diff options
Diffstat (limited to 'lib/Transforms/Scalar/JumpThreading.cpp')
| -rw-r--r-- | lib/Transforms/Scalar/JumpThreading.cpp | 735 |
1 files changed, 474 insertions, 261 deletions
diff --git a/lib/Transforms/Scalar/JumpThreading.cpp b/lib/Transforms/Scalar/JumpThreading.cpp index 10c9ec6..5864113 100644 --- a/lib/Transforms/Scalar/JumpThreading.cpp +++ b/lib/Transforms/Scalar/JumpThreading.cpp @@ -16,7 +16,8 @@ #include "llvm/IntrinsicInst.h" #include "llvm/LLVMContext.h" #include "llvm/Pass.h" -#include "llvm/Analysis/ConstantFolding.h" +#include "llvm/Analysis/InstructionSimplify.h" +#include "llvm/Analysis/LazyValueInfo.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include "llvm/Transforms/Utils/Local.h" #include "llvm/Transforms/Utils/SSAUpdater.h" @@ -40,6 +41,12 @@ Threshold("jump-threading-threshold", cl::desc("Max block size to duplicate for jump threading"), cl::init(6), cl::Hidden); +// Turn on use of LazyValueInfo. +static cl::opt<bool> +EnableLVI("enable-jump-threading-lvi", cl::ReallyHidden); + + + namespace { /// This pass performs 'jump threading', which looks at blocks that have /// multiple predecessors and multiple successors. If one or more of the @@ -59,6 +66,7 @@ namespace { /// class JumpThreading : public FunctionPass { TargetData *TD; + LazyValueInfo *LVI; #ifdef NDEBUG SmallPtrSet<BasicBlock*, 16> LoopHeaders; #else @@ -69,20 +77,31 @@ namespace { JumpThreading() : FunctionPass(&ID) {} bool runOnFunction(Function &F); - void FindLoopHeaders(Function &F); + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + if (EnableLVI) + AU.addRequired<LazyValueInfo>(); + } + + void FindLoopHeaders(Function &F); bool ProcessBlock(BasicBlock *BB); - bool ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB); + bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs, + BasicBlock *SuccBB); bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, BasicBlock *PredBB); - - BasicBlock *FactorCommonPHIPreds(PHINode *PN, Value *Val); + + typedef SmallVectorImpl<std::pair<ConstantInt*, + BasicBlock*> > PredValueInfo; + + bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, + PredValueInfo &Result); + bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB); + + bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB); bool ProcessJumpOnPHI(PHINode *PN); - bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd); - bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB); bool SimplifyPartiallyRedundantLoad(LoadInst *LI); }; @@ -100,6 +119,7 @@ FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } bool JumpThreading::runOnFunction(Function &F) { DEBUG(errs() << "Jump threading on function '" << F.getName() << "'\n"); TD = getAnalysisIfAvailable<TargetData>(); + LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0; FindLoopHeaders(F); @@ -109,6 +129,7 @@ bool JumpThreading::runOnFunction(Function &F) { bool Changed = false; for (Function::iterator I = F.begin(), E = F.end(); I != E;) { BasicBlock *BB = I; + // Thread all of the branches we can over this block. while (ProcessBlock(BB)) Changed = true; @@ -123,6 +144,29 @@ bool JumpThreading::runOnFunction(Function &F) { LoopHeaders.erase(BB); DeleteDeadBlock(BB); Changed = true; + } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { + // Can't thread an unconditional jump, but if the block is "almost + // empty", we can replace uses of it with uses of the successor and make + // this dead. + if (BI->isUnconditional() && + BB != &BB->getParent()->getEntryBlock()) { + BasicBlock::iterator BBI = BB->getFirstNonPHI(); + // Ignore dbg intrinsics. + while (isa<DbgInfoIntrinsic>(BBI)) + ++BBI; + // If the terminator is the only non-phi instruction, try to nuke it. + if (BBI->isTerminator()) { + // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the + // block, we have to make sure it isn't in the LoopHeaders set. We + // reinsert afterward in the rare case when the block isn't deleted. + bool ErasedFromLoopHeaders = LoopHeaders.erase(BB); + + if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) + Changed = true; + else if (ErasedFromLoopHeaders) + LoopHeaders.insert(BB); + } + } } } AnotherIteration = Changed; @@ -139,6 +183,10 @@ static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { /// Ignore PHI nodes, these will be flattened when duplication happens. BasicBlock::const_iterator I = BB->getFirstNonPHI(); + // FIXME: THREADING will delete values that are just used to compute the + // branch, so they shouldn't count against the duplication cost. + + // Sum up the