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/ADT/DenseMap.h"
17 #include "llvm/ADT/STLExtras.h"
18 #include "llvm/ADT/SmallPtrSet.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Analysis/InstructionSimplify.h"
21 #include "llvm/Analysis/MemoryBuiltins.h"
22 #include "llvm/Analysis/ValueTracking.h"
23 #include "llvm/DIBuilder.h"
24 #include "llvm/DebugInfo.h"
25 #include "llvm/IR/Constants.h"
26 #include "llvm/IR/DataLayout.h"
27 #include "llvm/IR/DerivedTypes.h"
28 #include "llvm/IR/Dominators.h"
29 #include "llvm/IR/GlobalAlias.h"
30 #include "llvm/IR/GlobalVariable.h"
31 #include "llvm/IR/IRBuilder.h"
32 #include "llvm/IR/Instructions.h"
33 #include "llvm/IR/IntrinsicInst.h"
34 #include "llvm/IR/Intrinsics.h"
35 #include "llvm/IR/MDBuilder.h"
36 #include "llvm/IR/Metadata.h"
37 #include "llvm/IR/Operator.h"
38 #include "llvm/Support/CFG.h"
39 #include "llvm/Support/Debug.h"
40 #include "llvm/Support/GetElementPtrTypeIterator.h"
41 #include "llvm/Support/MathExtras.h"
42 #include "llvm/Support/ValueHandle.h"
43 #include "llvm/Support/raw_ostream.h"
46 STATISTIC(NumRemoved, "Number of unreachable basic blocks removed");
48 //===----------------------------------------------------------------------===//
49 // Local constant propagation.
52 /// ConstantFoldTerminator - If a terminator instruction is predicated on a
53 /// constant value, convert it into an unconditional branch to the constant
54 /// destination. This is a nontrivial operation because the successors of this
55 /// basic block must have their PHI nodes updated.
56 /// Also calls RecursivelyDeleteTriviallyDeadInstructions() on any branch/switch
57 /// conditions and indirectbr addresses this might make dead if
58 /// DeleteDeadConditions is true.
59 bool llvm::ConstantFoldTerminator(BasicBlock *BB, bool DeleteDeadConditions,
60 const TargetLibraryInfo *TLI) {
61 TerminatorInst *T = BB->getTerminator();
62 IRBuilder<> Builder(T);
64 // Branch - See if we are conditional jumping on constant
65 if (BranchInst *BI = dyn_cast<BranchInst>(T)) {
66 if (BI->isUnconditional()) return false; // Can't optimize uncond branch
67 BasicBlock *Dest1 = BI->getSuccessor(0);
68 BasicBlock *Dest2 = BI->getSuccessor(1);
70 if (ConstantInt *Cond = dyn_cast<ConstantInt>(BI->getCondition())) {
71 // Are we branching on constant?
72 // YES. Change to unconditional branch...
73 BasicBlock *Destination = Cond->getZExtValue() ? Dest1 : Dest2;
74 BasicBlock *OldDest = Cond->getZExtValue() ? Dest2 : Dest1;
76 //cerr << "Function: " << T->getParent()->getParent()
77 // << "\nRemoving branch from " << T->getParent()
78 // << "\n\nTo: " << OldDest << endl;
80 // Let the basic block know that we are letting go of it. Based on this,
81 // it will adjust it's PHI nodes.
82 OldDest->removePredecessor(BB);
84 // Replace the conditional branch with an unconditional one.
85 Builder.CreateBr(Destination);
86 BI->eraseFromParent();
90 if (Dest2 == Dest1) { // Conditional branch to same location?
91 // This branch matches something like this:
92 // br bool %cond, label %Dest, label %Dest
93 // and changes it into: br label %Dest
95 // Let the basic block know that we are letting go of one copy of it.
96 assert(BI->getParent() && "Terminator not inserted in block!");
97 Dest1->removePredecessor(BI->getParent());
99 // Replace the conditional branch with an unconditional one.
100 Builder.CreateBr(Dest1);
101 Value *Cond = BI->getCondition();
102 BI->eraseFromParent();
103 if (DeleteDeadConditions)
104 RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
110 if (SwitchInst *SI = dyn_cast<SwitchInst>(T)) {
111 // If we are switching on a constant, we can convert the switch into a
112 // single branch instruction!
113 ConstantInt *CI = dyn_cast<ConstantInt>(SI->getCondition());
114 BasicBlock *TheOnlyDest = SI->getDefaultDest();
115 BasicBlock *DefaultDest = TheOnlyDest;
117 // Figure out which case it goes to.
118 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end();
120 // Found case matching a constant operand?
121 if (i.getCaseValue() == CI) {
122 TheOnlyDest = i.getCaseSuccessor();
126 // Check to see if this branch is going to the same place as the default
127 // dest. If so, eliminate it as an explicit compare.
128 if (i.getCaseSuccessor() == DefaultDest) {
129 MDNode* MD = SI->getMetadata(LLVMContext::MD_prof);
130 unsigned NCases = SI->getNumCases();
131 // Fold the case metadata into the default if there will be any branches
132 // left, unless the metadata doesn't match the switch.
133 if (NCases > 1 && MD && MD->getNumOperands() == 2 + NCases) {
134 // Collect branch weights into a vector.
135 SmallVector<uint32_t, 8> Weights;
136 for (unsigned MD_i = 1, MD_e = MD->getNumOperands(); MD_i < MD_e;
138 ConstantInt* CI = dyn_cast<ConstantInt>(MD->getOperand(MD_i));
140 Weights.push_back(CI->getValue().getZExtValue());
142 // Merge weight of this case to the default weight.
143 unsigned idx = i.getCaseIndex();
144 Weights[0] += Weights[idx+1];
145 // Remove weight for this case.
146 std::swap(Weights[idx+1], Weights.back());
148 SI->setMetadata(LLVMContext::MD_prof,
149 MDBuilder(BB->getContext()).
150 createBranchWeights(Weights));
152 // Remove this entry.
