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Diffstat (limited to 'contrib/llvm/lib/Transforms/Scalar/JumpThreading.cpp')
| -rw-r--r-- | contrib/llvm/lib/Transforms/Scalar/JumpThreading.cpp | 1534 |
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diff --git a/contrib/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/contrib/llvm/lib/Transforms/Scalar/JumpThreading.cpp new file mode 100644 index 0000000..df05b71 --- /dev/null +++ b/contrib/llvm/lib/Transforms/Scalar/JumpThreading.cpp @@ -0,0 +1,1534 @@ +//===- JumpThreading.cpp - Thread control through conditional blocks ------===// +// +// The LLVM Compiler Infrastructure +// +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. +// +//===----------------------------------------------------------------------===// +// +// This file implements the Jump Threading pass. +// +//===----------------------------------------------------------------------===// + +#define DEBUG_TYPE "jump-threading" +#include "llvm/Transforms/Scalar.h" +#include "llvm/IntrinsicInst.h" +#include "llvm/LLVMContext.h" +#include "llvm/Pass.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" +#include "llvm/Target/TargetData.h" +#include "llvm/ADT/DenseMap.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/ADT/STLExtras.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/SmallSet.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/Support/Debug.h" +#include "llvm/Support/ValueHandle.h" +#include "llvm/Support/raw_ostream.h" +using namespace llvm; + +STATISTIC(NumThreads, "Number of jumps threaded"); +STATISTIC(NumFolds, "Number of terminators folded"); +STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi"); + +static cl::opt<unsigned> +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 + /// predecessors of the block can be proven to always jump to one of the + /// successors, we forward the edge from the predecessor to the successor by + /// duplicating the contents of this block. + /// + /// An example of when this can occur is code like this: + /// + /// if () { ... + /// X = 4; + /// } + /// if (X < 3) { + /// + /// In this case, the unconditional branch at the end of the first if can be + /// revectored to the false side of the second if. + /// + class JumpThreading : public FunctionPass { + TargetData *TD; + LazyValueInfo *LVI; +#ifdef NDEBUG + SmallPtrSet<BasicBlock*, 16> LoopHeaders; +#else + SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders; +#endif + public: + static char ID; // Pass identification + JumpThreading() : FunctionPass(&ID) {} + + bool runOnFunction(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, const SmallVectorImpl<BasicBlock*> &PredBBs, + BasicBlock *SuccBB); + bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, + const SmallVectorImpl<BasicBlock *> &PredBBs); + + 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 ProcessBranchOnPHI(PHINode *PN); + bool ProcessBranchOnXOR(BinaryOperator *BO); + + bool SimplifyPartiallyRedundantLoad(LoadInst *LI); + }; +} + +char JumpThreading::ID = 0; +static RegisterPass<JumpThreading> +X("jump-threading", "Jump Threading"); + +// Public interface to the Jump Threading pass +FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); } + +/// runOnFunction - Top level algorithm. +/// +bool JumpThreading::runOnFunction(Function &F) { + DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n"); + TD = getAnalysisIfAvailable<TargetData>(); + LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0; + + FindLoopHeaders(F); + + bool Changed, EverChanged = false; + do { + 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; + + ++I; + + // If the block is trivially dead, zap it. This eliminates the successor + // edges which simplifies the CFG. + if (pred_begin(BB) == pred_end(BB) && + BB != &BB->getParent()->getEntryBlock()) { + DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName() + << "' with terminator: " << *BB->getTerminator() << '\n'); + 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 if needed. + bool ErasedFromLoopHeaders = LoopHeaders.erase(BB); + BasicBlock *Succ = BI->getSuccessor(0); + + if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) { + Changed = true; + // If we deleted BB and BB was the header of a loop, then the + // successor is now the header of the loop. + BB = Succ; + } + + if (ErasedFromLoopHeaders) + LoopHeaders.insert(BB); + } + } + } + } + EverChanged |= Changed; + } while (Changed); + + LoopHeaders.clear(); + return EverChanged; +} + +/// getJumpThreadDuplicationCost - Return the cost of duplicating this block to +/// thread across it. +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; + for (; !isa<TerminatorInst>(I); ++I) { + // Debugger intrinsics don't incur code size. + if (isa<DbgInfoIntrinsic>(I)) continue; + + // If this is a pointer->pointer bitcast, it is free. + if (isa<BitCastInst>(I) && I->getType()->isPointerTy()) + continue; + + // All