//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===// // // The LLVM Compiler Infrastructure // // This file is distributed under the University of Illinois Open Source // License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // // Peephole optimize the CFG. // //===----------------------------------------------------------------------===// #define DEBUG_TYPE "simplifycfg" #include "llvm/Transforms/Utils/Local.h" #include "llvm/Constants.h" #include "llvm/Instructions.h" #include "llvm/IntrinsicInst.h" #include "llvm/Type.h" #include "llvm/DerivedTypes.h" #include "llvm/GlobalVariable.h" #include "llvm/Analysis/InstructionSimplify.h" #include "llvm/Target/TargetData.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include "llvm/ADT/DenseMap.h" #include "llvm/ADT/SmallVector.h" #include "llvm/ADT/SmallPtrSet.h" #include "llvm/ADT/Statistic.h" #include "llvm/ADT/STLExtras.h" #include "llvm/Support/CFG.h" #include "llvm/Support/Debug.h" #include "llvm/Support/raw_ostream.h" #include #include #include using namespace llvm; STATISTIC(NumSpeculations, "Number of speculative executed instructions"); namespace { class SimplifyCFGOpt { const TargetData *const TD; Value *isValueEqualityComparison(TerminatorInst *TI); BasicBlock *GetValueEqualityComparisonCases(TerminatorInst *TI, std::vector > &Cases); bool SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI, BasicBlock *Pred); bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI); bool SimplifyReturn(ReturnInst *RI); bool SimplifyUnwind(UnwindInst *UI); bool SimplifyUnreachable(UnreachableInst *UI); bool SimplifySwitch(SwitchInst *SI); bool SimplifyIndirectBr(IndirectBrInst *IBI); bool SimplifyUncondBranch(BranchInst *BI); bool SimplifyCondBranch(BranchInst *BI); public: explicit SimplifyCFGOpt(const TargetData *td) : TD(td) {} bool run(BasicBlock *BB); }; } /// SafeToMergeTerminators - Return true if it is safe to merge these two /// terminator instructions together. /// static bool SafeToMergeTerminators(TerminatorInst *SI1, TerminatorInst *SI2) { if (SI1 == SI2) return false; // Can't merge with self! // It is not safe to merge these two switch instructions if they have a common // successor, and if that successor has a PHI node, and if *that* PHI node has // conflicting incoming values from the two switch blocks. BasicBlock *SI1BB = SI1->getParent(); BasicBlock *SI2BB = SI2->getParent(); SmallPtrSet SI1Succs(succ_begin(SI1BB), succ_end(SI1BB)); for (succ_iterator I = succ_begin(SI2BB), E = succ_end(SI2BB); I != E; ++I) if (SI1Succs.count(*I)) for (BasicBlock::iterator BBI = (*I)->begin(); isa(BBI); ++BBI) { PHINode *PN = cast(BBI); if (PN->getIncomingValueForBlock(SI1BB) != PN->getIncomingValueForBlock(SI2BB)) return false; } return true; } /// AddPredecessorToBlock - Update PHI nodes in Succ to indicate that there will /// now be entries in it from the 'NewPred' block. The values that will be /// flowing into the PHI nodes will be the same as those coming in from /// ExistPred, an existing predecessor of Succ. static void AddPredecessorToBlock(BasicBlock *Succ, BasicBlock *NewPred, BasicBlock *ExistPred) { if (!isa(Succ->begin())) return; // Quick exit if nothing to do PHINode *PN; for (BasicBlock::iterator I = Succ->begin(); (PN = dyn_cast(I)); ++I) PN->addIncoming(PN->getIncomingValueForBlock(ExistPred), NewPred); } /// GetIfCondition - Given a basic block (BB) with two predecessors (and at /// least one PHI node in it), check to see if the merge at this block is due /// to an "if condition". If so, return the boolean condition that determines /// which entry into BB will be taken. Also, return by references the block /// that will be entered from if the condition is true, and the block that will /// be entered if the condition is false. /// /// This does no checking to see if the true/false blocks have large or unsavory /// instructions in them. static Value *GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue, BasicBlock *&IfFalse) { PHINode *SomePHI = cast(BB->begin()); assert(SomePHI->getNumIncomingValues() == 2 && "Function can only handle blocks with 2 predecessors!"); BasicBlock *Pred1 = SomePHI->getIncomingBlock(0); BasicBlock *Pred2 = SomePHI->getIncomingBlock(1); // We can only handle branches. Other control flow will be lowered to // branches if possible anyway. BranchInst *Pred1Br = dyn_cast(Pred1->getTerminator()); BranchInst *Pred2Br = dyn_cast(Pred2->getTerminator()); if (Pred1Br == 0 || Pred2Br == 0) return 0; // Eliminate code duplication by ensuring that Pred1Br is conditional if // either are. if (Pred2Br->isConditional()) { // If both branches are conditional, we don't have an "if statement". In // reality, we could transform this case, but since the condition will be // required anyway, we stand no chance of eliminating it, so the xform is // probably not profitable. if (Pred1Br->isConditional()) return 0; std::swap(Pred1, Pred2); std::swap(Pred1Br, Pred2Br); } if (Pred1Br->isConditional()) { // The only thing we have to watch out for here is to make sure that Pred2 // doesn't have incoming edges from other blocks. If it does, the condition // doesn't dominate BB. if (Pred2->getSinglePredecessor() == 0) return 0; // If we found a conditional branch predecessor, make sure that it branches // to BB and Pred2Br. If it doesn't, this isn't an "if statement". if (Pred1Br->getSuccessor(0) == BB && Pred1Br->getSuccessor(1) == Pred2) { IfTrue = Pred1; IfFalse = Pred2; } else if (Pred1Br->getSuccessor(0) == Pred2 && Pred1Br->getSuccessor(1) == BB) { IfTrue = Pred2; IfFalse = Pred1; } else { // We know that one arm of the conditional goes to BB, so the other must // go somewhere unrelated, and this must not be an "if statement". return 0; } return Pred1Br->getCondition(); } // Ok, if we got here, both predecessors end with an unconditional branch to // BB. Don't panic! If both blocks only have a single (identical) // predecessor, and THAT is a conditional branch, then we're all ok! BasicBlock *CommonPred = Pred1->getSinglePredecessor(); if (CommonPred == 0 || CommonPred != Pred2->getSinglePredecessor()) return 0; // Otherwise, if this is a conditional branch, then we can use it! BranchInst *BI = dyn_cast(CommonPred->getTerminator()); if (BI == 0) return 0; assert(BI->isConditional() && "Two successors but not conditional?"); if (BI->getSuccessor(0) == Pred1) { IfTrue = Pred1; IfFalse = Pred2; } else { IfTrue = Pred2; IfFalse = Pred1; } return BI->getCondition(); } /// DominatesMergePoint - If we have a merge point of an "if condition" as /// accepted above, return true if the specified value dominates the block. We /// don't handle the true generality of domination here, just a special case /// which works well enough for us. /// /// If AggressiveInsts is non-null, and if V does not dominate BB, we check to /// see if V (which must be an instruction) is cheap to compute and is /// non-trapping. If both are true, the instruction is inserted into the set /// and true is returned. static bool DominatesMergePoint(Value *V, BasicBlock *BB, SmallPtrSet *AggressiveInsts) { Instruction *I = dyn_cast(V); if (!I) { // Non-instructions all dominate instructions, but not all constantexprs // can be executed unconditionally. if (ConstantExpr *C = dyn_cast(V)) if (C->canTrap()) return false; return true; } BasicBlock *PBB = I->getParent(); // We don't want to allow weird loops that might have the "if condition" in // the bottom of this block. if (PBB == BB) return false; // If this instruction is defined in a block that contains an unconditional // branch to BB, then it must be in the 'conditional' part of the "if // statement". If not, it definitely dominates the region. BranchInst *BI = dyn_cast(PBB->getTerminator()); if (BI == 0 || BI->isConditional() || BI->getSuccessor(0) != BB) return true; // If we aren't allowing aggressive promotion anymore, then don't consider // instructions in the 'if region'. if (AggressiveInsts == 0) return false; // Okay, it looks like the instruction IS in the "condition". Check to // see if it's a cheap instruction to unconditionally compute, and if it // only uses stuff defined outside of the condition. If so, hoist it out. if (!I->isSafeToSpeculativelyExecute()) return false; switch (I->getOpcode()) { default: return false; // Cannot hoist this out safely. case Instruction::Load: // We have to check to make sure there are no instructions before the // load in its basic block, as we are going to hoist the load out to its // predecessor. if (PBB->getFirstNonPHIOrDbg() != I) return false; break; case Instruction::Add: case Instruction::Sub: case Instruction::And: case Instruction::Or: case Instruction::Xor: case Instruction::Shl: case Instruction::LShr: case Instruction::AShr: case Instruction::ICmp: break; // These are all cheap and non-trapping instructions. } // Okay, we can only really hoist these out if their operands are not // defined in the conditional region. for (User::op_iterator i = I->op_begin(), e = I->op_end(); i != e; ++i) if (!DominatesMergePoint(*i, BB, 0)) return false; // Okay, it's safe to do this! Remember this instruction. AggressiveInsts->insert(I); return true; } /// GetConstantInt - Extract ConstantInt from value, looking through IntToPtr /// and PointerNullValue. Return NULL if value is not a constant int. static ConstantInt *GetConstantInt(Value *V, const TargetData *TD) { // Normal constant int. ConstantInt *CI = dyn_cast(V); if (CI || !TD || !isa(V) || !V->getType()->isPointerTy()) return CI; // This is some kind of pointer constant. Turn it into a pointer-sized // ConstantInt if possible. const IntegerType *PtrTy = TD->getIntPtrType(V->getContext()); // Null pointer means 0, see SelectionDAGBuilder::getValue(const Value*). if (isa(V)) return ConstantInt::get(PtrTy, 0); // IntToPtr const int. if (ConstantExpr *CE = dyn_cast(V)) if (CE->getOpcode() == Instruction::IntToPtr) if (ConstantInt *CI = dyn_cast(CE->getOperand(0))) { // The constant is very likely to have the right type already. if (CI->getType() == PtrTy) return CI; else return cast (ConstantExpr::getIntegerCast(CI, PtrTy, /*isSigned=*/false)); } return 0; } /// GatherConstantCompares - Given a potentially 'or'd or 'and'd together /// collection of icmp eq/ne instructions that compare a