153 DefaultDest->removePredecessor(SI->getParent());
159 // Otherwise, check to see if the switch only branches to one destination.
160 // We do this by reseting "TheOnlyDest" to null when we find two non-equal
162 if (i.getCaseSuccessor() != TheOnlyDest) TheOnlyDest = 0;
165 if (CI && !TheOnlyDest) {
166 // Branching on a constant, but not any of the cases, go to the default
168 TheOnlyDest = SI->getDefaultDest();
171 // If we found a single destination that we can fold the switch into, do so
174 // Insert the new branch.
175 Builder.CreateBr(TheOnlyDest);
176 BasicBlock *BB = SI->getParent();
178 // Remove entries from PHI nodes which we no longer branch to...
179 for (unsigned i = 0, e = SI->getNumSuccessors(); i != e; ++i) {
180 // Found case matching a constant operand?
181 BasicBlock *Succ = SI->getSuccessor(i);
182 if (Succ == TheOnlyDest)
183 TheOnlyDest = 0; // Don't modify the first branch to TheOnlyDest
185 Succ->removePredecessor(BB);
188 // Delete the old switch.
189 Value *Cond = SI->getCondition();
190 SI->eraseFromParent();
191 if (DeleteDeadConditions)
192 RecursivelyDeleteTriviallyDeadInstructions(Cond, TLI);
196 if (SI->getNumCases() == 1) {
197 // Otherwise, we can fold this switch into a conditional branch
198 // instruction if it has only one non-default destination.
199 SwitchInst::CaseIt FirstCase = SI->case_begin();
200 Value *Cond = Builder.CreateICmpEQ(SI->getCondition(),
201 FirstCase.getCaseValue(), "cond");
203 // Insert the new branch.
204 BranchInst *NewBr = Builder.CreateCondBr(Cond,
205 FirstCase.getCaseSuccessor(),
206 SI->getDefaultDest());
207 MDNode* MD = SI->getMetadata(LLVMContext::MD_prof);
208 if (MD && MD->getNumOperands() == 3) {
209 ConstantInt *SICase = dyn_cast<ConstantInt>(MD->getOperand(2));
210 ConstantInt *SIDef = dyn_cast<ConstantInt>(MD->getOperand(1));
211 assert(SICase && SIDef);
212 // The TrueWeight should be the weight for the single case of SI.
213 NewBr->setMetadata(LLVMContext::MD_prof,
214 MDBuilder(BB->getContext()).
215 createBranchWeights(SICase->getValue().getZExtValue(),
216 SIDef->getValue().getZExtValue()));
219 // Delete the old switch.
220 SI->eraseFromParent();
226 if (IndirectBrInst *IBI = dyn_cast<IndirectBrInst>(T)) {
227 // indirectbr blockaddress(@F, @BB) -> br label @BB
228 if (BlockAddress *BA =
229 dyn_cast<BlockAddress>(IBI->getAddress()->stripPointerCasts())) {
230 BasicBlock *TheOnlyDest = BA->getBasicBlock();
231 // Insert the new branch.
232 Builder.CreateBr(TheOnlyDest);
234 for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) {
235 if (IBI->getDestination(i) == TheOnlyDest)
238 IBI->getDestination(i)->removePredecessor(IBI->getParent());
240 Value *Address = IBI->getAddress();
241 IBI->eraseFromParent();
242 if (DeleteDeadConditions)
243 RecursivelyDeleteTriviallyDeadInstructions(Address, TLI);
245 // If we didn't find our destination in the IBI successor list, then we
246 // have undefined behavior. Replace the unconditional branch with an
247 // 'unreachable' instruction.
249 BB->getTerminator()->eraseFromParent();
250 new UnreachableInst(BB->getContext(), BB);
261 //===----------------------------------------------------------------------===//
262 // Local dead code elimination.
265 /// isInstructionTriviallyDead - Return true if the result produced by the
266 /// instruction is not used, and the instruction has no side effects.
268 bool llvm::isInstructionTriviallyDead(Instruction *I,
269 const TargetLibraryInfo *TLI) {
270 if (!I->use_empty() || isa<TerminatorInst>(I)) return false;
272 // We don't want the landingpad instruction removed by anything this general.
273 if (isa<LandingPadInst>(I))
276 // We don't want debug info removed by anything this general, unless
277 // debug info is empty.
278 if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(I)) {
279 if (DDI->getAddress())
283 if (DbgValueInst *DVI = dyn_cast<DbgValueInst>(I)) {
289 if (!I->mayHaveSideEffects()) return true;
291 // Special case intrinsics that "may have side effects" but can be deleted
293 if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
294 // Safe to delete llvm.stacksave if dead.
295 if (II->getIntrinsicID() == Intrinsic::stacksave)
298 // Lifetime intrinsics are dead when their right-hand is undef.
299 if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
300 II->getIntrinsicID() == Intrinsic::lifetime_end)
301 return isa<UndefValue>(II->getArgOperand(1));
304 if (isAllocLikeFn(I, TLI)) return true;
306 if (CallInst *CI = isFreeCall(I, TLI))
307 if (Constant *C = dyn_cast<Constant>(CI->getArgOperand(0)))
308 return C->isNullValue() || isa<UndefValue>(C);
313 /// RecursivelyDeleteTriviallyDeadInstructions - If the specified value is a
314 /// trivially dead instruction, delete it. If that makes any of its operands
315 /// trivially dead, delete them too, recursively. Return true if any
316 /// instructions were deleted.
318 llvm::RecursivelyDeleteTriviallyDeadInstructions(Value *V,
319 const TargetLibraryInfo *TLI) {
320 Instruction *I = dyn_cast<Instruction>(V);
321 if (!I || !I->use_empty() || !isInstructionTriviallyDead(I, TLI))
324 SmallVector<Instruction*, 16> DeadInsts;
325 DeadInsts.push_back(I);
328 I = DeadInsts.pop_back_val();
330 // Null out all of the instruction's operands to see if any operand becomes
332 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
333 Value *OpV = I->getOperand(i);
336 if (!OpV->use_empty()) continue;
338 // If the operand is an instruction that became dead as we nulled out the
339 // operand, and if it is 'trivially' dead, delete it in a future loop
341 if (Instruction *OpI = dyn_cast<Instruction>(OpV))
342 if (isInstructionTriviallyDead(OpI, TLI))
343 DeadInsts.push_back(OpI);
346 I->eraseFromParent();
347 } while (!DeadInsts.empty());
352 /// areAllUsesEqual - Check whether the uses of a value are all the same.
