X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FCodeGen%2FPHIElimination.cpp;h=6994aa58fbd5459099b3e5e5946e906eb3f79071;hb=df234353fb396e84e7a3a1cdd94f73681e65bd88;hp=e21869192d9f0f24817ddd33e8ac988eb8d1e4b1;hpb=576a27043d95d0b9b8a010bccfd38ed9c0afa739;p=oota-llvm.git diff --git a/lib/CodeGen/PHIElimination.cpp b/lib/CodeGen/PHIElimination.cpp index e21869192d9..6994aa58fbd 100644 --- a/lib/CodeGen/PHIElimination.cpp +++ b/lib/CodeGen/PHIElimination.cpp @@ -14,47 +14,45 @@ //===----------------------------------------------------------------------===// #define DEBUG_TYPE "phielim" +#include "PHIEliminationUtils.h" #include "llvm/CodeGen/LiveVariables.h" #include "llvm/CodeGen/Passes.h" -#include "llvm/CodeGen/MachineFunctionPass.h" +#include "llvm/CodeGen/MachineDominators.h" #include "llvm/CodeGen/MachineInstr.h" +#include "llvm/CodeGen/MachineInstrBuilder.h" +#include "llvm/CodeGen/MachineLoopInfo.h" #include "llvm/CodeGen/MachineRegisterInfo.h" #include "llvm/Target/TargetInstrInfo.h" +#include "llvm/Function.h" #include "llvm/Target/TargetMachine.h" #include "llvm/ADT/SmallPtrSet.h" #include "llvm/ADT/STLExtras.h" #include "llvm/ADT/Statistic.h" +#include "llvm/Support/CommandLine.h" #include "llvm/Support/Compiler.h" +#include "llvm/Support/Debug.h" #include -#include using namespace llvm; -STATISTIC(NumAtomic, "Number of atomic phis lowered"); +static cl::opt +DisableEdgeSplitting("disable-phi-elim-edge-splitting", cl::init(false), + cl::Hidden, cl::desc("Disable critical edge splitting " + "during PHI elimination")); namespace { - class VISIBILITY_HIDDEN PNE : public MachineFunctionPass { - MachineRegisterInfo *MRI; // Machine register information + class PHIElimination : public MachineFunctionPass { + MachineRegisterInfo *MRI; // Machine register information public: static char ID; // Pass identification, replacement for typeid - PNE() : MachineFunctionPass((intptr_t)&ID) {} + PHIElimination() : MachineFunctionPass(ID) { + initializePHIEliminationPass(*PassRegistry::getPassRegistry()); + } virtual bool runOnMachineFunction(MachineFunction &Fn); - - virtual void getAnalysisUsage(AnalysisUsage &AU) const { - AU.addPreserved(); - AU.addPreservedID(MachineLoopInfoID); - AU.addPreservedID(MachineDominatorsID); - MachineFunctionPass::getAnalysisUsage(AU); - } + virtual void getAnalysisUsage(AnalysisUsage &AU) const; private: - /// findInsertionPoint - Find a safe location to insert a move to copy - /// source of a PHI instruction. - MachineBasicBlock::iterator - findInsertionPoint(MachineBasicBlock &MBB, MachineInstr *DefMI, - unsigned DstReg, unsigned SrcReg) const; - /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions /// in predecessor basic blocks. /// @@ -70,302 +68,323 @@ namespace { /// void analyzePHINodes(const MachineFunction& Fn); - typedef std::pair BBVRegPair; - typedef std::map VRegPHIUse; + /// Split critical edges where necessary for good coalescer performance. + bool SplitPHIEdges(MachineFunction &MF, MachineBasicBlock &MBB, + LiveVariables &LV, MachineLoopInfo *MLI); + + typedef std::pair BBVRegPair; + typedef DenseMap VRegPHIUse; VRegPHIUse VRegPHIUseCount; // Defs of PHI sources which are implicit_def. SmallPtrSet ImpDefs; - }; - char PNE::ID = 0; - RegisterPass X("phi-node-elimination", - "Eliminate PHI nodes for register allocation"); + // Map reusable lowered PHI node -> incoming join register. + typedef DenseMap LoweredPHIMap; + LoweredPHIMap LoweredPHIs; + }; } -const PassInfo *llvm::PHIEliminationID = X.getPassInfo(); +STATISTIC(NumAtomic, "Number of atomic phis lowered"); +STATISTIC(NumCriticalEdgesSplit, "Number of critical edges split"); +STATISTIC(NumReused, "Number of reused lowered phis"); -bool PNE::runOnMachineFunction(MachineFunction &Fn) { - MRI = &Fn.getRegInfo(); +char PHIElimination::ID = 0; +INITIALIZE_PASS(PHIElimination, "phi-node-elimination", + "Eliminate PHI nodes for register