//===- SimplifyCFG.cpp - Code to perform CFG simplification ---------------===// // // The LLVM Compiler Infrastructure // // This file was developed by the LLVM research group and 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/Type.h" #include "llvm/Support/CFG.h" #include "llvm/Support/Debug.h" #include "llvm/Transforms/Utils/BasicBlockUtils.h" #include #include #include #include using namespace llvm; /// 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(); std::set 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) { assert(std::find(succ_begin(ExistPred), succ_end(ExistPred), Succ) != succ_end(ExistPred) && "ExistPred is not a predecessor of Succ!"); if (!isa(Succ->begin())) return; // Quick exit if nothing to do for (BasicBlock::iterator I = Succ->begin(); isa(I); ++I) { PHINode *PN = cast(I); Value *V = PN->getIncomingValueForBlock(ExistPred); PN->addIncoming(V, NewPred); } } // CanPropagatePredecessorsForPHIs - Return true if we can fold BB, an // almost-empty BB ending in an unconditional branch to Succ, into succ. // // Assumption: Succ is the single successor for BB. // static bool CanPropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!"); // Check to see if one of the predecessors of BB is already a predecessor of // Succ. If so, we cannot do the transformation if there are any PHI nodes // with incompatible values coming in from the two edges! // if (isa(Succ->front())) { std::set BBPreds(pred_begin(BB), pred_end(BB)); for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI) if (std::find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) { // Loop over all of the PHI nodes checking to see if there are // incompatible values coming in. for (BasicBlock::iterator I = Succ->begin(); isa(I); ++I) { PHINode *PN = cast(I); // Loop up the entries in the PHI node for BB and for *PI if the // values coming in are non-equal, we cannot merge these two blocks // (instead we should insert a conditional move or something, then // merge the blocks). if (PN->getIncomingValueForBlock(BB) != PN->getIncomingValueForBlock(*PI)) return false; // Values are not equal... } } } // Finally, if BB has PHI nodes that are used by things other than the PHIs in // Succ and Succ has predecessors that are not Succ and not Pred, we cannot // fold these blocks, as we don't know whether BB dominates Succ or not to // update the PHI nodes correctly. if (!isa(BB->begin()) || Succ->getSinglePredecessor()) return true; // If the predecessors of Succ are only BB and Succ itself, we can handle this. bool IsSafe = true; for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI) if (*PI != Succ && *PI != BB) { IsSafe = false; break; } if (IsSafe) return true; // If the PHI nodes in BB are only used by instructions in Succ, we are ok if // BB and Succ have no common predecessors. for (BasicBlock::iterator I = BB->begin(); isa(I); ++I) { PHINode *PN = cast(I); for (Value::use_iterator UI = PN->use_begin(), E = PN->use_end(); UI != E; ++UI) if (cast(*UI)->getParent() != Succ) return false; } // Scan the predecessor sets of BB and Succ, making sure there are no common // predecessors. Common predecessors would cause us to build a phi node with // differing incoming values, which is not legal. std::set BBPreds(pred_begin(BB), pred_end(BB)); for (pred_iterator PI = pred_begin(Succ), E = pred_end(Succ); PI != E; ++PI) if (BBPreds.count(*PI)) return false; return true; } /// TryToSimplifyUncondBranchFromEmptyBlock - BB contains an unconditional /// branch to Succ, and contains no instructions other than PHI nodes and the /// branch. If possible, eliminate BB. static bool TryToSimplifyUncondBranchFromEmptyBlock(BasicBlock *BB, BasicBlock *Succ) { // If our successor has PHI nodes, then we need to update them to include // entries for BB's predecessors, not for BB itself. Be careful though, // if this transformation fails (returns true) then we cannot do this // transformation! // if (!CanPropagatePredecessorsForPHIs(BB, Succ)) return false; DOUT << "Killing Trivial BB: \n" << *BB; if (isa(Succ->begin())) { // If there is more than one pred of succ, and there are PHI nodes in // the successor, then we need to add incoming edges for the PHI nodes // const std::vector BBPreds(pred_begin(BB), pred_end(BB)); // Loop over all of the PHI nodes in the successor of BB. for (BasicBlock::iterator I = Succ->begin(); isa(I); ++I) { PHINode *PN = cast(I); Value *OldVal = PN->removeIncomingValue(BB, false); assert(OldVal && "No entry in PHI for Pred BB!"); // If this incoming value is one of the PHI nodes in BB, the new entries // in the PHI node are the entries from the old PHI. if (isa(OldVal) && cast(OldVal)->getParent() == BB) { PHINode *OldValPN = cast(OldVal); for (unsigned i = 0, e = OldValPN->getNumIncomingValues(); i != e; ++i) PN->addIncoming(OldValPN->getIncomingValue(i), OldValPN->getIncomingBlock(i)); } else { for (std::vector::const_iterator PredI = BBPreds.begin(), End = BBPreds.end(); PredI != End; ++PredI) { // Add an incoming value for each of the new incoming values... PN->addIncoming(OldVal, *PredI); } } } } if (isa(&BB->front())) { std::vector OldSuccPreds(pred_begin(Succ), pred_end(Succ)); // Move all PHI nodes in BB to Succ if they are alive, otherwise // delete them. while (PHINode *PN = dyn_cast(&BB->front())) if (PN->use_empty()) { // Just remove the dead phi. This happens if Succ's PHIs were the only // users of the PHI nodes. PN->eraseFromParent(); } else { // The instruction is alive, so this means that Succ must have // *ONLY* had BB as a predecessor, and the PHI node is still valid // now. Simply move it into Succ, because we know that BB // strictly dominated Succ. Succ->getInstList().splice(Succ->begin(), BB->getInstList(), BB->begin()); // We need to add new entries for the PHI node to account for // predecessors of Succ that the PHI node does not take into // account. At this point, since we know that BB dominated succ, // this means that we should any newly added incoming edges should // use the PHI node as the value for these edges, because they are // loop back edges. for (unsigned i = 0, e = OldSuccPreds.size(); i != e; ++i) if (OldSuccPreds[i] != BB) PN->addIncoming(PN, OldSuccPreds[i]); } } // Everything that jumped to BB now goes to Succ. std::string OldName = BB->getName(); BB->replaceAllUsesWith(Succ); BB->eraseFromParent(); // Delete the old basic block. if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can Succ->setName(OldName); return true; } /// GetIfCondition - Given a basic block (BB) with two predecessors (and /// presumably PHI nodes 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. /// /// static Value *GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue, BasicBlock *&IfFalse) { assert(std::distance(pred_begin(BB), pred_end(BB)) == 2 && "Function can only handle blocks with 2 predecessors!"); BasicBlock *Pred1 = *pred_begin(BB); BasicBlock *Pred2 = *++pred_begin(BB); // We can only handle branches. Other control flow will be lowered to // branches if possible anyway. if (!isa(Pred1->getTerminator()) || !isa(Pred2->getTerminator())) return 0; BranchInst *Pred1Br = cast(Pred1->getTerminator()); BranchInst *Pred2Br = cast(Pred2->getTerminator()); // 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()) { // 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; } // 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 (++pred_begin(Pred2) != pred_end(Pred2)) 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! if (pred_begin(Pred1) == pred_end(Pred1) || ++pred_begin(Pred1) != pred_end(Pred1) || pred_begin(Pred2) == pred_end(Pred2) || ++pred_begin(Pred2) != pred_end(Pred2) || *pred_begin(Pred1) != *pred_begin(Pred2)) return 0; // Otherwise, if this is a conditional branch, then we can use it! BasicBlock *CommonPred = *pred_begin(Pred1); if (BranchInst *BI = dyn_cast(CommonPred->getTerminator())) { 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(); } return 0; } // 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, std::set *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 (BranchInst *BI = dyn_cast(PBB->getTerminator())) if (BI->isUnconditional() && BI->getSuccessor(0) == BB) { if (!AggressiveInsts) return false; // Okay, it looks like the instruction IS in the "condition". Check to // see if its a cheap instruction to unconditionally compute, and if it // only uses stuff defined outside of the condition. If so, hoist it out. switch (I->getOpcode()) { default: return false; // Cannot hoist this out safely. case Instruction::Load: // We can hoist loads that are non-volatile and obviously cannot trap. if (cast(I)->isVolatile()) return false; if (!isa(I->getOperand(0)) && !isa(I->getOperand(0))) return false; // Finally, 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 loop // out to its predecessor. if (PBB->begin() != BasicBlock::iterator(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::SetEQ: case Instruction::SetNE: case Instruction::SetLT: case Instruction::SetGT: case Instruction::SetLE: case Instruction::SetGE: 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 (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) if (!DominatesMergePoint(I->getOperand(i), BB, 0)) return false; // Okay, it's safe to do this! Remember this instruction. AggressiveInsts->insert(I); } return true; } // GatherConstantSetEQs - Given a potentially 'or'd together collection of seteq // instructions that compare a value against a constant, return the value being // compared, and stick the constant into the Values vector. static Value *GatherConstantSetEQs(Value *V, std::vector &Values){ if (Instruction *Inst = dyn_cast(V)) if (Inst->getOpcode() == Instruction::SetEQ) { if (ConstantInt *C = dyn_cast(Inst->getOperand(1))) { Values.push_back(C); return Inst->getOperand(0); } else if (ConstantInt *C = dyn_cast(Inst->getOperand(0))) { Values.push_back(C); return Inst->getOperand(1); } } else if (Inst->getOpcode() == Instruction::Or) { if (Value *LHS = GatherConstantSetEQs(Inst->getOperand(0), Values)) if (Value *RHS = GatherConstantSetEQs(Inst->getOperand(1), Values)) if (LHS == RHS) return LHS; } return 0; } // GatherConstantSetNEs - Given a potentially 'and'd together collection of // setne instructions that compare a value against a constant, return the value // being compared, and stick the constant into the Values vector. static Value *GatherConstantSetNEs(Value *V, std::vector &Values){ if (Instruction *Inst = dyn_cast(V)) if (Inst->getOpcode() == Instruction::SetNE) { if (ConstantInt *C = dyn_cast(Inst->getOperand(1))) { Values.push_back(C); return Inst->getOperand(0); } else if (ConstantInt *C = dyn_cast(Inst->getOperand(0))) { Values.push_back(C); return Inst->getOperand(1); } } else if (Inst->getOpcode() == Instruction::And) { if (Value *LHS = GatherConstantSetNEs(Inst->getOperand(0), Values)) if (Value *RHS = GatherConstantSetNEs(Inst->getOperand(1), Values)) if (LHS == RHS) return LHS; } return 0; } /// GatherValueComparisons - If the specified Cond is an 'and' or 'or' of a /// bunch of comparisons of one value against constants, return the value and /// the constants being compared. static bool GatherValueComparisons(Instruction *Cond, Value *&CompVal, std::vector &Values) { if (Cond->getOpcode() == Instruction::Or) { CompVal = GatherConstantSetEQs(Cond, Values); // Return true to indicate that the condition is true if the CompVal is // equal to one of the constants. return true; } else if (Cond->getOpcode() == Instruction::And) { CompVal = GatherConstantSetNEs(Cond, Values); // Return false to indicate that the condition is false if the CompVal is // equal to one of the constants. return false; } return false; } /// ErasePossiblyDeadInstructionTree - If the specified instruction is dead and /// has no side effects, nuke it. If it uses any instructions that become dead /// because the instruction is now gone, nuke them too. static void ErasePossiblyDeadInstructionTree(Instruction *I) { if (!isInstructionTriviallyDead(I)) return; std::vector InstrsToInspect; InstrsToInspect.push_back(I); while (!InstrsToInspect.empty()) { I = InstrsToInspect.back(); InstrsToInspect.pop_back(); if (!isInstructionTriviallyDead(I)) continue; // If I is in the work list multiple times, remove previous instances. for (unsigned i = 0, e = InstrsToInspect.size(); i != e; ++i) if (InstrsToInspect[i] == I) { InstrsToInspect.erase(InstrsToInspect.begin()+i); --i, --e; } // Add operands of dead instruction to worklist. for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) if (Instruction *OpI = dyn_cast(I->getOperand(i))) InstrsToInspect.push_back(OpI); // Remove dead instruction. I->eraseFromParent(); } } // isValueEqualityComparison - Return true if the specified terminator checks to // see if a value is equal to constant integer value. static Value *isValueEqualityComparison(TerminatorInst *TI) { 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) return 0; return SI->getCondition(); } if (BranchInst *BI = dyn_cast(TI)) if (BI->isConditional() && BI->getCondition()->hasOneUse()) if (SetCondInst *SCI = dyn_cast(BI->getCondition())) if ((SCI->getOpcode() == Instruction::SetEQ || SCI->getOpcode() == Instruction::SetNE) && isa(SCI->getOperand(1))) return SCI->getOperand(0); return 0; } // Given a value comparison instruction, decode all of the 'cases' that it // represents and return the 'default' block. static BasicBlock * 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); SetCondInst *SCI = cast(BI->getCondition()); Cases.push_back(std::make_pair(cast(SCI->getOperand(1)), BI->getSuccessor(SCI->getOpcode() == Instruction::SetNE))); return BI->getSuccessor(SCI->getOpcode() == Instruction::SetEQ); } // EliminateBlockCases - Given an 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. std::sort(V1->begin(), V1->end()); std::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. static bool 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)) { if (BranchInst *BTI = dyn_cast(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!"); Value *Cond = BTI->getCondition(); // Insert the new branch. Instruction *NI = new BranchInst(ThisDef, TI); // Remove PHI node entries for the dead edge. ThisCases[0].second->removePredecessor(TI->getParent()); DOUT << "Threading pred instr: " << *Pred->getTerminator() << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"; TI->eraseFromParent(); // Nuke the old one. // If condition is now dead, nuke it. if (Instruction *CondI = dyn_cast(Cond)) ErasePossiblyDeadInstructionTree(CondI); return true; } else { SwitchInst *SI = cast(TI); // Okay, TI has cases that are statically dead, prune them away. std::set DeadCases; for (unsigned i = 0, e = PredCases.size(); i != e; ++i) DeadCases.insert(PredCases[i].first); DOUT << "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); } DOUT << "Leaving: " << *TI << "\n"; return true; } } } else { // 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) TIV = PredCases[i].first; else return false; // Cannot handle multiple values coming to this block. 