| //===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===// |
| // |
| // 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. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This pass eliminates machine instruction PHI nodes by inserting copy |
| // instructions. This destroys SSA information, but is the desired input for |
| // some register allocators. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/CodeGen/LiveVariables.h" |
| #include "llvm/CodeGen/Passes.h" |
| #include "llvm/CodeGen/MachineFunctionPass.h" |
| #include "llvm/CodeGen/MachineInstr.h" |
| #include "llvm/CodeGen/SSARegMap.h" |
| #include "llvm/Target/TargetInstrInfo.h" |
| #include "llvm/Target/TargetMachine.h" |
| #include "llvm/ADT/DenseMap.h" |
| #include "llvm/ADT/STLExtras.h" |
| #include "llvm/ADT/Statistic.h" |
| #include <set> |
| #include <algorithm> |
| using namespace llvm; |
| |
| namespace { |
| Statistic<> NumAtomic("phielim", "Number of atomic phis lowered"); |
| Statistic<> NumSimple("phielim", "Number of simple phis lowered"); |
| |
| struct PNE : public MachineFunctionPass { |
| bool runOnMachineFunction(MachineFunction &Fn) { |
| bool Changed = false; |
| |
| // 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); |
| |
| return Changed; |
| } |
| |
| virtual void getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.addPreserved<LiveVariables>(); |
| MachineFunctionPass::getAnalysisUsage(AU); |
| } |
| |
| private: |
| /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions |
| /// in predecessor basic blocks. |
| /// |
| bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB); |
| void LowerAtomicPHINode(MachineBasicBlock &MBB, |
| MachineBasicBlock::iterator AfterPHIsIt, |
| DenseMap<unsigned, VirtReg2IndexFunctor> &VUC); |
| }; |
| |
| RegisterPass<PNE> X("phi-node-elimination", |
| "Eliminate PHI nodes for register allocation"); |
| } |
| |
| |
| const PassInfo *llvm::PHIEliminationID = X.getPassInfo(); |
| |
| /// 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) |
| return false; // Quick exit for basic blocks without PHIs. |
| |
| // VRegPHIUseCount - Keep track of the number of times each virtual register |
| // is used by PHI nodes in successors of this block. |
| DenseMap<unsigned, VirtReg2IndexFunctor> VRegPHIUseCount; |
| VRegPHIUseCount.grow(MF.getSSARegMap()->getLastVirtReg()); |
| |
| for (MachineBasicBlock::pred_iterator PI = MBB.pred_begin(), |
| E = MBB.pred_end(); PI != E; ++PI) |
| for (MachineBasicBlock::succ_iterator SI = (*PI)->succ_begin(), |
| E = (*PI)->succ_end(); SI != E; ++SI) |
| for (MachineBasicBlock::iterator BBI = (*SI)->begin(), E = (*SI)->end(); |
| BBI != E && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI) |
| for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) |
| VRegPHIUseCount[BBI->getOperand(i).getReg()]++; |
| |
| // 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... |
| |
| while (MBB.front().getOpcode() == TargetInstrInfo::PHI) { |
| LowerAtomicPHINode(MBB, AfterPHIsIt, VRegPHIUseCount); |
| } |
| return true; |
| } |
| |
| /// InstructionUsesRegister - Return true if the specified machine instr has a |
| /// use of the specified register. |
| static bool InstructionUsesRegister(MachineInstr *MI, unsigned SrcReg) { |
| for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) |
| if (MI->getOperand(0).isRegister() && |
| MI->getOperand(0).getReg() == SrcReg && |
| MI->getOperand(0).isUse()) |
| return true; |
| return false; |
| } |
| |
| /// 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, |
| DenseMap<unsigned, VirtReg2IndexFunctor> &VRegPHIUseCount) { |
| // Unlink the PHI node from the basic block, but don't delete the PHI yet. |
| MachineInstr *MPhi = MBB.remove(MBB.begin()); |
| |
| unsigned DestReg = MPhi->getOperand(0).getReg(); |
| |
| // Create a new register for the incoming PHI arguments/ |
| MachineFunction &MF = *MBB.getParent(); |