value against a /// constant, return the value being compared, and stick the constant into the /// Values vector. static Value * GatherConstantCompares(Value *V, std::vector &Vals, Value *&Extra, const TargetData *TD, bool isEQ) { Instruction *I = dyn_cast(V); if (I == 0) return 0; // If this is an icmp against a constant, handle this as one of the cases. if (ICmpInst *ICI = dyn_cast(I)) { if (ICI->getPredicate() == (isEQ ? ICmpInst::ICMP_EQ : ICmpInst::ICMP_NE)) if (ConstantInt *C = GetConstantInt(I->getOperand(1), TD)) { Vals.push_back(C); return I->getOperand(0); } return 0; } // Otherwise, we can only handle an | or &, depending on isEQ. if (I->getOpcode() != (isEQ ? Instruction::Or : Instruction::And)) return 0; unsigned NumValsBeforeLHS = Vals.size(); if (Value *LHS = GatherConstantCompares(I->getOperand(0), Vals, Extra, TD, isEQ)) { unsigned NumVals = Vals.size(); if (Value *RHS = GatherConstantCompares(I->getOperand(1), Vals, Extra, TD, isEQ)) { if (LHS == RHS) return LHS; Vals.resize(NumVals); } // The RHS of the or/and can't be folded in and we haven't used "Extra" yet, // set it and return success. if (Extra == 0 || Extra == I->getOperand(1)) { Extra = I->getOperand(1); return LHS; } Vals.resize(NumValsBeforeLHS); return 0; } // If the LHS can't be folded in, but Extra is available and RHS can, try to // use LHS as Extra. if (Extra == 0 || Extra == I->getOperand(0)) { Value *OldExtra = Extra; Extra = I->getOperand(0); if (Value *RHS = GatherConstantCompares(I->getOperand(1), Vals, Extra, TD, isEQ)) return RHS; assert(Vals.size() == NumValsBeforeLHS); Extra = OldExtra; } return 0; } static void EraseTerminatorInstAndDCECond(TerminatorInst *TI) { Instruction* Cond = 0; if (SwitchInst *SI = dyn_cast(TI)) { Cond = dyn_cast(SI->getCondition()); } else if (BranchInst *BI = dyn_cast(TI)) { if (BI->isConditional()) Cond = dyn_cast(BI->getCondition()); } else if (IndirectBrInst *IBI = dyn_cast(TI)) { Cond = dyn_cast(IBI->getAddress()); } TI->eraseFromParent(); if (Cond) RecursivelyDeleteTriviallyDeadInstructions(Cond); } /// isValueEqualityComparison - Return true if the specified terminator checks /// to see if a value is equal to constant integer value. Value *SimplifyCFGOpt::isValueEqualityComparison(TerminatorInst *TI) { Value *CV = 0; if (SwitchInst *SI = dyn_cast(TI)) { // Do not permit merging of large switch instructions into their // predecessors unless there is only one predecessor. if (SI->getNumSuccessors()*std::distance(pred_begin(SI->getParent()), pred_end(SI->getParent())) <= 128) CV = SI->getCondition(); } else if (BranchInst *BI = dyn_cast(TI)) if (BI->isConditional() && BI->getCondition()->hasOneUse()) if (ICmpInst *ICI = dyn_cast(BI->getCondition())) if ((ICI->getPredicate() == ICmpInst::ICMP_EQ || ICI->getPredicate() == ICmpInst::ICMP_NE) && GetConstantInt(ICI->getOperand(1), TD)) CV = ICI->getOperand(0); // Unwrap any lossless ptrtoint cast. if (TD && CV && CV->getType() == TD->getIntPtrType(CV->getContext())) if (PtrToIntInst *PTII = dyn_cast(CV)) CV = PTII->getOperand(0); return CV; } /// GetValueEqualityComparisonCases - Given a value comparison instruction, /// decode all of the 'cases' that it represents and return the 'default' block. BasicBlock *SimplifyCFGOpt:: GetValueEqualityComparisonCases(TerminatorInst *TI, std::vector > &Cases) { if (SwitchInst *SI = dyn_cast(TI)) { Cases.reserve(SI->getNumCases()); for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) Cases.push_back(std::make_pair(SI->getCaseValue(i), SI->getSuccessor(i))); return SI->getDefaultDest(); } BranchInst *BI = cast(TI); ICmpInst *ICI = cast(BI->getCondition()); Cases.push_back(std::make_pair(GetConstantInt(ICI->getOperand(1), TD), BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_NE))); return BI->getSuccessor(ICI->getPredicate() == ICmpInst::ICMP_EQ); } /// EliminateBlockCases - Given a vector of bb/value pairs, remove any entries /// in the list that match the specified block. static void EliminateBlockCases(BasicBlock *BB, std::vector > &Cases) { for (unsigned i = 0, e = Cases.size(); i != e; ++i) if (Cases[i].second == BB) { Cases.erase(Cases.begin()+i); --i; --e; } } /// ValuesOverlap - Return true if there are any keys in C1 that exist in C2 as /// well. static bool ValuesOverlap(std::vector > &C1, std::vector > &C2) { std::vector > *V1 = &C1, *V2 = &C2; // Make V1 be smaller than V2. if (V1->size() > V2->size()) std::swap(V1, V2); if (V1->size() == 0) return false; if (V1->size() == 1) { // Just scan V2. ConstantInt *TheVal = (*V1)[0].first; for (unsigned i = 0, e = V2->size(); i != e; ++i) if (TheVal == (*V2)[i].first) return true; } // Otherwise, just sort both lists and compare element by element. array_pod_sort(V1->begin(), V1->end()); array_pod_sort(V2->begin(), V2->end()); unsigned i1 = 0, i2 = 0, e1 = V1->size(), e2 = V2->size(); while (i1 != e1 && i2 != e2) { if ((*V1)[i1].first == (*V2)[i2].first) return true; if ((*V1)[i1].first < (*V2)[i2].first) ++i1; else ++i2; } return false; } /// SimplifyEqualityComparisonWithOnlyPredecessor - If TI is known to be a /// terminator instruction and its block is known to only have a single /// predecessor block, check to see if that predecessor is also a value /// comparison with the same value, and if that comparison determines the /// outcome of this comparison. If so, simplify TI. This does a very limited /// form of jump threading. bool SimplifyCFGOpt:: SimplifyEqualityComparisonWithOnlyPredecessor(TerminatorInst *TI, BasicBlock *Pred) { Value *PredVal = isValueEqualityComparison(Pred->getTerminator()); if (!PredVal) return false; // Not a value comparison in predecessor. Value *ThisVal = isValueEqualityComparison(TI); assert(ThisVal && "This isn't a value comparison!!"); if (ThisVal != PredVal) return false; // Different predicates. // Find out information about when control will move from Pred to TI's block. std::vector > PredCases; BasicBlock *PredDef = GetValueEqualityComparisonCases(Pred->getTerminator(), PredCases); EliminateBlockCases(PredDef, PredCases); // Remove default from cases. // Find information about how control leaves this block. std::vector > ThisCases; BasicBlock *ThisDef = GetValueEqualityComparisonCases(TI, ThisCases); EliminateBlockCases(ThisDef, ThisCases); // Remove default from cases. // If TI's block is the default block from Pred's comparison, potentially // simplify TI based on this knowledge. if (PredDef == TI->getParent()) { // If we are here, we know that the value is none of those cases listed in // PredCases. If there are any cases in ThisCases that are in PredCases, we // can simplify TI. if (!ValuesOverlap(PredCases, ThisCases)) return false; if (isa(TI)) { // Okay, one of the successors of this condbr is dead. Convert it to a // uncond br. assert(ThisCases.size() == 1 && "Branch can only have one case!"); // Insert the new branch. Instruction *NI = BranchInst::Create(ThisDef, TI); (void) NI; // Remove PHI node entries for the dead edge. ThisCases[0].second->removePredecessor(TI->getParent()); DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator() << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"); EraseTerminatorInstAndDCECond(TI); return true; } SwitchInst *SI = cast(TI); // Okay, TI has cases that are statically dead, prune them away. SmallPtrSet DeadCases; for (unsigned i = 0, e = PredCases.size(); i != e; ++i) DeadCases.insert(PredCases[i].first); DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator() << "Through successor TI: " << *TI); for (unsigned i = SI->getNumCases()-1; i != 0; --i) if (DeadCases.count(SI->getCaseValue(i))) { SI->getSuccessor(i)->removePredecessor(TI->getParent()); SI->removeCase(i); } DEBUG(dbgs() << "Leaving: " << *TI << "\n"); return true; } // Otherwise, TI's block must correspond to some matched value. Find out // which value (or set of values) this is. ConstantInt *TIV = 0; BasicBlock *TIBB = TI->getParent(); for (unsigned i = 0, e = PredCases.size(); i != e; ++i) if (PredCases[i].second == TIBB) { if (TIV != 0) return false; // Cannot handle multiple values coming to this block. TIV = PredCases[i].first; } assert(TIV && "No edge from pred to succ?"); // Okay, we found the one constant that our value can be if we get into TI's // BB. Find out which successor will unconditionally be branched to. BasicBlock *TheRealDest = 0; for (unsigned i = 0, e = ThisCases.size(); i != e; ++i) if (ThisCases[i].first == TIV) { TheRealDest = ThisCases[i].second; break; } // If not handled by any explicit cases, it is handled by the default case. if (TheRealDest == 0) TheRealDest = ThisDef; // Remove PHI node entries for dead edges. BasicBlock *CheckEdge = TheRealDest; for (succ_iterator SI = succ_begin(TIBB), e = succ_end(TIBB); SI != e; ++SI) if (*SI != CheckEdge) (*SI)->removePredecessor(TIBB); else CheckEdge = 0; // Insert the new branch. Instruction *NI = BranchInst::Create(TheRealDest, TI); (void) NI; DEBUG(dbgs() << "Threading pred instr: " << *Pred->getTerminator() << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"); EraseTerminatorInstAndDCECond(TI); return true; } namespace { /// ConstantIntOrdering - This class implements a stable ordering of constant /// integers that does not depend on their address. This is important for /// applications that sort ConstantInt's to ensure uniqueness. struct ConstantIntOrdering { bool operator()(const ConstantInt *LHS, const ConstantInt *RHS) const { return LHS->getValue().ult(RHS->getValue()); } }; } static int ConstantIntSortPredicate(const void *P1, const void *P2) { const ConstantInt *LHS = *(const ConstantInt**)P1; const ConstantInt *RHS = *(const ConstantInt**)P2; return LHS->getValue().ult(RHS->getValue()) ? 