this switch into a conditional branch 155 // instruction if it has only one non-default destination. 156 Value *Cond = new ICmpInst(SI, ICmpInst::ICMP_EQ, SI->getCondition(), 157 SI->getSuccessorValue(1), "cond"); 158 // Insert the new branch. 159 BranchInst::Create(SI->getSuccessor(1), SI->getSuccessor(0), Cond, SI); 160 161 // Delete the old switch. 162 SI->eraseFromParent(); 163 return true; 164 } 165 return false; 166 } 167 168 if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(T)) { 169 // indirectbr blockaddress(@F, @BB) -> br label @BB 170 if (BlockAddress *BA = 171 dyn_cast<BlockAddress>(IBI->getAddress()->stripPointerCasts())) { 172 BasicBlock *TheOnlyDest = BA->getBasicBlock(); 173 // Insert the new branch. 174 BranchInst::Create(TheOnlyDest, IBI); 175 176 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) { 177 if (IBI->getDestination(i) == TheOnlyDest) 178 TheOnlyDest = 0; 179 else 180 IBI->getDestination(i)->removePredecessor(IBI->getParent()); 181 } 182 IBI->eraseFromParent(); 183 184 // If we didn't find our destination in the IBI successor list, then we 185 // have undefined behavior. Replace the unconditional branch with an 186 // 'unreachable' instruction. 187 if (TheOnlyDest) { 188 BB->getTerminator()->eraseFromParent(); 189 new UnreachableInst(BB->getContext(), BB); 190 } 191 192 return true; 193 } 194 } 195 196 return false; 197} 198 199 200//===----------------------------------------------------------------------===// 201// Local dead code elimination. 202// 203 204/// isInstructionTriviallyDead - Return true if the result produced by the 205/// instruction is not used, and the instruction has no side effects. 206/// 207bool llvm::isInstructionTriviallyDead(Instruction *I) { 208 if (!I->use_empty() || isa<TerminatorInst>(I)) return false; 209 210 // We don't want debug info removed by anything this general. 211 if (isa<DbgInfoIntrinsic>(I)) return false; 212 213 if (!I->mayHaveSideEffects()) return true; 214 215 // Special case intrinsics that "may have side effects" but can be deleted 216 // when dead. 217 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) 218 // Safe to delete llvm.stacksave if dead. 219 if (II->getIntrinsicID() == Intrinsic::stacksave) 220 return true; 221 return false; 222} 223 224/// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a 225/// trivially dead instruction, delete it. If that makes any of its operands 226/// trivially dead, delete them too, recursively. Return true if any 227/// instructions were deleted. 228bool llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) { 229 Instruction *I = dyn_cast<Instruction>(V); 230 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I)) 231 return false; 232 233 SmallVector<Instruction*, 16> DeadInsts; 234 DeadInsts.push_back(I); 235 236 do { 237 I = DeadInsts.pop_back_val(); 238 239 // Null out all of the instruction's operands to see if any operand becomes 240 // dead as we go. 241 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { 242 Value *OpV = I->getOperand(i); 243 I->setOperand(i, 0); 244 245 if (!OpV->use_empty()) continue; 246 247 // If the operand is an instruction that became dead as we nulled out the 248 // operand, and if it is 'trivially' dead, delete it in a future loop 249 // iteration. 250 if (Instruction *OpI = dyn_cast<Instruction>(OpV)) 251 if (isInstructionTriviallyDead(OpI)) 252 DeadInsts.push_back(OpI); 253 } 254 255 I->eraseFromParent(); 256 } while (!DeadInsts.empty()); 257 258 return true; 259} 260 261/// RecursivelyDeleteDeadPHINode - If the specified value is an effectively 262/// dead PHI node, due to being a def-use chain of single-use nodes that 263/// either forms a cycle or is terminated by a trivially dead instruction, 264/// delete it. If that makes any of its operands trivially dead, delete them 265/// too, recursively. Return true if the PHI node is actually deleted. 266bool 267llvm::RecursivelyDeleteDeadPHINode(PHINode *PN) { 268 // We can remove a PHI if it is on a cycle in the def-use graph 269 // where each node in the cycle has degree one, i.e. only one use, 270 // and is an instruction with no side effects. 271 if (!PN->hasOneUse()) 272 return false; 273 274 bool Changed = false; 275 SmallPtrSet<PHINode *, 4> PHIs; 276 PHIs.insert(PN); 277 for (Instruction *J = cast<Instruction>(*PN->use_begin()); 278 J->hasOneUse() && !J->mayHaveSideEffects(); 279 J = cast<Instruction>(*J->use_begin())) 280 // If we find a PHI more than once, we're on a cycle that 281 // won't prove fruitful. 