allocation", false, false) - analyzePHINodes(Fn); +char& llvm::PHIEliminationID = PHIElimination::ID; + +void PHIElimination::getAnalysisUsage(AnalysisUsage &AU) const { + AU.addPreserved(); + AU.addPreserved(); + AU.addPreserved(); + MachineFunctionPass::getAnalysisUsage(AU); +} + +bool PHIElimination::runOnMachineFunction(MachineFunction &MF) { + MRI = &MF.getRegInfo(); bool Changed = false; + // This pass takes the function out of SSA form. + MRI->leaveSSA(); + + // Split critical edges to help the coalescer + if (!DisableEdgeSplitting) { + if (LiveVariables *LV = getAnalysisIfAvailable()) { + MachineLoopInfo *MLI = getAnalysisIfAvailable(); + for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) + Changed |= SplitPHIEdges(MF, *I, *LV, MLI); + } + } + + // Populate VRegPHIUseCount + analyzePHINodes(MF); + // Eliminate PHI instructions by inserting copies into predecessor blocks. - for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) - Changed |= EliminatePHINodes(Fn, *I); + for (MachineFunction::iterator I = MF.begin(), E = MF.end(); I != E; ++I) + Changed |= EliminatePHINodes(MF, *I); // Remove dead IMPLICIT_DEF instructions. - for (SmallPtrSet::iterator I = ImpDefs.begin(), + for (SmallPtrSet::iterator I = ImpDefs.begin(), E = ImpDefs.end(); I != E; ++I) { MachineInstr *DefMI = *I; unsigned DefReg = DefMI->getOperand(0).getReg(); - if (MRI->use_begin(DefReg) == MRI->use_end()) + if (MRI->use_nodbg_empty(DefReg)) DefMI->eraseFromParent(); } + // Clean up the lowered PHI instructions. + for (LoweredPHIMap::iterator I = LoweredPHIs.begin(), E = LoweredPHIs.end(); + I != E; ++I) + MF.DeleteMachineInstr(I->first); + + LoweredPHIs.clear(); ImpDefs.clear(); VRegPHIUseCount.clear(); + return Changed; } - /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in /// predecessor basic blocks. /// -bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) { - if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI) +bool PHIElimination::EliminatePHINodes(MachineFunction &MF, + MachineBasicBlock &MBB) { + if (MBB.empty() || !MBB.front().isPHI()) return false; // Quick exit for basic blocks without PHIs. // Get an iterator to the first instruction after the last PHI node (this may // also be the end of the basic block). - MachineBasicBlock::iterator AfterPHIsIt = MBB.begin(); - while (AfterPHIsIt != MBB.end() && - AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI) - ++AfterPHIsIt; // Skip over all of the PHI nodes... + MachineBasicBlock::iterator AfterPHIsIt = MBB.SkipPHIsAndLabels(MBB.begin()); - while (MBB.front().getOpcode() == TargetInstrInfo::PHI) + while (MBB.front().isPHI()) LowerAtomicPHINode(MBB, AfterPHIsIt); return true; } -/// findInsertionPoint - Find a safe location to insert a move to copy -/// source of a PHI instruction. -MachineBasicBlock::iterator -PNE::findInsertionPoint(MachineBasicBlock &MBB, MachineInstr *DefMI, - unsigned DstReg, unsigned SrcReg) const { - if (DefMI->getOpcode() == TargetInstrInfo::PHI || - DefMI->getParent() != &MBB) - return MBB.getFirstTerminator(); - - for (MachineRegisterInfo::use_iterator I = MRI->use_begin(SrcReg), - E = MRI->use_end(); I != E; ++I) - if (I->getParent() == &MBB) - return MBB.getFirstTerminator(); - for (MachineRegisterInfo::use_iterator I = MRI->use_begin(DstReg), - E = MRI->use_end(); I != E; ++I) - if (I->getParent() == &MBB) - return MBB.getFirstTerminator(); - - MachineBasicBlock::iterator I = DefMI; - return ++I; +/// isSourceDefinedByImplicitDef - Return true if all sources of the phi node +/// are implicit_def's. +static bool isSourceDefinedByImplicitDef(const MachineInstr *MPhi, + const MachineRegisterInfo *MRI) { + for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) { + unsigned SrcReg = MPhi->getOperand(i).getReg(); + const MachineInstr *DefMI = MRI->getVRegDef(SrcReg); + if (!DefMI || !DefMI->isImplicitDef()) + return false; + } + return true; } + + /// LowerAtomicPHINode - Lower