cost of each instruction until we get to the terminator. Don't // include the terminator because the copy won't include it. unsigned Size = 0; @@ -173,8 +221,6 @@ static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) { return Size; } - - /// FindLoopHeaders - We do not want jump threading to turn proper loop /// structures into irreducible loops. Doing this breaks up the loop nesting /// hierarchy and pessimizes later transformations. To prevent this from @@ -198,29 +244,181 @@ void JumpThreading::FindLoopHeaders(Function &F) { LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second)); } - -/// FactorCommonPHIPreds - If there are multiple preds with the same incoming -/// value for the PHI, factor them together so we get one block to thread for -/// the whole group. -/// This is important for things like "phi i1 [true, true, false, true, x]" -/// where we only need to clone the block for the true blocks once. +/// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see +/// if we can infer that the value is a known ConstantInt in any of our +/// predecessors. If so, return the known list of value and pred BB in the +/// result vector. If a value is known to be undef, it is returned as null. +/// +/// This returns true if there were any known values. /// -BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Value *Val) { - SmallVector<BasicBlock*, 16> CommonPreds; - for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) - if (PN->getIncomingValue(i) == Val) - CommonPreds.push_back(PN->getIncomingBlock(i)); - - if (CommonPreds.size() == 1) - return CommonPreds[0]; +bool JumpThreading:: +ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){ + // If V is a constantint, then it is known in all predecessors. + if (isa<ConstantInt>(V) || isa<UndefValue>(V)) { + ConstantInt *CI = dyn_cast<ConstantInt>(V); - DEBUG(errs() << " Factoring out " << CommonPreds.size() - << " common predecessors.\n"); - return SplitBlockPredecessors(PN->getParent(), - &CommonPreds[0], CommonPreds.size(), - ".thr_comm", this); -} + for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) + Result.push_back(std::make_pair(CI, *PI)); + return true; + } + + // If V is a non-instruction value, or an instruction in a different block, + // then it can't be derived from a PHI. + Instruction *I = dyn_cast<Instruction>(V); + if (I == 0 || I->getParent() != BB) { + + // Okay, if this is a live-in value, see if it has a known value at the end + // of any of our predecessors. + // + // FIXME: This should be an edge property, not a block end property. + /// TODO: Per PR2563, we could infer value range information about a + /// predecessor based on its terminator. + // + if (LVI) { + // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if + // "I" is a non-local compare-with-a-constant instruction. This would be + // able to handle value inequalities better, for example if the compare is + // "X < 4" and "X < 3" is known true but "X < 4" itself is not available. + // Perhaps getConstantOnEdge should be smart enough to do this? + + for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { + // If the value is known by LazyValueInfo to be a constant in a + // predecessor, use that information to try to thread this block. + Constant *PredCst = LVI->getConstantOnEdge(V, *PI, BB); + if (PredCst == 0 || + (!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst))) + continue; + + Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), *PI)); + } + + return !Result.empty(); + } + + return false; + } + + /// If I is a PHI node, then we know the incoming values for any constants. + if (PHINode *PN = dyn_cast<PHINode>(I)) { + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { + Value *InVal = PN->getIncomingValue(i); + if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) { + ConstantInt *CI = dyn_cast<ConstantInt>(InVal); + Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i))); + } + } + return !Result.empty(); + } + + SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals; + + // Handle some boolean conditions. + if (I->getType()->getPrimitiveSizeInBits() == 1) { + // X | true -> true + // X & false -> false + if (I->getOpcode() == Instruction::Or || + I->getOpcode() == Instruction::And) { + ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals); + ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals); + + if (LHSVals.empty() && RHSVals.empty()) + return false; + + ConstantInt *InterestingVal; + if (I->getOpcode() == Instruction::Or) + InterestingVal = ConstantInt::getTrue(I->getContext()); + else + InterestingVal = ConstantInt::getFalse(I->getContext()); + + // Scan for the sentinel. + for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) + if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) + Result.push_back(LHSVals[i]); + for (unsigned i = 0, e = RHSVals.size(); i != e; ++i) + if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) + Result.push_back(RHSVals[i]); + return !Result.empty(); + } + + // Handle the NOT form of XOR. + if (I->getOpcode() == Instruction::Xor && + isa<ConstantInt>(I->getOperand(1)) && + cast<ConstantInt>(I->getOperand(1))->isOne()) { + ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result); + if (Result.empty()) + return false; + + // Invert the known values. + for (unsigned i = 0, e = Result.size(); i != e; ++i) + if (Result[i].first) + Result[i].first = + cast<ConstantInt>(ConstantExpr::getNot(Result[i].first)); + return true; + } + } + // Handle compare with phi operand, where the PHI is defined in this block. + if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) { + PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0)); + if (PN && PN->getParent() == BB) { + // We can do this simplification if any comparisons fold to true or false. + // See if any do. + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { + BasicBlock *PredBB = PN->getIncomingBlock(i); + Value *LHS = PN->getIncomingValue(i); + Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB); + + Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD); + if (Res == 0) { + if (!LVI || !isa<Constant>(RHS)) + continue; + + LazyValueInfo::Tristate + ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS, + cast<Constant>(RHS), PredBB, BB); + if (ResT == LazyValueInfo::Unknown) + continue; + Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT); + } + + if (isa<UndefValue>(Res)) + Result.push_back(std::make_pair((ConstantInt*)0, PredBB)); + else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res)) + Result.push_back(std::make_pair(CI, PredBB)); + } + + return !Result.empty(); + } + + + // If comparing a live-in value against a constant, see if we know the + // live-in value on any predecessors. + if (LVI && isa<Constant>(Cmp->getOperand(1)) && + Cmp->getType()->isInteger() && // Not vector compare. + (!isa<Instruction>(Cmp->getOperand(0)) || + cast<Instruction>(Cmp->getOperand(0))->getParent() != BB)) { + Constant *RHSCst = cast<Constant>(Cmp->getOperand(1)); + + for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { + // If the value is known by LazyValueInfo to be a constant in a + // predecessor, use that information to try to thread this block. + LazyValueInfo::Tristate + Res = LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0), + RHSCst, *PI, BB); + if (Res == LazyValueInfo::Unknown) + continue; + + Constant *ResC = ConstantInt::get(Cmp->getType(), Res); + Result.push_back(std::make_pair(cast<ConstantInt>(ResC), *PI)); + } + + return !Result.empty(); + } + } + return false; +} + + /// GetBestDestForBranchOnUndef - If we determine that the specified block ends /// in an undefined jump, decide which block is best to revector to. @@ -251,7 +449,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) { // successor, merge the blocks. This encourages recursive jump threading // because now the condition in this block can be threaded through // predecessors of our predecessor block. - if (BasicBlock *SinglePred = BB->getSinglePredecessor()) + if (BasicBlock *SinglePred = BB->getSinglePredecessor()) { if (SinglePred->getTerminator()->getNumSuccessors() == 1 && SinglePred != BB) { // If SinglePred was a loop header, BB becomes one. @@ -267,10 +465,10 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) { BB->moveBefore(&BB->getParent()->getEntryBlock()); return true; } - - // See if this block ends with a branch or switch. If so, see if the - // condition is a phi node. If so, and if an entry of the phi node is a - // constant, we can thread the block. + } + + // Look to see if the terminator is a branch of switch, if not we can't thread + // it. Value *Condition; if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) { // Can't thread an unconditional jump. @@ -301,7 +499,7 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) { TerminatorInst *BBTerm = BB->getTerminator(); for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { if (i == BestSucc) continue; - BBTerm->getSuccessor(i)->removePredecessor(BB); + RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD); } DEBUG(errs() << " In