//===- 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. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Utils/Local.h" #include "llvm/Constants.h" #include "llvm/Instructions.h" #include "llvm/Type.h" #include "llvm/Support/CFG.h" #include #include #include using namespace llvm; // PropagatePredecessorsForPHIs - This gets "Succ" ready to have the // predecessors from "BB". This is a little tricky because "Succ" has PHI // nodes, which need to have extra slots added to them to hold the merge edges // from BB's predecessors, and BB itself might have had PHI nodes in it. This // function returns true (failure) if the Succ BB already has a predecessor that // is a predecessor of BB and incoming PHI arguments would not be discernible. // // Assumption: Succ is the single successor for BB. // static bool PropagatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { assert(*succ_begin(BB) == Succ && "Succ is not successor of BB!"); if (!isa(Succ->front())) return false; // We can make the transformation, no problem. // If there is more than one predecessor, 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)); // 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! // for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); PI != PE; ++PI) if (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(); PHINode *PN = dyn_cast(I); ++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). int Idx1 = PN->getBasicBlockIndex(BB); int Idx2 = PN->getBasicBlockIndex(*PI); assert(Idx1 != -1 && Idx2 != -1 && "Didn't have entries for my predecessors??"); if (PN->getIncomingValue(Idx1) != PN->getIncomingValue(Idx2)) return true; // Values are not equal... } } // Loop over all of the PHI nodes in the successor BB for (BasicBlock::iterator I = Succ->begin(); PHINode *PN = dyn_cast(I); ++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... if (isa(OldVal) && cast(OldVal)->getParent() == BB) { PHINode *OldValPN = cast(OldVal); for (std::vector::const_iterator PredI = BBPreds.begin(), End = BBPreds.end(); PredI != End; ++PredI) { PN->addIncoming(OldValPN->getIncomingValueForBlock(*PredI), *PredI); } } 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); } } } return false; } /// 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. static bool DominatesMergePoint(Value *V, BasicBlock *BB) { if (Instruction *I = dyn_cast(V)) { BasicBlock *PBB = I->getParent(); // 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 (isa(PBB->getTerminator()) && cast(PBB->getTerminator())->isUnconditional() && cast(PBB->getTerminator())->getSuccessor(0) == BB) return false; // We also don't want to allow wierd loops that might have the "if // condition" in the bottom of this block. if (PBB == BB) return false; } // Non-instructions all dominate instructions. 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 (Constant *C = dyn_cast(Inst->getOperand(1))) { Values.push_back(C); return Inst->getOperand(0); } else if (Constant *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 (Constant *C = dyn_cast(Inst->getOperand(1))) { Values.push_back(C); return Inst->getOperand(0); } else if (Constant *C = dyn_cast(Inst->getOperand(0))) { Values.push_back(C); return Inst->getOperand(1); } } else if (Inst->getOpcode() == Instruction::Cast) { // Cast of X to bool is really a comparison against zero. assert(Inst->getType() == Type::BoolTy && "Can only handle bool values!"); Values.push_back(Constant::getNullValue(Inst->getOperand(0)->getType())); return Inst->getOperand(0); } 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)) { std::vector Operands(I->op_begin(), I->op_end()); I->getParent()->getInstList().erase(I); for (unsigned i = 0, e = Operands.size(); i != e; ++i) if (Instruction *OpI = dyn_cast(Operands[i])) ErasePossiblyDeadInstructionTree(OpI); } } /// 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(); PHINode *PN = dyn_cast(BBI); ++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, and 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(); PHINode *PN = dyn_cast(I); ++I) { Value *V = PN->getIncomingValueForBlock(ExistPred); PN->addIncoming(V, NewPred); } } // 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)) 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(cast(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); } // 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, PTI); for (unsigned i = 0, e = PredCases.size(); i != e; ++i) NewSI->addCase(PredCases[i].first, PredCases[i].second); Pred->getInstList().erase(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 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; } // 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)) { //cerr << "Removing BB: \n" << BB; // Loop through all of our successors and make sure they know that one // of their predecessors is going away. for_each(succ_begin(BB), succ_end(BB), std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB)); while (!BB->empty()) { Instruction &I = BB->back(); // If this instruction is used, replace uses with an arbitrary // constant 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 reference the constant instead I.replaceAllUsesWith(Constant::getNullValue(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); // Check to see if this block has no non-phi instructions and only a single // successor. If so, replace references to this basic block with references // to the successor. succ_iterator SI(succ_begin(BB)); if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ? BasicBlock::iterator BBI = BB->begin(); // Skip over phi nodes... while (isa(*BBI)) ++BBI; if (BBI->isTerminator()) { // Terminator is the only non-phi instruction! BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor if (Succ != BB) { // Arg, don't hurt infinite loops! // 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 (!PropagatePredecessorsForPHIs(BB, Succ)) { //cerr << "Killing Trivial BB: \n" << BB; std::string OldName = BB->getName(); 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()) BB->getInstList().erase(BB->begin()); // Nuke instruction... 