PHIElimination.cpp revision 015959ee38e4fd4a920f6b0065c50e524762f580
1//===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This pass eliminates machine instruction PHI nodes by inserting copy 11// instructions. This destroys SSA information, but is the desired input for 12// some register allocators. 13// 14//===----------------------------------------------------------------------===// 15 16#include "llvm/CodeGen/Passes.h" 17#include "llvm/CodeGen/MachineFunctionPass.h" 18#include "llvm/CodeGen/MachineInstr.h" 19#include "llvm/CodeGen/SSARegMap.h" 20#include "llvm/CodeGen/LiveVariables.h" 21#include "llvm/Target/TargetInstrInfo.h" 22#include "llvm/Target/TargetMachine.h" 23#include "Support/STLExtras.h" 24using namespace llvm; 25 26namespace { 27 struct PNE : public MachineFunctionPass { 28 bool runOnMachineFunction(MachineFunction &Fn) { 29 bool Changed = false; 30 31 // Eliminate PHI instructions by inserting copies into predecessor blocks. 32 // 33 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) 34 Changed |= EliminatePHINodes(Fn, *I); 35 36 //std::cerr << "AFTER PHI NODE ELIM:\n"; 37 //Fn.dump(); 38 return Changed; 39 } 40 41 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 42 AU.addPreserved<LiveVariables>(); 43 MachineFunctionPass::getAnalysisUsage(AU); 44 } 45 46 private: 47 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions 48 /// in predecessor basic blocks. 49 /// 50 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB); 51 }; 52 53 RegisterPass<PNE> X("phi-node-elimination", 54 "Eliminate PHI nodes for register allocation"); 55} 56 57 58const PassInfo *llvm::PHIEliminationID = X.getPassInfo(); 59 60/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in 61/// predecessor basic blocks. 62/// 63bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) { 64 if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI) 65 return false; // Quick exit for normal case... 66 67 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>(); 68 const TargetInstrInfo &MII = MF.getTarget().getInstrInfo(); 69 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo(); 70 71 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) { 72 // Unlink the PHI node from the basic block... but don't delete the PHI yet 73 MachineInstr *MI = MBB.remove(MBB.begin()); 74 75 assert(MRegisterInfo::isVirtualRegister(MI->getOperand(0).getReg()) && 76 "PHI node doesn't write virt reg?"); 77 78 unsigned DestReg = MI->getOperand(0).getReg(); 79 80 // Create a new register for the incoming PHI arguments 81 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg); 82 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC); 83 84 // Insert a register to register copy in the top of the current block (but 85 // after any remaining phi nodes) which copies the new incoming register 86 // into the phi node destination. 87 // 88 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin(); 89 while (AfterPHIsIt != MBB.end() && 90 AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI) 91 ++AfterPHIsIt; // Skip over all of the PHI nodes... 92 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC); 93 94 // Update live variable information if there is any... 95 if (LV) { 96 MachineInstr *PHICopy = --AfterPHIsIt; 97 98 // Add information to LiveVariables to know that the incoming value is 99 // killed. Note that because the value is defined in several places (once 100 // each for each incoming block), the "def" block and instruction fields 101 // for the VarInfo is not filled in. 102 // 103 LV->addVirtualRegisterKilled(IncomingReg, &MBB, PHICopy); 104 105 // Since we are going to be deleting the PHI node, if it is the last use 106 // of any registers, or if the value itself is dead, we need to move this 107 // information over to the new copy we just inserted... 108 // 109 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator> 110 RKs = LV->killed_range(MI); 111 std::vector<std::pair<MachineInstr*, unsigned> > Range; 112 if (RKs.first != RKs.second) { 113 // Copy the range into a vector... 114 Range.assign(RKs.first, RKs.second); 115 116 // Delete the range... 117 LV->removeVirtualRegistersKilled(RKs.first, RKs.second); 118 119 // Add all of the kills back, which will update the appropriate info... 120 for (unsigned i = 0, e = Range.size(); i != e; ++i) 121 LV->addVirtualRegisterKilled(Range[i].second, &MBB, PHICopy); 122 } 123 124 RKs = LV->dead_range(MI); 125 if (RKs.first != RKs.second) { 126 // Works as above... 