PHIElimination.cpp revision b52e0241c03a257f96bd9c77788eff5b1a7fd437
1//===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===// 2// 3// This pass eliminates machine instruction PHI nodes by inserting copy 4// instructions. This destroys SSA information, but is the desired input for 5// some register allocators. 6// 7//===----------------------------------------------------------------------===// 8 9#include "llvm/CodeGen/MachineFunctionPass.h" 10#include "llvm/CodeGen/MachineInstr.h" 11#include "llvm/CodeGen/SSARegMap.h" 12#include "llvm/CodeGen/LiveVariables.h" 13#include "llvm/Target/TargetInstrInfo.h" 14#include "llvm/Target/TargetMachine.h" 15#include "llvm/Support/CFG.h" 16 17namespace { 18 struct PNE : public MachineFunctionPass { 19 bool runOnMachineFunction(MachineFunction &Fn) { 20 bool Changed = false; 21 22 // Eliminate PHI instructions by inserting copies into predecessor blocks. 23 // 24 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) 25 Changed |= EliminatePHINodes(Fn, *I); 26 27 //std::cerr << "AFTER PHI NODE ELIM:\n"; 28 //Fn.dump(); 29 return Changed; 30 } 31 32 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 33 AU.addPreserved<LiveVariables>(); 34 MachineFunctionPass::getAnalysisUsage(AU); 35 } 36 37 private: 38 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions 39 /// in predecessor basic blocks. 40 /// 41 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB); 42 }; 43 44 RegisterPass<PNE> X("phi-node-elimination", 45 "Eliminate PHI nodes for register allocation"); 46} 47 48const PassInfo *PHIEliminationID = X.getPassInfo(); 49 50/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in 51/// predecessor basic blocks. 52/// 53bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) { 54 if (MBB.empty() || MBB.front()->getOpcode() != TargetInstrInfo::PHI) 55 return false; // Quick exit for normal case... 56 57 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>(); 58 const TargetInstrInfo &MII = MF.getTarget().getInstrInfo(); 59 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo(); 60 61 while (MBB.front()->getOpcode() == TargetInstrInfo::PHI) { 62 MachineInstr *MI = MBB.front(); 63 // Unlink the PHI node from the basic block... but don't delete the PHI yet 64 MBB.erase(MBB.begin()); 65 66 assert(MI->getOperand(0).isVirtualRegister() && 67 "PHI node doesn't write virt reg?"); 68 69 unsigned DestReg = MI->getOperand(0).getAllocatedRegNum(); 70 71 // Create a new register for the incoming PHI arguments 72 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg); 73 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC); 74 75 // Insert a register to register copy in the top of the current block (but 76 // after any remaining phi nodes) which copies the new incoming register 77 // into the phi node destination. 78 // 79 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin(); 80 while (AfterPHIsIt != MBB.end() && 81 (*AfterPHIsIt)->getOpcode() == TargetInstrInfo::PHI) 82 ++AfterPHIsIt; // Skip over all of the PHI nodes... 83 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC); 84 85 // Update live variable information if there is any... 86 if (LV) { 87 MachineInstr *PHICopy = *(AfterPHIsIt-1); 88 89 // Add information to LiveVariables to know that the incoming value is 90 // killed. Note that because the value is defined in several places (once 91 // each for each incoming block), the "def" block and instruction fields 92 // for the VarInfo is not filled in. 93 // 94 LV->addVirtualRegisterKilled(IncomingReg, &MBB, PHICopy); 95 96 // Since we are going to be deleting the PHI node, if it is the last use 97 // of any registers, or if the value itself is dead, we need to move this 98 // information over to the new copy we just inserted... 99 // 100 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator> 101 RKs = LV->killed_range(MI); 102 std::vector<std::pair<MachineInstr*, unsigned> > Range; 103 if (RKs.first != RKs.second) { 104 // Copy the range into a vector... 105 Range.assign(RKs.first, RKs.second); 106 107 // Delete the range... 108 LV->removeVirtualRegistersKilled(RKs.first, RKs.second); 109 110 // Add all of the kills back, which will update the appropriate info... 111 for (unsigned i = 0, e = Range.size(); i != e; ++i) 112 LV->addVirtualRegisterKilled(Range[i].second, &MBB, PHICopy); 113 } 114 115 RKs = LV->dead_range(MI); 116 if (RKs.first != RKs.second) { 117 // Works as above... 118 Range.assign(RKs.first, RKs.second); 119 LV->removeVirtualRegistersDead(RKs.first, RKs.second); 120 for (unsigned i = 0, e = Range.size(); i != e; ++i) 121 LV->addVirtualRegisterDead(Range[i].second, &MBB, PHICopy); 122 } 123 } 124 125 // Now loop over all of the incoming arguments, changing them to copy into 126 // the IncomingReg register in the corresponding predecessor basic block. 