PHIElimination.cpp revision 44b94c2185f4c1b826ec6003d25370cf2efac219
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/LiveVariables.h" 17#include "llvm/CodeGen/Passes.h" 18#include "llvm/CodeGen/MachineFunctionPass.h" 19#include "llvm/CodeGen/MachineInstr.h" 20#include "llvm/CodeGen/SSARegMap.h" 21#include "llvm/Target/TargetInstrInfo.h" 22#include "llvm/Target/TargetMachine.h" 23#include "llvm/ADT/DenseMap.h" 24#include "llvm/ADT/STLExtras.h" 25using namespace llvm; 26 27namespace { 28 struct PNE : public MachineFunctionPass { 29 bool runOnMachineFunction(MachineFunction &Fn) { 30 bool Changed = false; 31 32 // Eliminate PHI instructions by inserting copies into predecessor blocks. 33 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) 34 Changed |= EliminatePHINodes(Fn, *I); 35 36 return Changed; 37 } 38 39 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 40 AU.addPreserved<LiveVariables>(); 41 MachineFunctionPass::getAnalysisUsage(AU); 42 } 43 44 private: 45 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions 46 /// in predecessor basic blocks. 47 /// 48 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB); 49 }; 50 51 RegisterPass<PNE> X("phi-node-elimination", 52 "Eliminate PHI nodes for register allocation"); 53} 54 55 56const PassInfo *llvm::PHIEliminationID = X.getPassInfo(); 57 58/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in 59/// predecessor basic blocks. 60/// 61bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) { 62 if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI) 63 return false; // Quick exit for normal case... 64 65 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>(); 66 const TargetInstrInfo &MII = *MF.getTarget().getInstrInfo(); 67 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo(); 68 69 // VRegPHIUseCount - Keep track of the number of times each virtual register 70 // is used by PHI nodes in successors of this block. 71 DenseMap<unsigned, VirtReg2IndexFunctor> VRegPHIUseCount; 72 VRegPHIUseCount.grow(MF.getSSARegMap()->getLastVirtReg()); 73 74 unsigned BBIsSuccOfPreds = 0; // Number of times MBB is a succ of preds 75 for (MachineBasicBlock::pred_iterator PI = MBB.pred_begin(), 76 E = MBB.pred_end(); PI != E; ++PI) 77 for (MachineBasicBlock::succ_iterator SI = (*PI)->succ_begin(), 78 E = (*PI)->succ_end(); SI != E; ++SI) { 79 BBIsSuccOfPreds += *SI == &MBB; 80 for (MachineBasicBlock::iterator BBI = (*SI)->begin(); BBI !=(*SI)->end() && 81 BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI) 82 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) 83 VRegPHIUseCount[BBI->getOperand(i).getReg()]++; 84 } 85 86 // Get an iterator to the first instruction after the last PHI node (this may 87 // also be the end of the basic block). While we are scanning the PHIs, 88 // populate the VRegPHIUseCount map. 89 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin(); 90 while (AfterPHIsIt != MBB.end() && 91 AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI) 92 ++AfterPHIsIt; // Skip over all of the PHI nodes... 93 94 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) { 95 // Unlink the PHI node from the basic block, but don't delete the PHI yet. 96 MachineInstr *MPhi = MBB.remove(MBB.begin()); 97 98 assert(MRegisterInfo::isVirtualRegister(MPhi->getOperand(0).getReg()) && 99 "PHI node doesn't write virt reg?"); 100 101 unsigned DestReg = MPhi->getOperand(0).getReg(); 102 103 // Create a new register for the incoming PHI arguments 104 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg); 105 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC); 106 107 // Insert a register to register copy in the top of the current block (but 108 // after any remaining phi nodes) which copies the new incoming register 109 // into the phi node destination. 110 // 111 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC); 112 113 // Update live variable information if there is any... 114 if (LV) { 115 MachineInstr *PHICopy = prior(AfterPHIsIt); 116 117 // Add information to LiveVariables to know that the incoming value is 118 // killed. Note that because the value is defined in several places (once 119 // each for each incoming block), the "def" block and instruction fields 120 // for the VarInfo is not filled in. 121 // 122 LV->addVirtualRegisterKilled(IncomingReg, PHICopy); 123 124 // Since we are going to be deleting the PHI node, if it is the last use 125 // of any registers, or if the value itself is dead, we need to move this 126 // information over to the new copy we just inserted. 