127 Range.assign(RKs.first, RKs.second); 128 LV->removeVirtualRegistersDead(RKs.first, RKs.second); 129 for (unsigned i = 0, e = Range.size(); i != e; ++i) 130 LV->addVirtualRegisterDead(Range[i].second, &MBB, PHICopy); 131 } 132 } 133 134 // Now loop over all of the incoming arguments, changing them to copy into 135 // the IncomingReg register in the corresponding predecessor basic block. 136 // 137 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) { 138 MachineOperand &opVal = MI->getOperand(i-1); 139 140 // Get the MachineBasicBlock equivalent of the BasicBlock that is the 141 // source path the PHI. 142 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock(); 143 144 MachineBasicBlock::iterator I = opBlock.getFirstTerminator(); 145 146 // Check to make sure we haven't already emitted the copy for this block. 147 // This can happen because PHI nodes may have multiple entries for the 148 // same basic block. It doesn't matter which entry we use though, because 149 // all incoming values are guaranteed to be the same for a particular bb. 150 // 151 // If we emitted a copy for this basic block already, it will be right 152 // where we want to insert one now. Just check for a definition of the 153 // register we are interested in! 154 // 155 bool HaveNotEmitted = true; 156 157 if (I != opBlock.begin()) { 158 MachineBasicBlock::iterator PrevInst = prior(I); 159 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) { 160 MachineOperand &MO = PrevInst->getOperand(i); 161 if (MO.isRegister() && MO.getReg() == IncomingReg) 162 if (MO.isDef()) { 163 HaveNotEmitted = false; 164 break; 165 } 166 } 167 } 168 169 if (HaveNotEmitted) { // If the copy has not already been emitted, do it. 170 assert(MRegisterInfo::isVirtualRegister(opVal.getReg()) && 171 "Machine PHI Operands must all be virtual registers!"); 172 unsigned SrcReg = opVal.getReg(); 173 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC); 174 175 // Now update live variable information if we have it. 176 if (LV) { 177 // We want to be able to insert a kill of the register if this PHI 178 // (aka, the copy we just inserted) is the last use of the source 179 // value. Live variable analysis conservatively handles this by 180 // saying that the value is live until the end of the block the PHI 181 // entry lives in. If the value really is dead at the PHI copy, there 182 // will be no successor blocks which have the value live-in. 183 // 184 // Check to see if the copy is the last use, and if so, update the 185 // live variables information so that it knows the copy source 186 // instruction kills the incoming value. 187 // 188 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg); 189 190 // Loop over all of the successors of the basic block, checking to see 191 // if the value is either live in the block, or if it is killed in the 192 // block. Also check to see if this register is in use by another PHI 193 // node which has not yet been eliminated. If so, it will be killed 194 // at an appropriate point later. 195 // 196 bool ValueIsLive = false; 197 for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(), 198 E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) { 199 MachineBasicBlock *MBB = *SI; 200 201 // Is it alive in this successor? 202 unsigned SuccIdx = LV->getMachineBasicBlockIndex(MBB); 203 if (SuccIdx < InRegVI.AliveBlocks.size() && 204 InRegVI.AliveBlocks[SuccIdx]) { 205 ValueIsLive = true; 206 break; 207 } 208 209 // Is it killed in this successor? 210 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i) 211 if (InRegVI.Kills[i].first == MBB) { 212 ValueIsLive = true; 213 break; 214 } 215 216 // Is it used by any PHI instructions in this block? 217 if (ValueIsLive) break; 218 219 // Loop over all of the PHIs in this successor, checking to see if 220 // the register is being used... 221 for (MachineBasicBlock::iterator BBI = MBB->begin(), E=MBB->end(); 222 BBI != E && BBI->getOpcode() == TargetInstrInfo::PHI; 223 ++BBI) 224 for (unsigned i = 1, e = BBI->getNumOperands(); i < e; i += 2) 225 if (BBI->getOperand(i).getReg() == SrcReg) { 226 ValueIsLive = true; 227 break; 228 } 229 } 230 231 // Okay, if we now know that the value is not live out of the block, 232 // we can add a kill marker to the copy we inserted saying that it 233 // kills the incoming value! 234 // 235 if (!ValueIsLive) { 236 MachineBasicBlock::iterator Prev = prior(I); 237 LV->addVirtualRegisterKilled(SrcReg, &opBlock, Prev); 238 } 239 } 240 } 241 } 242 243 // really delete the PHI instruction now! 244 delete MI; 245 } 246 return true; 247} 248