282 if (PHINode *JP = dyn_cast<PHINode>(J)) 283 if (!PHIs.insert(cast<PHINode>(JP))) { 284 // Break the cycle and delete the PHI and its operands. 285 JP->replaceAllUsesWith(UndefValue::get(JP->getType())); 286 (void)RecursivelyDeleteTriviallyDeadInstructions(JP); 287 Changed = true; 288 break; 289 } 290 return Changed; 291} 292 293/// SimplifyInstructionsInBlock - Scan the specified basic block and try to 294/// simplify any instructions in it and recursively delete dead instructions. 295/// 296/// This returns true if it changed the code, note that it can delete 297/// instructions in other blocks as well in this block. 298bool llvm::SimplifyInstructionsInBlock(BasicBlock *BB, const TargetData *TD) { 299 bool MadeChange = false; 300 for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) { 301 Instruction *Inst = BI++; 302 303 if (Value *V = SimplifyInstruction(Inst, TD)) { 304 WeakVH BIHandle(BI); 305 ReplaceAndSimplifyAllUses(Inst, V, TD); 306 MadeChange = true; 307 if (BIHandle != BI) 308 BI = BB->begin(); 309 continue; 310 } 311 312 MadeChange |= RecursivelyDeleteTriviallyDeadInstructions(Inst); 313 } 314 return MadeChange; 315} 316 317//===----------------------------------------------------------------------===// 318// Control Flow Graph Restructuring. 319// 320 321 322/// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this 323/// method is called when we're about to delete Pred as a predecessor of BB. If 324/// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred. 325/// 326/// Unlike the removePredecessor method, this attempts to simplify uses of PHI 327/// nodes that collapse into identity values. For example, if we have: 328/// x = phi(1, 0, 0, 0) 329/// y = and x, z 330/// 331/// .. and delete the predecessor corresponding to the '1', this will attempt to 332/// recursively fold the and to 0. 333void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred, 334 TargetData *TD) { 335 // This only adjusts blocks with PHI nodes. 336 if (!isa<PHINode>(BB->begin())) 337 return; 338 339 // Remove the entries for Pred from the PHI nodes in BB, but do not simplify 340 // them down. This will leave us with single entry phi nodes and other phis 341 // that can be removed. 342 BB->removePredecessor(Pred, true); 343 344 WeakVH PhiIt = &BB->front(); 345 while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) { 346 PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt)); 347 348 Value *PNV = SimplifyInstruction(PN, TD); 349 if (PNV == 0) continue; 350 351 // If we're able to simplify the phi to a single value, substitute the new 352 // value into all of its uses. 353 assert(PNV != PN && "SimplifyInstruction broken!"); 354 355 Value *OldPhiIt = PhiIt; 356 ReplaceAndSimplifyAllUses(PN, PNV, TD); 357 358 // If recursive simplification ended up deleting the next PHI node we would 359 // iterate to, then our iterator is invalid, restart scanning from the top 360 // of the block. 361 if (PhiIt != OldPhiIt) PhiIt = &BB->front(); 362 } 363} 364 365 366/// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its 367/// predecessor is known to have one successor (DestBB!). Eliminate the edge 368/// between them, moving the instructions in the predecessor into DestBB and 369/// deleting the predecessor block. 370/// 371void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) { 372 // If BB has single-entry PHI nodes, fold them. 373 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) { 374 Value *NewVal = PN->getIncomingValue(0); 375 // Replace self referencing PHI with undef, it must be dead. 376 if (NewVal == PN) NewVal = UndefValue::get(PN->getType()); 377 PN->replaceAllUsesWith(NewVal); 378 PN->eraseFromParent(); 379 } 380 381 BasicBlock *PredBB = DestBB->getSinglePredecessor(); 382 assert(PredBB && "Block doesn't have a single predecessor!"); 383 384 // Splice all the instructions from PredBB to DestBB. 385 PredBB->getTerminator()->eraseFromParent(); 386 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList()); 387 388 // Zap anything that took the address of DestBB. Not doing this will give the 389 // address an invalid value. 390 if (DestBB->hasAddressTaken()) { 391 BlockAddress *BA = BlockAddress::get(DestBB); 392 Constant *Replacement = 393 ConstantInt::get(llvm::Type::getInt32Ty(BA->getContext()), 1); 394 BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(Replacement, 395 BA->getType())); 396 BA->destroyConstant(); 397 } 398 399 // Anything that branched to PredBB now branches to DestBB. 