block '" << BB->getName() @@ -318,7 +516,8 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) { // br COND, BBX, BBY // BBX: // br COND, BBZ, BBW - if (!Condition->hasOneUse() && // Multiple uses. + if (!LVI && + !Condition->hasOneUse() && // Multiple uses. (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition. pred_iterator PI = pred_begin(BB), E = pred_end(BB); if (isa<BranchInst>(BB->getTerminator())) { @@ -338,52 +537,40 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) { } // All the rest of our checks depend on the condition being an instruction. - if (CondInst == 0) + if (CondInst == 0) { + // FIXME: Unify this with code below. + if (LVI && ProcessThreadableEdges(Condition, BB)) + return true; return false; + } + // See if this is a phi node in the current block. if (PHINode *PN = dyn_cast<PHINode>(CondInst)) if (PN->getParent() == BB) return ProcessJumpOnPHI(PN); - // If this is a conditional branch whose condition is and/or of a phi, try to - // simplify it. - if ((CondInst->getOpcode() == Instruction::And || - CondInst->getOpcode() == Instruction::Or) && - isa<BranchInst>(BB->getTerminator()) && - ProcessBranchOnLogical(CondInst, BB, - CondInst->getOpcode() == Instruction::And)) - return true; - if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) { - if (isa<PHINode>(CondCmp->getOperand(0))) { - // If we have "br (phi != 42)" and the phi node has any constant values - // as operands, we can thread through this block. - // - // If we have "br (cmp phi, x)" and the phi node contains x such that the - // comparison uniquely identifies the branch target, we can thread - // through this block. - - if (ProcessBranchOnCompare(CondCmp, BB)) - return true; - } - - // If we have a comparison, loop over the predecessors to see if there is - // a condition with the same value. - pred_iterator PI = pred_begin(BB), E = pred_end(BB); - for (; PI != E; ++PI) - if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) - if (PBI->isConditional() && *PI != BB) { - if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) { - if (CI->getOperand(0) == CondCmp->getOperand(0) && - CI->getOperand(1) == CondCmp->getOperand(1) && - CI->getPredicate() == CondCmp->getPredicate()) { - // TODO: Could handle things like (x != 4) --> (x == 17) - if (ProcessBranchOnDuplicateCond(*PI, BB)) - return true; + if (!LVI && + (!isa<PHINode>(CondCmp->getOperand(0)) || + cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) { + // If we have a comparison, loop over the predecessors to see if there is + // a condition with a lexically identical value. + pred_iterator PI = pred_begin(BB), E = pred_end(BB); + for (; PI != E; ++PI) + if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) + if (PBI->isConditional() && *PI != BB) { + if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) { + if (CI->getOperand(0) == CondCmp->getOperand(0) && + CI->getOperand(1) == CondCmp->getOperand(1) && + CI->getPredicate() == CondCmp->getPredicate()) { + // TODO: Could handle things like (x != 4) --> (x == 17) + if (ProcessBranchOnDuplicateCond(*PI, BB)) + return true; + } } } - } + } } // Check for some cases that are worth simplifying. Right now we want to look @@ -398,10 +585,21 @@ bool JumpThreading::ProcessBlock(BasicBlock *BB) { if (isa<Constant>(CondCmp->getOperand(1))) SimplifyValue = CondCmp->getOperand(0); + // TODO: There are other places where load PRE would be profitable, such as + // more complex comparisons. if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue)) if (SimplifyPartiallyRedundantLoad(LI)) return true; + + // Handle a variety of cases where we are branching on something derived from + // a PHI node in the current block. If we can prove that any predecessors + // compute a predictable value based on a PHI node, thread those predecessors. + // + if (ProcessThreadableEdges(CondInst, BB)) + return true; + + // TODO: If we have: "br (X > 0)" and we have a predecessor where we know // "(X == 4)" thread through this block. @@ -459,8 +657,11 @@ bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB, // Next, figure out which successor we are threading to. BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir); + SmallVector<BasicBlock*, 2> Preds; + Preds.push_back(PredBB); + // Ok, try to thread it! - return ThreadEdge(BB, PredBB, SuccBB); + return ThreadEdge(BB, Preds, SuccBB); } /// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that @@ -553,7 +754,7 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { Value *LoadedPtr = LI->getOperand(0); // If the loaded operand is defined in the LoadBB, it can't be available. - // FIXME: Could do PHI translation, that would be fun :) + // TODO: Could do simple PHI translation, that would be fun :) if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) if (PtrOp->getParent() == LoadBB) return false; @@ -562,8 +763,8 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { // the entry to its block. BasicBlock::iterator BBIt = LI; - if (Value *AvailableVal = FindAvailableLoadedValue(LoadedPtr, LoadBB, - BBIt, 6)) { + if (Value *AvailableVal = + FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) { // If the value if the load is locally available within the block, just use // it. This frequently occurs for reg2mem'd allocas. //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n"; @@ -646,7 +847,7 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { // Split them out to their own block. UnavailablePred = SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), - "thread-split", this); + "thread-pre-split", this); } // If the value isn't available in all predecessors, then there will be @@ -655,7 +856,8 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { if (UnavailablePred) { assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && "Can't handle critical edge here!"); - Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", + Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false, + LI->getAlignment(), UnavailablePred->getTerminator()); AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); } @@ -690,55 +892,183 @@ bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { return true; } - -/// ProcessJumpOnPHI - We have a conditional branch or switch on a PHI node in -/// the current block. See if there are any simplifications we can do based on -/// inputs to the phi node. -/// -bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) { - BasicBlock *BB = PN->getParent(); +/// FindMostPopularDest - The specified list contains multiple possible +/// threadable destinations. Pick the one that occurs the most frequently in +/// the list. +static BasicBlock * +FindMostPopularDest(BasicBlock *BB, + const SmallVectorImpl<std::pair<BasicBlock*, + BasicBlock*> > &PredToDestList) { + assert(!PredToDestList.empty()); + + // Determine popularity. If there are multiple possible destinations, we + // explicitly choose to ignore 'undef' destinations. We prefer to thread + // blocks with known and real destinations to threading undef. We'll handle + // them later if interesting. + DenseMap<BasicBlock*, unsigned> DestPopularity; + for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) + if (PredToDestList[i].second) + DestPopularity[PredToDestList[i].second]++; + + // Find the most popular dest. + DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin(); + BasicBlock *MostPopularDest = DPI->first; + unsigned Popularity = DPI->second; + SmallVector<BasicBlock*, 4> SamePopularity; + + for (++DPI; DPI != DestPopularity.end(); ++DPI) { + // If the popularity of this entry isn't higher than the popularity we've + // seen so far, ignore it. + if (DPI->second < Popularity) + ; // ignore. + else if (DPI->second == Popularity) { + // If it is the same as what we've seen so far, keep track of it. + SamePopularity.push_back(DPI->first); + } else { + // If it is more popular, remember it. + SamePopularity.clear(); + MostPopularDest = DPI->first; + Popularity = DPI->second; + } + } - // See if the phi node has any constant integer or undef values. If so, we - // can determine where the corresponding predecessor will branch. - for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { - Value *PredVal = PN->getIncomingValue(i); - - // Check to see if this input is a constant integer. If so, the direction - // of the branch is predictable. - if (ConstantInt *CI = dyn_cast<ConstantInt>(PredVal)) { - // Merge any common predecessors that will act the same. - BasicBlock *PredBB = FactorCommonPHIPreds(PN, CI); + // Okay, now we know the most popular destination. If there is more than + // destination, we need to determine one. This is arbitrary, but we need + // to make a deterministic decision. Pick the first one that appears in the + // successor list. + if (!SamePopularity.empty()) { + SamePopularity.push_back(MostPopularDest); + TerminatorInst *TI = BB->getTerminator(); + for (unsigned i = 0; ; ++i) { + assert(i != TI->getNumSuccessors() && "Didn't find any successor!"); - BasicBlock *SuccBB; - if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) - SuccBB = BI->getSuccessor(CI->isZero()); - else { - SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); - SuccBB = SI->getSuccessor(SI->findCaseValue(CI)); - } + if (std::find(SamePopularity.begin(), SamePopularity.end(), + TI->getSuccessor(i)) == SamePopularity.end()) + continue; - // Ok, try to thread it! - return ThreadEdge(BB, PredBB, SuccBB); + MostPopularDest = TI->getSuccessor(i); + break; } + } + + // Okay, we have finally picked the most popular destination. + return MostPopularDest; +} + +bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) { + // If threading this would thread across a loop header, don't even try to + // thread the edge. + if (LoopHeaders.count(BB)) + return false; + + SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues; + if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues)) + return false; + assert(!PredValues.empty() && + "ComputeValueKnownInPredecessors returned true with no values"); + + DEBUG(errs() << "IN BB: " << *BB; + for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { + errs() << " BB '" << BB->getName() << "': FOUND condition = "; + if (PredValues[i].first) + errs() << *PredValues[i].first; + else + errs() << "UNDEF"; + errs() << " for pred '" << PredValues[i].second->getName() + << "'.\n"; + }); + + // Decide what we want to thread through. Convert our list of known values to + // a list of known destinations for each pred. This also discards duplicate + // predecessors and keeps track of the undefined inputs (which are represented + // as a null dest in the PredToDestList). + SmallPtrSet<BasicBlock*, 16> SeenPreds; + SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList; + + BasicBlock *OnlyDest = 0; + BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL; + + for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { + BasicBlock *Pred = PredValues[i].second; + if (!SeenPreds.insert(Pred)) + continue; // Duplicate predecessor entry. - // If the input is an undef, then it doesn't matter which way it will go. - // Pick an arbitrary dest and thread the edge. - if (UndefValue *UV = dyn_cast<UndefValue>(PredVal)) { - // Merge any common predecessors that will act the same. - BasicBlock *PredBB = FactorCommonPHIPreds(PN, UV); - BasicBlock *SuccBB = - BB->getTerminator()->getSuccessor(GetBestDestForJumpOnUndef(BB)); - - // Ok, try to thread it! - return ThreadEdge(BB, PredBB, SuccBB); + // If the predecessor ends with an indirect goto, we can't change its + // destination. + if (isa<IndirectBrInst>(Pred->getTerminator())) + continue; + + ConstantInt *Val = PredValues[i].first; + + BasicBlock *DestBB; + if (Val == 0) // Undef. + DestBB = 0; + else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) + DestBB = BI->getSuccessor(Val->isZero()); + else { + SwitchInst *SI = cast<SwitchInst>(BB->getTerminator()); + DestBB = SI->getSuccessor(SI->findCaseValue(Val)); } + + // If we have exactly one destination, remember it for efficiency below. + if (i == 0) + OnlyDest = DestBB; + else if (OnlyDest != DestBB) + OnlyDest = MultipleDestSentinel; + + PredToDestList.push_back(std::make_pair(Pred, DestBB)); } - // If the incoming values are all variables, we don't know the destination of - // any predecessors. However, if any of the predecessor blocks end in an - // unconditional branch, we can *duplicate* the jump into that block in order - // to further encourage jump threading and to eliminate cases where we have - // branch on a phi of an icmp (branch on icmp is much better). + // If all edges were unthreadable, we fail. + if (PredToDestList.empty()) + return false; + + // Determine which is the most common successor. If we have many inputs and + // this block is a switch, we want to start by threading the batch that goes + // to the most popular destination first. If we only know about one + // threadable destination (the common case) we can avoid this. + BasicBlock *MostPopularDest = OnlyDest; + + if (MostPopularDest == MultipleDestSentinel) + MostPopularDest = FindMostPopularDest(BB, PredToDestList); + + // Now that we know what the most popular destination is, factor all + // predecessors that will jump to it into a single predecessor. + SmallVector<BasicBlock*, 16> PredsToFactor; + for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i) + if (PredToDestList[i].second == MostPopularDest) { + BasicBlock *Pred = PredToDestList[i].first; + + // This predecessor may be a switch or something else that has multiple + // edges to the block. Factor each of these edges by listing them + // according to # occurrences in PredsToFactor. + TerminatorInst *PredTI = Pred->getTerminator(); + for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i) + if (PredTI->getSuccessor(i) == BB) + PredsToFactor.push_back(Pred); + } + + // If the threadable edges are branching on an undefined value, we get to pick + // the destination that these predecessors should get to. + if (MostPopularDest == 0) + MostPopularDest = BB->getTerminator()-> + getSuccessor(GetBestDestForJumpOnUndef(BB)); + + // Ok, try to thread it! + return ThreadEdge(BB, PredsToFactor, MostPopularDest); +} + +/// ProcessJumpOnPHI - We have a conditional branch or switch on a PHI node in +/// the current block. See if there are any simplifications we can do based on +/// inputs to the phi node. +/// +bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) { + BasicBlock *BB = PN->getParent(); + + // If any of the predecessor blocks end in an unconditional branch, we can + // *duplicate* the jump into that block in order to further encourage jump + // threading and to eliminate cases where we have branch on a phi of an icmp + // (branch on icmp is much better). // We don't want to do this tranformation for switches, because we don't // really want to duplicate a switch. @@ -759,137 +1089,6 @@ bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) { } -/// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch -/// whose condition is an AND/OR where one side is PN. If PN has constant -/// operands that permit us to evaluate the condition for some operand, thread -/// through the block. For example with: -/// br (and X, phi(Y, Z, false)) -/// the predecessor corresponding to the 'false' will always jump to the false -/// destination of the branch. -/// -bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB, - bool isAnd) { - // If this is a binary operator tree of the same AND/OR opcode, check the - // LHS/RHS. - if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V)) - if ((isAnd && BO->getOpcode() == Instruction::And) || - (!isAnd && BO->getOpcode() == Instruction::Or)) { - if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd)) - return true; - if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd)) - return true; - } - - // If this isn't a PHI node, we can't handle it. - PHINode *PN = dyn_cast<PHINode>(V); - if (!PN || PN->getParent() != BB) return false; - - // We can only do the simplification for phi nodes of 'false' with AND or - // 'true' with OR. See if we have any entries in the phi for this. - unsigned PredNo = ~0U; - ConstantInt *PredCst = ConstantInt::get(Type::getInt1Ty(BB->getContext()), - !isAnd); - for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { - if (PN->getIncomingValue(i) == PredCst) { - PredNo = i; - break; - } - } - - // If no match, bail out. - if (PredNo == ~0U) - return false; - - // If so, we can actually do this threading. Merge any common predecessors - // that will act the same. - BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst); - - // Next, figure out which successor we are threading to. If this was an AND, - // the constant must be FALSE, and we must be targeting the 'false' block. - // If this is an OR, the constant must be TRUE, and we must be targeting the - // 'true' block. - BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd); - - // Ok, try to thread it! - return ThreadEdge(BB, PredBB, SuccBB); -} - -/// GetResultOfComparison - Given an icmp/fcmp predicate and the left and right -/// hand sides of the compare instruction, try to determine the result. If the -/// result can not be determined, a null pointer is returned. -static Constant *GetResultOfComparison(CmpInst::Predicate pred, - Value *LHS, Value *RHS, - LLVMContext &Context) { - if (Constant *CLHS = dyn_cast<Constant>(LHS)) - if (Constant *CRHS = dyn_cast<Constant>(RHS)) - return ConstantExpr::getCompare(pred, CLHS, CRHS); - - if (LHS == RHS) - if (isa<IntegerType>(LHS->getType()) || isa<PointerType>(LHS->getType())) - return ICmpInst::isTrueWhenEqual(pred) ? - ConstantInt::getTrue(Context) : ConstantInt::getFalse(Context); - - return 0; -} - -/// ProcessBranchOnCompare - We found a branch on a comparison between a phi -/// node and a value. If we can identify when the comparison is true between -/// the phi inputs and the value, we can fold the compare for that edge and -/// thread through it. -bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) { - PHINode *PN = cast<PHINode>(Cmp->getOperand(0)); - Value *RHS = Cmp->getOperand(1); - - // If the phi