1 : -1; } /// FoldValueComparisonIntoPredecessors - The specified terminator is a value /// equality comparison instruction (either a switch or a branch on "X == c"). /// See if any of the predecessors of the terminator block are value comparisons /// on the same value. If so, and if safe to do so, fold them together. bool SimplifyCFGOpt::FoldValueComparisonIntoPredecessors(TerminatorInst *TI) { BasicBlock *BB = TI->getParent(); Value *CV = isValueEqualityComparison(TI); // CondVal assert(CV && "Not a comparison?"); bool Changed = false; SmallVector Preds(pred_begin(BB), pred_end(BB)); while (!Preds.empty()) { BasicBlock *Pred = Preds.pop_back_val(); // See if the predecessor is a comparison with the same value. TerminatorInst *PTI = Pred->getTerminator(); Value *PCV = isValueEqualityComparison(PTI); // PredCondVal if (PCV == CV && SafeToMergeTerminators(TI, PTI)) { // Figure out which 'cases' to copy from SI to PSI. std::vector > BBCases; BasicBlock *BBDefault = GetValueEqualityComparisonCases(TI, BBCases); std::vector > PredCases; BasicBlock *PredDefault = GetValueEqualityComparisonCases(PTI, PredCases); // Based on whether the default edge from PTI goes to BB or not, fill in // PredCases and PredDefault with the new switch cases we would like to // build. SmallVector NewSuccessors; if (PredDefault == BB) { // If this is the default destination from PTI, only the edges in TI // that don't occur in PTI, or that branch to BB will be activated. std::set PTIHandled; for (unsigned i = 0, e = PredCases.size(); i != e; ++i) if (PredCases[i].second != BB) PTIHandled.insert(PredCases[i].first); else { // The default destination is BB, we don't need explicit targets. std::swap(PredCases[i], PredCases.back()); PredCases.pop_back(); --i; --e; } // Reconstruct the new switch statement we will be building. if (PredDefault != BBDefault) { PredDefault->removePredecessor(Pred); PredDefault = BBDefault; NewSuccessors.push_back(BBDefault); } for (unsigned i = 0, e = BBCases.size(); i != e; ++i) if (!PTIHandled.count(BBCases[i].first) && BBCases[i].second != BBDefault) { PredCases.push_back(BBCases[i]); NewSuccessors.push_back(BBCases[i].second); } } else { // If this is not the default destination from PSI, only the edges // in SI that occur in PSI with a destination of BB will be // activated. std::set PTIHandled; for (unsigned i = 0, e = PredCases.size(); i != e; ++i) if (PredCases[i].second == BB) { PTIHandled.insert(PredCases[i].first); std::swap(PredCases[i], PredCases.back()); PredCases.pop_back(); --i; --e; } // Okay, now we know which constants were sent to BB from the // predecessor. Figure out where they will all go now. for (unsigned i = 0, e = BBCases.size(); i != e; ++i) if (PTIHandled.count(BBCases[i].first)) { // If this is one we are capable of getting... PredCases.push_back(BBCases[i]); NewSuccessors.push_back(BBCases[i].second); PTIHandled.erase(BBCases[i].first);// This constant is taken care of } // If there are any constants vectored to BB that TI doesn't handle, // they must go to the default destination of TI. for (std::set::iterator I = PTIHandled.begin(), E = PTIHandled.end(); I != E; ++I) { PredCases.push_back(std::make_pair(*I, BBDefault)); NewSuccessors.push_back(BBDefault); } } // Okay, at this point, we know which new successor Pred will get. Make // sure we update the number of entries in the PHI nodes for these // successors. for (unsigned i = 0, e = NewSuccessors.size(); i != e; ++i) AddPredecessorToBlock(NewSuccessors[i], Pred, BB); // Convert pointer to int before we switch. if (CV->getType()->isPointerTy()) { assert(TD && "Cannot switch on pointer without TargetData"); CV = new PtrToIntInst(CV, TD->getIntPtrType(CV->getContext()), "magicptr", PTI); } // Now that the successors are updated, create the new Switch instruction. SwitchInst *NewSI = SwitchInst::Create(CV, PredDefault, PredCases.size(), PTI); for (unsigned i = 0, e = PredCases.size(); i != e; ++i) NewSI->addCase(PredCases[i].first, PredCases[i].second); EraseTerminatorInstAndDCECond(PTI); // Okay, last check. If BB is still a successor of PSI, then we must // have an infinite loop case. If so, add an infinitely looping block // to handle the case to preserve the behavior of the code. BasicBlock *InfLoopBlock = 0; for (unsigned i = 0, e = NewSI->getNumSuccessors(); i != e; ++i) if (NewSI->getSuccessor(i) == BB) { if (InfLoopBlock == 0) { // Insert it at the end of the function, because it's either code, // or it won't matter if it's hot. :) InfLoopBlock = BasicBlock::Create(BB->getContext(), "infloop", BB->getParent()); BranchInst::Create(InfLoopBlock, InfLoopBlock); } NewSI->setSuccessor(i, InfLoopBlock); } Changed = true; } } return Changed; } // isSafeToHoistInvoke - If we would need to insert a select that uses the // value of this invoke (comments in HoistThenElseCodeToIf explain why we // would need to do this), we can't hoist the invoke, as there is nowhere // to put the select in this case. static bool isSafeToHoistInvoke(BasicBlock *BB1, BasicBlock *BB2, Instruction *I1, Instruction *I2) { for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) { PHINode *PN; for (BasicBlock::iterator BBI = SI->begin(); (PN = dyn_cast(BBI)); ++BBI) { Value *BB1V = PN->getIncomingValueForBlock(BB1); Value *BB2V = PN->getIncomingValueForBlock(BB2); if (BB1V != BB2V && (BB1V==I1 || BB2V==I2)) { return false; } } } return true; } /// HoistThenElseCodeToIf - Given a conditional branch that goes to BB1 and /// BB2, hoist any common code in the two blocks up into the branch block. The /// caller of this function guarantees that BI's block dominates BB1 and BB2. static bool HoistThenElseCodeToIf(BranchInst *BI) { // This does very trivial matching, with limited scanning, to find identical // instructions in the two blocks. In particular, we don't want to get into // O(M*N) situations here where M and N are the sizes of BB1 and BB2. As // such, we currently just scan for obviously identical instructions in an // identical order. BasicBlock *BB1 = BI->getSuccessor(0); // The true destination. BasicBlock *BB2 = BI->getSuccessor(1); // The false destination BasicBlock::iterator BB1_Itr = BB1->begin(); BasicBlock::iterator BB2_Itr = BB2->begin(); Instruction *I1 = BB1_Itr++, *I2 = BB2_Itr++; while (isa(I1)) I1 = BB1_Itr++; while (isa(I2)) I2 = BB2_Itr++; if (I1->getOpcode() != I2->getOpcode() || isa(I1) || !I1->isIdenticalToWhenDefined(I2) || (isa(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2))) return false; // If we get here, we can hoist at least one instruction. BasicBlock *BIParent = BI->getParent(); do { // If we are hoisting the terminator instruction, don't move one (making a // broken BB), instead clone it, and remove BI. if (isa(I1)) goto HoistTerminator; // For a normal instruction, we just move one to right before the branch, // then replace all uses of the other with the first. Finally, we remove // the now redundant second instruction. BIParent->getInstList().splice(BI, BB1->getInstList(), I1); if (!I2->use_empty()) I2->replaceAllUsesWith(I1); I1->intersectOptionalDataWith(I2); I2->eraseFromParent(); I1 = BB1_Itr++; while (isa(I1)) I1 = BB1_Itr++; I2 = BB2_Itr++; while (isa(I2)) I2 = BB2_Itr++; } while (I1->getOpcode() == I2->getOpcode() && I1->isIdenticalToWhenDefined(I2)); return true; HoistTerminator: // It may not be possible to hoist an invoke. if (isa(I1) && !isSafeToHoistInvoke(BB1, BB2, I1, I2)) return true; // Okay, it is safe to hoist the terminator. Instruction *NT = I1->clone(); BIParent->getInstList().insert(BI, NT); if (!NT->getType()->isVoidTy()) { I1->replaceAllUsesWith(NT); I2->replaceAllUsesWith(NT); NT->takeName(I1); } // Hoisting one of the terminators from our successor is a great thing. // Unfortunately, the successors of the if/else blocks may have PHI nodes in // them. If they do, all PHI entries for BB1/BB2 must agree for all PHI // nodes, so we insert select instruction to compute the final result. std::map, SelectInst*> InsertedSelects; for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) { PHINode *PN; for (BasicBlock::iterator BBI = SI->begin(); (PN = dyn_cast(BBI)); ++BBI) { Value *BB1V = PN->getIncomingValueForBlock(BB1); Value *BB2V = PN->getIncomingValueForBlock(BB2); if (BB1V == BB2V) continue; // These values do not agree. Insert a select instruction before NT // that determines the right value. SelectInst *&SI = InsertedSelects[std::make_pair(BB1V, BB2V)]; if (SI == 0) SI = SelectInst::Create(BI->getCondition(), BB1V, BB2V, BB1V->getName()+"."+BB2V->getName(), NT); // Make the PHI node use the select for all incoming values for BB1/BB2 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) if (PN->getIncomingBlock(i) == BB1 || PN->getIncomingBlock(i) == BB2) PN->setIncomingValue(i, SI); } } // Update any PHI nodes in our new successors. for (succ_iterator SI = succ_begin(BB1), E = succ_end(BB1); SI != E; ++SI) AddPredecessorToBlock(*SI, BIParent, BB1); EraseTerminatorInstAndDCECond(BI); return true; } /// SpeculativelyExecuteBB - Given a conditional branch that goes to BB1 /// and an BB2 and the only successor of BB1 is BB2, hoist simple code /// (for now, restricted to a single instruction that's side effect free) from /// the BB1 into the branch block to speculatively execute it. static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) { // Only speculatively execution a single instruction (not counting the // terminator) for now. Instruction *HInst = NULL; Instruction *Term = BB1->getTerminator(); for (BasicBlock::iterator BBI = BB1->begin(), BBE = BB1->end(); BBI != BBE; ++BBI) { Instruction *I = BBI; // Skip debug info. if (isa(I)) continue; if (I == Term) break; if (HInst) return false; HInst = I; } if (!HInst) return false; // Be conservative for now. FP select instruction can often be expensive. Value *BrCond = BI->getCondition(); if (isa(BrCond)) return false; // If BB1 is actually on the false edge of the conditional branch, remember // to swap the select operands later. bool Invert = false; if (BB1 != BI->getSuccessor(0)) { assert(BB1 == BI->getSuccessor(1) && "No edge from 'if' block?"); Invert = true; } // Turn // BB: // %t1 = icmp // br i1 %t1, label %BB1, label %BB2 // BB1: // %t3 = add %t2, c // br label BB2 // BB2: // => // BB: // %t1 = icmp // %t4 = add %t2, c // %t3 = select i1 %t1, %t2, %t3 switch (HInst->getOpcode()) { default: return false; // Not safe / profitable to hoist. case Instruction::Add: case Instruction::Sub: // Not worth doing for vector ops. if (HInst->getType()->isVectorTy()) return false; break; case Instruction::And: case Instruction::Or: case Instruction::Xor: case Instruction::Shl: case Instruction::LShr: case Instruction::AShr: // Don't mess with vector operations. if (HInst->getType()->isVectorTy()) return false; break; // These are all cheap and non-trapping instructions. } // If the instruction is