127 // 128 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) { 129 MachineOperand &opVal = MI->getOperand(i-1); 130 131 // Get the MachineBasicBlock equivalent of the BasicBlock that is the 132 // source path the PHI. 133 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock(); 134 135 // Figure out where to insert the copy, which is at the end of the 136 // predecessor basic block, but before any terminator/branch 137 // instructions... 138 MachineBasicBlock::iterator I = opBlock.end(); 139 if (I != opBlock.begin()) { // Handle empty blocks 140 --I; 141 // must backtrack over ALL the branches in the previous block 142 while (MII.isTerminatorInstr((*I)->getOpcode()) && 143 I != opBlock.begin()) 144 --I; 145 146 // move back to the first branch instruction so new instructions 147 // are inserted right in front of it and not in front of a non-branch 148 if (!MII.isTerminatorInstr((*I)->getOpcode())) 149 ++I; 150 } 151 152 // Check to make sure we haven't already emitted the copy for this block. 153 // This can happen because PHI nodes may have multiple entries for the 154 // same basic block. It doesn't matter which entry we use though, because 155 // all incoming values are guaranteed to be the same for a particular bb. 156 // 157 // If we emitted a copy for this basic block already, it will be right 158 // where we want to insert one now. Just check for a definition of the 159 // register we are interested in! 160 // 161 bool HaveNotEmitted = true; 162 163 if (I != opBlock.begin()) { 164 MachineInstr *PrevInst = *(I-1); 165 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) { 166 MachineOperand &MO = PrevInst->getOperand(i); 167 if (MO.isVirtualRegister() && MO.getReg() == IncomingReg) 168 if (MO.opIsDef() || MO.opIsDefAndUse()) { 169 HaveNotEmitted = false; 170 break; 171 } 172 } 173 } 174 175 if (HaveNotEmitted) { // If the copy has not already been emitted, do it. 176 assert(opVal.isVirtualRegister() && 177 "Machine PHI Operands must all be virtual registers!"); 178 unsigned SrcReg = opVal.getReg(); 179 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC); 180 181 // Now update live variable information if we have it. 182 if (LV) { 183 // We want to be able to insert a kill of the register if this PHI 184 // (aka, the copy we just inserted) is the last use of the source 185 // value. Live variable analysis conservatively handles this by 186 // saying that the value is live until the end of the block the PHI 187 // entry lives in. If the value really is dead at the PHI copy, there 188 // will be no successor blocks which have the value live-in. 189 // 190 // Check to see if the copy is the last use, and if so, update the 191 // live variables information so that it knows the copy source 192 // instruction kills the incoming value. 193 // 194 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg); 195 196 // Loop over all of the successors of the basic block, checking to 197 // see if the value is either live in the block, or if it is killed 198 // in the block. 199 // 200 bool ValueIsLive = false; 201 BasicBlock *BB = opBlock.getBasicBlock(); 202 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); 203 SI != E; ++SI) { 204 const std::pair<MachineBasicBlock*, unsigned> & 205 SuccInfo = LV->getBasicBlockInfo(*SI); 206 207 // Is it alive in this successor? 208 unsigned SuccIdx = SuccInfo.second; 209 if (SuccIdx < InRegVI.AliveBlocks.size() && 210 InRegVI.AliveBlocks[SuccIdx]) { 211 ValueIsLive = true; 212 break; 213 } 214 215 // Is it killed in this successor? 216 MachineBasicBlock *MBB = SuccInfo.first; 217 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i) 218 if (InRegVI.Kills[i].first == MBB) { 219 ValueIsLive = true; 220 break; 221 } 222 } 223 224 // Okay, if we now know that the value is not live out of the block, 225 // we can add a kill marker to the copy we inserted saying that it 226 // kills the incoming value! 227 // 228 if (!ValueIsLive) { 229 // One more complication to worry about. There may actually be 230 // multiple PHI nodes using this value on this branch. If we aren't 231 // careful, the first PHI node will end up killing the value, not 232 // letting it get the to the copy for the final PHI node in the 233 // block. Therefore we have to check to see if there is already a 234 // kill in this block, and if so, extend the lifetime to our new 235 // copy. 236 // 237 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i) 238 if (InRegVI.Kills[i].first == &opBlock) { 239 std::pair<LiveVariables::killed_iterator, 240 LiveVariables::killed_iterator> Range 241 = LV->killed_range(InRegVI.Kills[i].second); 242 LV->removeVirtualRegistersKilled(Range.first, Range.second); 243 break; 244 } 245 246 LV->addVirtualRegisterKilled(SrcReg, &opBlock, *(I-1)); 247 } 248 } 249 } 250 } 251 252 // really delete the PHI instruction now! 253 delete MI; 254 } 255 256 return true; 257} 258