| const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg); |
| unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC); |
| |
| // Insert a register to register copy in the top of the current block (but |
| // after any remaining phi nodes) which copies the new incoming register |
| // into the phi node destination. |
| // |
| const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo(); |
| RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC); |
| |
| // Update live variable information if there is any... |
| LiveVariables *LV = getAnalysisToUpdate<LiveVariables>(); |
| if (LV) { |
| MachineInstr *PHICopy = prior(AfterPHIsIt); |
| |
| // 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 |
| // information over to the new copy we just inserted. |
| // |
| LV->removeVirtualRegistersKilled(MPhi); |
| |
| // If the result is dead, update LV. |
| if (LV->RegisterDefIsDead(MPhi, DestReg)) { |
| LV->addVirtualRegisterDead(DestReg, PHICopy); |
| LV->removeVirtualRegistersDead(MPhi); |
| } |
| |
| // Realize that the destination register is defined by the PHI copy now, not |
| // the PHI itself. |
| LV->getVarInfo(DestReg).DefInst = PHICopy; |
| } |
| |
| // Adjust the VRegPHIUseCount map to account for the removal of this PHI |
| // node. |
| unsigned NumPreds = (MPhi->getNumOperands()-1)/2; |
| for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) |
| VRegPHIUseCount[MPhi->getOperand(i).getReg()] -= NumPreds; |
| |
| // Now loop over all of the incoming arguments, changing them to copy into |
| // the IncomingReg register in the corresponding predecessor basic block. |
| // |
| std::set<MachineBasicBlock*> MBBsInsertedInto; |
| for (int i = MPhi->getNumOperands() - 1; i >= 2; i-=2) { |
| unsigned SrcReg = MPhi->getOperand(i-1).getReg(); |
| assert(MRegisterInfo::isVirtualRegister(SrcReg) && |
| "Machine PHI Operands must all be virtual registers!"); |
| |
| // Get the MachineBasicBlock equivalent of the BasicBlock that is the |
| // source path the PHI. |
| MachineBasicBlock &opBlock = *MPhi->getOperand(i).getMachineBasicBlock(); |
| |
| // 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. |
| if (!MBBsInsertedInto.insert(&opBlock).second) |
| continue; // If the copy has already been emitted, we're done. |
| |
| // Get an iterator pointing to the first terminator in the block (or end()). |
| // This is the point where we can insert a copy if we'd like to. |
| MachineBasicBlock::iterator I = opBlock.getFirstTerminator(); |
| |
| // Insert the copy. |
| RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC); |
| |
| // 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); |
| |
| // 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[SrcReg] != 0; |
| |
| std::vector<MachineBasicBlock*> 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; |
| } |
| |
| OpSuccBlocks.push_back(SuccMBB); |
| } |
| |
| // 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; |
| } |
| } |
| } |
| |
| // 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) { |
| // In our final twist, we have to decide which instruction kills the |
| // 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. |
| bool FirstTerminatorUsesValue = false; |
| if (I != opBlock.end()) { |
| FirstTerminatorUsesValue = InstructionUsesRegister(I, SrcReg); |
| |
| // Check that no other terminators use values. |
| #ifndef NDEBUG |
| for (MachineBasicBlock::iterator TI = next(I); TI != opBlock.end(); |
| ++TI) { |
| assert(!InstructionUsesRegister(TI, SrcReg) && |
| "Terminator instructions cannot use virtual registers unless" |
| "they are the first terminator in a block!"); |
| } |
| #endif |
| } |
| |
| MachineBasicBlock::iterator KillInst; |
| if (!FirstTerminatorUsesValue) |
| KillInst = prior(I); |
| else |
| KillInst = I; |
| |
| // 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; |
| } |
| } |
| |
| // Really delete the PHI instruction now! |
| delete MPhi; |
| ++NumAtomic; |
| } |