353 /// This is similar to Instruction::hasOneUse() except this will also return
354 /// true when there are no uses or multiple uses that all refer to the same
356 static bool areAllUsesEqual(Instruction *I) {
357 Value::use_iterator UI = I->use_begin();
358 Value::use_iterator UE = I->use_end();
363 for (++UI; UI != UE; ++UI) {
370 /// RecursivelyDeleteDeadPHINode - If the specified value is an effectively
371 /// dead PHI node, due to being a def-use chain of single-use nodes that
372 /// either forms a cycle or is terminated by a trivially dead instruction,
373 /// delete it. If that makes any of its operands trivially dead, delete them
374 /// too, recursively. Return true if a change was made.
375 bool llvm::RecursivelyDeleteDeadPHINode(PHINode *PN,
376 const TargetLibraryInfo *TLI) {
377 SmallPtrSet<Instruction*, 4> Visited;
378 for (Instruction *I = PN; areAllUsesEqual(I) && !I->mayHaveSideEffects();
379 I = cast<Instruction>(*I->use_begin())) {
381 return RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
383 // If we find an instruction more than once, we're on a cycle that
384 // won't prove fruitful.
385 if (!Visited.insert(I)) {
386 // Break the cycle and delete the instruction and its operands.
387 I->replaceAllUsesWith(UndefValue::get(I->getType()));
388 (void)RecursivelyDeleteTriviallyDeadInstructions(I, TLI);
395 /// SimplifyInstructionsInBlock - Scan the specified basic block and try to
396 /// simplify any instructions in it and recursively delete dead instructions.
398 /// This returns true if it changed the code, note that it can delete
399 /// instructions in other blocks as well in this block.
400 bool llvm::SimplifyInstructionsInBlock(BasicBlock *BB, const DataLayout *TD,
401 const TargetLibraryInfo *TLI) {
402 bool MadeChange = false;
405 // In debug builds, ensure that the terminator of the block is never replaced
406 // or deleted by these simplifications. The idea of simplification is that it
407 // cannot introduce new instructions, and there is no way to replace the
408 // terminator of a block without introducing a new instruction.
409 AssertingVH<Instruction> TerminatorVH(--BB->end());
412 for (BasicBlock::iterator BI = BB->begin(), E = --BB->end(); BI != E; ) {
413 assert(!BI->isTerminator());
414 Instruction *Inst = BI++;
417 if (recursivelySimplifyInstruction(Inst, TD, TLI)) {
424 MadeChange |= RecursivelyDeleteTriviallyDeadInstructions(Inst, TLI);
431 //===----------------------------------------------------------------------===//
432 // Control Flow Graph Restructuring.
436 /// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this
437 /// method is called when we're about to delete Pred as a predecessor of BB. If
438 /// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred.
440 /// Unlike the removePredecessor method, this attempts to simplify uses of PHI
441 /// nodes that collapse into identity values. For example, if we have:
442 /// x = phi(1, 0, 0, 0)
445 /// .. and delete the predecessor corresponding to the '1', this will attempt to
446 /// recursively fold the and to 0.
447 void llvm::RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred,
449 // This only adjusts blocks with PHI nodes.
450 if (!isa<PHINode>(BB->begin()))
453 // Remove the entries for Pred from the PHI nodes in BB, but do not simplify
454 // them down. This will leave us with single entry phi nodes and other phis
455 // that can be removed.
456 BB->removePredecessor(Pred, true);
458 WeakVH PhiIt = &BB->front();
459 while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) {
460 PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt));
461 Value *OldPhiIt = PhiIt;
463 if (!recursivelySimplifyInstruction(PN, TD))
466 // If recursive simplification ended up deleting the next PHI node we would
467 // iterate to, then our iterator is invalid, restart scanning from the top
469 if (PhiIt != OldPhiIt) PhiIt = &BB->front();
474 /// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
475 /// predecessor is known to have one successor (DestBB!). Eliminate the edge
476 /// between them, moving the instructions in the predecessor into DestBB and
477 /// deleting the predecessor block.
479 void llvm::MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB, Pass *P) {
480 // If BB has single-entry PHI nodes, fold them.
481 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
482 Value *NewVal = PN->getIncomingValue(0);
483 // Replace self referencing PHI with undef, it must be dead.
484 if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
485 PN->replaceAllUsesWith(NewVal);
486 PN->eraseFromParent();
489 BasicBlock *PredBB = DestBB->getSinglePredecessor();
490 assert(PredBB && "Block doesn't have a single predecessor!");
492 // Zap anything that took the address of DestBB. Not doing this will give the
493 // address an invalid value.
494 if (DestBB->hasAddressTaken()) {
495 BlockAddress *BA = BlockAddress::get(DestBB);
496 Constant *Replacement =
497 ConstantInt::get(llvm::Type::getInt32Ty(BA->getContext()), 1);
498 BA->replaceAllUsesWith(ConstantExpr::getIntToPtr(Replacement,
500 BA->destroyConstant();
503 // Anything that branched to PredBB now branches to DestBB.
504 PredBB->replaceAllUsesWith(DestBB);
506 // Splice all the instructions from PredBB to DestBB.
507 PredBB->getTerminator()->eraseFromParent();
508 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
511 DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>();
513 BasicBlock *PredBBIDom = DT->getNode(PredBB)->getIDom()->getBlock();
514 DT->changeImmediateDominator(DestBB, PredBBIDom);
515 DT->eraseNode(PredBB);
519 PredBB->eraseFromParent();
522 /// CanMergeValues - Return true if we can choose one of these values to use
523 /// in place of the other. Note that we will always choose the non-undef
525 static bool CanMergeValues(Value *First, Value *Second) {
526 return First == Second || isa<UndefValue>(First) || isa<UndefValue>(Second);
529 /// CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an
530 /// almost-empty BB ending in an unconditional branch to Succ, into Succ.