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 = new BranchInst(TheRealDest, TI); DOUT << "Threading pred instr: " << *Pred->getTerminator() << "Through successor TI: " << *TI << "Leaving: " << *NI << "\n"; Instruction *Cond = 0; if (BranchInst *BI = dyn_cast(TI)) Cond = dyn_cast(BI->getCondition()); TI->eraseFromParent(); // Nuke the old one. if (Cond) ErasePossiblyDeadInstructionTree(Cond); return true; } return false; } // 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. static bool FoldValueComparisonIntoPredecessors(TerminatorInst *TI) { BasicBlock *BB = TI->getParent(); Value *CV = isValueEqualityComparison(TI); // CondVal assert(CV && "Not a comparison?"); bool Changed = false; std::vector Preds(pred_begin(BB), pred_end(BB)); while (!Preds.empty()) { BasicBlock *Pred = Preds.back(); Preds.pop_back(); // 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. std::vector 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); // Now that the successors are updated, create the new Switch instruction. SwitchInst *NewSI = new SwitchInst(CV, PredDefault, PredCases.size(),PTI); for (unsigned i = 0, e = PredCases.size(); i != e; ++i) NewSI->addCase(PredCases[i].first, PredCases[i].second); Instruction *DeadCond = 0; if (BranchInst *BI = dyn_cast(PTI)) // If PTI is a branch, remember the condition. DeadCond = dyn_cast(BI->getCondition()); Pred->getInstList().erase(PTI); // If the condition is dead now, remove the instruction tree. if (DeadCond) ErasePossiblyDeadInstructionTree(DeadCond); // 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 loop, because it's either code, // or it won't matter if it's hot. :) InfLoopBlock = new BasicBlock("infloop", BB->getParent()); new BranchInst(InfLoopBlock, InfLoopBlock); } NewSI->setSuccessor(i, InfLoopBlock); } Changed = true; } } return Changed; } /// 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 Instruction *I1 = BB1->begin(), *I2 = BB2->begin(); if (I1->getOpcode() != I2->getOpcode() || !I1->isIdenticalTo(I2) || isa(I1) || isa(I1)) 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); BB2->getInstList().erase(I2); I1 = BB1->begin(); I2 = BB2->begin(); } while (I1->getOpcode() == I2->getOpcode() && I1->isIdenticalTo(I2)); return true; HoistTerminator: // Okay, it is safe to hoist the terminator. Instruction *NT = I1->clone(); BIParent->getInstList().insert(BI, NT); if (NT->getType() != Type::VoidTy) { I1->replaceAllUsesWith(NT); I2->replaceAllUsesWith(NT); NT->setName(I1->getName()); } // 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) { // 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 = new SelectInst(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); BI->eraseFromParent(); 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; // If this basic block contains anything other than a PHI (which controls the // branch) and branch itself, bail out. FIXME: improve this in the future. for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI, ++Size) { if (Size > 10) return false; // Don't clone large BB's. // We can only support instructions that are 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) { 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) { if (PN->getIncomingValue(0) != PN) PN->replaceAllUsesWith(PN->getIncomingValue(0)); else PN->replaceAllUsesWith(UndefValue::get(PN->getType())); PN->eraseFromParent(); 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) if (ConstantBool *CB = dyn_cast(PN->getIncomingValue(i))) { // 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->getValue()); 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 = new BasicBlock(RealDest->getName()+".critedge", RealDest->getParent(), RealDest); new BranchInst(RealDest, EdgeBB); PHINode *PN; for (BasicBlock::iterator BBI = RealDest->begin(); (PN = dyn_cast(BBI)); ++BBI) { Value *V = PN->getIncomingValueForBlock(BB); PN->addIncoming(V, EdgeBB); } // 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(); std::map TranslateMap; // Track translated values. for (BasicBlock::iterator BBI = BB->begin(); &*BBI != BI; ++BBI) { if (PHINode *PN = dyn_cast(BBI)) { TranslateMap[PN] = PN->getIncomingValueForBlock(PredBB); } else { // Clone the instruction. Instruction *N = BBI->clone(); if (BBI->hasName()) N->setName(BBI->getName()+".c"); // Update operands