Mercurial > hg > Members > tobaru > cbc > CbC_llvm
view lib/Transforms/Scalar/JumpThreading.cpp @ 83:60c9769439b8
LLVM 3.7
author | Tatsuki IHA <e125716@ie.u-ryukyu.ac.jp> |
---|---|
date | Wed, 18 Feb 2015 14:55:36 +0900 |
parents | 54457678186b |
children | afa8332a0e37 |
line wrap: on
line source
//===- 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. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" #include "llvm/ADT/DenseMap.h" #include "llvm/ADT/DenseSet.h" #include "llvm/ADT/STLExtras.h" #include "llvm/ADT/SmallPtrSet.h" #include "llvm/ADT/SmallSet.h" #include "llvm/ADT/Statistic.h" #include "llvm/Analysis/CFG.h" #include "llvm/Analysis/ConstantFolding.h" #include "llvm/Analysis/InstructionSimplify.h" #include "llvm/Analysis/LazyValueInfo.h" #include "llvm/Analysis/Loads.h" #include "llvm/IR/DataLayout.h" #include "llvm/IR/IntrinsicInst.h" #include "llvm/IR/LLVMContext.h" #include "llvm/IR/Metadata.h" #include "llvm/IR/ValueHandle.h" #include "llvm/Pass.h" #include "llvm/Support/CommandLine.h" #include "llvm/Support/Debug.h" #include "llvm/Support/raw_ostream.h" #include "llvm/Analysis/TargetLibraryInfo.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include "llvm/Transforms/Utils/Local.h" #include "llvm/Transforms/Utils/SSAUpdater.h" using namespace llvm; #define DEBUG_TYPE "jump-threading" 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> BBDuplicateThreshold("jump-threading-threshold", cl::desc("Max block size to duplicate for jump threading"), cl::init(6), cl::Hidden); namespace { // These are at global scope so static functions can use them too. typedef SmallVectorImpl<std::pair<Constant*, BasicBlock*> > PredValueInfo; typedef SmallVector<std::pair<Constant*, BasicBlock*>, 8> PredValueInfoTy; // This is used to keep track of what kind of constant we're currently hoping // to find. enum ConstantPreference { WantInteger, WantBlockAddress }; /// 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 { const DataLayout *DL; TargetLibraryInfo *TLI; LazyValueInfo *LVI; #ifdef NDEBUG SmallPtrSet<BasicBlock*, 16> LoopHeaders; #else SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders; #endif DenseSet<std::pair<Value*, BasicBlock*> > RecursionSet; unsigned BBDupThreshold; // RAII helper for updating the recursion stack. struct RecursionSetRemover { DenseSet<std::pair<Value*, BasicBlock*> > &TheSet; std::pair<Value*, BasicBlock*> ThePair; RecursionSetRemover(DenseSet<std::pair<Value*, BasicBlock*> > &S, std::pair<Value*, BasicBlock*> P) : TheSet(S), ThePair(P) { } ~RecursionSetRemover() { TheSet.erase(ThePair); } }; public: static char ID; // Pass identification JumpThreading(int T = -1) : FunctionPass(ID) { BBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T); initializeJumpThreadingPass(*PassRegistry::getPassRegistry()); } bool runOnFunction(Function &F) override; void getAnalysisUsage(AnalysisUsage &AU) const override { AU.addRequired<LazyValueInfo>(); AU.addPreserved<LazyValueInfo>(); AU.addRequired<TargetLibraryInfoWrapperPass>(); } 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); bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result, ConstantPreference Preference, Instruction *CxtI = nullptr); bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB, ConstantPreference Preference, Instruction *CxtI = nullptr); bool ProcessBranchOnPHI(PHINode *PN); bool ProcessBranchOnXOR(BinaryOperator *BO); bool SimplifyPartiallyRedundantLoad(LoadInst *LI); bool TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB); }; } char JumpThreading::ID = 0; INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading", "Jump Threading", false, false) INITIALIZE_PASS_DEPENDENCY(LazyValueInfo) INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass) INITIALIZE_PASS_END(JumpThreading, "jump-threading", "Jump Threading", false, false) // Public interface to the Jump Threading pass FunctionPass *llvm::createJumpThreadingPass(int Threshold) { return new JumpThreading(Threshold); } /// runOnFunction - Top level algorithm. /// bool JumpThreading::runOnFunction(Function &F) { if (skipOptnoneFunction(F)) return false; DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n"); DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>(); DL = DLP ? &DLP->getDataLayout() : nullptr; TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(); LVI = &getAnalysis<LazyValueInfo>(); // Remove unreachable blocks from function as they may result in infinite // loop. We do threading if we found something profitable. Jump threading a // branch can create other opportunities. If these opportunities form a cycle // i.e. if any jump treading is undoing previous threading in the path, then // we will loop forever. We take care of this issue by not jump threading for // back edges. This works for normal cases but not for unreachable blocks as // they may have cycle with no back edge. removeUnreachableBlocks(F); 