the PHI node at the top of the specified block, /// under the assuption that it needs to be lowered in a way that supports /// atomic execution of PHIs. This lowering method is always correct all of the /// time. -void PNE::LowerAtomicPHINode(MachineBasicBlock &MBB, - MachineBasicBlock::iterator AfterPHIsIt) { +/// +void PHIElimination::LowerAtomicPHINode( + MachineBasicBlock &MBB, + MachineBasicBlock::iterator AfterPHIsIt) { + ++NumAtomic; // Unlink the PHI node from the basic block, but don't delete the PHI yet. MachineInstr *MPhi = MBB.remove(MBB.begin()); + unsigned NumSrcs = (MPhi->getNumOperands() - 1) / 2; unsigned DestReg = MPhi->getOperand(0).getReg(); + assert(MPhi->getOperand(0).getSubReg() == 0 && "Can't handle sub-reg PHIs"); + bool isDead = MPhi->getOperand(0).isDead(); // Create a new register for the incoming PHI arguments. MachineFunction &MF = *MBB.getParent(); - const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg); - unsigned IncomingReg = MF.getRegInfo().createVirtualRegister(RC); + unsigned IncomingReg = 0; + bool reusedIncoming = false; // Is IncomingReg reused from an earlier PHI? - // Insert a register to register copy in the top of the current block (but + // Insert a register to register copy at the top of the current block (but // after any remaining phi nodes) which copies the new incoming register // into the phi node destination. - // const TargetInstrInfo *TII = MF.getTarget().getInstrInfo(); - TII->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC, RC); + if (isSourceDefinedByImplicitDef(MPhi, MRI)) + // If all sources of a PHI node are implicit_def, just emit an + // implicit_def instead of a copy. + BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(), + TII->get(TargetOpcode::IMPLICIT_DEF), DestReg); + else { + // Can we reuse an earlier PHI node? This only happens for critical edges, + // typically those created by tail duplication. + unsigned &entry = LoweredPHIs[MPhi]; + if (entry) { + // An identical PHI node was already lowered. Reuse the incoming register. + IncomingReg = entry; + reusedIncoming = true; + ++NumReused; + DEBUG(dbgs() << "Reusing " << PrintReg(IncomingReg) << " for " << *MPhi); + } else { + const TargetRegisterClass *RC = MF.getRegInfo().getRegClass(DestReg); + entry = IncomingReg = MF.getRegInfo().createVirtualRegister(RC); + } + BuildMI(MBB, AfterPHIsIt, MPhi->getDebugLoc(), + TII->get(TargetOpcode::COPY), DestReg) + .addReg(IncomingReg); + } - // Update live variable information if there is any... - LiveVariables *LV = getAnalysisToUpdate(); + // Update live variable information if there is any. + LiveVariables *LV = getAnalysisIfAvailable(); if (LV) { MachineInstr *PHICopy = prior(AfterPHIsIt); - // Increment use count of the newly created virtual register. - LV->getVarInfo(IncomingReg).NumUses++; + if (IncomingReg) { + LiveVariables::VarInfo &VI = LV->getVarInfo(IncomingReg); + + // Increment use count of the newly created virtual register. + VI.NumUses++; + LV->setPHIJoin(IncomingReg); + + // When we are reusing the incoming register, it may already have been + // killed in this block. The old kill will also have been inserted at + // AfterPHIsIt, so it appears before the current PHICopy. + if (reusedIncoming) + if (MachineInstr *OldKill = VI.findKill(&MBB)) { + DEBUG(dbgs() << "Remove old kill from " << *OldKill); + LV->removeVirtualRegisterKilled(IncomingReg, OldKill); + DEBUG(MBB.dump()); + } - // Add information to LiveVariables to know that the incoming value is - // killed. Note that because the value is defined in several places (once - // each for each incoming block), the "def" block and instruction fields - // for the VarInfo is not filled in. - // - LV->addVirtualRegisterKilled(IncomingReg, PHICopy); + // Add information to LiveVariables to know that the incoming value is + // killed. Note that because the value is defined in several places (once + // each for each