due to translation. for (unsigned i = 0, e = N->getNumOperands(); i != e; ++i) { std::map::iterator PI = TranslateMap.find(N->getOperand(i)); if (PI != TranslateMap.end()) N->setOperand(i, PI->second); } // Check for trivial simplification. if (Constant *C = ConstantFoldInstruction(N)) { TranslateMap[BBI] = C; delete N; // Constant 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) | 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) { // 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) 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; DOUT << "FOUND IF CONDITION! " << *IfCond << " T: " << IfTrue->getName() << " F: " << IfFalse->getName() << "\n"; // 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. std::set AggressiveInsts; BasicBlock::iterator AfterPHIIt = BB->begin(); while (isa(AfterPHIIt)) { PHINode *PN = cast(AfterPHIIt++); if (PN->getIncomingValue(0) == PN->getIncomingValue(1)) { if (PN->getIncomingValue(0) != PN) PN->replaceAllUsesWith(PN->getIncomingValue(0)); else PN->replaceAllUsesWith(UndefValue::get(PN->getType())); } else if (!DominatesMergePoint(PN->getIncomingValue(0), BB, &AggressiveInsts) || !DominatesMergePoint(PN->getIncomingValue(1), BB, &AggressiveInsts)) { 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, *IfBlock1 = 0, *IfBlock2 = 0; PN = cast(BB->begin()); BasicBlock *Pred = PN->getIncomingBlock(0); if (cast(Pred->getTerminator())->isUnconditional()) { IfBlock1 = Pred; DomBlock = *pred_begin(Pred); for (BasicBlock::iterator I = Pred->begin(); !isa(I); ++I) if (!AggressiveInsts.count(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; } } Pred = PN->getIncomingBlock(1); if (cast(Pred->getTerminator())->isUnconditional()) { IfBlock2 = Pred; DomBlock = *pred_begin(Pred); for (BasicBlock::iterator I = Pred->begin(); !isa(I); ++I) if (!AggressiveInsts.count(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 we can still promote the PHI nodes after this gauntlet of tests, // do all of the PHI's now. // 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(DomBlock->getTerminator(), IfBlock1->getInstList(), IfBlock1->begin(), IfBlock1->getTerminator()); } if (IfBlock2) { DomBlock->getInstList().splice(DomBlock->getTerminator(), 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); std::string Name = PN->getName(); PN->setName(""); PN->replaceAllUsesWith(new SelectInst(IfCond, TrueVal, FalseVal, Name, AfterPHIIt)); BB->getInstList().erase(PN); } 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->getZExtValue() < RHS->getZExtValue(); } }; } // 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. // // WARNING: The entry node of a function may not be simplified. // bool llvm::SimplifyCFG(BasicBlock *BB) { bool Changed = false; Function *M = BB->getParent(); assert(BB && BB->getParent() && "Block not embedded in function!"); assert(BB->getTerminator() && "Degenerate basic block encountered!"); assert(&BB->getParent()->front() != BB && "Can't Simplify entry block!"); // Remove basic blocks that have no predecessors... which are unreachable. if (pred_begin(BB) == pred_end(BB) || *pred_begin(BB) == BB && ++pred_begin(BB) == pred_end(BB)) { DOUT << "Removing BB: \n" << *BB; // Loop through all of our successors and make sure they know that one // of their predecessors is going away. for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) SI->removePredecessor(BB); while (!BB->empty()) { Instruction &I = BB->back(); // If this instruction is used, replace uses with an arbitrary // value. Because control flow can't get here, we don't care // what we replace the value with. Note that since this block is // unreachable, and all values contained within it must dominate their // uses, that all uses will eventually be removed. if (!I.use_empty()) // Make all users of this instruction use undef instead I.replaceAllUsesWith(UndefValue::get(I.getType())); // Remove the instruction from the basic block BB->getInstList().pop_back(); } M->getBasicBlockList().erase(BB); return true; } // Check to see if we can constant propagate this terminator instruction // away... Changed |= ConstantFoldTerminator(BB); // If this is a returning block with only PHI nodes in it, fold the return // instruction into any unconditional branch predecessors. // // If any predecessor is a conditional branch that just selects among // different return values, fold the replace the branch/return with a select // and return. if (ReturnInst *RI = dyn_cast(BB->getTerminator())) { BasicBlock::iterator BBI = BB->getTerminator(); if (BBI == BB->begin() || isa(--BBI)) { // Find predecessors that end with branches. std::vector UncondBranchPreds; std::vector CondBranchPreds; for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) { TerminatorInst *PTI = (*PI)->getTerminator(); if (BranchInst *BI = dyn_cast(PTI)) if (BI->isUnconditional()) UncondBranchPreds.push_back(*PI); else CondBranchPreds.push_back(BI); } // If we found some, do the transformation! if (!UncondBranchPreds.empty()) { while (!UncondBranchPreds.empty()) { BasicBlock *Pred = UncondBranchPreds.back(); DOUT << "FOLDING: " << *BB << "INTO UNCOND BRANCH PRED: " << *Pred; UncondBranchPreds.pop_back(); 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. if (NewRet->getNumOperands() == 1) if (PHINode *PN = dyn_cast(NewRet->getOperand(0))) if (PN->getParent() == BB) NewRet->setOperand(0, PN->getIncomingValueForBlock(Pred)); // Update any PHI nodes in the returning block to realize that we no // longer branch to them. BB->removePredecessor(Pred); Pred->getInstList().erase(UncondBranch); } // 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. M->getBasicBlockList().erase(BB); 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.back(); CondBranchPreds.pop_back(); BasicBlock *TrueSucc = BI->getSuccessor(0); BasicBlock *FalseSucc = BI->getSuccessor(1); BasicBlock *OtherSucc = TrueSucc == BB ? FalseSucc : TrueSucc; // Check to see if the non-BB successor is also a return block. if (isa(OtherSucc->getTerminator())) { // Check to see if there are only PHI instructions in this block. BasicBlock::iterator OSI = OtherSucc->getTerminator(); if (OSI == OtherSucc->begin() || isa(--OSI)) { // 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 (RI->getNumOperands() == 0) { TrueSucc->removePredecessor(BI->getParent()); FalseSucc->removePredecessor(BI->getParent()); new ReturnInst(0, BI); BI->getParent()->getInstList().erase(BI); return true; } // Otherwise, figure out what the true and false return values are // so we can insert a new select instruction. Value *TrueValue = TrueSucc->getTerminator()->getOperand(0); Value *FalseValue = FalseSucc->getTerminator()->getOperand(0); // Unwrap any PHI nodes in the return blocks. if (PHINode *TVPN = dyn_cast(TrueValue)) if (TVPN->getParent() == TrueSucc) TrueValue = TVPN->getIncomingValueForBlock(BI->getParent()); if (PHINode *FVPN = dyn_cast(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 ((!isa(TrueValue) || !cast(TrueValue)->canTrap()) && (!isa(TrueValue) || !cast(TrueValue)->canTrap())) { TrueSucc->removePredecessor(BI->getParent()); FalseSucc->removePredecessor(BI->getParent()); // Insert a new select instruction. Value *NewRetVal; Value *BrCond = BI->getCondition(); if (TrueValue != FalseValue) NewRetVal = new SelectInst(BrCond, TrueValue, FalseValue, "retval", BI); else NewRetVal = TrueValue; DOUT << "\nCHANGING BRANCH TO TWO RETURNS INTO SELECT:" << "\n " << *BI << "Select = " << *NewRetVal << "TRUEBLOCK: " << *TrueSucc << "FALSEBLOCK: "<< *FalseSucc; new ReturnInst(NewRetVal, BI); BI->eraseFromParent(); if (Instruction *BrCondI = dyn_cast(BrCond)) if (isInstructionTriviallyDead(BrCondI)) BrCondI->eraseFromParent(); return true; } } } } } } else if (isa(BB->begin())) { // 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, and any unconditional branch // predecessor with an unwind. // std::vector Preds(pred_begin(BB), pred_end(BB)); while (!Preds.empty()) { BasicBlock *Pred = Preds.back(); if (BranchInst *BI = dyn_cast(Pred->getTerminator())) { if (BI->isUnconditional()) { Pred->getInstList().pop_back(); // nuke uncond branch new UnwindInst(Pred); // Use unwind. Changed = true; } } else if (InvokeInst *II = dyn_cast(Pred->getTerminator())) if (II->getUnwindDest() == BB) { // Insert a new branch instruction before the invoke, because this // is now a fall through... BranchInst *BI = new BranchInst(II->getNormalDest(), II); Pred->getInstList().remove(II); // Take out of symbol table // Insert the call now... std::vector Args(II->op_begin()+3, II->op_end()); CallInst *CI = new CallInst(II->getCalledValue(), Args, II->getName(), BI); CI->setCallingConv(II->getCallingConv()); // 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, remove it. if (pred_begin(BB) == pred_end(BB)) { // We know there are no successors, so just nuke the block. M->getBasicBlockList().erase(BB); return true; } } else if (SwitchInst *SI = dyn_cast(BB->getTerminator())) { if (isValueEqualityComparison(SI)) { // 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) || 1; // If the block only contains the switch, see if we can fold the block // away into any preds. if (SI == &BB->front()) if (FoldValueComparisonIntoPredecessors(SI)) return SimplifyCFG(BB) || 1; } } else if (BranchInst *BI = dyn_cast(BB->getTerminator())) { if (BI->isUnconditional()) { BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes... while (isa(*BBI)) ++BBI; BasicBlock *Succ = BI->getSuccessor(0); if (BBI->isTerminator() && // Terminator is the only non-phi instruction! Succ != BB) // Don't hurt infinite loops! if (TryToSimplifyUncondBranchFromEmptyBlock(BB, Succ)) return 1; } else { // 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) || 1; // This block must be empty, except for the setcond inst, if it exists. BasicBlock::iterator I = BB->begin(); if (&*I == BI || (&*I == cast(BI->getCondition()) && &*++I == BI)) if (FoldValueComparisonIntoPredecessors(BI)) 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)) 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. BasicBlock *TrueDest = BI->getSuccessor(0); BasicBlock *FalseDest = BI->getSuccessor(1); if (BinaryOperator *Cond = dyn_cast(BI->getCondition())) if (Cond->getParent() == BB && &BB->front() == Cond && Cond->getNext() == BI && Cond->hasOneUse() && TrueDest != BB && FalseDest != BB) for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI!=E; ++PI) if (BranchInst *PBI = dyn_cast((*PI)->getTerminator())) if (PBI->isConditional() && SafeToMergeTerminators(BI, PBI)) { BasicBlock *PredBlock = *PI; if (PBI->getSuccessor(0) == FalseDest || PBI->getSuccessor(1) == TrueDest) { // Invert the predecessors condition test (xor it with true), // which allows us to write this code once. Value *NewCond = BinaryOperator::createNot(PBI->getCondition(), 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 ((PBI->getSuccessor(0) == TrueDest && FalseDest != BB) || (PBI->getSuccessor(1) == FalseDest && TrueDest != BB)) { // Clone Cond into the predecessor basic block, and or/and the // two conditions together. Instruction *New = Cond->clone(); New->setName(Cond->getName()); Cond->setName(Cond->getName()+".old"); PredBlock->getInstList().insert(PBI, New); Instruction::BinaryOps Opcode = PBI->getSuccessor(0) == TrueDest ? Instruction::Or : Instruction::And; Value *NewCond = BinaryOperator::create(Opcode, PBI->getCondition(), New, "bothcond", 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 SimplifyCFG(BB) | 1; } } // Scan predessor blocks for conditional branchs. 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 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(ConstantBool::get(CondIsTrue)); return SimplifyCFG(BB); // Nuke the branch on constant. } // Otherwise, if there are multiple predecessors, insert a PHI that // merges in the constant and simplify the block result. if (BlockIsSimpleEnoughToThreadThrough(BB)) { PHINode *NewPN = new PHINode(Type::BoolTy, BI->getCondition()->getName()+".pr", BB->begin()); for (PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) if ((PBI = dyn_cast((*PI)->getTerminator())) && PBI != BI && PBI->isConditional() && PBI->getCondition() == BI->getCondition() && PBI->getSuccessor(0) != PBI->getSuccessor(1)) { bool CondIsTrue = PBI->getSuccessor(0) == BB; NewPN->addIncoming(ConstantBool::get(CondIsTrue), *PI); } else { NewPN->addIncoming(BI->getCondition(), *PI); } BI->setCondition(NewPN); // This will thread the branch. return SimplifyCFG(BB) | 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. if (&BB->front() == BI) { 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 { PBIOp = BIOp = -1; } // 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 (PBIOp != -1 && PBI->getSuccessor(PBIOp) == BB) PBIOp = BIOp = -1; // 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. if (PBIOp != -1) { BasicBlock *CommonDest = PBI->getSuccessor(PBIOp); unsigned NumPhis = 0; for (BasicBlock::iterator II = CommonDest->begin(); isa(II); ++II, ++NumPhis) { if (NumPhis > 2) { // Disable this xform. PBIOp = -1; break; } } } // Finally, if everything is ok, fold the branches to logical ops. if (PBIOp != -1) { BasicBlock *CommonDest = PBI->getSuccessor(PBIOp); BasicBlock *OtherDest = BI->getSuccessor(BIOp ^ 1); // If OtherDest *is* BB, then this is a basic block with just // a conditional branch in it, where one edge (OtherDesg) goes // back to the block. We know that the program doesn't get // stuck in the infinite loop, so the condition must be such // that OtherDest isn't branched through. Forward to CommonDest, // and avoid an infinite loop at optimizer time. if (OtherDest == BB) OtherDest = CommonDest; DOUT << "FOLDING BRs:" << *PBI->getParent() << "AND: " << *BI->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. PHINode *PN; for (BasicBlock::iterator II = OtherDest->begin(); (PN = dyn_cast(II)); ++II) { Value *V = PN->getIncomingValueForBlock(BB); PN->addIncoming(V, PBI->getParent()); } // 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. 