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_empty(BB) && BB != &BB->getParent()->getEntryBlock()) { DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName() << "' with terminator: " << *BB->getTerminator() << '\n'); LoopHeaders.erase(BB); LVI->eraseBlock(BB); DeleteDeadBlock(BB); Changed = true; continue; } 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 && BI->isUnconditional() && BB != &BB->getParent()->getEntryBlock() && // If the terminator is the only non-phi instruction, try to nuke it. BB->getFirstNonPHIOrDbg()->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); // FIXME: It is always conservatively correct to drop the info // for a block even if it doesn't get erased. This isn't totally // awesome, but it allows us to use AssertingVH to prevent nasty // dangling pointer issues within LazyValueInfo. LVI->eraseBlock(BB); 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. Stop scanning the block when passing the threshold. static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB, unsigned Threshold) { /// 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) { // Stop scanning the block if we've reached the threshold. if (Size > Threshold) return Size; // 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 (CI->cannotDuplicate()) // Blocks with NoDuplicate are modelled as having infinite cost, so they // are never duplicated. return ~0U; else 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; // The same holds for indirect branches, but slightly more so. if (isa<IndirectBrInst>(I)) Size = Size > 8 ? Size-8 : 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)); } /// getKnownConstant - Helper method to determine if we can thread over a /// terminator with the given value as its condition, and if so what value to /// use for that. What kind of value this is depends on whether we want an /// integer or a block address, but an undef is always accepted. /// Returns null if Val is null or not an appropriate constant. static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) { if (!Val) return nullptr; // Undef is "known" enough. if (UndefValue *U = dyn_cast<UndefValue>(Val)) return U; if (Preference == WantBlockAddress) return dyn_cast<BlockAddress>(Val->stripPointerCasts()); return dyn_cast<ConstantInt>(Val); } /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see /// if we can infer that the value is a known ConstantInt/BlockAddress or undef /// in any of our predecessors. If so, return the known list of value and pred /// BB in the result vector. /// /// This returns true if there were any known values. /// bool JumpThreading:: ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result, ConstantPreference Preference, Instruction *CxtI) { // This method walks up use-def chains recursively. Because of this, we could // get into an infinite loop going around loops in the use-def chain. To // prevent this, keep track of what (value, block) pairs we've already visited // and terminate the search if we loop back to them if (!RecursionSet.insert(std::make_pair(V, BB)).second) return false; // An RAII help to remove this pair from the recursion set once the recursion // stack pops back out again. RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB)); // If V is a constant, then it is known in all predecessors. if (Constant *KC = getKnownConstant(V, Preference)) { for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) Result.push_back(std::make_pair(KC, *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 || 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. // // 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) { BasicBlock *P = *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, P, BB, CxtI); if (Constant *KC = getKnownConstant(PredCst, Preference)) Result.push_back(std::make_pair(KC, P)); } return !Result.empty(); } /// 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 (Constant *KC = getKnownConstant(InVal, Preference)) { Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i))); } else { Constant *CI = LVI->getConstantOnEdge(InVal, PN->getIncomingBlock(i), BB, CxtI); if (Constant *KC = getKnownConstant(CI, Preference)) Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i))); } } return !Result.empty(); } PredValueInfoTy LHSVals, RHSVals; // Handle some boolean conditions. if (I->getType()->getPrimitiveSizeInBits() == 1) { assert(Preference == WantInteger && "One-bit non-integer type?"); // X | true -> true // X & false -> false if (I->getOpcode() == Instruction::Or || I->getOpcode() == Instruction::And) { ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals, WantInteger, CxtI); ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals, WantInteger, CxtI); 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()); SmallPtrSet<BasicBlock*, 4> LHSKnownBBs; // 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 || isa<UndefValue>(LHSVals[i].first)) { Result.push_back(LHSVals[i]); Result.back().first = InterestingVal; LHSKnownBBs.insert(LHSVals[i].second); } for (unsigned i = 0, e = RHSVals.size(); i != e; ++i) if (RHSVals[i].first == InterestingVal || isa<UndefValue>(RHSVals[i].first)) { // If we already inferred a value for this block on the LHS, don't // re-add it. if (!LHSKnownBBs.count(RHSVals[i].second)) { 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, WantInteger, CxtI); if (Result.empty()) return false; // Invert the known values. for (unsigned i = 0, e = Result.size(); i != e; ++i) Result[i].first = ConstantExpr::getNot(Result[i].first); return true; } // Try to simplify some other binary operator values. } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) { assert(Preference != WantBlockAddress && "A binary operator creating a block address?"); if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) { PredValueInfoTy LHSVals; ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals, WantInteger, CxtI); // Try to use constant folding to simplify the binary operator. for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) { Constant *V = LHSVals[i].first; Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI); if (Constant *KC = getKnownConstant(Folded, WantInteger)) Result.push_back(std::make_pair(KC, LHSVals[i].second)); } } return !Result.empty(); } // Handle compare with phi operand, where the PHI is defined in this block. if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) { assert(Preference == WantInteger && "Compares only produce integers"); 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, DL); if (!Res) { if (!isa<Constant>(RHS)) continue; LazyValueInfo::Tristate ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS, cast<Constant>(RHS), PredBB, BB, CxtI ? CxtI : Cmp); if (ResT == LazyValueInfo::Unknown) continue; Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT); } if (Constant *KC = getKnownConstant(Res, WantInteger)) Result.push_back(std::make_pair(KC, 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 (isa<Constant>(Cmp->getOperand(1)) && Cmp->getType()->isIntegerTy()) { if (!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){ BasicBlock *P = *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, P, BB, CxtI ? CxtI : Cmp); if (Res == LazyValueInfo::Unknown) continue; Constant *ResC = ConstantInt::get(Cmp->getType(), Res); Result.push_back(std::make_pair(ResC, P)); } return !Result.empty(); } // Try to find a constant value for the LHS of a comparison, // and evaluate it statically if we can. if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) { PredValueInfoTy LHSVals; ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals, WantInteger, CxtI); for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) { Constant *V = LHSVals[i].first; Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(), V, CmpConst); if (Constant *KC = getKnownConstant(Folded, WantInteger)) Result.push_back(std::make_pair(KC, LHSVals[i].second)); } return !Result.empty(); } } } if (SelectInst *SI = dyn_cast<SelectInst>(I)) { // Handle select instructions where at least one operand is a known constant // and we can figure out the condition value for any predecessor block. Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference); Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference); PredValueInfoTy Conds; if ((TrueVal || FalseVal) && ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds, WantInteger, CxtI)) { for (unsigned i = 0, e = Conds.size(); i != e; ++i) { Constant *Cond = Conds[i].first; // Figure out what value to use for the condition. bool KnownCond; if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) { // A known boolean. KnownCond = CI->isOne(); } else { assert(isa<UndefValue>(Cond) && "Unexpected condition value"); // Either operand will do, so be sure to pick the one that's a known // constant. // FIXME: Do this more cleverly if both values are known constants? KnownCond = (TrueVal != nullptr); } // See if the select has a known constant value for this predecessor. if (Constant *Val = KnownCond ? TrueVal : FalseVal) Result.push_back(std::make_pair(Val, Conds[i].second)); } return !Result.empty(); } } // If all else fails, see if LVI can figure out a constant value for us. Constant *CI = LVI->getConstant(V, BB, CxtI); if (Constant *KC = getKnownConstant(CI, Preference)) { for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) Result.push_back(std::make_pair(KC, *PI)); } return !Result.empty(); } /// 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; MinNumPreds = NumPreds; } } return MinSucc; } static bool hasAddressTakenAndUsed(BasicBlock *BB) { if (!BB->hasAddressTaken()) return false; // If the block has its address taken, it may be a tree of dead constants // hanging off of it. These shouldn't keep the block alive. BlockAddress *BA = BlockAddress::get(BB); BA->removeDeadConstantUsers(); return !BA->use_empty(); } /// 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_empty(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 && !hasAddressTakenAndUsed(BB)) { // If SinglePred was a loop header, BB becomes one. if (LoopHeaders.erase(SinglePred)) LoopHeaders.insert(BB); LVI->eraseBlock(SinglePred); MergeBasicBlockIntoOnlyPred(BB); return true; } } // What kind of constant we're looking for. ConstantPreference Preference = WantInteger; // Look to see if the terminator is a conditional branch, switch or indirect // branch, if not we can't thread it. Value *Condition; Instruction *Terminator = BB->getTerminator(); if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) { // Can't thread an unconditional jump. if (BI->isUnconditional()) return false; Condition = BI->getCondition(); } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) { Condition = SI->getCondition(); } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) { // Can't thread indirect branch with no successors. if (IB->getNumSuccessors() == 0) return false; Condition = IB->getAddress()->stripPointerCasts(); Preference = WantBlockAddress; } else { return false; // Must be an invoke. } // Run constant folding to see if we can reduce the condition to a simple // constant. if (Instruction *I = dyn_cast<Instruction>(Condition)) { Value *SimpleVal = ConstantFoldInstruction(I, DL, TLI); if (SimpleVal) { I->replaceAllUsesWith(SimpleVal); I->eraseFromParent(); Condition = SimpleVal; } } // 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; BBTerm->getSuccessor(i)->removePredecessor(BB, true); } DEBUG(dbgs() << " In block '" << BB->getName() << "' folding undef terminator: " << *BBTerm << '\n'); BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm); BBTerm->eraseFromParent(); return true; } // 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 (getKnownConstant(Condition, Preference)) { DEBUG(dbgs() << " In block '" << BB->getName() << "' folding terminator: " << *BB->getTerminator() << '\n'); ++NumFolds; ConstantFoldTerminator(BB, true); return true; } Instruction *CondInst = dyn_cast<Instruction>(Condition); // All the rest of our checks depend on the condition being an instruction. if (!CondInst) { // FIXME: Unify this with code below. if (ProcessThreadableEdges(Condition, BB, Preference, Terminator)) return true; return false; } if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) { // For a comparison where the LHS is outside this block, it's possible // that we've branched on it before. Used LVI to see if we can simplify // the branch based on that. BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1)); pred_iterator PI = pred_begin(BB), PE = pred_end(BB); if (CondBr && CondConst && CondBr->isConditional() && PI != PE && (!isa<Instruction>(CondCmp->getOperand(0)) || cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) { // For predecessor edge, determine if the comparison is true or false // on that edge. If they're all true or all false, we can simplify the // branch. // FIXME: We could handle mixed true/false by duplicating code. LazyValueInfo::Tristate Baseline = LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0), CondConst, *PI, BB, CondCmp); if (Baseline != LazyValueInfo::Unknown) { // Check that all remaining incoming values match the first one. while (++PI != PE) { LazyValueInfo::Tristate Ret = LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0), CondConst, *PI, BB, CondCmp); if (Ret != Baseline) break; } // If we terminated early, then one of the values didn't match. if (PI == PE) { unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0; unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1; CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true); BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr); CondBr->eraseFromParent(); return true; } } } else if (CondBr && CondConst && CondBr->isConditional()) { // There might be an invariant in the same block with the conditional // that can determine the predicate. LazyValueInfo::Tristate Ret = LVI->getPredicateAt(CondCmp->getPredicate(), CondCmp->getOperand(0), CondConst, CondCmp); if (Ret != LazyValueInfo::Unknown) { unsigned ToRemove = Ret == LazyValueInfo::True ? 1 : 0; unsigned ToKeep = Ret == LazyValueInfo::True ? 