400 PredBB->replaceAllUsesWith(DestBB); 401 402 if (P) { 403 ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>(); 404 if (PI) { 405 PI->replaceAllUses(PredBB, DestBB); 406 PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB)); 407 } 408 } 409 // Nuke BB. 410 PredBB->eraseFromParent(); 411} 412 413/// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an 414/// almost-empty BB ending in an unconditional branch to Succ, into succ. 415/// 416/// Assumption: Succ is the single successor for BB. 417/// 418static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { 419 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!"); 420 421 DEBUG(dbgs() << "Looking to fold " << BB->getName() << " into " 422 << Succ->getName() << "\n"); 423 // Shortcut, if there is only a single predecessor it must be BB and merging 424 // is always safe 425 if (Succ->getSinglePredecessor()) return true; 426 427 // Make a list of the predecessors of BB 428 typedef SmallPtrSet<BasicBlock*, 16> BlockSet; 429 BlockSet BBPreds(pred_begin(BB), pred_end(BB)); 430 431 // Use that list to make another list of common predecessors of BB and Succ 432 BlockSet CommonPreds; 433 for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); 434 PI != PE; ++PI) { 435 BasicBlock *P = *PI; 436 if (BBPreds.count(P)) 437 CommonPreds.insert(P); 438 } 439 440 // Shortcut, if there are no common predecessors, merging is always safe 441 if (CommonPreds.empty()) 442 return true; 443 444 // Look at all the phi nodes in Succ, to see if they present a conflict when 445 // merging these blocks 446 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 447 PHINode *PN = cast<PHINode>(I); 448 449 // If the incoming value from BB is again a PHINode in 450 // BB which has the same incoming value for *PI as PN does, we can 451 // merge the phi nodes and then the blocks can still be merged 452 PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB)); 453 if (BBPN && BBPN->getParent() == BB) { 454 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end(); 455 PI != PE; PI++) { 456 if (BBPN->getIncomingValueForBlock(*PI) 457 != PN->getIncomingValueForBlock(*PI)) { 458 DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in " 459 << Succ->getName() << " is conflicting with " 460 << BBPN->getName() << " with regard to common predecessor " 461 << (*PI)->getName() << "\n"); 462 return false; 463 } 464 } 465 } else { 466 Value* Val = PN->getIncomingValueForBlock(BB); 467 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end(); 468 PI != PE; PI++) { 469 // See if the incoming value for the common predecessor is equal to the 470 // one for BB, in which case this phi node will not prevent the merging 471 // of the block. 472 if (Val != PN->getIncomingValueForBlock(*PI)) { 473 DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in " 474 << Succ->getName() << " is conflicting with regard to common " 475 << "predecessor " << (*PI)->getName() << "\n"); 476 return false; 477 } 478 } 479 } 480 } 481 482 return true; 483} 484 485/// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an 486/// unconditional branch, and contains no instructions other than PHI nodes, 487/// potential debug intrinsics and the branch. If possible, eliminate BB by 488/// rewriting all the predecessors to branch to the successor block and return 489/// true. If we can't transform, return false. 490bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) { 491 assert(BB != &BB->getParent()->getEntryBlock() && 492 "TryToSimplifyUncondBranchFromEmptyBlock called on entry block!"); 493 494 // We can't eliminate infinite loops. 495 BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0); 496 if (BB == Succ) return false; 497 498 // Check to see if merging these blocks would cause conflicts for any of the 499 // phi nodes in BB or Succ. If not, we can safely merge. 500 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false; 501 502 // Check for cases where Succ has multiple predecessors and a PHI node in BB 503 // has uses which will not disappear when the PHI nodes are merged. It is 504 // possible to handle such cases, but difficult: it requires checking whether 505 // BB dominates Succ, which is non-trivial to calculate in the case where 506 // Succ has multiple predecessors. Also, it requires checking whether 507 // constructing the necessary self-referential PHI node doesn't intoduce any 508 // conflicts; this isn't too difficult, but the previous code for doing this 509 // was incorrect. 