127 // 128 LV->removeVirtualRegistersKilled(MPhi); 129 130 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator> 131 RKs = LV->dead_range(MPhi); 132 if (RKs.first != RKs.second) { 133 for (LiveVariables::killed_iterator I = RKs.first; I != RKs.second; ++I) 134 LV->addVirtualRegisterDead(*I, PHICopy); 135 LV->removeVirtualRegistersDead(MPhi); 136 } 137 } 138 139 // Adjust the VRegPHIUseCount map to account for the removal of this PHI 140 // node. 141 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) 142 VRegPHIUseCount[MPhi->getOperand(i).getReg()] -= BBIsSuccOfPreds; 143 144 // Now loop over all of the incoming arguments, changing them to copy into 145 // the IncomingReg register in the corresponding predecessor basic block. 146 // 147 for (int i = MPhi->getNumOperands() - 1; i >= 2; i-=2) { 148 MachineOperand &opVal = MPhi->getOperand(i-1); 149 150 // Get the MachineBasicBlock equivalent of the BasicBlock that is the 151 // source path the PHI. 152 MachineBasicBlock &opBlock = *MPhi->getOperand(i).getMachineBasicBlock(); 153 154 MachineBasicBlock::iterator I = opBlock.getFirstTerminator(); 155 156 // Check to make sure we haven't already emitted the copy for this block. 157 // This can happen because PHI nodes may have multiple entries for the 158 // same basic block. It doesn't matter which entry we use though, because 159 // all incoming values are guaranteed to be the same for a particular bb. 160 // 161 // If we emitted a copy for this basic block already, it will be right 162 // where we want to insert one now. Just check for a definition of the 163 // register we are interested in! 164 // 165 bool HaveNotEmitted = true; 166 167 if (I != opBlock.begin()) { 168 MachineBasicBlock::iterator PrevInst = prior(I); 169 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) { 170 MachineOperand &MO = PrevInst->getOperand(i); 171 if (MO.isRegister() && MO.getReg() == IncomingReg) 172 if (MO.isDef()) { 173 HaveNotEmitted = false; 174 break; 175 } 176 } 177 } 178 179 if (HaveNotEmitted) { // If the copy has not already been emitted, do it. 180 assert(MRegisterInfo::isVirtualRegister(opVal.getReg()) && 181 "Machine PHI Operands must all be virtual registers!"); 182 unsigned SrcReg = opVal.getReg(); 183 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC); 184 185 // Now update live variable information if we have it. 186 if (LV) { 187 // We want to be able to insert a kill of the register if this PHI 188 // (aka, the copy we just inserted) is the last use of the source 189 // value. Live variable analysis conservatively handles this by 190 // saying that the value is live until the end of the block the PHI 191 // entry lives in. If the value really is dead at the PHI copy, there 192 // will be no successor blocks which have the value live-in. 193 // 194 // Check to see if the copy is the last use, and if so, update the 195 // live variables information so that it knows the copy source 196 // instruction kills the incoming value. 197 // 198 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg); 199 200 // Loop over all of the successors of the basic block, checking to see 201 // if the value is either live in the block, or if it is killed in the 202 // block. Also check to see if this register is in use by another PHI 203 // node which has not yet been eliminated. If so, it will be killed 204 // at an appropriate point later. 205 // 206 bool ValueIsLive = false; 207 for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(), 208 E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) { 209 MachineBasicBlock *SuccMBB = *SI; 210 211 // Is it alive in this successor? 212 unsigned SuccIdx = SuccMBB->getNumber(); 213 if (SuccIdx < InRegVI.AliveBlocks.size() && 214 InRegVI.AliveBlocks[SuccIdx]) { 215 ValueIsLive = true; 216 break; 217 } 218 219 // Is it killed in this successor? 220 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i) 221 if (InRegVI.Kills[i]->getParent() == SuccMBB) { 222 ValueIsLive = true; 223 break; 224 } 225 226 // Is it used by any PHI instructions in this block? 227 if (!ValueIsLive) 228 ValueIsLive = VRegPHIUseCount[SrcReg] != 0; 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, Prev); 238 239 // This vreg no longer lives all of the way through opBlock. 240 unsigned opBlockNum = opBlock.getNumber(); 241 if (opBlockNum < InRegVI.AliveBlocks.size()) 242 InRegVI.AliveBlocks[opBlockNum] = false; 243 } 244 } 245 } 246 } 247 248 // Really delete the PHI instruction now! 249 delete MPhi; 250 } 251 return true; 252} 253