0 : 1; CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true); BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr); CondBr->eraseFromParent(); return true; } } if (CondBr && CondConst && TryToUnfoldSelect(CondCmp, 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, Preference, Terminator)) 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; } /// 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/atomic loads. if (!LI->isSimple()) 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; // If the load is defined in a landing pad, it can't be partially redundant, // because the edges between the invoke and the landing pad cannot have other // instructions between them. if (LoadBB->isLandingPad()) 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()); if (AvailableVal->getType() != LI->getType()) AvailableVal = CastInst::CreateBitOrPointerCast(AvailableVal, LI->getType(), "", LI); 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; // If all of the loads and stores that feed the value have the same AA tags, // then we can propagate them onto any newly inserted loads. AAMDNodes AATags; LI->getAAMetadata(AATags); SmallPtrSet<BasicBlock*, 8> PredsScanned; typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy; AvailablePredsTy AvailablePreds; BasicBlock *OneUnavailablePred = nullptr; // 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).second) continue; // Scan the predecessor to see if the value is available in the pred. BBIt = PredBB->end(); AAMDNodes ThisAATags; Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6, nullptr, &ThisAATags); if (!PredAvailable) { OneUnavailablePred = PredBB; continue; } // If AA tags disagree or are not present, forget about them. if (AATags != ThisAATags) AATags = AAMDNodes(); // 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 = nullptr; // 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) { BasicBlock *P = *PI; // If the predecessor is an indirect goto, we can't split the edge. if (isa<IndirectBrInst>(P->getTerminator())) return false; if (!AvailablePredSet.count(P)) PredsToSplit.push_back(P); } // Split them out to their own block. UnavailablePred = SplitBlockPredecessors(LoadBB, PredsToSplit, "thread-pre-split"); } // 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!"); LoadInst *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false, LI->getAlignment(), UnavailablePred->getTerminator()); NewVal->setDebugLoc(LI->getDebugLoc()); if (AATags) NewVal->setAAMetadata(AATags); 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. pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB); PHINode *PN = PHINode::Create(LI->getType(), std::distance(PB, PE), "", LoadBB->begin()); PN->takeName(LI); PN->setDebugLoc(LI->getDebugLoc()); // Insert new entries into the PHI for each predecessor. A single block may // have multiple entries here. for (pred_iterator PI = PB; PI != PE; ++PI) { BasicBlock *P = *PI; AvailablePredsTy::iterator I = std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(), std::make_pair(P, (Value*)nullptr)); assert(I != AvailablePreds.end() && I->first == P && "Didn't find entry for predecessor!"); // If we have an available predecessor but it requires casting, insert the // cast in the predecessor and use the cast. Note that we have to update the // AvailablePreds vector as we go so that all of the PHI entries for this // predecessor use the same bitcast. Value *&PredV = I->second; if (PredV->getType() != LI->getType()) PredV = CastInst::CreateBitOrPointerCast(PredV, LI->getType(), "", P->getTerminator()); PN->addIncoming(PredV, 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 one // 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, ConstantPreference Preference, Instruction *CxtI) { // If threading this would thread across a loop header, don't even try to // thread the edge. if (LoopHeaders.count(BB)) return false; PredValueInfoTy PredValues; if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference, CxtI)) 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 = " << *PredValues[i].first << " 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 = nullptr; 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).second) continue; // Duplicate predecessor entry. // If the predecessor ends with an indirect goto, we can't change its // destination. if (isa<IndirectBrInst>(Pred->getTerminator())) continue; Constant *Val = PredValues[i].first; BasicBlock *DestBB; if (isa<UndefValue>(Val)) DestBB = nullptr; else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero()); else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) { DestBB = SI->findCaseValue(cast<ConstantInt>(Val)).getCaseSuccessor(); } else { assert(isa<IndirectBrInst>(BB->getTerminator()) && "Unexpected terminator"); DestBB = cast<BlockAddress>(Val)->getBasicBlock(); } // If we have exactly one destination, remember it for efficiency below. if (PredToDestList.empty()) 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) 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, ... PredValueInfoTy XorOpValues; bool isLHS = true; if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues, WantInteger, BO)) { assert(XorOpValues.empty()); if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues, WantInteger, BO)) 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 (isa<UndefValue>(XorOpValues[i].first)) // Ignore undefs for the count. continue; if (cast<ConstantInt>(XorOpValues[i].first)->isZero()) ++NumFalse; else ++NumTrue; } // Determine which value to split on, true, false, or undef if neither. ConstantInt *SplitVal = nullptr; 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 && !isa<UndefValue>(XorOpValues[i].first)) 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) { // 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, BBDupThreshold); if (JumpThreadCost > BBDupThreshold) { 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, ".thr_comm"); } // And finally, do it! DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '" << SuccBB->getName() << "' with cost: " << JumpThreadCost << ", across block:\n " << *BB << "\n"); LVI->threadEdge(PredBB, BB, SuccBB); // 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 *NewBI =BranchInst::Create(SuccBB, NewBB); NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc()); // 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 (Use &U : I->uses()) { Instruction *User = cast<Instruction>(U.getUser()); if (PHINode *UserPN = dyn_cast<PHINode>(User)) { if (UserPN->getIncomingBlock(U) == BB) continue; } else if (User->getParent() == BB) continue; UsesToRename.push_back(&U); } // 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->getType(), I->getName()); 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) { BB->removePredecessor(PredBB, true); 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, DL, TLI); // 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, BBDupThreshold); if (DuplicationCost > BBDupThreshold) { 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, ".thr_comm"); } // 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 || !OldPredBranch->isUnconditional()) { PredBB = SplitEdge(PredBB, BB); 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, DL)) { 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 (Use &U : I->uses()) { Instruction *User = cast<Instruction>(U.getUser()); if (PHINode *UserPN = dyn_cast<PHINode>(User)) { if (UserPN->getIncomingBlock(U) == BB) continue; } else if (User->getParent() == BB) continue; UsesToRename.push_back(&U); } // 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->getType(), I->getName()); 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. BB->removePredecessor(PredBB, true); // Remove the unconditional branch at the end of the PredBB block. OldPredBranch->eraseFromParent(); ++NumDupes; return true; } /// TryToUnfoldSelect - Look for blocks of the form /// bb1: /// %a = select /// br bb /// /// bb2: /// %p = phi [%a, %bb] ... /// %c = icmp %p /// br i1 %c /// /// And expand the select into a branch structure if one of its arms allows %c /// to be folded. This later enables threading from bb1 over bb2. bool JumpThreading::TryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) { BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator()); PHINode *CondLHS = dyn_cast<PHINode>(CondCmp->getOperand(0)); Constant *CondRHS = cast<Constant>(CondCmp->getOperand(1)); if (!CondBr || !CondBr->isConditional() || !CondLHS || CondLHS->getParent() != BB) return false; for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) { BasicBlock *Pred = CondLHS->getIncomingBlock(I); SelectInst *SI = dyn_cast<SelectInst>(CondLHS->getIncomingValue(I)); // Look if one of the incoming values is a select in the corresponding // predecessor. if (!SI || SI->getParent() != Pred || !SI->hasOneUse()) continue; BranchInst *PredTerm = dyn_cast<BranchInst>(Pred->getTerminator()); if (!PredTerm || !PredTerm->isUnconditional()) continue; // Now check if one of the select values would allow us to constant fold the // terminator in BB. We don't do the transform if both sides fold, those // cases will be threaded in any case. LazyValueInfo::Tristate LHSFolds = LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(1), CondRHS, Pred, BB, CondCmp); LazyValueInfo::Tristate RHSFolds = LVI->getPredicateOnEdge(CondCmp->getPredicate(), SI->getOperand(2), CondRHS, Pred, BB, CondCmp); if ((LHSFolds != LazyValueInfo::Unknown || RHSFolds != LazyValueInfo::Unknown) && LHSFolds != RHSFolds) { // Expand the select. // // Pred -- // | v // | NewBB // | | // |----- // v // BB BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "select.unfold", BB->getParent(), BB); // Move the unconditional branch to NewBB. PredTerm->removeFromParent(); NewBB->getInstList().insert(NewBB->end(), PredTerm); // Create a conditional branch and update PHI nodes. BranchInst::Create(NewBB, BB, SI->getCondition(), Pred); CondLHS->setIncomingValue(I, SI->getFalseValue()); CondLHS->addIncoming(SI->getTrueValue(), NewBB); // The select is now dead. SI->eraseFromParent(); // Update any other PHI nodes in BB. for (BasicBlock::iterator BI = BB->begin(); PHINode *Phi = dyn_cast<PHINode>(BI); ++BI) if (Phi != CondLHS) Phi->addIncoming(Phi->getIncomingValueForBlock(Pred), NewBB); return true; } } return false; }