obviously dead, don't try to predicate it. if (HInst->use_empty()) { HInst->eraseFromParent(); return true; } // Can we speculatively execute the instruction? And what is the value // if the condition is false? Consider the phi uses, if the incoming value // from the "if" block are all the same V, then V is the value of the // select if the condition is false. BasicBlock *BIParent = BI->getParent(); SmallVector PHIUses; Value *FalseV = NULL; BasicBlock *BB2 = BB1->getTerminator()->getSuccessor(0); for (Value::use_iterator UI = HInst->use_begin(), E = HInst->use_end(); UI != E; ++UI) { // Ignore any user that is not a PHI node in BB2. These can only occur in // unreachable blocks, because they would not be dominated by the instr. PHINode *PN = dyn_cast(*UI); if (!PN || PN->getParent() != BB2) return false; PHIUses.push_back(PN); Value *PHIV = PN->getIncomingValueForBlock(BIParent); if (!FalseV) FalseV = PHIV; else if (FalseV != PHIV) return false; // Inconsistent value when condition is false. } assert(FalseV && "Must have at least one user, and it must be a PHI"); // Do not hoist the instruction if any of its operands are defined but not // used in this BB. The transformation will prevent the operand from // being sunk into the use block. for (User::op_iterator i = HInst->op_begin(), e = HInst->op_end(); i != e; ++i) { Instruction *OpI = dyn_cast(*i); if (OpI && OpI->getParent() == BIParent && !OpI->isUsedInBasicBlock(BIParent)) return false; } // If we get here, we can hoist the instruction. Try to place it // before the icmp instruction preceding the conditional branch. BasicBlock::iterator InsertPos = BI; if (InsertPos != BIParent->begin()) --InsertPos; // Skip debug info between condition and branch. while (InsertPos != BIParent->begin() && isa(InsertPos)) --InsertPos; if (InsertPos == BrCond && !isa(BrCond)) { SmallPtrSet BB1Insns; for(BasicBlock::iterator BB1I = BB1->begin(), BB1E = BB1->end(); BB1I != BB1E; ++BB1I) BB1Insns.insert(BB1I); for(Value::use_iterator UI = BrCond->use_begin(), UE = BrCond->use_end(); UI != UE; ++UI) { Instruction *Use = cast(*UI); if (!BB1Insns.count(Use)) continue; // If BrCond uses the instruction that place it just before // branch instruction. InsertPos = BI; break; } } else InsertPos = BI; BIParent->getInstList().splice(InsertPos, BB1->getInstList(), HInst); // Create a select whose true value is the speculatively executed value and // false value is the previously determined FalseV. SelectInst *SI; if (Invert) SI = SelectInst::Create(BrCond, FalseV, HInst, FalseV->getName() + "." + HInst->getName(), BI); else SI = SelectInst::Create(BrCond, HInst, FalseV, HInst->getName() + "." + FalseV->getName(), BI); // Make the PHI node use the select for all incoming values for "then" and // "if" blocks. for (unsigned i = 0, e = PHIUses.size(); i != e; ++i) { PHINode *PN = PHIUses[i]; for (unsigned j = 0, ee = PN->getNumIncomingValues(); j != ee; ++j) if (PN->getIncomingBlock(j) == BB1 || PN->getIncomingBlock(j) == BIParent) PN->setIncomingValue(j, SI); } ++NumSpeculations; return true; } /// BlockIsSimpleEnoughToThreadThrough - Return true if we can thread a branch /// across this block. static bool BlockIsSimpleEnoughToThreadThrough(BasicBlock *BB) { BranchInst *BI = cast(BB->getTerminator()); unsigned Size = 0; for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) { if (isa(BBI)) continue; if (Size > 10) return false; // Don't clone large BB's. ++Size; // We can only support instructions that do not define values that are // live outside of the current basic block. for (Value::use_iterator UI = BBI->use_begin(), E = BBI->use_end(); UI != E; ++UI) { Instruction *U = cast(*UI); if (U->getParent() != BB || isa(U)) return false; } // Looks ok, continue checking. } return true; } /// FoldCondBranchOnPHI - If we have a conditional branch on a PHI node value /// that is defined in the same block as the branch and if any PHI entries are /// constants, thread edges corresponding to that entry to be branches to their /// ultimate destination. static bool FoldCondBranchOnPHI(BranchInst *BI, const TargetData *TD) { BasicBlock *BB = BI->getParent(); PHINode *PN = dyn_cast(BI->getCondition()); // NOTE: we currently cannot transform this case if the PHI node is used // outside of the block. if (!PN || PN->getParent() != BB || !PN->hasOneUse()) return false; // Degenerate case of a single entry PHI. if (PN->getNumIncomingValues() == 1) { FoldSingleEntryPHINodes(PN->getParent()); return true; } // Now we know that this block has multiple preds and two succs. if (!BlockIsSimpleEnoughToThreadThrough(BB)) return false; // Okay, this is a simple enough basic block. See if any phi values are // constants. for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { ConstantInt *CB = dyn_cast(PN->getIncomingValue(i)); if (CB == 0 || !CB->getType()->isIntegerTy(1)) continue; // Okay, we now know that all edges from PredBB should be revectored to // branch to RealDest. BasicBlock *PredBB = PN->getIncomingBlock(i); BasicBlock *RealDest = BI->getSuccessor(!CB->getZExtValue()); if (RealDest == BB) continue; // Skip self loops. // The dest block might have PHI nodes, other predecessors and other // difficult cases. Instead of being smart about this, just insert a new // block that jumps to the destination block, effectively splitting // the edge we are about to create. BasicBlock *EdgeBB = BasicBlock::Create(BB->getContext(), RealDest->getName()+".critedge", RealDest->getParent(), RealDest); BranchInst::Create(RealDest, EdgeBB); // Update PHI nodes. AddPredecessorToBlock(RealDest, EdgeBB, BB); // BB may have instructions that are being threaded over. Clone these // instructions into EdgeBB. We know that there will be no uses of the // cloned instructions outside of EdgeBB. BasicBlock::iterator InsertPt = EdgeBB->begin(); DenseMap TranslateMap; // Track translated values. for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) { if (PHINode *PN = dyn_cast(BBI)) { TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB); continue; } // Clone the instruction. Instruction *N = BBI->clone(); if (BBI->hasName()) N->setName(BBI->getName()+".c"); // Update operands due to translation. for (User::op_iterator i = N->op_begin(), e = N->op_end(); i != e; ++i) { DenseMap::iterator PI = TranslateMap.find(*i); if (PI != TranslateMap.end()) *i = PI->second; } // Check for trivial simplification. if (Value *V = SimplifyInstruction(N, TD)) { TranslateMap[BBI] = V; delete N; // Instruction folded away, don't need actual inst } else { // Insert the new instruction into its new home. EdgeBB->getInstList().insert(InsertPt, N); if (!BBI->use_empty()) TranslateMap[BBI] = N; } } // Loop over all of the edges from PredBB to BB, changing them to branch // to EdgeBB instead. TerminatorInst *PredBBTI = PredBB->getTerminator(); for (unsigned i = 0, e = PredBBTI->getNumSuccessors(); i != e; ++i) if (PredBBTI->getSuccessor(i) == BB) { BB->removePredecessor(PredBB); PredBBTI->setSuccessor(i, EdgeBB); } // Recurse, simplifying any other constants. return FoldCondBranchOnPHI(BI, TD) | true; } return false; } /// FoldTwoEntryPHINode - Given a BB that starts with the specified two-entry /// PHI node, see if we can eliminate it. static bool FoldTwoEntryPHINode(PHINode *PN, const TargetData *TD) { // Ok, this is a two entry PHI node. Check to see if this is a simple "if // statement", which has a very simple dominance structure. Basically, we // are trying to find the condition that is being branched on, which // subsequently causes this merge to happen. We really want control // dependence information for this check, but simplifycfg can't keep it up // to date, and this catches most of the cases we care about anyway. BasicBlock *BB = PN->getParent(); BasicBlock *IfTrue, *IfFalse; Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse); if (!IfCond || // Don't bother if the branch will be constant folded trivially. isa(IfCond)) return false; // Okay, we found that we can merge this two-entry phi node into a select. // Doing so would require us to fold *all* two entry phi nodes in this block. // At some point this becomes non-profitable (particularly if the target // doesn't support cmov's). Only do this transformation if there are two or // fewer PHI nodes in this block. unsigned NumPhis = 0; for (BasicBlock::iterator I = BB->begin(); isa(I); ++NumPhis, ++I) if (NumPhis > 2) return false; // Loop over the PHI's seeing if we can promote them all to select // instructions. While we are at it, keep track of the instructions // that need to be moved to the dominating block. SmallPtrSet AggressiveInsts; for (BasicBlock::iterator II = BB->begin(); isa(II);) { PHINode *PN = cast(II++); if (Value *V = SimplifyInstruction(PN, TD)) { PN->replaceAllUsesWith(V); PN->eraseFromParent(); continue; } if (!DominatesMergePoint(PN->getIncomingValue(0), BB, &AggressiveInsts) || !DominatesMergePoint(PN->getIncomingValue(1), BB, &AggressiveInsts)) return false; } // If we folded the the first phi, PN dangles at this point. Refresh it. If // we ran out of PHIs then we simplified them all. PN = dyn_cast(BB->begin()); if (PN == 0) return true; // Don't fold i1 branches on PHIs which contain binary operators. These can // often be turned into switches and other things. if (PN->getType()->isIntegerTy(1) && (isa(PN->getIncomingValue(0)) || isa(PN->getIncomingValue(1)) || isa(IfCond))) return false; // If we all PHI nodes are promotable, check to make sure that all // instructions in the predecessor blocks can be promoted as well. If // not, we won't be able to get rid of the control flow, so it's not // worth promoting to select instructions. BasicBlock *DomBlock = 0; BasicBlock *IfBlock1 = PN->getIncomingBlock(0); BasicBlock *IfBlock2 = PN->getIncomingBlock(1); if (cast(IfBlock1->getTerminator())->isConditional()) { IfBlock1 = 0; } else { DomBlock = *pred_begin(IfBlock1); for (BasicBlock::iterator I = IfBlock1->begin();!isa(I);++I) if (!AggressiveInsts.count(I) && !isa(I)) { // This is not an aggressive instruction that we can promote. // Because of this, we won't be able to get rid of the control // flow, so the xform is not worth it. return false; } } if (cast(IfBlock2->getTerminator())->isConditional()) { IfBlock2 = 0; } else { DomBlock = *pred_begin(IfBlock2); for (BasicBlock::iterator I = IfBlock2->begin();!isa(I);++I) if (!AggressiveInsts.count(I) && !isa(I)) { // This is not an aggressive instruction that we can promote. // Because of this, we won't be able to get rid of the control // flow, so the xform is not worth it. return false; } } DEBUG(dbgs() << "FOUND IF CONDITION! " << *IfCond << " T: " << IfTrue->getName() << " F: " << IfFalse->getName() << "\n"); // If we can still promote the PHI nodes after this gauntlet of tests, // do all of the PHI's now. Instruction *InsertPt = DomBlock->getTerminator(); // Move all 'aggressive' instructions, which are defined in the // conditional parts of the if's up to the dominating block. if (IfBlock1) DomBlock->getInstList().splice(InsertPt, IfBlock1->getInstList(), IfBlock1->begin(), IfBlock1->getTerminator()); if (IfBlock2) DomBlock->getInstList().splice(InsertPt, IfBlock2->getInstList(), IfBlock2->begin(), IfBlock2->getTerminator()); while (PHINode *PN = dyn_cast(BB->begin())) { // Change the PHI node into a select instruction. Value *TrueVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse); Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue); Value *NV = SelectInst::Create(IfCond, TrueVal, FalseVal, "", InsertPt); PN->replaceAllUsesWith(NV); NV->takeName(PN); PN->eraseFromParent(); } // At this point, IfBlock1 and IfBlock2 are both empty, so our if statement // has been flattened. Change DomBlock to jump directly to our new block to // avoid other simplifycfg's kicking in on the diamond. TerminatorInst *OldTI = DomBlock->getTerminator(); BranchInst::Create(BB, OldTI); OldTI->eraseFromParent(); return true; } /// SimplifyCondBranchToTwoReturns - If we found a conditional branch that goes /// to two returning blocks, try to merge them together into one return, /// introducing a select if the return values disagree. static bool SimplifyCondBranchToTwoReturns(BranchInst *BI) { assert(BI->isConditional() && "Must be a conditional branch"); BasicBlock *TrueSucc = BI->getSuccessor(0); BasicBlock *FalseSucc = BI->getSuccessor(1); ReturnInst *TrueRet = cast(TrueSucc->getTerminator()); ReturnInst *FalseRet = cast(FalseSucc->getTerminator()); // Check to ensure both blocks are empty (just a return) or optionally empty // with PHI nodes. If there are other instructions, merging would cause extra // computation on one path or the other. if (!TrueSucc->getFirstNonPHIOrDbg()->isTerminator()) return false; if (!FalseSucc->getFirstNonPHIOrDbg()->isTerminator()) return false; // Okay, we found a branch that is going to two return nodes. If // there is no return value for this function, just change the // branch into a return. if (FalseRet->getNumOperands() == 0) { TrueSucc->removePredecessor(BI->getParent()); FalseSucc->removePredecessor(BI->getParent()); ReturnInst::Create(BI->getContext(), 0, BI); EraseTerminatorInstAndDCECond(BI); return true; } // Otherwise, figure out what the true and false return values are // so we can insert a new select instruction. Value *TrueValue = TrueRet->getReturnValue(); Value *FalseValue = FalseRet->getReturnValue(); // Unwrap any PHI nodes in the return blocks. if (PHINode *TVPN = dyn_cast_or_null(TrueValue)) if (TVPN->getParent() == TrueSucc) TrueValue = TVPN->getIncomingValueForBlock(BI->getParent()); if (PHINode *FVPN = dyn_cast_or_null(FalseValue)) if (FVPN->getParent() == FalseSucc) FalseValue = FVPN->getIncomingValueForBlock(BI->getParent()); // In order for this transformation to be safe, we must be able to // unconditionally execute both operands to the return. This is // normally the case, but we could have a potentially-trapping // constant expression that prevents this transformation from being // safe. if (ConstantExpr *TCV = dyn_cast_or_null(TrueValue)) if (TCV->canTrap()) return false; if (ConstantExpr *FCV = dyn_cast_or_null(FalseValue)) if (FCV->canTrap()) return false; // Okay, we collected all the mapped values and checked them for sanity, and // defined to really do this transformation. First, update the CFG. TrueSucc->removePredecessor(BI->getParent()); FalseSucc->removePredecessor(BI->getParent()); // Insert select instructions where needed. Value *BrCond = BI->getCondition(); if (TrueValue) { // Insert a select if the results differ. if (TrueValue == FalseValue || isa(FalseValue)) { } else if (isa(TrueValue)) { TrueValue = FalseValue; } else { TrueValue = SelectInst::Create(BrCond, TrueValue, FalseValue, "retval", BI); } } Value *RI = !TrueValue ? ReturnInst::Create(BI->getContext(), BI) : ReturnInst::Create(BI->getContext(), TrueValue, BI); (void) RI; DEBUG(dbgs() << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:" << "\n " << *BI << "NewRet = " << *RI << "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc); EraseTerminatorInstAndDCECond(BI); return true; } /// FoldBranchToCommonDest - If this basic block is ONLY a setcc and a branch, /// and if a predecessor branches to us and one of our successors, fold the /// setcc into the predecessor and use logical operations to pick the right /// destination. bool llvm::FoldBranchToCommonDest(BranchInst *BI) { BasicBlock *BB = BI->getParent(); Instruction *Cond = dyn_cast(BI->getCondition()); if (Cond == 0 || (!isa(Cond) && !isa(Cond)) || Cond->getParent() != BB || !Cond->hasOneUse()) return false; // Only allow this if the condition is a simple instruction that can be // executed unconditionally. It must be in the same block as the branch, and // must be at the front of the block. BasicBlock::iterator FrontIt = BB->front(); // Ignore dbg intrinsics. while (isa(FrontIt)) ++FrontIt; // Allow a single instruction to be hoisted in addition to the compare // that feeds the branch. We later ensure that any values that _it_ uses // were also live in the predecessor, so that we don't unnecessarily create // register pressure or inhibit out-of-order execution. Instruction *BonusInst = 0; if (&*FrontIt != Cond && FrontIt->hasOneUse() && *FrontIt->use_begin() == Cond && FrontIt->isSafeToSpeculativelyExecute()) { BonusInst = &*FrontIt; ++FrontIt; } // Only a single bonus inst is allowed. if (&*FrontIt != Cond) return false; // Make sure the instruction after the condition is the cond branch. BasicBlock::iterator CondIt = Cond; ++CondIt; // Ingore dbg intrinsics. while(isa(CondIt)) ++CondIt; if (&*CondIt != BI) { assert (!isa(CondIt) && "Hey do not forget debug info!"); return false; } // Cond is known to be a compare or binary operator. Check to make sure that // neither operand is a potentially-trapping constant expression. if (ConstantExpr *CE = dyn_cast(Cond->getOperand(0))) if (CE->canTrap()) return false; if (ConstantExpr *CE = dyn_cast(Cond->getOperand(1))) if (CE->canTrap()) return false; // Finally, don't infinitely unroll conditional loops. BasicBlock *TrueDest = BI->getSuccessor(0); BasicBlock *FalseDest = BI->getSuccessor(1); if (TrueDest == BB || FalseDest == BB) return false; for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { BasicBlock *PredBlock = *PI; BranchInst *PBI = dyn_cast(PredBlock->getTerminator()); // Check that we have two conditional branches. If there is a PHI node in // the common successor, verify that the same value flows in from both // blocks. if (PBI == 0 || PBI->isUnconditional() || !SafeToMergeTerminators(BI, PBI)) continue; // Ensure that any values used in the bonus instruction are also used // by the terminator of the predecessor. This means that those values // must already have been resolved, so we won't be inhibiting the // out-of-order core by speculating them earlier. if (BonusInst) { // Collect the values used by the bonus inst SmallPtrSet UsedValues; for (Instruction::op_iterator OI = BonusInst->op_begin(), OE = BonusInst->op_end(); OI != OE; ++OI) { Value* V = *OI; if (!isa(V)) UsedValues.insert(V); } SmallVector, 4> Worklist; Worklist.push_back(std::make_pair(PBI->getOperand(0), 0)); // Walk up to four levels back up the use-def chain of the predecessor's // terminator to see if all those values were used. The choice of four // levels is arbitrary, to provide a compile-time-cost bound. while (!Worklist.empty()) { std::pair Pair = Worklist.back(); Worklist.pop_back(); if (Pair.second >= 4) continue; UsedValues.erase(Pair.first); if (UsedValues.empty()) break; if (Instruction *I = dyn_cast(Pair.first)) { for (Instruction::op_iterator OI = I->op_begin(), OE = I->op_end(); OI != OE; ++OI) Worklist.push_back(std::make_pair(OI->get(), Pair.second+1)); } } if (!UsedValues.empty()) return false; } Instruction::BinaryOps Opc; bool InvertPredCond = false; if (PBI->getSuccessor(0) == TrueDest) Opc = Instruction::Or; else if (PBI->getSuccessor(1) == FalseDest) Opc = Instruction::And; else if (PBI->getSuccessor(0) == FalseDest) Opc = Instruction::And, InvertPredCond = true; else if (PBI->getSuccessor(1) == TrueDest) Opc = Instruction::Or, InvertPredCond = true; else continue; DEBUG(dbgs() << "FOLDING BRANCH TO COMMON DEST:\n" << *PBI << *BB); // If we need to invert the condition in the pred block to match, do so now. if (InvertPredCond) { Value *NewCond = PBI->getCondition(); if (NewCond->hasOneUse() && isa(NewCond)) { CmpInst *CI = cast(NewCond); CI->setPredicate(CI->getInversePredicate()); } else { NewCond = BinaryOperator::CreateNot(NewCond, PBI->getCondition()->getName()+".not", PBI); } PBI->setCondition(NewCond); BasicBlock *OldTrue = PBI->getSuccessor(0); BasicBlock *OldFalse = PBI->getSuccessor(1); PBI->setSuccessor(0, OldFalse); PBI->setSuccessor(1, OldTrue); } // If we have a bonus inst, clone it into the predecessor block. Instruction *NewBonus = 0; if (BonusInst) { NewBonus = BonusInst->clone(); PredBlock->getInstList().insert(PBI, NewBonus); NewBonus->takeName(BonusInst); BonusInst->setName(BonusInst->getName()+".old"); } // Clone Cond into the predecessor basic block, and or/and the // two conditions together. Instruction *New = Cond->clone(); if (BonusInst) New->replaceUsesOfWith(BonusInst, NewBonus); PredBlock->getInstList().insert(PBI, New); New->takeName(Cond); Cond->setName(New->getName()+".old"); Value *NewCond = BinaryOperator::Create(Opc, PBI->getCondition(), New, "or.cond", PBI); PBI->setCondition(NewCond); if (PBI->getSuccessor(0) == BB) { AddPredecessorToBlock(TrueDest, PredBlock, BB); PBI->setSuccessor(0, TrueDest); } if (PBI->getSuccessor(1) == BB) { AddPredecessorToBlock(FalseDest, PredBlock, BB); PBI->setSuccessor(1, FalseDest); } return true; } return false; } /// SimplifyCondBranchToCondBranch - If we have a conditional branch as a /// predecessor of another block, this function tries to simplify it. We know /// that PBI and BI are both conditional branches, and BI is in one of the /// successor blocks of PBI - PBI branches to BI. static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI) { assert(PBI->isConditional() && BI->isConditional()); BasicBlock *BB = BI->getParent(); // If this block ends with a branch instruction, and if there is a // predecessor that ends on a branch of the same condition, make // this conditional branch redundant. if (PBI->getCondition() == BI->getCondition() && PBI->getSuccessor(0) != PBI->getSuccessor(1)) { // Okay, the outcome of this conditional branch is statically // knowable. If this block had a single pred, handle specially. if (BB->getSinglePredecessor()) { // Turn this into a branch on constant. bool CondIsTrue = PBI->getSuccessor(0) == BB; BI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue)); return true; // Nuke the branch on constant. } // Otherwise, if there are multiple predecessors, insert a PHI that merges // in the constant and simplify the block result. Subsequent passes of // simplifycfg will thread the block. if (BlockIsSimpleEnoughToThreadThrough(BB)) { PHINode *NewPN = PHINode::Create(Type::getInt1Ty(BB->getContext()), BI->getCondition()->getName() + ".pr", BB->begin()); // Okay, we're going to insert the PHI node. Since PBI is not the only // predecessor, compute the PHI'd conditional value for all of the preds. // Any predecessor where the condition is not computable we keep symbolic. for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { BasicBlock *P = *PI; if ((PBI = dyn_cast(P->getTerminator())) && PBI != BI && PBI->isConditional() && PBI->getCondition() == BI->getCondition() && PBI->getSuccessor(0) != PBI->getSuccessor(1)) { bool CondIsTrue = PBI->getSuccessor(0) == BB; NewPN->addIncoming(ConstantInt::get(Type::getInt1Ty(BB->getContext()), CondIsTrue), P); } else { NewPN->addIncoming(BI->getCondition(), P); } } BI->setCondition(NewPN); return true; } } // If this is a conditional branch in an empty block, and if any // predecessors is a conditional branch to one of our destinations, // fold the conditions into logical ops and one cond br. BasicBlock::iterator BBI = BB->begin(); // Ignore dbg intrinsics. while (isa(BBI)) ++BBI; if (&*BBI != BI) return false; if (ConstantExpr *CE = dyn_cast(BI->getCondition())) if (CE->canTrap()) return false; int PBIOp, BIOp; if (PBI->getSuccessor(0) == BI->getSuccessor(0)) PBIOp = BIOp = 0; else if (PBI->getSuccessor(0) == BI->getSuccessor(1)) PBIOp = 0, BIOp = 1; else if (PBI->getSuccessor(1) == BI->getSuccessor(0)) PBIOp = 1, BIOp = 0; else if (PBI->getSuccessor(1) == BI->getSuccessor(1)) PBIOp = BIOp = 1; else return false; // Check to make sure that the other destination of this branch // isn't BB itself. If so, this is an infinite loop that will // keep getting unwound. if (PBI->getSuccessor(PBIOp) == BB) return false; // Do not perform this transformation if it would require // insertion of a large number of select instructions. For targets // without predication/cmovs, this is a big pessimization. BasicBlock *CommonDest = PBI->getSuccessor(PBIOp); unsigned NumPhis = 0; for (BasicBlock::iterator II = CommonDest->begin(); isa(II); ++II, ++NumPhis) if (NumPhis > 2) // Disable this xform. return false; // Finally, if everything is ok, fold the branches to logical ops. BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1); DEBUG(dbgs() << "FOLDING BRs:" << *PBI->getParent() << "AND: " << *BI->getParent()); // If OtherDest *is* BB, then BB is a basic block with a single conditional // branch in it, where one edge (OtherDest) goes back to itself but the other // exits. We don't *know* that the program avoids the infinite loop // (even though that seems likely). If we do this xform naively, we'll end up // recursively unpeeling the loop. Since we know that (after the xform is // done) that the block *is* infinite if reached, we just make it an obviously // infinite loop with no cond branch. if (OtherDest == BB) { // Insert it at the end of the function, because it's either code, // or it won't matter if it's hot. :) BasicBlock *InfLoopBlock = BasicBlock::Create(BB->getContext(), "infloop", BB->getParent()); BranchInst::Create(InfLoopBlock, InfLoopBlock); OtherDest = InfLoopBlock; } DEBUG(dbgs() << *PBI->getParent()->getParent()); // BI may have other predecessors. Because of this, we leave // it alone, but modify PBI. // Make sure we get to CommonDest on True&True directions. Value *PBICond = PBI->getCondition(); if (PBIOp) PBICond = BinaryOperator::CreateNot(PBICond, PBICond->getName()+".not", PBI); Value *BICond = BI->getCondition(); if (BIOp) BICond = BinaryOperator::CreateNot(BICond, BICond->getName()+".not", PBI); // Merge the conditions. Value *Cond = BinaryOperator::CreateOr(PBICond, BICond, "brmerge", PBI); // Modify PBI to branch on the new condition to the new dests. PBI->setCondition(Cond); PBI->setSuccessor(0, CommonDest); PBI->setSuccessor(1, OtherDest); // OtherDest may have phi nodes. If so, add an entry from PBI's // block that are identical to the entries for BI's block. AddPredecessorToBlock(OtherDest, PBI->getParent(), BB); // We know that the CommonDest already had an edge from PBI to // it. If it has PHIs though, the PHIs may have different // entries for BB and PBI's BB. If so, insert a select to make // them agree. PHINode *PN; for (BasicBlock::iterator II = CommonDest->begin(); (PN = dyn_cast(II)); ++II) { Value *BIV = PN->getIncomingValueForBlock(BB); unsigned PBBIdx = PN->getBasicBlockIndex(PBI->getParent()); Value *PBIV = PN->getIncomingValue(PBBIdx); if (BIV != PBIV) { // Insert a select in PBI to pick the right value. Value *NV = SelectInst::Create(PBICond, PBIV, BIV, PBIV->getName()+".mux", PBI); PN->setIncomingValue(PBBIdx, NV); } } DEBUG(dbgs() << "INTO: " << *PBI->getParent()); DEBUG(dbgs() << *PBI->getParent()->getParent()); // This basic block is probably dead. We know it has at least // one fewer predecessor. return true; } // SimplifyIndirectBrOnSelect - Replaces // (indirectbr (select cond, blockaddress(@fn, BlockA), // blockaddress(@fn, BlockB))) // with // (br cond, BlockA, BlockB). static bool SimplifyIndirectBrOnSelect(IndirectBrInst *IBI, SelectInst *SI) { // Check that both operands of the select are block addresses. BlockAddress *TBA = dyn_cast(SI->getTrueValue()); BlockAddress *FBA = dyn_cast(SI->getFalseValue()); if (!TBA || !FBA) return false; // Extract the actual blocks. BasicBlock *TrueBB = TBA->getBasicBlock(); BasicBlock *FalseBB = FBA->getBasicBlock(); // Remove any superfluous successor edges from the CFG. // First, figure out which successors to preserve. // If TrueBB and FalseBB are equal, only try to preserve one copy of that // successor. BasicBlock *KeepEdge1 = TrueBB; BasicBlock *KeepEdge2 = TrueBB != FalseBB ? FalseBB : 0; // Then remove the rest. for (unsigned I = 0, E = IBI->getNumSuccessors(); I != E; ++I) { BasicBlock *Succ = IBI->getSuccessor(I); // Make sure only to keep exactly one copy of each edge. if (Succ == KeepEdge1) KeepEdge1 = 0; else if (Succ == KeepEdge2) KeepEdge2 = 0; else Succ->removePredecessor(IBI->getParent()); } // Insert an appropriate new terminator. if ((KeepEdge1 == 0) && (KeepEdge2 == 0)) { if (TrueBB == FalseBB) // We were only looking for one successor, and it was present. // Create an unconditional branch to it. BranchInst::Create(TrueBB, IBI); else // We found both of the successors we were looking for. // Create a conditional branch sharing the condition of the select. BranchInst::Create(TrueBB, FalseBB, SI->getCondition(), IBI); } else if (KeepEdge1 && (KeepEdge2 || TrueBB == FalseBB)) { // Neither of the selected blocks were successors, so this // indirectbr must be unreachable. new UnreachableInst(IBI->getContext(), IBI); } else { // One of the selected values was a successor, but the other wasn't. // Insert an unconditional branch to the one that was found; // the edge to the one that wasn't must be unreachable. if (KeepEdge1 == 0) // Only TrueBB was found. BranchInst::Create(TrueBB, IBI); else // Only FalseBB was found. BranchInst::Create(FalseBB, IBI); } EraseTerminatorInstAndDCECond(IBI); return true; } /// TryToSimplifyUncondBranchWithICmpInIt - This is called when we find an icmp /// instruction (a seteq/setne with a constant) as the only instruction in a /// block that ends with an uncond branch. We are looking for a very specific /// pattern that occurs when "A == 1 || A == 2 || A == 3" gets simplified. In /// this case, we merge the first two "or's of icmp" into a switch, but then the /// default value goes to an uncond block with a seteq in it, we get something /// like: /// /// switch i8 %A, label %DEFAULT [ i8 1, label %end i8 2, label %end ] /// DEFAULT: /// %tmp = icmp eq i8 %A, 92 /// br label %end /// end: /// ... = phi i1 [ true, %entry ], [ %tmp, %DEFAULT ], [ true, %entry ] /// /// We prefer to split the edge to 'end' so that there is a true/false entry to /// the PHI, merging the third icmp into the switch. static bool TryToSimplifyUncondBranchWithICmpInIt(ICmpInst *ICI, const TargetData *TD) { BasicBlock *BB = ICI->getParent(); // If the block has any PHIs in it or the icmp has multiple uses, it is too // complex. if (isa(BB->begin()) || !ICI->hasOneUse()) return false; Value *V = ICI->getOperand(0); ConstantInt *Cst = cast(ICI->getOperand(1)); // The pattern we're looking for is where our only predecessor is a switch on // 'V' and this block is the default case for the switch. In this case we can // fold the compared value into the switch to simplify things. BasicBlock *Pred = BB->getSinglePredecessor(); if (Pred == 0 || !isa(Pred->getTerminator())) return false; SwitchInst *SI = cast(Pred->getTerminator()); if (SI->getCondition() != V) return false; // If BB is reachable on a non-default case, then we simply know the value of // V in this block. Substitute it and constant fold the icmp instruction // away. if (SI->getDefaultDest() != BB) { ConstantInt *VVal = SI->findCaseDest(BB); assert(VVal && "Should have a unique destination value"); ICI->setOperand(0, VVal); if (Value *V = SimplifyInstruction(ICI, TD)) { ICI->replaceAllUsesWith(V); ICI->eraseFromParent(); } // BB is now empty, so it is likely to simplify away. return SimplifyCFG(BB) | true; } // Ok, the block is reachable from the default dest. If the constant we're // comparing exists in one of the other edges, then we can constant fold ICI // and zap it. if (SI->findCaseValue(Cst) != 0) { Value *V; if (ICI->getPredicate() == ICmpInst::ICMP_EQ) V = ConstantInt::getFalse(BB->getContext()); else V = ConstantInt::getTrue(BB->getContext()); ICI->replaceAllUsesWith(V); ICI->eraseFromParent(); // BB is now empty, so it is likely to simplify away. return SimplifyCFG(BB) | true; } // The use of the icmp has to be in the 'end' block, by the only PHI node in // the block. BasicBlock *SuccBlock = BB->getTerminator()->getSuccessor(0); PHINode *PHIUse = dyn_cast(ICI->use_back()); if (PHIUse == 0 || PHIUse != &SuccBlock->front() || isa(++BasicBlock::iterator(PHIUse))) return false; // If the icmp is a SETEQ, then the default dest gets false, the new edge gets // true in the PHI. Constant *DefaultCst = ConstantInt::getTrue(BB->getContext()); Constant *NewCst = ConstantInt::getFalse(BB->getContext()); if (ICI->getPredicate() == ICmpInst::ICMP_EQ) std::swap(DefaultCst, NewCst); // Replace ICI (which is used by the PHI for the default value) with true or // false depending on if it is EQ or NE. ICI->replaceAllUsesWith(DefaultCst); ICI->eraseFromParent(); // Okay, the switch goes to this block on a default value. Add an edge from // the switch to the merge point on the compared value. BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), "switch.edge", BB->getParent(), BB); SI->addCase(Cst, NewBB); // NewBB branches to the phi block, add the uncond branch and the phi entry. BranchInst::Create(SuccBlock, NewBB); PHIUse->addIncoming(NewCst, NewBB); return true; } /// SimplifyBranchOnICmpChain - The specified branch is a conditional branch. /// Check to see if it is branching on an or/and chain of icmp instructions, and /// fold it into a switch instruction if so. static bool SimplifyBranchOnICmpChain(BranchInst *BI, const TargetData *TD) { Instruction *Cond = dyn_cast(BI->getCondition()); if (Cond == 0) return false; // Change br (X == 0 | X == 1), T, F into a switch instruction. // If this is a bunch of seteq's or'd together, or if it's a bunch of // 'setne's and'ed together, collect them. Value *CompVal = 0; std::vector Values; bool TrueWhenEqual = true; Value *ExtraCase = 0; if (Cond->getOpcode() == Instruction::Or) { CompVal = GatherConstantCompares(Cond, Values, ExtraCase, TD, true); } else if (Cond->getOpcode() == Instruction::And) { CompVal = GatherConstantCompares(Cond, Values, ExtraCase, TD, false); TrueWhenEqual = false; } // If we didn't have a multiply compared value, fail. if (CompVal == 0) return false; // There might be duplicate constants in the list, which the switch // instruction can't handle, remove them now. array_pod_sort(Values.begin(), Values.end(), ConstantIntSortPredicate); Values.erase(std::unique(Values.begin(), Values.end()), Values.end()); // If Extra was used, we require at least two switch values to do the // transformation. A switch with one value is just an cond branch. if (ExtraCase && Values.size() < 2) return false; // Figure out which block is which destination. BasicBlock *DefaultBB = BI->getSuccessor(1); BasicBlock *EdgeBB = BI->getSuccessor(0); if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB); BasicBlock *BB = BI->getParent(); DEBUG(dbgs() << "Converting 'icmp' chain with " << Values.size() << " cases into SWITCH. BB is:\n" << *BB); // If there are any extra values that couldn't be folded into the switch // then we evaluate them with an explicit branch first. Split the block // right before the condbr to handle it. if (ExtraCase) { BasicBlock *NewBB = BB->splitBasicBlock(BI, "switch.early.test"); // Remove the uncond branch added to the old block. TerminatorInst *OldTI = BB->getTerminator(); if (TrueWhenEqual) BranchInst::Create(EdgeBB, NewBB, ExtraCase, OldTI); else BranchInst::Create(NewBB, EdgeBB, ExtraCase, OldTI); OldTI->eraseFromParent(); // If there are PHI nodes in EdgeBB, then we need to add a new entry to them // for the edge we just added. AddPredecessorToBlock(EdgeBB, BB, NewBB); DEBUG(dbgs() << " ** 'icmp' chain unhandled condition: " << *ExtraCase << "\nEXTRABB = " << *BB); BB = NewBB; } // Convert pointer to int before we switch. if (CompVal->getType()->isPointerTy()) { assert(TD && "Cannot switch on pointer without TargetData"); CompVal = new PtrToIntInst(CompVal, TD->getIntPtrType(CompVal->getContext()), "magicptr", BI); } // Create the new switch instruction now. SwitchInst *New = SwitchInst::Create(CompVal, DefaultBB, Values.size(), BI); // Add all of the 'cases' to the switch instruction. for (unsigned i = 0, e = Values.size(); i != e; ++i) New->addCase(Values[i], EdgeBB); // We added edges from PI to the EdgeBB. As such, if there were any // PHI nodes in EdgeBB, they need entries to be added corresponding to // the number of edges added. for (BasicBlock::iterator BBI = EdgeBB->begin(); isa(BBI); ++BBI) { PHINode *PN = cast(BBI); Value *InVal = PN->getIncomingValueForBlock(BB); for (unsigned i = 0, e = Values.size()-1; i != e; ++i) PN->addIncoming(InVal, BB); } // Erase the old branch instruction. EraseTerminatorInstAndDCECond(BI); DEBUG(dbgs() << " ** 'icmp' chain result is:\n" << *BB << '\n'); return true; } bool SimplifyCFGOpt::SimplifyReturn(ReturnInst *RI) { BasicBlock *BB = RI->getParent(); if (!BB->getFirstNonPHIOrDbg()->isTerminator()) return false; // Find predecessors that end with branches. SmallVector UncondBranchPreds; SmallVector CondBranchPreds; for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { BasicBlock *P = *PI; TerminatorInst *PTI = P->getTerminator(); if (BranchInst *BI = dyn_cast(PTI)) { if (BI->isUnconditional()) UncondBranchPreds.push_back(P); else CondBranchPreds.push_back(BI); } } // If we found some, do the transformation! if (!UncondBranchPreds.empty()) { while (!UncondBranchPreds.empty()) { BasicBlock *Pred = UncondBranchPreds.pop_back_val(); DEBUG(dbgs() << "FOLDING: " << *BB << "INTO UNCOND BRANCH PRED: " << *Pred); Instruction *UncondBranch = Pred->getTerminator(); // Clone the return and add it to the end of the predecessor. Instruction *NewRet = RI->clone(); Pred->getInstList().push_back(NewRet); // If the return instruction returns a value, and if the value was a // PHI node in "BB", propagate the right value into the return. for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end(); i != e; ++i) if (PHINode *PN = dyn_cast(*i)) if (PN->getParent() == BB) *i = PN->getIncomingValueForBlock(Pred); // Update any PHI nodes in the returning block to realize that we no // longer branch to them. BB->removePredecessor(Pred); UncondBranch->eraseFromParent(); } // If we eliminated all predecessors of the block, delete the block now. if (pred_begin(BB) == pred_end(BB)) // We know there are no successors, so just nuke the block. BB->eraseFromParent(); return true; } // Check out all of the conditional branches going to this return // instruction. If any of them just select between returns, change the // branch itself into a select/return pair. while (!CondBranchPreds.empty()) { BranchInst *BI = CondBranchPreds.pop_back_val(); // Check to see if the non-BB successor is also a return block. if (isa(BI->getSuccessor(0)->getTerminator()) && isa(BI->getSuccessor(1)->getTerminator()) && SimplifyCondBranchToTwoReturns(BI)) return true; } return false; } bool SimplifyCFGOpt::SimplifyUnwind(UnwindInst *UI) { // Check to see if the first instruction in this block is just an unwind. // If so, replace any invoke instructions which use this as an exception // destination with call instructions. BasicBlock *BB = UI->getParent(); if (!BB->getFirstNonPHIOrDbg()->isTerminator()) return false; bool Changed = false; SmallVector Preds(pred_begin(BB), pred_end(BB)); while (!Preds.empty()) { BasicBlock *Pred = Preds.back(); InvokeInst *II = dyn_cast(Pred->getTerminator()); if (II && II->getUnwindDest() == BB) { // Insert a new branch instruction before the invoke, because this // is now a fall through. BranchInst *BI = BranchInst::Create(II->getNormalDest(), II); Pred->getInstList().remove(II); // Take out of symbol table // Insert the call now. SmallVector Args(II->op_begin(), II->op_end()-3); CallInst *CI = CallInst::Create(II->getCalledValue(), Args.begin(), Args.end(), II->getName(), BI); CI->setCallingConv(II->getCallingConv()); CI->setAttributes(II->getAttributes()); // If the invoke produced a value, the Call now does instead. II->replaceAllUsesWith(CI); delete II; Changed = true; } Preds.pop_back(); } // If this block is now dead (and isn't the entry block), remove it. if (pred_begin(BB) == pred_end(BB) && BB != &BB->getParent()->getEntryBlock()) { // We know there are no successors, so just nuke the block. BB->eraseFromParent(); return true; } return Changed; } bool SimplifyCFGOpt::SimplifyUnreachable(UnreachableInst *UI) { BasicBlock *BB = UI->getParent(); bool Changed = false; // If there are any instructions immediately before the unreachable that can // be removed, do so. while (UI != BB->begin()) { BasicBlock::iterator BBI = UI; --BBI; // Do not delete instructions that can have side effects, like calls // (which may never return) and volatile loads and stores. if (isa(BBI) && !isa(BBI)) break; if (StoreInst *SI = dyn_cast(BBI)) if (SI->isVolatile()) break; if (LoadInst *LI = dyn_cast(BBI)) if (LI->isVolatile()) break; // Delete this instruction BBI->eraseFromParent(); Changed = true; } // If the unreachable instruction is the first in the block, take a gander // at all of the predecessors of this instruction, and simplify them. if (&BB->front() != UI) return Changed; SmallVector Preds(pred_begin(BB), pred_end(BB)); for (unsigned i = 0, e = Preds.size(); i != e; ++i) { TerminatorInst *TI = Preds[i]->getTerminator(); if (BranchInst *BI = dyn_cast(TI)) { if (BI->isUnconditional()) { if (BI->getSuccessor(0) == BB) { new UnreachableInst(TI->getContext(), TI); TI->eraseFromParent(); Changed = true; } } else { if (BI->getSuccessor(0) == BB) { BranchInst::Create(BI->getSuccessor(1), BI); EraseTerminatorInstAndDCECond(BI); } else if (BI->getSuccessor(1) == BB) { BranchInst::Create(BI->getSuccessor(0), BI); EraseTerminatorInstAndDCECond(BI); Changed = true; } } } else if (SwitchInst *SI = dyn_cast(TI)) { for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) if (SI->getSuccessor(i) == BB) { BB->removePredecessor(SI->getParent()); SI->removeCase(i); --i; --e; Changed = true; } // If the default value is unreachable, figure out the most popular // destination and make it the default. if (SI->getSuccessor(0) == BB) { std::map Popularity; for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) Popularity[SI->getSuccessor(i)]++; // Find the most popular block. unsigned MaxPop = 0; BasicBlock *MaxBlock = 0; for (std::map::iterator I = Popularity.begin(), E = Popularity.end(); I != E; ++I) { if (I->second > MaxPop) { MaxPop = I->second; MaxBlock = I->first; } } if (MaxBlock) { // Make this the new default, allowing us to delete any explicit // edges to it. SI->setSuccessor(0, MaxBlock); Changed = true; // If MaxBlock has phinodes in it, remove MaxPop-1 entries from // it. if (isa(MaxBlock->begin())) for (unsigned i = 0; i != MaxPop-1; ++i) MaxBlock->removePredecessor(SI->getParent()); for (unsigned i = 1, e = SI->getNumCases(); i != e; ++i) if (SI->getSuccessor(i) == MaxBlock) { SI->removeCase(i); --i; --e; } } } } else if (InvokeInst *II = dyn_cast(TI)) { if (II->getUnwindDest() == BB) { // Convert the invoke to a call instruction. This would be a good // place to note that the call does not throw though. BranchInst *BI = BranchInst::Create(II->getNormalDest(), II); II->removeFromParent(); // Take out of symbol table // Insert the call now... SmallVector Args(II->op_begin(), II->op_end()-3); CallInst *CI = CallInst::Create(II->getCalledValue(), Args.begin(), Args.end(), II->getName(), BI); CI->setCallingConv(II->getCallingConv()); CI->setAttributes(II->getAttributes()); // If the invoke produced a value, the call does now instead. II->replaceAllUsesWith(CI); delete II; Changed = true; } } } // If this block is now dead, remove it. if (pred_begin(BB) == pred_end(BB) && BB != &BB->getParent()->getEntryBlock()) { // We know there are no successors, so just nuke the block. BB->eraseFromParent(); return true; } return Changed; } bool SimplifyCFGOpt::SimplifySwitch(SwitchInst *SI) { // If this switch is too complex to want to look at, ignore it. if (!isValueEqualityComparison(SI)) return false; BasicBlock *BB = SI->getParent(); // If we only have one predecessor, and if it is a branch on this value, // see if that predecessor totally determines the outcome of this switch. if (BasicBlock *OnlyPred = BB->getSinglePredecessor()) if (SimplifyEqualityComparisonWithOnlyPredecessor(SI, OnlyPred)) return SimplifyCFG(BB) | true; // If the block only contains the switch, see if we can fold the block // away into any preds. BasicBlock::iterator BBI = BB->begin(); // Ignore dbg intrinsics. while (isa(BBI)) ++BBI; if (SI == &*BBI) if (FoldValueComparisonIntoPredecessors(SI)) return SimplifyCFG(BB) | true; return false; } bool SimplifyCFGOpt::SimplifyIndirectBr(IndirectBrInst *IBI) { BasicBlock *BB = IBI->getParent(); bool Changed = false; // Eliminate redundant destinations. SmallPtrSet Succs; for (unsigned i = 0, e = IBI->getNumDestinations(); i != e; ++i) { BasicBlock *Dest = IBI->getDestination(i); if (!Dest->hasAddressTaken() || !Succs.insert(Dest)) { Dest->removePredecessor(BB); IBI->removeDestination(i); --i; --e; Changed = true; } } if (IBI->getNumDestinations() == 0) { // If the indirectbr has no successors, change it to unreachable. new UnreachableInst(IBI->getContext(), IBI); EraseTerminatorInstAndDCECond(IBI); return true; } if (IBI->getNumDestinations() == 1) { // If the indirectbr has one successor, change it to a direct branch. BranchInst::Create(IBI->getDestination(0), IBI); EraseTerminatorInstAndDCECond(IBI); return true; } if (SelectInst *SI = dyn_cast(IBI->getAddress())) { if (SimplifyIndirectBrOnSelect(IBI, SI)) return SimplifyCFG(BB) | true; } return Changed; } bool SimplifyCFGOpt::SimplifyUncondBranch(BranchInst *BI) { BasicBlock *BB = BI->getParent(); // If the Terminator is the only non-phi instruction, simplify the block. BasicBlock::iterator I = BB->getFirstNonPHIOrDbg(); if (I->isTerminator() && BB != &BB->getParent()->getEntryBlock() && TryToSimplifyUncondBranchFromEmptyBlock(BB)) return true; // If the only instruction in the block is a seteq/setne comparison // against a constant, try to simplify the block. if (ICmpInst *ICI = dyn_cast(I)) if (ICI->isEquality() && isa(ICI->getOperand(1))) { for (++I; isa(I); ++I) ; if (I->isTerminator() && TryToSimplifyUncondBranchWithICmpInIt(ICI, TD)) return true; } return false; } bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI) { BasicBlock *BB = BI->getParent(); // Conditional branch if (isValueEqualityComparison(BI)) { // If we only have one predecessor, and if it is a branch on this value, // see if that predecessor totally determines the outcome of this // switch. if (BasicBlock *OnlyPred = BB->getSinglePredecessor()) if (SimplifyEqualityComparisonWithOnlyPredecessor(BI, OnlyPred)) return SimplifyCFG(BB) | true; // This block must be empty, except for the setcond inst, if it exists. // Ignore dbg intrinsics. BasicBlock::iterator I = BB->begin(); // Ignore dbg intrinsics. while (isa(I)) ++I; if (&*I == BI) { if (FoldValueComparisonIntoPredecessors(BI)) return SimplifyCFG(BB) | true; } else if (&*I == cast(BI->getCondition())){ ++I; // Ignore dbg intrinsics. while (isa(I)) ++I; if (&*I == BI && FoldValueComparisonIntoPredecessors(BI)) return SimplifyCFG(BB) | true; } } // Try to turn "br (X == 0 | X == 1), T, F" into a switch instruction. if (SimplifyBranchOnICmpChain(BI, TD)) return true; // We have a conditional branch to two blocks that are only reachable // from BI. We know that the condbr dominates the two blocks, so see if // there is any identical code in the "then" and "else" blocks. If so, we // can hoist it up to the branching block. if (BI->getSuccessor(0)->getSinglePredecessor() != 0) { if (BI->getSuccessor(1)->getSinglePredecessor() != 0) { if (HoistThenElseCodeToIf(BI)) return SimplifyCFG(BB) | true; } else { // If Successor #1 has multiple preds, we may be able to conditionally // execute Successor #0 if it branches to successor #1. TerminatorInst *Succ0TI = BI->getSuccessor(0)->getTerminator(); if (Succ0TI->getNumSuccessors() == 1 && Succ0TI->getSuccessor(0) == BI->getSuccessor(1)) if (SpeculativelyExecuteBB(BI, BI->getSuccessor(0))) return SimplifyCFG(BB) | true; } } else if (BI->getSuccessor(1)->getSinglePredecessor() != 0) { // If Successor #0 has multiple preds, we may be able to conditionally // execute Successor #1 if it branches to successor #0. TerminatorInst *Succ1TI = BI->getSuccessor(1)->getTerminator(); if (Succ1TI->getNumSuccessors() == 1 && Succ1TI->getSuccessor(0) == BI->getSuccessor(0)) if (SpeculativelyExecuteBB(BI, BI->getSuccessor(1))) return SimplifyCFG(BB) | true; } // If this is a branch on a phi node in the current block, thread control // through this block if any PHI node entries are constants. if (PHINode *PN = dyn_cast(BI->getCondition())) if (PN->getParent() == BI->getParent()) if (FoldCondBranchOnPHI(BI, TD)) return SimplifyCFG(BB) | true; // If this basic block is ONLY a setcc and a branch, and if a predecessor // branches to us and one of our successors, fold the setcc into the // predecessor and use logical operations to pick the right destination. if (FoldBranchToCommonDest(BI)) return SimplifyCFG(BB) | true; // Scan predecessor blocks for conditional branches. for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) if (BranchInst *PBI = dyn_cast((*PI)->getTerminator())) if (PBI != BI && PBI->isConditional()) if (SimplifyCondBranchToCondBranch(PBI, BI)) return SimplifyCFG(BB) | true; return false; } bool SimplifyCFGOpt::run(BasicBlock *BB) { bool Changed = false; assert(BB && BB->getParent() && "Block not embedded in function!"); assert(BB->getTerminator() && "Degenerate basic block encountered!"); // Remove basic blocks that have no predecessors (except the entry block)... // or that just have themself as a predecessor. These are unreachable. if ((pred_begin(BB) == pred_end(BB) && BB != &BB->getParent()->getEntryBlock()) || BB->getSinglePredecessor() == BB) { DEBUG(dbgs() << "Removing BB: \n" << *BB); DeleteDeadBlock(BB); return true; } // Check to see if we can constant propagate this terminator instruction // away... Changed |= ConstantFoldTerminator(BB); // Check for and eliminate duplicate PHI nodes in this block. Changed |= EliminateDuplicatePHINodes(BB); // Merge basic blocks into their predecessor if there is only one distinct // pred, and if there is only one distinct successor of the predecessor, and // if there are no PHI nodes. // if (MergeBlockIntoPredecessor(BB)) return true; // If there is a trivial two-entry PHI node in this basic block, and we can // eliminate it, do so now. if (PHINode *PN = dyn_cast(BB->begin())) if (PN->getNumIncomingValues() == 2) Changed |= FoldTwoEntryPHINode(PN, TD); if (BranchInst *BI = dyn_cast(BB->getTerminator())) { if (BI->isUnconditional()) { if (SimplifyUncondBranch(BI)) return true; } else { if (SimplifyCondBranch(BI)) return true; } } else if (ReturnInst *RI = dyn_cast(BB->getTerminator())) { if (SimplifyReturn(RI)) return true; } else if (SwitchInst *SI = dyn_cast(BB->getTerminator())) { if (SimplifySwitch(SI)) return true; } else if (UnreachableInst *UI = dyn_cast(BB->getTerminator())) { if (SimplifyUnreachable(UI)) return true; } else if (UnwindInst *UI = dyn_cast(BB->getTerminator())) { if (SimplifyUnwind(UI)) return true; } else if (IndirectBrInst *IBI = dyn_cast(BB->getTerminator())) { if (SimplifyIndirectBr(IBI)) return true; } return Changed; } /// SimplifyCFG - This function is used to do simplification of a CFG. For /// example, it adjusts branches to branches to eliminate the extra hop, it /// eliminates unreachable basic blocks, and does other "peephole" optimization /// of the CFG. It returns true if a modification was made. /// bool llvm::SimplifyCFG(BasicBlock *BB, const TargetData *TD) { return SimplifyCFGOpt(TD).run(BB); }