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 = new SelectInst(PBICond, PBIV, BIV, PBIV->getName()+".mux", PBI); PN->setIncomingValue(PBBIdx, NV); } } DOUT << "INTO: " << *PBI->getParent(); // This basic block is probably dead. We know it has at least // one fewer predecessor. return SimplifyCFG(BB) | true; } } } } } else if (isa(BB->getTerminator())) { // If there are any instructions immediately before the unreachable that can // be removed, do so. Instruction *Unreachable = BB->getTerminator(); while (Unreachable != BB->begin()) { BasicBlock::iterator BBI = Unreachable; --BBI; if (isa(BBI)) break; // Delete this instruction BB->getInstList().erase(BBI); 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() == Unreachable) { std::vector 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); TI->eraseFromParent(); Changed = true; } } else { if (BI->getSuccessor(0) == BB) { new BranchInst(BI->getSuccessor(1), BI); BI->eraseFromParent(); } else if (BI->getSuccessor(1) == BB) { new BranchInst(BI->getSuccessor(0), BI); BI->eraseFromParent(); 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 = new BranchInst(II->getNormalDest(), II); II->removeFromParent(); // Take out of symbol table // Insert the call now... std::vector Args(II->op_begin()+3, II->op_end()); CallInst *CI = new CallInst(II->getCalledValue(), Args, II->getName(), BI); CI->setCallingConv(II->getCallingConv()); // 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)) { // We know there are no successors, so just nuke the block. M->getBasicBlockList().erase(BB); return true; } } } // 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. // pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); BasicBlock *OnlyPred = *PI++; for (; PI != PE; ++PI) // Search all predecessors, see if they are all same if (*PI != OnlyPred) { OnlyPred = 0; // There are multiple different predecessors... break; } BasicBlock *OnlySucc = 0; if (OnlyPred && OnlyPred != BB && // Don't break self loops OnlyPred->getTerminator()->getOpcode() != Instruction::Invoke) { // Check to see if there is only one distinct successor... succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred)); OnlySucc = BB; for (; SI != SE; ++SI) if (*SI != OnlySucc) { OnlySucc = 0; // There are multiple distinct successors! break; } } if (OnlySucc) { DOUT << "Merging: " << *BB << "into: " << *OnlyPred; // Resolve any PHI nodes at the start of the block. They are all // guaranteed to have exactly one entry if they exist, unless there are // multiple duplicate (but guaranteed to be equal) entries for the // incoming edges. This occurs when there are multiple edges from // OnlyPred to OnlySucc. // while (PHINode *PN = dyn_cast(&BB->front())) { PN->replaceAllUsesWith(PN->getIncomingValue(0)); BB->getInstList().pop_front(); // Delete the phi node... } // Delete the unconditional branch from the predecessor... OnlyPred->getInstList().pop_back(); // Move all definitions in the successor to the predecessor... OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList()); // Make all PHI nodes that referred to BB now refer to Pred as their // source... BB->replaceAllUsesWith(OnlyPred); std::string OldName = BB->getName(); // Erase basic block from the function... M->getBasicBlockList().erase(BB); // Inherit predecessors name if it exists... if (!OldName.empty() && !OnlyPred->hasName()) OnlyPred->setName(OldName); return true; } // Otherwise, if this block only has a single predecessor, and if that block // is a conditional branch, see if we can hoist any code from this block up // into our predecessor. if (OnlyPred) if (BranchInst *BI = dyn_cast(OnlyPred->getTerminator())) if (BI->isConditional()) { // Get the other block. BasicBlock *OtherBB = BI->getSuccessor(BI->getSuccessor(0) == BB); PI = pred_begin(OtherBB); ++PI; if (PI == pred_end(OtherBB)) { // We have a conditional branch to two blocks that are only reachable // from the condbr. 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. Changed |= HoistThenElseCodeToIf(BI); } } for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) if (BranchInst *BI = dyn_cast((*PI)->getTerminator())) // Change br (X == 0 | X == 1), T, F into a switch instruction. if (BI->isConditional() && isa(BI->getCondition())) { Instruction *Cond = cast(BI->getCondition()); // 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 = GatherValueComparisons(Cond, CompVal, Values); if (CompVal && CompVal->getType()->isInteger()) { // There might be duplicate constants in the list, which the switch // instruction can't handle, remove them now. std::sort(Values.begin(), Values.end(), ConstantIntOrdering()); Values.erase(std::unique(Values.begin(), Values.end()), Values.end()); // Figure out which block is which destination. BasicBlock *DefaultBB = BI->getSuccessor(1); BasicBlock *EdgeBB = BI->getSuccessor(0); if (!TrueWhenEqual) std::swap(DefaultBB, EdgeBB); // Create the new switch instruction now. SwitchInst *New = new SwitchInst(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(*PI); for (unsigned i = 0, e = Values.size()-1; i != e; ++i) PN->addIncoming(InVal, *PI); } // Erase the old branch instruction. (*PI)->getInstList().erase(BI); // Erase the potentially condition tree that was used to computed the // branch condition. ErasePossiblyDeadInstructionTree(Cond); 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); return Changed; }