532 /// Assumption: Succ is the single successor for BB.
534 static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) {
535 assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!");
537 DEBUG(dbgs() << "Looking to fold " << BB->getName() << " into "
538 << Succ->getName() << "\n");
539 // Shortcut, if there is only a single predecessor it must be BB and merging
541 if (Succ->getSinglePredecessor()) return true;
543 // Make a list of the predecessors of BB
544 SmallPtrSet<BasicBlock*, 16> BBPreds(pred_begin(BB), pred_end(BB));
546 // Look at all the phi nodes in Succ, to see if they present a conflict when
547 // merging these blocks
548 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
549 PHINode *PN = cast<PHINode>(I);
551 // If the incoming value from BB is again a PHINode in
552 // BB which has the same incoming value for *PI as PN does, we can
553 // merge the phi nodes and then the blocks can still be merged
554 PHINode *BBPN = dyn_cast<PHINode>(PN->getIncomingValueForBlock(BB));
555 if (BBPN && BBPN->getParent() == BB) {
556 for (unsigned PI = 0, PE = PN->getNumIncomingValues(); PI != PE; ++PI) {
557 BasicBlock *IBB = PN->getIncomingBlock(PI);
558 if (BBPreds.count(IBB) &&
559 !CanMergeValues(BBPN->getIncomingValueForBlock(IBB),
560 PN->getIncomingValue(PI))) {
561 DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in "
562 << Succ->getName() << " is conflicting with "
563 << BBPN->getName() << " with regard to common predecessor "
564 << IBB->getName() << "\n");
569 Value* Val = PN->getIncomingValueForBlock(BB);
570 for (unsigned PI = 0, PE = PN->getNumIncomingValues(); PI != PE; ++PI) {
571 // See if the incoming value for the common predecessor is equal to the
572 // one for BB, in which case this phi node will not prevent the merging
574 BasicBlock *IBB = PN->getIncomingBlock(PI);
575 if (BBPreds.count(IBB) &&
576 !CanMergeValues(Val, PN->getIncomingValue(PI))) {
577 DEBUG(dbgs() << "Can't fold, phi node " << PN->getName() << " in "
578 << Succ->getName() << " is conflicting with regard to common "
579 << "predecessor " << IBB->getName() << "\n");
589 typedef SmallVector<BasicBlock *, 16> PredBlockVector;
590 typedef DenseMap<BasicBlock *, Value *> IncomingValueMap;
592 /// \brief Determines the value to use as the phi node input for a block.
594 /// Select between \p OldVal any value that we know flows from \p BB
595 /// to a particular phi on the basis of which one (if either) is not
596 /// undef. Update IncomingValues based on the selected value.
598 /// \param OldVal The value we are considering selecting.
599 /// \param BB The block that the value flows in from.
600 /// \param IncomingValues A map from block-to-value for other phi inputs
601 /// that we have examined.
603 /// \returns the selected value.
604 static Value *selectIncomingValueForBlock(Value *OldVal, BasicBlock *BB,
605 IncomingValueMap &IncomingValues) {
606 if (!isa<UndefValue>(OldVal)) {
607 assert((!IncomingValues.count(BB) ||
608 IncomingValues.find(BB)->second == OldVal) &&
609 "Expected OldVal to match incoming value from BB!");
611 IncomingValues.insert(std::make_pair(BB, OldVal));
615 IncomingValueMap::const_iterator It = IncomingValues.find(BB);
616 if (It != IncomingValues.end()) return It->second;
621 /// \brief Create a map from block to value for the operands of a
624 /// Create a map from block to value for each non-undef value flowing
627 /// \param PN The phi we are collecting the map for.
628 /// \param IncomingValues [out] The map from block to value for this phi.
629 static void gatherIncomingValuesToPhi(PHINode *PN,
630 IncomingValueMap &IncomingValues) {
631 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
632 BasicBlock *BB = PN->getIncomingBlock(i);
633 Value *V = PN->getIncomingValue(i);
635 if (!isa<UndefValue>(V))
636 IncomingValues.insert(std::make_pair(BB, V));
640 /// \brief Replace the incoming undef values to a phi with the values
641 /// from a block-to-value map.
643 /// \param PN The phi we are replacing the undefs in.
644 /// \param IncomingValues A map from block to value.
645 static void replaceUndefValuesInPhi(PHINode *PN,
646 const IncomingValueMap &IncomingValues) {
647 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
648 Value *V = PN->getIncomingValue(i);
650 if (!isa<UndefValue>(V)) continue;
652 BasicBlock *BB = PN->getIncomingBlock(i);
653 IncomingValueMap::const_iterator It = IncomingValues.find(BB);
654 if (It == IncomingValues.end()) continue;
656 PN->setIncomingValue(i, It->second);
660 /// \brief Replace a value flowing from a block to a phi with
661 /// potentially multiple instances of that value flowing from the
662 /// block's predecessors to the phi.
664 /// \param BB The block with the value flowing into the phi.
665 /// \param BBPreds The predecessors of BB.
666 /// \param PN The phi that we are updating.
667 static void redirectValuesFromPredecessorsToPhi(BasicBlock *BB,
668 const PredBlockVector &BBPreds,
670 Value *OldVal = PN->removeIncomingValue(BB, false);
671 assert(OldVal && "No entry in PHI for Pred BB!");
673 IncomingValueMap IncomingValues;
675 // We are merging two blocks - BB, and the block containing PN - and
676 // as a result we need to redirect edges from the predecessors of BB
677 // to go to the block containing PN, and update PN
678 // accordingly. Since we allow merging blocks in the case where the
679 // predecessor and successor blocks both share some predecessors,
680 // and where some of those common predecessors might have undef
681 // values flowing into PN, we want to rewrite those values to be
682 // consistent with the non-undef values.