isn't in the current block, an incoming edge to this block - // doesn't control the destination. - if (PN->getParent() != BB) - return false; - - // We can do this simplification if any comparisons fold to true or false. - // See if any do. - Value *PredVal = 0; - bool TrueDirection = false; - for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { - PredVal = PN->getIncomingValue(i); - - Constant *Res = GetResultOfComparison(Cmp->getPredicate(), PredVal, - RHS, Cmp->getContext()); - if (!Res) { - PredVal = 0; - continue; - } - - // If this folded to a constant expr, we can't do anything. - if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) { - TrueDirection = ResC->getZExtValue(); - break; - } - // If this folded to undef, just go the false way. - if (isa<UndefValue>(Res)) { - TrueDirection = false; - break; - } - - // Otherwise, we can't fold this input. - PredVal = 0; - } - - // If no match, bail out. - if (PredVal == 0) - return false; - - // If so, we can actually do this threading. Merge any common predecessors - // that will act the same. - BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredVal); - - // Next, get our successor. - BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection); - - // Ok, try to thread it! - return ThreadEdge(BB, PredBB, SuccBB); -} - - /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new /// predecessor to the PHIBB block. If it has PHI nodes, add entries for /// NewPred using the entries from OldPred (suitably mapped). @@ -914,10 +1113,11 @@ static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB, } } -/// ThreadEdge - We have decided that it is safe and profitable to thread an -/// edge from PredBB to SuccBB across BB. Transform the IR to reflect this -/// change. -bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, +/// ThreadEdge - We have decided that it is safe and profitable to factor the +/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB +/// across BB. Transform the IR to reflect this change. +bool JumpThreading::ThreadEdge(BasicBlock *BB, + const SmallVectorImpl<BasicBlock*> &PredBBs, BasicBlock *SuccBB) { // If threading to the same block as we come from, we would infinite loop. if (SuccBB == BB) { @@ -929,8 +1129,7 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, // If threading this would thread across a loop header, don't thread the edge. // See the comments above FindLoopHeaders for justifications and caveats. if (LoopHeaders.count(BB)) { - DEBUG(errs() << " Not threading from '" << PredBB->getName() - << "' across loop header BB '" << BB->getName() + DEBUG(errs() << " Not threading across loop header BB '" << BB->getName() << "' to dest BB '" << SuccBB->getName() << "' - it might create an irreducible loop!\n"); return false; @@ -943,6 +1142,17 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, return false; } + // And finally, do it! Start by factoring the predecessors is needed. + BasicBlock *PredBB; + if (PredBBs.size() == 1) + PredBB = PredBBs[0]; + else { + DEBUG(errs() << " Factoring out " << PredBBs.size() + << " common predecessors.\n"); + PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(), + ".thr_comm", this); + } + // And finally, do it! DEBUG(errs() << " Threading edge from '" << PredBB->getName() << "' to '" << SuccBB->getName() << "' with cost: " << JumpThreadCost @@ -1034,7 +1244,7 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, TerminatorInst *PredTerm = PredBB->getTerminator(); for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) if (PredTerm->getSuccessor(i) == BB) { - BB->removePredecessor(PredBB); + RemovePredecessorAndSimplify(BB, PredBB, TD); PredTerm->setSuccessor(i, NewBB); } @@ -1044,9 +1254,12 @@ bool JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BI = NewBB->begin(); for (BasicBlock::iterator E = NewBB->end(); BI != E; ) { Instruction *Inst = BI++; - if (Constant *C = ConstantFoldInstruction(Inst, BB->getContext(), TD)) { - Inst->replaceAllUsesWith(C); - Inst->eraseFromParent(); + + if (Value *V = SimplifyInstruction(Inst, TD)) { + WeakVH BIHandle(BI); + ReplaceAndSimplifyAllUses(Inst, V, TD); + if (BIHandle == 0) + BI = NewBB->begin(); continue; } @@ -1164,7 +1377,7 @@ bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, // PredBB no longer jumps to BB, remove entries in the PHI node for the edge // that we nuked. - BB->removePredecessor(PredBB); + RemovePredecessorAndSimplify(BB, PredBB, TD); // Remove the unconditional branch at the end of the PredBB block. OldPredBranch->eraseFromParent(); |