other instructions count for at least one unit. + ++Size; + + // Calls are more expensive. If they are non-intrinsic calls, we model them + // as having cost of 4. If they are a non-vector intrinsic, we model them + // as having cost of 2 total, and if they are a vector intrinsic, we model + // them as having cost 1. + if (const CallInst *CI = dyn_cast<CallInst>(I)) { + if (!isa<IntrinsicInst>(CI)) + Size += 3; + else if (!CI->getType()->isVectorTy()) + Size += 1; + } + } + + // Threading through a switch statement is particularly profitable. If this + // block ends in a switch, decrease its cost to make it more likely to happen. + if (isa<SwitchInst>(I)) + Size = Size > 6 ? Size-6 : 0; + + 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 +/// happening, we first have to find the loop headers. Here we approximate this +/// by finding targets of backedges in the CFG. +/// +/// Note that there definitely are cases when we want to allow threading of +/// edges across a loop header. For example, threading a jump from outside the +/// loop (the preheader) to an exit block of the loop is definitely profitable. +/// It is also almost always profitable to thread backedges from within the loop +/// to exit blocks, and is often profitable to thread backedges to other blocks +/// within the loop (forming a nested loop). This simple analysis is not rich +/// enough to track all of these properties and keep it up-to-date as the CFG +/// mutates, so we don't allow any of these transformations. +/// +void JumpThreading::FindLoopHeaders(Function &F) { + SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges; + FindFunctionBackedges(F, Edges); + + for (unsigned i = 0, e = Edges.size(); i != e; ++i) + LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second)); +} + +/// 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. +/// +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); + + 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. If we find an undef, force it to the + // interesting value: x|undef -> true and x&undef -> false. + for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) + if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) { + Result.push_back(LHSVals[i]); + Result.back().first = InterestingVal; + } + for (unsigned i = 0, e = RHSVals.size(); i != e; ++i) + if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) { + Result.push_back(RHSVals[i]); + Result.back().first = InterestingVal; + } + 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()->isIntegerTy() && // 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. +/// +/// Since we can pick an arbitrary destination, we pick the successor with the +/// fewest predecessors. This should reduce the in-degree of the others. +/// +static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) { + TerminatorInst *BBTerm = BB->getTerminator(); + unsigned MinSucc = 0; + BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc); + // Compute the successor with the minimum number of predecessors. + unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); + for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) { + TestBB = BBTerm->getSuccessor(i); + unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB)); + if (NumPreds < MinNumPreds) + MinSucc = i; + } + + return MinSucc; +} + +/// ProcessBlock - If there are any predecessors whose control can be threaded +/// through to a successor, transform them now. +bool JumpThreading::ProcessBlock(BasicBlock *BB) { + // If the block is trivially dead, just return and let the caller nuke it. + // This simplifies other transformations. + if (pred_begin(BB) == pred_end(BB) && + BB != &BB->getParent()->getEntryBlock()) + return false; + + // If this block has a single predecessor, and if that pred has a single + // 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 (SinglePred->getTerminator()->getNumSuccessors() == 1 && + SinglePred != BB) { + // If SinglePred was a loop header, BB becomes one. + if (LoopHeaders.erase(SinglePred)) + LoopHeaders.insert(BB); + + // Remember if SinglePred was the entry block of the function. If so, we + // will need to move BB back to the entry position. + bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock(); + MergeBasicBlockIntoOnlyPred(BB); + + if (isEntry && BB != &BB->getParent()->getEntryBlock()) + BB->moveBefore(&BB->getParent()->getEntryBlock()); + return true; + } + } + + // 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. + if (BI->isUnconditional()) return false; + Condition = BI->getCondition(); + } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) + Condition = SI->getCondition(); + else + return false; // Must be an invoke. + + // If the terminator of this block is branching on a constant, simplify the + // terminator to an unconditional branch. This can occur due to threading in + // other blocks. + if (isa<ConstantInt>(Condition)) { + DEBUG(dbgs() << " In