684 gatherIncomingValuesToPhi(PN, IncomingValues);
686 // If this incoming value is one of the PHI nodes in BB, the new entries
687 // in the PHI node are the entries from the old PHI.
688 if (isa<PHINode>(OldVal) && cast<PHINode>(OldVal)->getParent() == BB) {
689 PHINode *OldValPN = cast<PHINode>(OldVal);
690 for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) {
691 // Note that, since we are merging phi nodes and BB and Succ might
692 // have common predecessors, we could end up with a phi node with
693 // identical incoming branches. This will be cleaned up later (and
694 // will trigger asserts if we try to clean it up now, without also
695 // simplifying the corresponding conditional branch).
696 BasicBlock *PredBB = OldValPN->getIncomingBlock(i);
697 Value *PredVal = OldValPN->getIncomingValue(i);
698 Value *Selected = selectIncomingValueForBlock(PredVal, PredBB,
701 // And add a new incoming value for this predecessor for the
702 // newly retargeted branch.
703 PN->addIncoming(Selected, PredBB);
706 for (unsigned i = 0, e = BBPreds.size(); i != e; ++i) {
707 // Update existing incoming values in PN for this
708 // predecessor of BB.
709 BasicBlock *PredBB = BBPreds[i];
710 Value *Selected = selectIncomingValueForBlock(OldVal, PredBB,
713 // And add a new incoming value for this predecessor for the
714 // newly retargeted branch.
715 PN->addIncoming(Selected, PredBB);
719 replaceUndefValuesInPhi(PN, IncomingValues);
722 /// TryToSimplifyUncondBranchFromEmptyBlock - BB is known to contain an
723 /// unconditional branch, and contains no instructions other than PHI nodes,
724 /// potential side-effect free intrinsics and the branch. If possible,
725 /// eliminate BB by rewriting all the predecessors to branch to the successor
726 /// block and return true. If we can't transform, return false.
727 bool llvm::TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB) {
728 assert(BB != &BB->getParent()->getEntryBlock() &&
729 "TryToSimplifyUncondBranchFromEmptyBlock called on entry block!");
731 // We can't eliminate infinite loops.
732 BasicBlock *Succ = cast<BranchInst>(BB->getTerminator())->getSuccessor(0);
733 if (BB == Succ) return false;
735 // Check to see if merging these blocks would cause conflicts for any of the
736 // phi nodes in BB or Succ. If not, we can safely merge.
737 if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false;
739 // Check for cases where Succ has multiple predecessors and a PHI node in BB
740 // has uses which will not disappear when the PHI nodes are merged. It is
741 // possible to handle such cases, but difficult: it requires checking whether
742 // BB dominates Succ, which is non-trivial to calculate in the case where
743 // Succ has multiple predecessors. Also, it requires checking whether
744 // constructing the necessary self-referential PHI node doesn't introduce any
745 // conflicts; this isn't too difficult, but the previous code for doing this
748 // Note that if this check finds a live use, BB dominates Succ, so BB is
749 // something like a loop pre-header (or rarely, a part of an irreducible CFG);
750 // folding the branch isn't profitable in that case anyway.
751 if (!Succ->getSinglePredecessor()) {
752 BasicBlock::iterator BBI = BB->begin();
753 while (isa<PHINode>(*BBI)) {
754 for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end();
756 if (PHINode* PN = dyn_cast<PHINode>(*UI)) {
757 if (PN->getIncomingBlock(UI) != BB)
767 DEBUG(dbgs() << "Killing Trivial BB: \n" << *BB);
769 if (isa<PHINode>(Succ->begin())) {
770 // If there is more than one pred of succ, and there are PHI nodes in
771 // the successor, then we need to add incoming edges for the PHI nodes
773 const PredBlockVector BBPreds(pred_begin(BB), pred_end(BB));
775 // Loop over all of the PHI nodes in the successor of BB.
776 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
777 PHINode *PN = cast<PHINode>(I);
779 redirectValuesFromPredecessorsToPhi(BB, BBPreds, PN);
783 if (Succ->getSinglePredecessor()) {
784 // BB is the only predecessor of Succ, so Succ will end up with exactly
785 // the same predecessors BB had.
787 // Copy over any phi, debug or lifetime instruction.
788 BB->getTerminator()->eraseFromParent();
789 Succ->getInstList().splice(Succ->getFirstNonPHI(), BB->getInstList());
791 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
792 // We explicitly check for such uses in CanPropagatePredecessorsForPHIs.
793 assert(PN->use_empty() && "There shouldn't be any uses here!");
794 PN->eraseFromParent();
798 // Everything that jumped to BB now goes to Succ.
799 BB->replaceAllUsesWith(Succ);
800 if (!Succ->hasName()) Succ->takeName(BB);
801 BB->eraseFromParent(); // Delete the old basic block.
805 /// EliminateDuplicatePHINodes - Check for and eliminate duplicate PHI
806 /// nodes in this block. This doesn't try to be clever about PHI nodes
807 /// which differ only in the order of the incoming values, but instcombine
808 /// orders them so it usually won't matter.
810 bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
811 bool Changed = false;
813 // This implementation doesn't currently consider undef operands
814 // specially. Theoretically, two phis which are identical except for
815 // one having an undef where the other doesn't could be collapsed.
817 // Map from PHI hash values to PHI nodes. If multiple PHIs have
818 // the same hash value, the element is the first PHI in the
819 // linked list in CollisionMap.
820 DenseMap<uintptr_t, PHINode *> HashMap;
822 // Maintain linked lists of PHI nodes with common hash values.
823 DenseMap<PHINode *, PHINode *> CollisionMap;
826 for (BasicBlock::iterator I = BB->begin();
827 PHINode *PN = dyn_cast<PHINode>(I++); ) {
828 // Compute a hash value on the operands. Instcombine will likely have sorted
829 // them, which helps expose duplicates, but we have to check all the
830 // operands to be safe in case instcombine hasn't run.
