1 //===-- Local.cpp - Functions to perform local transformations ------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This family of functions perform various local transformations to the
13 //===----------------------------------------------------------------------===//
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"
37 //===----------------------------------------------------------------------===//
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.
45 bool 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;
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();
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();
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))
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;
80 //===----------------------------------------------------------------------===//
81 // Local constant propagation.
84 // ConstantFoldTerminator - If a terminator instruction is predicated on a
85 // constant value, convert it into an unconditional branch to the constant
88 bool llvm::ConstantFoldTerminator(BasicBlock *BB) {
89 TerminatorInst *T = BB->getTerminator();
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);
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;
103 //cerr << "Function: " << T->getParent()->getParent()
104 // << "\nRemoving branch from " << T->getParent()
105 // << "\n\nTo: " << OldDest << endl;
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());
112 // Set the unconditional destination, and change the insn to be an
113 // unconditional branch.
114 BI->setUnconditionalDest(Destination);
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
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());
127 // Change a conditional branch to unconditional.
128 BI->setUnconditionalDest(Dest1);
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?");
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);
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());
157 --i; --e; // Don't skip an entry...
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
164 if (SI->getSuccessor(i) != TheOnlyDest) TheOnlyDest = 0;
167 if (CI && !TheOnlyDest) {
168 // Branching on a constant, but not any of the cases, go to the default
170 TheOnlyDest = SI->getDefaultDest();
173 // If we found a single destination that we can fold the switch into, do so
176 // Insert the new branch.
177 BranchInst::Create(TheOnlyDest, SI);
178 BasicBlock *BB = SI->getParent();
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
187 Succ->removePredecessor(BB);
190 // Delete the old switch.
191 BB->getInstList().erase(SI);
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);
203 // Delete the old switch.
204 SI->eraseFromParent();
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);
218 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
219 if (IBI->getDestination(i) == TheOnlyDest)
222 IBI->getDestination(i)->removePredecessor(IBI->getParent());
224 IBI->eraseFromParent();
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.
230 BB->getTerminator()->eraseFromParent();
231 new UnreachableInst(BB->getContext(), BB);
242 //===----------------------------------------------------------------------===//
243 // Local dead code elimination.
246 /// isInstructionTriviallyDead - Return true if the result produced by the
247 /// instruction is not used, and the instruction has no side effects.
249 bool llvm::isInstructionTriviallyDead(Instruction *I) {
250 if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
252 // We don't want debug info removed by anything this general.
253 if (isa<DbgInfoIntrinsic>(I)) return false;
255 // Likewise for memory use markers.
256 if (isa<MemoryUseIntrinsic>(I)) return false;
258 if (!I->mayHaveSideEffects()) return true;
260 // Special case intrinsics that "may have side effects" but can be deleted
262 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
263 // Safe to delete llvm.stacksave if dead.
264 if (II->getIntrinsicID() == Intrinsic::stacksave)
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.
272 void llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V) {
273 Instruction *I = dyn_cast<Instruction>(V);
274 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I))
277 SmallVector<Instruction*, 16> DeadInsts;
278 DeadInsts.push_back(I);
280 while (!DeadInsts.empty()) {
281 I = DeadInsts.pop_back_val();
283 // Null out all of the instruction's operands to see if any operand becomes
285 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
286 Value *OpV = I->getOperand(i);
289 if (!OpV->use_empty()) continue;
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
294 if (Instruction *OpI = dyn_cast<Instruction>(OpV))
295 if (isInstructionTriviallyDead(OpI))
296 DeadInsts.push_back(OpI);
299 I->eraseFromParent();
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.
309 llvm::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())
316 SmallPtrSet<PHINode *, 4> PHIs;
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);
332 //===----------------------------------------------------------------------===//
333 // Control Flow Graph Restructuring.
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.
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)
346 /// .. and delete the predecessor corresponding to the '1', this will attempt to
347 /// recursively fold the and to 0.
348 void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred,
350 // This only adjusts blocks with PHI nodes.
351 if (!isa<PHINode>(BB->begin()))
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);
359 WeakVH PhiIt = &BB->front();
360 while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) {
361 PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt));
363 Value *PNV = PN->hasConstantValue();
364 if (PNV == 0) continue;
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");
370 ReplaceAndSimplifyAllUses(PN, PNV, TD);
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
375 if (PhiIt == 0) PhiIt = &BB->front();
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.
385 void 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();
395 BasicBlock *PredBB = DestBB->getSinglePredecessor();
396 assert(PredBB && "Block doesn't have a single predecessor!");
398 // Splice all the instructions from PredBB to DestBB.
399 PredBB->getTerminator()->eraseFromParent();
400 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
402 // Anything that branched to PredBB now branches to DestBB.
403 PredBB->replaceAllUsesWith(DestBB);
406 ProfileInfo *PI = P->getAnalysisIfAvailable<ProfileInfo>();
408 PI->replaceAllUses(PredBB, DestBB);
409 PI->removeEdge(ProfileInfo::getEdge(PredBB, DestBB));
413 PredBB->eraseFromParent();
416 /// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
417 /// almost-empty BB ending in an unconditional branch to Succ, into succ.
419 /// Assumption: Succ is the single successor for BB.
421 static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
422 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
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
428 if (Succ->getSinglePredecessor()) return true;
430 // Make a list of the predecessors of BB
431 typedef SmallPtrSet<BasicBlock*, 16> BlockSet;
432 BlockSet BBPreds(pred_begin(BB), pred_end(BB));
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);
438 if (BBPreds.count(*PI))
439 CommonPreds.insert(*PI);
441 // Shortcut, if there are no common predecessors, merging is always safe
442 if (CommonPreds.empty())
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);
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();
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");
467 Value* Val = PN->getIncomingValueForBlock(BB);
468 for (BlockSet::iterator PI = CommonPreds.begin(), PE = CommonPreds.end();
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
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");
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.
491 bool 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;
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;
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
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();
517 if (PHINode* PN = dyn_cast<PHINode>(*UI)) {
518 if (PN->getIncomingBlock(UI) != BB)
528 DEBUG(errs() << "Killing Trivial BB: \n" << *BB);
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
534 const SmallVector<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
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!");
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));
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]);
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());
569 // We explicitly check for such uses in CanPropagatePredecessorsForPHIs.
570 assert(PN->use_empty() && "There shouldn't be any uses here!");
571 PN->eraseFromParent();
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.
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.
587 bool llvm::OnlyUsedByDbgInfoIntrinsics(Instruction *I,
588 SmallVectorImpl<DbgInfoIntrinsic *> *DbgInUses) {
592 for (Value::use_iterator UI = I->use_begin(), UE = I->use_end(); UI != UE;
594 if (DbgInfoIntrinsic *DI = dyn_cast<DbgInfoIntrinsic>(*UI)) {
596 DbgInUses->push_back(DI);
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.
611 bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
612 bool Changed = false;
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.
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;
623 // Maintain linked lists of PHI nodes with common hash values.
624 DenseMap<PHINode *, PHINode *> CollisionMap;
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.
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));
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();
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;
661 // Procede to the next PHI in the list.