block '" << BB->getName() + << "' folding terminator: " << *BB->getTerminator() << '\n'); + ++NumFolds; + ConstantFoldTerminator(BB); + return true; + } + + // If the terminator is branching on an undef, we can pick any of the + // successors to branch to. Let GetBestDestForJumpOnUndef decide. + if (isa<UndefValue>(Condition)) { + unsigned BestSucc = GetBestDestForJumpOnUndef(BB); + + // Fold the branch/switch. + TerminatorInst *BBTerm = BB->getTerminator(); + for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) { + if (i == BestSucc) continue; + RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD); + } + + DEBUG(dbgs() << " In block '" << BB->getName() + << "' folding undef terminator: " << *BBTerm << '\n'); + BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm); + BBTerm->eraseFromParent(); + return true; + } + + Instruction *CondInst = dyn_cast<Instruction>(Condition); + + // If the condition is an instruction defined in another block, see if a + // predecessor has the same condition: + // br COND, BBX, BBY + // BBX: + // br COND, BBZ, BBW + 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())) { + for (; PI != E; ++PI) + if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator())) + if (PBI->isConditional() && PBI->getCondition() == Condition && + ProcessBranchOnDuplicateCond(*PI, BB)) + return true; + } else { + assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator"); + for (; PI != E; ++PI) + if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator())) + if (PSI->getCondition() == Condition && + ProcessSwitchOnDuplicateCond(*PI, BB)) + return true; + } + } + + // All the rest of our checks depend on the condition being an instruction. + if (CondInst == 0) { + // FIXME: Unify this with code below. + if (LVI && ProcessThreadableEdges(Condition, BB)) + return true; + return false; + } + + + if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) { + 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 + // for loads that are used by a switch or by the condition for the branch. If + // we see one, check to see if it's partially redundant. If so, insert a PHI + // which can then be used to thread the values. + // + Value *SimplifyValue = CondInst; + if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue)) + 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; + + // If this is an otherwise-unfoldable branch on a phi node in the current + // block, see if we can simplify. + if (PHINode *PN = dyn_cast<PHINode>(CondInst)) + if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator())) + return ProcessBranchOnPHI(PN); + + + // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify. + if (CondInst->getOpcode() == Instruction::Xor && + CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator())) + return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst)); + + + // TODO: If we have: "br (X > 0)" and we have a predecessor where we know + // "(X == 4)", thread through this block. + + return false; +} + +/// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that +/// block that jump on exactly the same condition. This means that we almost +/// always know the direction of the edge in the DESTBB: +/// PREDBB: +/// br COND, DESTBB, BBY +/// DESTBB: +/// br COND, BBZ, BBW +/// +/// If DESTBB has multiple predecessors, we can't just constant fold the branch +/// in DESTBB, we have to thread over it. +bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB, + BasicBlock *BB) { + BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator()); + + // If both successors of PredBB go to DESTBB, we don't know anything. We can + // fold the branch to an unconditional one, which allows other recursive + // simplifications. + bool BranchDir; + if (PredBI->getSuccessor(1) != BB) + BranchDir = true; + else if (PredBI->getSuccessor(0) != BB) + BranchDir = false; + else { + DEBUG(dbgs() << " In block '" << PredBB->getName() + << "' folding terminator: " << *PredBB->getTerminator() << '\n'); + ++NumFolds; + ConstantFoldTerminator(PredBB); + return true; + } + + BranchInst *DestBI = cast<BranchInst>(BB->getTerminator()); + + // If the dest block has one predecessor, just fix the branch condition to a + // constant and fold it. + if (BB->getSinglePredecessor()) { + DEBUG(dbgs() << " In block '" << BB->getName() + << "' folding condition to '" << BranchDir << "': " + << *BB->getTerminator() << '\n'); + ++NumFolds; + Value *OldCond = DestBI->getCondition(); + DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()), + BranchDir)); + // Delete dead instructions before we fold the branch. Folding the branch + // can eliminate edges from the CFG which can end up deleting OldCond. + RecursivelyDeleteTriviallyDeadInstructions(OldCond); + ConstantFoldTerminator(BB); + return true; + } + + + // 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, Preds, SuccBB); +} + +/// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that +/// block that switch on exactly the same condition. This means that we almost +/// always know the direction of the edge in the DESTBB: +/// PREDBB: +/// switch COND [... DESTBB, BBY ... ] +/// DESTBB: +/// switch COND [... BBZ, BBW ] +/// +/// Optimizing switches like this is very important, because simplifycfg builds +/// switches out of repeated 'if' conditions. +bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, + BasicBlock *DestBB) { + // Can't thread edge to self. + if (PredBB == DestBB) + return false; + + SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator()); + SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator()); + + // There are a variety of optimizations that we can potentially do on these + // blocks: we order them from most to least preferable. + + // If DESTBB *just* contains the switch, then we can forward edges from PREDBB + // directly to their destination. This does not introduce *any* code size + // growth. Skip debug info first. + BasicBlock::iterator BBI = DestBB->begin(); + while (isa<DbgInfoIntrinsic>(BBI)) + BBI++; + + // FIXME: Thread if it just contains a PHI. + if (isa<SwitchInst>(BBI)) { + bool MadeChange = false; + // Ignore the default edge for now. + for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) { + ConstantInt *DestVal = DestSI->getCaseValue(i); + BasicBlock *DestSucc = DestSI->getSuccessor(i); + + // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if + // PredSI has an explicit case for it. If so, forward. If it is covered + // by the default case, we can't update PredSI. + unsigned PredCase = PredSI->findCaseValue(DestVal); + if (PredCase == 0) continue; + + // If PredSI doesn't go to DestBB on this value, then it won't reach the + // case on this condition. + if (PredSI->getSuccessor(PredCase) != DestBB && + DestSI->getSuccessor(i) != DestBB) + continue; + + // Do not forward this if it already goes to this destination, this would + // be an infinite loop. + if (PredSI->getSuccessor(PredCase) == DestSucc) + continue; + + // Otherwise, we're safe to make the change. Make sure that the edge from + // DestSI to DestSucc is not critical and has no PHI nodes. + DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI); + DEBUG(dbgs() << "THROUGH: " << *DestSI); + + // If the destination has PHI nodes, just split the edge for updating + // simplicity. + if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){ + SplitCriticalEdge(DestSI, i, this); + DestSucc = DestSI->getSuccessor(i); + } + FoldSingleEntryPHINodes(DestSucc); + PredSI->setSuccessor(PredCase, DestSucc); + MadeChange = true; + } + + if (MadeChange) + return true; + } + + return false; +} + + +/// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant +/// load instruction, eliminate it by replacing it with a PHI node. This is an +/// important optimization that encourages jump threading, and needs to be run +/// interlaced with other jump threading tasks. +bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) { + // Don't hack volatile loads. + if (LI->isVolatile()) return false; + + // If the load is defined in a block with exactly one predecessor, it can't be + // partially redundant. + BasicBlock *LoadBB = LI->getParent(); + if (LoadBB->getSinglePredecessor()) + return false; + + Value *LoadedPtr = LI->getOperand(0); + + // If the loaded operand is defined in the LoadBB, it can't be available. + // TODO: Could do simple PHI translation, that would be fun :) + if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr)) + if (PtrOp->getParent() == LoadBB) + return false; + + // Scan a few instructions up from the load, to see if it is obviously live at + // the entry to its block. + BasicBlock::iterator BBIt = LI; + + 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"; + + // If the returned value is the load itself, replace with an undef. This can + // only happen in dead loops. + if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType()); + LI->replaceAllUsesWith(AvailableVal); + LI->eraseFromParent(); + return true; + } + + // Otherwise, if we scanned the whole block and got to the top of the block, + // we know the block is locally transparent to the load. If not, something + // might clobber its value. + if (BBIt != LoadBB->begin()) + return false; + + + SmallPtrSet<BasicBlock*, 8> PredsScanned; + typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; + AvailablePredsTy AvailablePreds; + BasicBlock *OneUnavailablePred = 0; + + // If we got here, the loaded value is transparent through to the start of the + // block. Check to see if it is available in any of the predecessor blocks. + for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); + PI != PE; ++PI) { + BasicBlock *PredBB = *PI; + + // If we already scanned this predecessor, skip it. + if (!PredsScanned.insert(PredBB)) + continue; + + // Scan the predecessor to see if the value is available in the pred. + BBIt = PredBB->end(); + Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6); + if (!PredAvailable) { + OneUnavailablePred = PredBB; + continue; + } + + // If so, this load is partially redundant. Remember this info so that we + // can create a PHI node. + AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable)); + } + + // If the loaded value isn't available in any predecessor, it isn't partially + // redundant. + if (AvailablePreds.empty()) return false; + + // Okay, the loaded value is available in at least one (and maybe all!) + // predecessors. If the value is unavailable in more than one unique + // predecessor, we want to insert a merge block for those common predecessors. + // This ensures that we only have to insert one reload, thus not increasing + // code size. + BasicBlock *UnavailablePred = 0; + + // If there is exactly one predecessor where the value is unavailable, the + // already computed 'OneUnavailablePred' block is it. If it ends in an + // unconditional branch, we know that it isn't a critical edge. + if (PredsScanned.size() == AvailablePreds.size()+1 && + OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) { + UnavailablePred = OneUnavailablePred; + } else if (PredsScanned.size() != AvailablePreds.size()) { + // Otherwise, we had multiple unavailable predecessors or we had a critical + // edge from the one. + SmallVector<BasicBlock*, 8> PredsToSplit; + SmallPtrSet<BasicBlock*, 8> AvailablePredSet; + + for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i) + AvailablePredSet.insert(AvailablePreds[i].first); + + // Add all the unavailable predecessors to the PredsToSplit list. + for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB); + PI != PE; ++PI) + if (!AvailablePredSet.count(*PI)) + PredsToSplit.push_back(*PI); + + // Split them out to their own block. + UnavailablePred = + SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(), + "thread-pre-split", this); + } + + // If the value isn't available in all predecessors, then there will be + // exactly one where it isn't available. Insert a load on that edge and add + // it to the AvailablePreds list. + if (UnavailablePred) { + assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 && + "Can't handle critical edge here!"); + Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false, + LI->getAlignment(), + UnavailablePred->getTerminator()); + AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal)); + } + + // Now we know that each predecessor of this block has a value in + // AvailablePreds, sort them for efficient access as we're walking the preds. + array_pod_sort(AvailablePreds.begin(), AvailablePreds.end()); + + // Create a PHI node at the start of the block for the PRE'd load value. + PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin()); + PN->takeName(LI); + + // Insert new entries into the PHI for each predecessor. A single block may + // have multiple entries here. + for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E; + ++PI) { + AvailablePredsTy::iterator I = + std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), + std::make_pair(*PI, (Value*)0)); + + assert(I != AvailablePreds.end() && I->first == *PI && + "Didn't find entry for predecessor!"); + + PN->addIncoming(I->second, I->first); + } + + //cerr << "PRE: " << *LI << *PN << "\n"; + + LI->replaceAllUsesWith(PN); + LI->eraseFromParent(); + + return true; +} + +/// 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; + } + } + + // 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!"); + + if (std::find(SamePopularity.begin(), SamePopularity.end(), + TI->getSuccessor(i)) == SamePopularity.end()) + continue; + + 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(dbgs() << "IN BB: " << *BB; + for (unsigned i = 0, e = PredValues.size(); i != e; ++i) { + dbgs() << " BB '" << BB->getName() << "': FOUND condition = "; + if (PredValues[i].first) + dbgs() << *PredValues[i].first; + else + dbgs() << "UNDEF"; + dbgs() << " 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 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 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); +} + +/// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch 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::ProcessBranchOnPHI(PHINode *PN) { + BasicBlock *BB = PN->getParent(); + + // TODO: We could make use of this to do it once for blocks with common PHI + // values. + SmallVector<BasicBlock*, 1> PredBBs; + PredBBs.resize(1); + + // If any of the predecessor blocks end in an unconditional branch, we can + // *duplicate* the conditional branch 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). + for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { + BasicBlock *PredBB = PN->getIncomingBlock(i); + if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator())) + if (PredBr->isUnconditional()) { + PredBBs[0] = PredBB; + // Try to duplicate BB into