incoming block), the "def" block and instruction fields + // for the VarInfo is not filled in. + LV->addVirtualRegisterKilled(IncomingReg, PHICopy); + } - // Since we are going to be deleting the PHI node, if it is the last use - // of any registers, or if the value itself is dead, we need to move this + // Since we are going to be deleting the PHI node, if it is the last use of + // any registers, or if the value itself is dead, we need to move this // information over to the new copy we just inserted. - // LV->removeVirtualRegistersKilled(MPhi); // If the result is dead, update LV. - if (MPhi->registerDefIsDead(DestReg)) { + if (isDead) { LV->addVirtualRegisterDead(DestReg, PHICopy); - LV->removeVirtualRegistersDead(MPhi); + LV->removeVirtualRegisterDead(DestReg, MPhi); } - - LV->getVarInfo(IncomingReg).UsedBlocks[MBB.getNumber()] = true; } - // Adjust the VRegPHIUseCount map to account for the removal of this PHI - // node. + // Adjust the VRegPHIUseCount map to account for the removal of this PHI node. for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) - --VRegPHIUseCount[BBVRegPair(MPhi->getOperand(i + 1).getMBB(), + --VRegPHIUseCount[BBVRegPair(MPhi->getOperand(i+1).getMBB()->getNumber(), MPhi->getOperand(i).getReg())]; - // Now loop over all of the incoming arguments, changing them to copy into - // the IncomingReg register in the corresponding predecessor basic block. - // + // Now loop over all of the incoming arguments, changing them to copy into the + // IncomingReg register in the corresponding predecessor basic block. SmallPtrSet MBBsInsertedInto; - for (int i = MPhi->getNumOperands() - 1; i >= 2; i-=2) { - unsigned SrcReg = MPhi->getOperand(i-1).getReg(); + for (int i = NumSrcs - 1; i >= 0; --i) { + unsigned SrcReg = MPhi->getOperand(i*2+1).getReg(); + unsigned SrcSubReg = MPhi->getOperand(i*2+1).getSubReg(); + assert(TargetRegisterInfo::isVirtualRegister(SrcReg) && "Machine PHI Operands must all be virtual registers!"); - // If source is defined by an implicit def, there is no need to insert - // a copy. + // Get the MachineBasicBlock equivalent of the BasicBlock that is the source + // path the PHI. + MachineBasicBlock &opBlock = *MPhi->getOperand(i*2+2).getMBB(); + + // If source is defined by an implicit def, there is no need to insert a + // copy. MachineInstr *DefMI = MRI->getVRegDef(SrcReg); - if (DefMI->getOpcode() == TargetInstrInfo::IMPLICIT_DEF) { + if (DefMI->isImplicitDef()) { ImpDefs.insert(DefMI); continue; } - // Get the MachineBasicBlock equivalent of the BasicBlock that is the - // source path the PHI. - MachineBasicBlock &opBlock = *MPhi->getOperand(i).getMBB(); - // Check to make sure we haven't already emitted the copy for this block. - // This can happen because PHI nodes may have multiple entries for the - // same basic block. + // This can happen because PHI nodes may have multiple entries for the same + // basic block. if (!MBBsInsertedInto.insert(&opBlock)) continue; // If the copy has already been emitted, we're done. - - // Find a safe location to insert the copy, this may be the first - // terminator in the block (or end()). + + // Find a safe location to insert the copy, this may be the first terminator + // in the block (or end()). MachineBasicBlock::iterator InsertPos = - findInsertionPoint(opBlock, DefMI, IncomingReg, SrcReg); - + findPHICopyInsertPoint(&opBlock, &MBB, SrcReg); + // Insert the copy. - TII->copyRegToReg(opBlock, InsertPos, IncomingReg, SrcReg, RC, RC); + if (!reusedIncoming && IncomingReg) + BuildMI(opBlock, InsertPos, MPhi->getDebugLoc(), + TII->get(TargetOpcode::COPY), IncomingReg).addReg(SrcReg, 0, SrcSubReg); // Now update live variable information if we have it. Otherwise we're done if (!LV) continue; - - // We want to be able to insert a kill of the register if this PHI - // (aka, the copy we just inserted) is the last use of the source - // value. Live variable analysis