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. BB->getInstList().remove(BB->begin()); Succ->getInstList().push_front(PN); // 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... BB->replaceAllUsesWith(Succ); // Delete the old basic block... M->getBasicBlockList().erase(BB); if (!OldName.empty() && !Succ->hasName()) // Transfer name if we can Succ->setName(OldName); //cerr << "Function after removal: \n" << M; return true; } } } } // If this is a returning block with only PHI nodes in it, fold the return // instruction into any unconditional branch predecessors. if (ReturnInst *RI = dyn_cast(BB->getTerminator())) { BasicBlock::iterator BBI = BB->getTerminator(); if (BBI == BB->begin() || isa(--BBI)) { // Find predecessors that end with unconditional branches. std::vector UncondBranchPreds; 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); } // If we found some, do the transformation! if (!UncondBranchPreds.empty()) { while (!UncondBranchPreds.empty()) { BasicBlock *Pred = UncondBranchPreds.back(); 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; } } } else if (UnwindInst *UI = dyn_cast(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. // std::vector Preds(pred_begin(BB), pred_end(BB)); while (!Preds.empty()) { BasicBlock *Pred = Preds.back(); 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); // 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->begin())) { if (FoldValueComparisonIntoPredecessors(SI)) return SimplifyCFG(BB) || 1; } else if (BranchInst *BI = dyn_cast(BB->getTerminator())) { if (Value *CompVal = isValueEqualityComparison(BB->getTerminator())) { // 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) || 1; } } // 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) { //cerr << "Merging: " << BB << "into: " << OnlyPred; TerminatorInst *Term = OnlyPred->getTerminator(); // 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; } 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()); 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, 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(); PHINode *PN = dyn_cast(BBI); ++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) { // 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 *IfTrue, *IfFalse; if (Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse)) { //std::cerr << "FOUND IF CONDITION! " << *IfCond << " T: " // << IfTrue->getName() << " F: " << IfFalse->getName() << "\n"; // Figure out where to insert instructions as necessary. BasicBlock::iterator AfterPHIIt = BB->begin(); while (isa(AfterPHIIt)) ++AfterPHIIt; BasicBlock::iterator I = BB->begin(); while (PHINode *PN = dyn_cast(I)) { ++I; // If we can eliminate this PHI by directly computing it based on the // condition, do so now. We can't eliminate PHI nodes where the // incoming values are defined in the conditional parts of the branch, // so check for this. // if (DominatesMergePoint(PN->getIncomingValue(0), BB) && DominatesMergePoint(PN->getIncomingValue(1), BB)) { Value *TrueVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfFalse); Value *FalseVal = PN->getIncomingValue(PN->getIncomingBlock(0) == IfTrue); // FIXME: when we have a 'select' statement, we can be completely // generic and clean here and let the instcombine pass clean up // after us, by folding the select instructions away when possible. // if (TrueVal == FalseVal) { // Degenerate case... PN->replaceAllUsesWith(TrueVal); BB->getInstList().erase(PN); Changed = true; } else if (isa(TrueVal) && isa(FalseVal)) { if (TrueVal == ConstantBool::True) { // The PHI node produces the same thing as the condition. PN->replaceAllUsesWith(IfCond); } else { // The PHI node produces the inverse of the condition. Insert a // "NOT" instruction, which is really a XOR. Value *InverseCond = BinaryOperator::createNot(IfCond, IfCond->getName()+".inv", AfterPHIIt); PN->replaceAllUsesWith(InverseCond); } BB->getInstList().erase(PN); Changed = true; } else if (isa(TrueVal) && isa(FalseVal)){ // If this is a PHI of two constant integers, we insert a cast of // the boolean to the integer type in question, giving us 0 or 1. // Then we multiply this by the difference of the two constants, // giving us 0 if false, and the difference if true. We add this // result to the base constant, giving us our final value. We // rely on the instruction combiner to eliminate many special // cases, like turning multiplies into shifts when possible. std::string Name = PN->getName(); PN->setName(""); Value *TheCast = new CastInst(IfCond, TrueVal->getType(), Name, AfterPHIIt); Constant *TheDiff = ConstantExpr::get(Instruction::Sub, cast(TrueVal), cast(FalseVal)); Value *V = TheCast; if (TheDiff != ConstantInt::get(TrueVal->getType(), 1)) V = BinaryOperator::create(Instruction::Mul, TheCast, TheDiff, TheCast->getName()+".scale", AfterPHIIt); if (!cast(FalseVal)->isNullValue()) V = BinaryOperator::create(Instruction::Add, V, FalseVal, V->getName()+".offs", AfterPHIIt); PN->replaceAllUsesWith(V); BB->getInstList().erase(PN); Changed = true; } else if (isa(FalseVal) && cast(FalseVal)->isNullValue()) { // If the false condition is an integral zero value, we can // compute the PHI by multiplying the condition by the other // value. std::string Name = PN->getName(); PN->setName(""); Value *TheCast = new CastInst(IfCond, TrueVal->getType(), Name+".c", AfterPHIIt); Value *V = BinaryOperator::create(Instruction::Mul, TrueVal, TheCast, Name, AfterPHIIt); PN->replaceAllUsesWith(V); BB->getInstList().erase(PN); Changed = true; } else if (isa(TrueVal) && cast(TrueVal)->isNullValue()) { // If the true condition is an integral zero value, we can compute // the PHI by multiplying the inverse condition by the other // value. std::string Name = PN->getName(); PN->setName(""); Value *NotCond = BinaryOperator::createNot(IfCond, Name+".inv", AfterPHIIt); Value *TheCast = new CastInst(NotCond, TrueVal->getType(), Name+".inv", AfterPHIIt); Value *V = BinaryOperator::create(Instruction::Mul, FalseVal, TheCast, Name, AfterPHIIt); PN->replaceAllUsesWith(V); BB->getInstList().erase(PN); Changed = true; } } } } } return Changed; }