PredBB. + if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs)) + return true; + } + } + + return false; +} + +/// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on +/// a xor instruction in the current block. See if there are any +/// simplifications we can do based on inputs to the xor. +/// +bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) { + BasicBlock *BB = BO->getParent(); + + // If either the LHS or RHS of the xor is a constant, don't do this + // optimization. + if (isa<ConstantInt>(BO->getOperand(0)) || + isa<ConstantInt>(BO->getOperand(1))) + return false; + + // If the first instruction in BB isn't a phi, we won't be able to infer + // anything special about any particular predecessor. + if (!isa<PHINode>(BB->front())) + return false; + + // If we have a xor as the branch input to this block, and we know that the + // LHS or RHS of the xor in any predecessor is true/false, then we can clone + // the condition into the predecessor and fix that value to true, saving some + // logical ops on that path and encouraging other paths to simplify. + // + // This copies something like this: + // + // BB: + // %X = phi i1 [1], [%X'] + // %Y = icmp eq i32 %A, %B + // %Z = xor i1 %X, %Y + // br i1 %Z, ... + // + // Into: + // BB': + // %Y = icmp ne i32 %A, %B + // br i1 %Z, ... + + SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues; + bool isLHS = true; + if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) { + assert(XorOpValues.empty()); + if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues)) + return false; + isLHS = false; + } + + assert(!XorOpValues.empty() && + "ComputeValueKnownInPredecessors returned true with no values"); + + // Scan the information to see which is most popular: true or false. The + // predecessors can be of the set true, false, or undef. + unsigned NumTrue = 0, NumFalse = 0; + for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { + if (!XorOpValues[i].first) continue; // Ignore undefs for the count. + if (XorOpValues[i].first->isZero()) + ++NumFalse; + else + ++NumTrue; + } + + // Determine which value to split on, true, false, or undef if neither. + ConstantInt *SplitVal = 0; + if (NumTrue > NumFalse) + SplitVal = ConstantInt::getTrue(BB->getContext()); + else if (NumTrue != 0 || NumFalse != 0) + SplitVal = ConstantInt::getFalse(BB->getContext()); + + // Collect all of the blocks that this can be folded into so that we can + // factor this once and clone it once. + SmallVector<BasicBlock*, 8> BlocksToFoldInto; + for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) { + if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue; + + BlocksToFoldInto.push_back(XorOpValues[i].second); + } + + // If we inferred a value for all of the predecessors, then duplication won't + // help us. However, we can just replace the LHS or RHS with the constant. + if (BlocksToFoldInto.size() == + cast<PHINode>(BB->front()).getNumIncomingValues()) { + if (SplitVal == 0) { + // If all preds provide undef, just nuke the xor, because it is undef too. + BO->replaceAllUsesWith(UndefValue::get(BO->getType())); + BO->eraseFromParent(); + } else if (SplitVal->isZero()) { + // If all preds provide 0, replace the xor with the other input. + BO->replaceAllUsesWith(BO->getOperand(isLHS)); + BO->eraseFromParent(); + } else { + // If all preds provide 1, set the computed value to 1. + BO->setOperand(!isLHS, SplitVal); + } + + return true; + } + + // Try to duplicate BB into PredBB. + return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto); +} + + +/// 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). +static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB, + BasicBlock *OldPred, + BasicBlock *NewPred, + DenseMap<Instruction*, Value*> &ValueMap) { + for (BasicBlock::iterator PNI = PHIBB->begin(); + PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) { + // Ok, we have a PHI node. Figure out what the incoming value was for the + // DestBlock. + Value *IV = PN->getIncomingValueForBlock(OldPred); + + // Remap the value if necessary. + if (Instruction *Inst = dyn_cast<Instruction>(IV)) { + DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst); + if (I != ValueMap.end()) + IV = I->second; + } + + PN->addIncoming(IV, NewPred); + } +} + +/// 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) { + DEBUG(dbgs() << " Not threading across BB '" << BB->getName() + << "' - would thread to self!\n"); + return false; + } + + // 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(dbgs() << " Not threading across loop header BB '" << BB->getName() + << "' to dest BB '" << SuccBB->getName() + << "' - it might create an irreducible loop!\n"); + return false; + } + + unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB); + if (JumpThreadCost > Threshold) { + DEBUG(dbgs() << " Not threading BB '" << BB->getName() + << "' - Cost is too high: " << JumpThreadCost << "\n"); + return false; + } + + // And finally, do it! Start by factoring the predecessors is needed. + BasicBlock *PredBB; + if (PredBBs.size() == 1) + PredBB = PredBBs[0]; + else { + DEBUG(dbgs() << " Factoring out " << PredBBs.size() + << " common predecessors.