conservatively handles this by - // saying that the value is live until the end of the block the PHI - // entry lives in. If the value really is dead at the PHI copy, there - // will be no successor blocks which have the value live-in. - // - // Check to see if the copy is the last use, and if so, update the - // live variables information so that it knows the copy source - // instruction kills the incoming value. - // - LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg); - InRegVI.UsedBlocks[opBlock.getNumber()] = true; - - // Loop over all of the successors of the basic block, checking to see - // if the value is either live in the block, or if it is killed in the - // block. Also check to see if this register is in use by another PHI - // node which has not yet been eliminated. If so, it will be killed - // at an appropriate point later. - // - // Is it used by any PHI instructions in this block? - bool ValueIsLive = VRegPHIUseCount[BBVRegPair(&opBlock, SrcReg)] != 0; - - std::vector OpSuccBlocks; - - // Otherwise, scan successors, including the BB the PHI node lives in. - for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(), - E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) { - MachineBasicBlock *SuccMBB = *SI; - - // Is it alive in this successor? - unsigned SuccIdx = SuccMBB->getNumber(); - if (SuccIdx < InRegVI.AliveBlocks.size() && - InRegVI.AliveBlocks[SuccIdx]) { - ValueIsLive = true; - break; - } + // We want to be able to insert a kill of the register if this PHI (aka, the + // copy we just inserted) is the last use of the source value. Live + // variable analysis conservatively handles this by saying that the value is + // live until the end of the block the PHI entry lives in. If the value + // really is dead at the PHI copy, there will be no successor blocks which + // have the value live-in. - OpSuccBlocks.push_back(SuccMBB); - } + // Also check to see if this register is in use by another PHI node which + // has not yet been eliminated. If so, it will be killed at an appropriate + // point later. - // Check to see if this value is live because there is a use in a successor - // that kills it. - if (!ValueIsLive) { - switch (OpSuccBlocks.size()) { - case 1: { - MachineBasicBlock *MBB = OpSuccBlocks[0]; - for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i) - if (InRegVI.Kills[i]->getParent() == MBB) { - ValueIsLive = true; - break; - } - break; - } - case 2: { - MachineBasicBlock *MBB1 = OpSuccBlocks[0], *MBB2 = OpSuccBlocks[1]; - for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i) - if (InRegVI.Kills[i]->getParent() == MBB1 || - InRegVI.Kills[i]->getParent() == MBB2) { - ValueIsLive = true; - break; - } - break; - } - default: - std::sort(OpSuccBlocks.begin(), OpSuccBlocks.end()); - for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i) - if (std::binary_search(OpSuccBlocks.begin(), OpSuccBlocks.end(), - InRegVI.Kills[i]->getParent())) { - ValueIsLive = true; - break; - } - } - } + // Is it used by any PHI instructions in this block? + bool ValueIsUsed = VRegPHIUseCount[BBVRegPair(opBlock.getNumber(), SrcReg)]; - // Okay, if we now know that the value is not live out of the block, - // we can add a kill marker in this block saying that it kills the incoming - // value! - if (!ValueIsLive) { + // Okay, if we now know that the value is not live out of the block, we can + // add a kill marker in this block saying that it kills the incoming value! + if (!ValueIsUsed && !LV->isLiveOut(SrcReg, opBlock)) { // In our final twist, we have to decide which instruction kills the - // register. In most cases this is the copy, however, the first + // register. In most cases this is the copy, however, the first // terminator instruction at the end of the block may also use the value. // In this case, we should mark *it* as being the killing block, not the // copy. - MachineBasicBlock::iterator KillInst = prior(InsertPos); + MachineBasicBlock::iterator KillInst; MachineBasicBlock::iterator Term = opBlock.getFirstTerminator(); - if (Term != opBlock.end()) { - if (Term->readsRegister(SrcReg)) - KillInst = Term; - + if (Term != opBlock.end() && Term->readsRegister(SrcReg)) { + KillInst = Term; + // Check that no other terminators use values. #ifndef NDEBUG - for (MachineBasicBlock::iterator TI = next(Term); TI != opBlock.end(); - ++TI) { + for (MachineBasicBlock::iterator TI = llvm::next(Term); + TI != opBlock.end(); ++TI) { + if (TI->isDebugValue()) + continue; assert(!TI->readsRegister(SrcReg) && "Terminator instructions cannot use virtual registers unless" "they are the first terminator in a block!"); } #endif + } else if (reusedIncoming || !IncomingReg) { + // We may have to rewind a bit if we didn't insert a copy this time. + KillInst = Term; + while (KillInst != opBlock.begin()) { + --KillInst; + if (KillInst->isDebugValue()) + continue; + if (KillInst->readsRegister(SrcReg)) + break; + } + } else { + // We just inserted this copy. + KillInst = prior(InsertPos); } - + assert(KillInst->readsRegister(SrcReg) && "Cannot find kill instruction"); + // Finally, mark it killed. LV->addVirtualRegisterKilled(SrcReg, KillInst); // This vreg no longer lives all of the way through opBlock. unsigned opBlockNum = opBlock.getNumber(); - if (opBlockNum < InRegVI.AliveBlocks.size()) - InRegVI.AliveBlocks[opBlockNum] = false; + LV->getVarInfo(SrcReg).AliveBlocks.reset(opBlockNum); } } - - // Really delete the PHI instruction now! - delete MPhi; - ++NumAtomic; + + // Really delete the PHI instruction now, if it is not in the LoweredPHIs map. + if (reusedIncoming || !IncomingReg) + MF.DeleteMachineInstr(MPhi); } /// analyzePHINodes - Gather information about the PHI nodes in here. In @@ -373,12 +392,46 @@ void PNE::LowerAtomicPHINode(MachineBasicBlock &MBB, /// used in a PHI node. We map that to the BB the vreg is coming from. This is /// used later to determine when the vreg is killed in the BB. /// -void PNE::analyzePHINodes(const MachineFunction& Fn) { - for (MachineFunction::const_iterator I = Fn.begin(), E = Fn.end(); +void PHIElimination::analyzePHINodes(const MachineFunction& MF) { + for (MachineFunction::const_iterator I = MF.begin(), E = MF.end(); I != E; ++I) for (MachineBasicBlock::const_iterator BBI = I->begin(), BBE = I->end(); - BBI != BBE && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI) + BBI != BBE && BBI->isPHI(); ++BBI) for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) - ++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i + 1).getMBB(), + ++VRegPHIUseCount[BBVRegPair(BBI->getOperand(i+1).getMBB()->getNumber(), BBI->getOperand(i).getReg())]; } + +bool PHIElimination::SplitPHIEdges(MachineFunction &MF, + MachineBasicBlock &MBB, + LiveVariables &LV, + MachineLoopInfo *MLI) { + if (MBB.empty() || !MBB.front().isPHI() || MBB.isLandingPad()) + return false; // Quick exit for basic blocks without PHIs. + + bool Changed = false; + for (MachineBasicBlock::const_iterator BBI = MBB.begin(), BBE = MBB.end(); + BBI != BBE && BBI->isPHI(); ++BBI) { + for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) { + unsigned Reg = BBI->getOperand(i).getReg(); + MachineBasicBlock *PreMBB = BBI->getOperand(i+1).getMBB(); + // We break edges when registers are live out from the predecessor block + // (not considering PHI nodes). If the register is live in to this block + // anyway, we would gain nothing from splitting. + // Avoid splitting backedges of loops. It would introduce small + // out-of-line blocks into the loop which is very bad for code placement. + if (PreMBB != &MBB && + !LV.isLiveIn(Reg, MBB) && LV.isLiveOut(Reg, *PreMBB)) { + if (!MLI || + !(MLI->getLoopFor(PreMBB) == MLI->getLoopFor(&MBB) && + MLI->isLoopHeader(&MBB))) { + if (PreMBB->SplitCriticalEdge(&MBB, this)) { + Changed = true; + ++NumCriticalEdgesSplit; + } + } + } + } + } + return Changed; +}