510 // 511 // Note that if this check finds a live use, BB dominates Succ, so BB is 512 // something like a loop pre-header (or rarely, a part of an irreducible CFG); 513 // folding the branch isn't profitable in that case anyway. 514 if (!Succ->getSinglePredecessor()) { 515 BasicBlock::iterator BBI = BB->begin(); 516 while (isa<PHINode>(*BBI)) { 517 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end(); 518 UI != E; ++UI) { 519 if (PHINode* PN = dyn_cast<PHINode>(*UI)) { 520 if (PN->getIncomingBlock(UI) != BB) 521 return false; 522 } else { 523 return false; 524 } 525 } 526 ++BBI; 527 } 528 } 529 530 DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB); 531 532 if (isa<PHINode>(Succ->begin())) { 533 // If there is more than one pred of succ, and there are PHI nodes in 534 // the successor, then we need to add incoming edges for the PHI nodes 535 // 536 const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB)); 537 538 // Loop over all of the PHI nodes in the successor of BB. 539 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 540 PHINode *PN = cast<PHINode>(I); 541 Value *OldVal = PN->removeIncomingValue(BB, false); 542 assert(OldVal && "No entry in PHI for Pred BB!"); 543 544 // If this incoming value is one of the PHI nodes in BB, the new entries 545 // in the PHI node are the entries from the old PHI. 546 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) { 547 PHINode *OldValPN = cast<PHINode>(OldVal); 548 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) 549 // Note that, since we are merging phi nodes and BB and Succ might 550 // have common predecessors, we could end up with a phi node with 551 // identical incoming branches. This will be cleaned up later (and 552 // will trigger asserts if we try to clean it up now, without also 553 // simplifying the corresponding conditional branch). 554 PN->addIncoming(OldValPN->getIncomingValue(i), 555 OldValPN->getIncomingBlock(i)); 556 } else { 557 // Add an incoming value for each of the new incoming values. 558 for (unsigned i = 0, e = BBPreds.size(); i != e; ++i) 559 PN->addIncoming(OldVal, BBPreds[i]); 560 } 561 } 562 } 563 564 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) { 565 if (Succ->getSinglePredecessor()) { 566 // BB is the only predecessor of Succ, so Succ will end up with exactly 567 // the same predecessors BB had. 568 Succ->getInstList().splice(Succ->begin(), 569 BB->getInstList(), BB->begin()); 570 } else { 571 // We explicitly check for such uses in CanPropagatePredecessorsForPHIs. 572 assert(PN->use_empty() && "There shouldn't be any uses here!"); 573 PN->eraseFromParent(); 574 } 575 } 576 577 // Everything that jumped to BB now goes to Succ. 578 BB->replaceAllUsesWith(Succ); 579 if (!Succ->hasName()) Succ->takeName(BB); 580 BB->eraseFromParent(); // Delete the old basic block. 581 return true; 582} 583 584/// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI 585/// nodes in this block. This doesn't try to be clever about PHI nodes 586/// which differ only in the order of the incoming values, but instcombine 587/// orders them so it usually won't matter. 588/// 589bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) { 590 bool Changed = false; 591 592 // This implementation doesn't currently consider undef operands 593 // specially. Theroetically, two phis which are identical except for 594 // one having an undef where the other doesn't could be collapsed. 595 596 // Map from PHI hash values to PHI nodes. If multiple PHIs have 597 // the same hash value, the element is the first PHI in the 598 // linked list in CollisionMap. 599 DenseMap<uintptr_t, PHINode *> HashMap; 600 601 // Maintain linked lists of PHI nodes with common hash values. 602 DenseMap<PHINode *, PHINode *> CollisionMap; 603 604 // Examine each PHI. 605 for (BasicBlock::iterator I = BB->begin(); 606 PHINode *PN = dyn_cast<PHINode>(I++); ) { 607 // Compute a hash value on the operands. Instcombine will likely have sorted 608 // them, which helps expose duplicates, but we have to check all the 609 // operands to be safe in case instcombine hasn't run. 610 uintptr_t Hash = 0; 611 for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) { 612 // This hash algorithm is quite weak as hash functions go, but it seems 613 // to do a good enough job for this particular purpose, and is very quick. 