\n"); + PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(), + ".thr_comm", this); + } + + // And finally, do it! + DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '" + << SuccBB->getName() << "' with cost: " << JumpThreadCost + << ", across block:\n " + << *BB << "\n"); + + // We are going to have to map operands from the original BB block to the new + // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to + // account for entry from PredBB. + DenseMap<Instruction*, Value*> ValueMapping; + + BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), + BB->getName()+".thread", + BB->getParent(), BB); + NewBB->moveAfter(PredBB); + + BasicBlock::iterator BI = BB->begin(); + for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) + ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); + + // Clone the non-phi instructions of BB into NewBB, keeping track of the + // mapping and using it to remap operands in the cloned instructions. + for (; !isa<TerminatorInst>(BI); ++BI) { + Instruction *New = BI->clone(); + New->setName(BI->getName()); + NewBB->getInstList().push_back(New); + ValueMapping[BI] = New; + + // Remap operands to patch up intra-block references. + for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) + if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { + DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); + if (I != ValueMapping.end()) + New->setOperand(i, I->second); + } + } + + // We didn't copy the terminator from BB over to NewBB, because there is now + // an unconditional jump to SuccBB. Insert the unconditional jump. + BranchInst::Create(SuccBB, NewBB); + + // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the + // PHI nodes for NewBB now. + AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping); + + // If there were values defined in BB that are used outside the block, then we + // now have to update all uses of the value to use either the original value, + // the cloned value, or some PHI derived value. This can require arbitrary + // PHI insertion, of which we are prepared to do, clean these up now. + SSAUpdater SSAUpdate; + SmallVector<Use*, 16> UsesToRename; + for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { + // Scan all uses of this instruction to see if it is used outside of its + // block, and if so, record them in UsesToRename. + for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; + ++UI) { + Instruction *User = cast<Instruction>(*UI); + if (PHINode *UserPN = dyn_cast<PHINode>(User)) { + if (UserPN->getIncomingBlock(UI) == BB) + continue; + } else if (User->getParent() == BB) + continue; + + UsesToRename.push_back(&UI.getUse()); + } + + // If there are no uses outside the block, we're done with this instruction. + if (UsesToRename.empty()) + continue; + + DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); + + // We found a use of I outside of BB. Rename all uses of I that are outside + // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks + // with the two values we know. + SSAUpdate.Initialize(I); + SSAUpdate.AddAvailableValue(BB, I); + SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]); + + while (!UsesToRename.empty()) + SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); + DEBUG(dbgs() << "\n"); + } + + + // Ok, NewBB is good to go. Update the terminator of PredBB to jump to + // NewBB instead of BB. This eliminates predecessors from BB, which requires + // us to simplify any PHI nodes in BB. + TerminatorInst *PredTerm = PredBB->getTerminator(); + for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i) + if (PredTerm->getSuccessor(i) == BB) { + RemovePredecessorAndSimplify(BB, PredBB, TD); + PredTerm->setSuccessor(i, NewBB); + } + + // At this point, the IR is fully up to date and consistent. Do a quick scan + // over the new instructions and zap any that are constants or dead. This + // frequently happens because of phi translation. + SimplifyInstructionsInBlock(NewBB, TD); + + // Threaded an edge! + ++NumThreads; + return true; +} + +/// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch +/// to BB which contains an i1 PHI node and a conditional branch on that PHI. +/// If we can duplicate the contents of BB up into PredBB do so now, this +/// improves the odds that the branch will be on an analyzable instruction like +/// a compare. +bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB, + const SmallVectorImpl<BasicBlock *> &PredBBs) { + assert(!PredBBs.empty() && "Can't handle an empty set"); + + // If BB is a loop header, then duplicating this block outside the loop would + // cause us to transform this into an irreducible loop, don't do this. + // See the comments above FindLoopHeaders for justifications and caveats. + if (LoopHeaders.count(BB)) { + DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName() + << "' into predecessor block '" << PredBBs[0]->getName() + << "' - it might create an irreducible loop!