832 // This hash algorithm is quite weak as hash functions go, but it seems
833 // to do a good enough job for this particular purpose, and is very quick.
834 for (User::op_iterator I = PN->op_begin(), E = PN->op_end(); I != E; ++I) {
835 Hash ^= reinterpret_cast<uintptr_t>(static_cast<Value *>(*I));
836 Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7));
838 for (PHINode::block_iterator I = PN->block_begin(), E = PN->block_end();
840 Hash ^= reinterpret_cast<uintptr_t>(static_cast<BasicBlock *>(*I));
841 Hash = (Hash << 7) | (Hash >> (sizeof(uintptr_t) * CHAR_BIT - 7));
843 // Avoid colliding with the DenseMap sentinels ~0 and ~0-1.
845 // If we've never seen this hash value before, it's a unique PHI.
846 std::pair<DenseMap<uintptr_t, PHINode *>::iterator, bool> Pair =
847 HashMap.insert(std::make_pair(Hash, PN));
848 if (Pair.second) continue;
849 // Otherwise it's either a duplicate or a hash collision.
850 for (PHINode *OtherPN = Pair.first->second; ; ) {
851 if (OtherPN->isIdenticalTo(PN)) {
852 // A duplicate. Replace this PHI with its duplicate.
853 PN->replaceAllUsesWith(OtherPN);
854 PN->eraseFromParent();
858 // A non-duplicate hash collision.
859 DenseMap<PHINode *, PHINode *>::iterator I = CollisionMap.find(OtherPN);
860 if (I == CollisionMap.end()) {
861 // Set this PHI to be the head of the linked list of colliding PHIs.
862 PHINode *Old = Pair.first->second;
863 Pair.first->second = PN;
864 CollisionMap[PN] = Old;
867 // Proceed to the next PHI in the list.
875 /// enforceKnownAlignment - If the specified pointer points to an object that
876 /// we control, modify the object's alignment to PrefAlign. This isn't
877 /// often possible though. If alignment is important, a more reliable approach
878 /// is to simply align all global variables and allocation instructions to
879 /// their preferred alignment from the beginning.
881 static unsigned enforceKnownAlignment(Value *V, unsigned Align,
882 unsigned PrefAlign, const DataLayout *TD) {
883 V = V->stripPointerCasts();
885 if (AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
886 // If the preferred alignment is greater than the natural stack alignment
887 // then don't round up. This avoids dynamic stack realignment.
888 if (TD && TD->exceedsNaturalStackAlignment(PrefAlign))
890 // If there is a requested alignment and if this is an alloca, round up.
891 if (AI->getAlignment() >= PrefAlign)
892 return AI->getAlignment();
893 AI->setAlignment(PrefAlign);
897 if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
898 // If there is a large requested alignment and we can, bump up the alignment
900 if (GV->isDeclaration()) return Align;
901 // If the memory we set aside for the global may not be the memory used by
902 // the final program then it is impossible for us to reliably enforce the
903 // preferred alignment.
904 if (GV->isWeakForLinker()) return Align;
906 if (GV->getAlignment() >= PrefAlign)
907 return GV->getAlignment();
908 // We can only increase the alignment of the global if it has no alignment
909 // specified or if it is not assigned a section. If it is assigned a
910 // section, the global could be densely packed with other objects in the
911 // section, increasing the alignment could cause padding issues.
912 if (!GV->hasSection() || GV->getAlignment() == 0)
913 GV->setAlignment(PrefAlign);
914 return GV->getAlignment();
920 /// getOrEnforceKnownAlignment - If the specified pointer has an alignment that
921 /// we can determine, return it, otherwise return 0. If PrefAlign is specified,
922 /// and it is more than the alignment of the ultimate object, see if we can
923 /// increase the alignment of the ultimate object, making this check succeed.
924 unsigned llvm::getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign,
925 const DataLayout *DL) {
926 assert(V->getType()->isPointerTy() &&
927 "getOrEnforceKnownAlignment expects a pointer!");
928 unsigned BitWidth = DL ? DL->getPointerTypeSizeInBits(V->getType()) : 64;
930 APInt KnownZero(BitWidth, 0), KnownOne(BitWidth, 0);
931 ComputeMaskedBits(V, KnownZero, KnownOne, DL);
932 unsigned TrailZ = KnownZero.countTrailingOnes();
934 // Avoid trouble with ridiculously large TrailZ values, such as
935 // those computed from a null pointer.
936 TrailZ = std::min(TrailZ, unsigned(sizeof(unsigned) * CHAR_BIT - 1));
938 unsigned Align = 1u << std::min(BitWidth - 1, TrailZ);
940 // LLVM doesn't support alignments larger than this currently.
941 Align = std::min(Align, +Value::MaximumAlignment);
943 if (PrefAlign > Align)
944 Align = enforceKnownAlignment(V, Align, PrefAlign, DL);
946 // We don't need to make any adjustment.
950 ///===---------------------------------------------------------------------===//
951 /// Dbg Intrinsic utilities
954 /// See if there is a dbg.value intrinsic for DIVar before I.
955 static bool LdStHasDebugValue(DIVariable &DIVar, Instruction *I) {
956 // Since we can't guarantee that the original dbg.declare instrinsic
957 // is removed by LowerDbgDeclare(), we need to make sure that we are
958 // not inserting the same dbg.value intrinsic over and over.
959 llvm::BasicBlock::InstListType::iterator PrevI(I);
960 if (PrevI != I->getParent()->getInstList().begin()) {
962 if (DbgValueInst *DVI = dyn_cast<DbgValueInst>(PrevI))
963 if (DVI->getValue() == I->getOperand(0) &&
964 DVI->getOffset() == 0 &&
965 DVI->getVariable() == DIVar)
971 /// Inserts a llvm.dbg.value intrinsic before a store to an alloca'd value
972 /// that has an associated llvm.dbg.decl intrinsic.