614 Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I)); 615 Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7)); 616 } 617 // If we've never seen this hash value before, it's a unique PHI. 618 std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair = 619 HashMap.insert(std::make_pair(Hash, PN)); 620 if (Pair.second) continue; 621 // Otherwise it's either a duplicate or a hash collision. 622 for (PHINode *OtherPN = Pair.first->second; ; ) { 623 if (OtherPN->isIdenticalTo(PN)) { 624 // A duplicate. Replace this PHI with its duplicate. 625 PN->replaceAllUsesWith(OtherPN); 626 PN->eraseFromParent(); 627 Changed = true; 628 break; 629 } 630 // A non-duplicate hash collision. 631 DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN); 632 if (I == CollisionMap.end()) { 633 // Set this PHI to be the head of the linked list of colliding PHIs. 634 PHINode *Old = Pair.first->second; 635 Pair.first->second = PN; 636 CollisionMap[PN] = Old; 637 break; 638 } 639 // Procede to the next PHI in the list. 640 OtherPN = I->second; 641 } 642 } 643 644 return Changed; 645} 646 647/// enforceKnownAlignment - If the specified pointer points to an object that 648/// we control, modify the object's alignment to PrefAlign. This isn't 649/// often possible though. If alignment is important, a more reliable approach 650/// is to simply align all global variables and allocation instructions to 651/// their preferred alignment from the beginning. 652/// 653unsigned enforceKnownAlignment(Value *V, unsigned Align, unsigned PrefAlign) { 654 655 User *U = dyn_cast<User>(V); 656 if (!U) return Align; 657 658 switch (Operator::getOpcode(U)) { 659 default: break; 660 case Instruction::BitCast: 661 return enforceKnownAlignment(U->getOperand(0), Align, PrefAlign); 662 case Instruction::GetElementPtr: { 663 // If all indexes are zero, it is just the alignment of the base pointer. 664 bool AllZeroOperands = true; 665 for (User::op_iterator i = U->op_begin() + 1, e = U->op_end(); i != e; ++i) 666 if (!isa<Constant>(*i) || 667 !cast<Constant>(*i)->isNullValue()) { 668 AllZeroOperands = false; 669 break; 670 } 671 672 if (AllZeroOperands) { 673 // Treat this like a bitcast. 674 return enforceKnownAlignment(U->getOperand(0), Align, PrefAlign); 675 } 676 return Align; 677 } 678 case Instruction::Alloca: { 679 AllocaInst *AI = cast<AllocaInst>(V); 680 // If there is a requested alignment and if this is an alloca, round up. 681 if (AI->getAlignment() >= PrefAlign) 682 return AI->getAlignment(); 683 AI->setAlignment(PrefAlign); 684 return PrefAlign; 685 } 686 } 687 688 if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) { 689 // If there is a large requested alignment and we can, bump up the alignment 690 // of the global. 691 if (GV->isDeclaration()) return Align; 692 693 if (GV->getAlignment() >= PrefAlign) 694 return GV->getAlignment(); 695 // We can only increase the alignment of the global if it has no alignment 696 // specified or if it is not assigned a section. If it is assigned a 697 // section, the global could be densely packed with other objects in the 698 // section, increasing the alignment could cause padding issues. 699 if (!GV->hasSection() || GV->getAlignment() == 0) 700 GV->setAlignment(PrefAlign); 701 return GV->getAlignment(); 702 } 703 704 return Align; 705} 706 707/// getOrEnforceKnownAlignment - If the specified pointer has an alignment that 708/// we can determine, return it, otherwise return 0. If PrefAlign is specified, 709/// and it is more than the alignment of the ultimate object, see if we can 710/// increase the alignment of the ultimate object, making this check succeed. 711unsigned llvm::getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign, 712 const TargetData *TD) { 713 assert(V->getType()->isPointerTy() && 714 "getOrEnforceKnownAlignment expects a pointer!"); 715 unsigned BitWidth = TD ? TD->getPointerSizeInBits() : 64; 716 APInt Mask = APInt::getAllOnesValue(BitWidth); 717 APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0); 718 ComputeMaskedBits(V, Mask, KnownZero, KnownOne, TD); 719 unsigned TrailZ = KnownZero.countTrailingOnes(); 720 721 // Avoid trouble with rediculously large TrailZ values, such as 722 // those computed from a null pointer. 723 TrailZ = std::min(TrailZ, unsigned(sizeof(unsigned) * CHAR_BIT - 1)); 724 725 unsigned Align = 1u << std::min(BitWidth - 1, TrailZ); 726 727 // LLVM doesn't support alignments larger than this currently. 728 Align = std::min(Align, +Value::MaximumAlignment); 729 730 if (PrefAlign > Align) 731 Align = enforceKnownAlignment(V, Align, PrefAlign); 732 733 // We don't need to make any adjustment. 734 return Align; 735} 736 737