\n"); + return false; + } + + unsigned DuplicationCost = getJumpThreadDuplicationCost(BB); + if (DuplicationCost > Threshold) { + DEBUG(dbgs() << " Not duplicating BB '" << BB->getName() + << "' - Cost is too high: " << DuplicationCost << "\n"); + return false; + } + + // And finally, do it! Start by factoring the predecessors is needed. + BasicBlock *PredBB; + if (PredBBs.size() == 1) + PredBB = PredBBs[0]; + else { + DEBUG(dbgs() << " Factoring out " << PredBBs.size() + << " common predecessors.\n"); + PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(), + ".thr_comm", this); + } + + // Okay, we decided to do this! Clone all the instructions in BB onto the end + // of PredBB. + DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '" + << PredBB->getName() << "' to eliminate branch on phi. Cost: " + << DuplicationCost << " block is:" << *BB << "\n"); + + // Unless PredBB ends with an unconditional branch, split the edge so that we + // can just clone the bits from BB into the end of the new PredBB. + BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator()); + + if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) { + PredBB = SplitEdge(PredBB, BB, this); + OldPredBranch = cast<BranchInst>(PredBB->getTerminator()); + } + + // We are going to have to map operands from the original BB block into the + // PredBB block. Evaluate PHI nodes in BB. + DenseMap<Instruction*, Value*> ValueMapping; + + BasicBlock::iterator BI = BB->begin(); + for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI) + ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB); + + // Clone the non-phi instructions of BB into PredBB, keeping track of the + // mapping and using it to remap operands in the cloned instructions. + for (; BI != BB->end(); ++BI) { + Instruction *New = BI->clone(); + + // Remap operands to patch up intra-block references. + for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i) + if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) { + DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst); + if (I != ValueMapping.end()) + New->setOperand(i, I->second); + } + + // If this instruction can be simplified after the operands are updated, + // just use the simplified value instead. This frequently happens due to + // phi translation. + if (Value *IV = SimplifyInstruction(New, TD)) { + delete New; + ValueMapping[BI] = IV; + } else { + // Otherwise, insert the new instruction into the block. + New->setName(BI->getName()); + PredBB->getInstList().insert(OldPredBranch, New); + ValueMapping[BI] = New; + } + } + + // Check to see if the targets of the branch had PHI nodes. If so, we need to + // add entries to the PHI nodes for branch from PredBB now. + BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator()); + AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB, + ValueMapping); + AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB, + ValueMapping); + + // If there were values defined in BB that are used outside the block, then we + // now have to update all uses of the value to use either the original value, + // the cloned value, or some PHI derived value. This can require arbitrary + // PHI insertion, of which we are prepared to do, clean these up now. + SSAUpdater SSAUpdate; + SmallVector<Use*, 16> UsesToRename; + for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) { + // Scan all uses of this instruction to see if it is used outside of its + // block, and if so, record them in UsesToRename. + for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; + ++UI) { + Instruction *User = cast<Instruction>(*UI); + if (PHINode *UserPN = dyn_cast<PHINode>(User)) { + if (UserPN->getIncomingBlock(UI) == BB) + continue; + } else if (User->getParent() == BB) + continue; + + UsesToRename.push_back(&UI.getUse()); + } + + // If there are no uses outside the block, we're done with this instruction. + if (UsesToRename.empty()) + continue; + + DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n"); + + // We found a use of I outside of BB. Rename all uses of I that are outside + // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks + // with the two values we know. + SSAUpdate.Initialize(I); + SSAUpdate.AddAvailableValue(BB, I); + SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]); + + while (!UsesToRename.empty()) + SSAUpdate.RewriteUse(*UsesToRename.pop_back_val()); + DEBUG(dbgs() << "\n"); + } + + // PredBB no longer jumps to BB, remove entries in the PHI node for the edge + // that we nuked. + RemovePredecessorAndSimplify(BB, PredBB, TD); + + // Remove the unconditional branch at the end of the PredBB block. + OldPredBranch->eraseFromParent(); + + ++NumDupes; + return true; +} + + |