973 bool llvm::ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI,
974 StoreInst *SI, DIBuilder &Builder) {
975 DIVariable DIVar(DDI->getVariable());
976 assert((!DIVar || DIVar.isVariable()) &&
977 "Variable in DbgDeclareInst should be either null or a DIVariable.");
981 if (LdStHasDebugValue(DIVar, SI))
984 Instruction *DbgVal = NULL;
985 // If an argument is zero extended then use argument directly. The ZExt
986 // may be zapped by an optimization pass in future.
987 Argument *ExtendedArg = NULL;
988 if (ZExtInst *ZExt = dyn_cast<ZExtInst>(SI->getOperand(0)))
989 ExtendedArg = dyn_cast<Argument>(ZExt->getOperand(0));
990 if (SExtInst *SExt = dyn_cast<SExtInst>(SI->getOperand(0)))
991 ExtendedArg = dyn_cast<Argument>(SExt->getOperand(0));
993 DbgVal = Builder.insertDbgValueIntrinsic(ExtendedArg, 0, DIVar, SI);
995 DbgVal = Builder.insertDbgValueIntrinsic(SI->getOperand(0), 0, DIVar, SI);
997 // Propagate any debug metadata from the store onto the dbg.value.
998 DebugLoc SIDL = SI->getDebugLoc();
999 if (!SIDL.isUnknown())
1000 DbgVal->setDebugLoc(SIDL);
1001 // Otherwise propagate debug metadata from dbg.declare.
1003 DbgVal->setDebugLoc(DDI->getDebugLoc());
1007 /// Inserts a llvm.dbg.value intrinsic before a load of an alloca'd value
1008 /// that has an associated llvm.dbg.decl intrinsic.
1009 bool llvm::ConvertDebugDeclareToDebugValue(DbgDeclareInst *DDI,
1010 LoadInst *LI, DIBuilder &Builder) {
1011 DIVariable DIVar(DDI->getVariable());
1012 assert((!DIVar || DIVar.isVariable()) &&
1013 "Variable in DbgDeclareInst should be either null or a DIVariable.");
1017 if (LdStHasDebugValue(DIVar, LI))
1020 Instruction *DbgVal =
1021 Builder.insertDbgValueIntrinsic(LI->getOperand(0), 0,
1024 // Propagate any debug metadata from the store onto the dbg.value.
1025 DebugLoc LIDL = LI->getDebugLoc();
1026 if (!LIDL.isUnknown())
1027 DbgVal->setDebugLoc(LIDL);
1028 // Otherwise propagate debug metadata from dbg.declare.
1030 DbgVal->setDebugLoc(DDI->getDebugLoc());
1034 /// LowerDbgDeclare - Lowers llvm.dbg.declare intrinsics into appropriate set
1035 /// of llvm.dbg.value intrinsics.
1036 bool llvm::LowerDbgDeclare(Function &F) {
1037 DIBuilder DIB(*F.getParent());
1038 SmallVector<DbgDeclareInst *, 4> Dbgs;
1039 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI)
1040 for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ++BI) {
1041 if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(BI))
1042 Dbgs.push_back(DDI);
1047 for (SmallVectorImpl<DbgDeclareInst *>::iterator I = Dbgs.begin(),
1048 E = Dbgs.end(); I != E; ++I) {
1049 DbgDeclareInst *DDI = *I;
1050 AllocaInst *AI = dyn_cast_or_null<AllocaInst>(DDI->getAddress());
1051 // If this is an alloca for a scalar variable, insert a dbg.value
1052 // at each load and store to the alloca and erase the dbg.declare.
1053 if (AI && !AI->isArrayAllocation()) {
1055 // We only remove the dbg.declare intrinsic if all uses are
1056 // converted to dbg.value intrinsics.
1057 bool RemoveDDI = true;
1058 for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end();
1060 if (StoreInst *SI = dyn_cast<StoreInst>(*UI))
1061 ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
1062 else if (LoadInst *LI = dyn_cast<LoadInst>(*UI))
1063 ConvertDebugDeclareToDebugValue(DDI, LI, DIB);
1067 DDI->eraseFromParent();
1073 /// FindAllocaDbgDeclare - Finds the llvm.dbg.declare intrinsic describing the
1074 /// alloca 'V', if any.
1075 DbgDeclareInst *llvm::FindAllocaDbgDeclare(Value *V) {
1076 if (MDNode *DebugNode = MDNode::getIfExists(V->getContext(), V))
1077 for (Value::use_iterator UI = DebugNode->use_begin(),
1078 E = DebugNode->use_end(); UI != E; ++UI)
1079 if (DbgDeclareInst *DDI = dyn_cast<DbgDeclareInst>(*UI))
1085 bool llvm::replaceDbgDeclareForAlloca(AllocaInst *AI, Value *NewAllocaAddress,
1086 DIBuilder &Builder) {
1087 DbgDeclareInst *DDI = FindAllocaDbgDeclare(AI);
1090 DIVariable DIVar(DDI->getVariable());
1091 assert((!DIVar || DIVar.isVariable()) &&
1092 "Variable in DbgDeclareInst should be either null or a DIVariable.");
1096 // Create a copy of the original DIDescriptor for user variable, appending
1097 // "deref" operation to a list of address elements, as new llvm.dbg.declare
1098 // will take a value storing address of the memory for variable, not
1100 Type *Int64Ty = Type::getInt64Ty(AI->getContext());
1101 SmallVector<Value*, 4> NewDIVarAddress;
1102 if (DIVar.hasComplexAddress()) {
1103 for (unsigned i = 0, n = DIVar.getNumAddrElements(); i < n; ++i) {
1104 NewDIVarAddress.push_back(
1105 ConstantInt::get(Int64Ty, DIVar.getAddrElement(i)));
1108 NewDIVarAddress.push_back(ConstantInt::get(Int64Ty, DIBuilder::OpDeref));
1109 DIVariable NewDIVar = Builder.createComplexVariable(
1110 DIVar.getTag(), DIVar.getContext(), DIVar.getName(),
1111 DIVar.getFile(), DIVar.getLineNumber(), DIVar.getType(),
1112 NewDIVarAddress, DIVar.getArgNumber());
1114 // Insert llvm.dbg.declare in the same basic block as the original alloca,
1115 // and remove old llvm.dbg.declare.
1116 BasicBlock *BB = AI->getParent();
1117 Builder.insertDeclare(NewAllocaAddress, NewDIVar, BB);
1118 DDI->eraseFromParent();
1122 /// changeToUnreachable - Insert an unreachable instruction before the specified
1123 /// instruction, making it and the rest of the code in the block dead.
1124 static void changeToUnreachable(Instruction *I, bool UseLLVMTrap) {
1125 BasicBlock *BB = I->getParent();
1126 // Loop over all of the successors, removing BB's entry from any PHI
1128 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
1129 (*SI)->removePredecessor(BB);
1131 // Insert a call to llvm.trap right before this. This turns the undefined
1132 // behavior into a hard fail instead of falling through into random code.
1135 Intrinsic::getDeclaration(BB->getParent()->getParent(), Intrinsic::trap);
1136 CallInst *CallTrap = CallInst::Create(TrapFn, "", I);
1137 CallTrap->setDebugLoc(I->getDebugLoc());
1139 new UnreachableInst(I->getContext(), I);
1141 // All instructions after this are dead.
1142 BasicBlock::iterator BBI = I, BBE = BB->end();
1143 while (BBI != BBE) {
1144 if (!BBI->use_empty())
1145 BBI->replaceAllUsesWith(UndefValue::get(BBI->getType()));
1146 BB->getInstList().erase(BBI++);
1150 /// changeToCall - Convert the specified invoke into a normal call.
1151 static void changeToCall(InvokeInst *II) {
1152 SmallVector<Value*, 8> Args(II->op_begin(), II->op_end() - 3);
1153 CallInst *NewCall = CallInst::Create(II->getCalledValue(), Args, "", II);
1154 NewCall->takeName(II);
1155 NewCall->setCallingConv(II->getCallingConv());
1156 NewCall->setAttributes(II->getAttributes());
1157 NewCall->setDebugLoc(II->getDebugLoc());
1158 II->replaceAllUsesWith(NewCall);
1160 // Follow the call by a branch to the normal destination.
1161 BranchInst::Create(II->getNormalDest(), II);
1163 // Update PHI nodes in the unwind destination
1164 II->getUnwindDest()->removePredecessor(II->getParent());
1165 II->eraseFromParent();
1168 static bool markAliveBlocks(BasicBlock *BB,
1169 SmallPtrSet<BasicBlock*, 128> &Reachable) {
1171 SmallVector<BasicBlock*, 128> Worklist;
1172 Worklist.push_back(BB);
1173 Reachable.insert(BB);
1174 bool Changed = false;
1176 BB = Worklist.pop_back_val();
1178 // Do a quick scan of the basic block, turning any obviously unreachable
1179 // instructions into LLVM unreachable insts. The instruction combining pass
1180 // canonicalizes unreachable insts into stores to null or undef.
1181 for (BasicBlock::iterator BBI = BB->begin(), E = BB->end(); BBI != E;++BBI){
1182 if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
1183 if (CI->doesNotReturn()) {
1184 // If we found a call to a no-return function, insert an unreachable
1185 // instruction after it. Make sure there isn't *already* one there
1188 if (!isa<UnreachableInst>(BBI)) {
1189 // Don't insert a call to llvm.trap right before the unreachable.
1190 changeToUnreachable(BBI, false);
1197 // Store to undef and store to null are undefined and used to signal that
1198 // they should be changed to unreachable by passes that can't modify the
1200 if (StoreInst *SI = dyn_cast<StoreInst>(BBI)) {
1201 // Don't touch volatile stores.
1202 if (SI->isVolatile()) continue;
1204 Value *Ptr = SI->getOperand(1);
1206 if (isa<UndefValue>(Ptr) ||
1207 (isa<ConstantPointerNull>(Ptr) &&
1208 SI->getPointerAddressSpace() == 0)) {
1209 changeToUnreachable(SI, true);
1216 // Turn invokes that call 'nounwind' functions into ordinary calls.
1217 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator())) {
1218 Value *Callee = II->getCalledValue();
1219 if (isa<ConstantPointerNull>(Callee) || isa<UndefValue>(Callee)) {
1220 changeToUnreachable(II, true);
1222 } else if (II->doesNotThrow()) {
1223 if (II->use_empty() && II->onlyReadsMemory()) {
1224 // jump to the normal destination branch.
1225 BranchInst::Create(II->getNormalDest(), II);
1226 II->getUnwindDest()->removePredecessor(II->getParent());
1227 II->eraseFromParent();
1234 Changed |= ConstantFoldTerminator(BB, true);
1235 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
1236 if (Reachable.insert(*SI))
1237 Worklist.push_back(*SI);
1238 } while (!Worklist.empty());
1242 /// removeUnreachableBlocksFromFn - Remove blocks that are not reachable, even
1243 /// if they are in a dead cycle. Return true if a change was made, false
1245 bool llvm::removeUnreachableBlocks(Function &F) {
1246 SmallPtrSet<BasicBlock*, 128> Reachable;
1247 bool Changed = markAliveBlocks(F.begin(), Reachable);
1249 // If there are unreachable blocks in the CFG...
1250 if (Reachable.size() == F.size())
1253 assert(Reachable.size() < F.size());
1254 NumRemoved += F.size()-Reachable.size();
1256 // Loop over all of the basic blocks that are not reachable, dropping all of
1257 // their internal references...
1258 for (Function::iterator BB = ++F.begin(), E = F.end(); BB != E; ++BB) {
1259 if (Reachable.count(BB))
1262 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
1263 if (Reachable.count(*SI))
1264 (*SI)->removePredecessor(BB);
1265 BB->dropAllReferences();
1268 for (Function::iterator I = ++F.begin(); I != F.end();)
1269 if (!Reachable.count(I))
1270 I = F.getBasicBlockList().erase(I);