PHIElimination.cpp revision 53a79aaae988d9dc9d12af8970f8b8fe58cc478d
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" 25#include <set> 26using namespace llvm; 27 28namespace { 29 struct PNE : public MachineFunctionPass { 30 bool runOnMachineFunction(MachineFunction &Fn) { 31 bool Changed = false; 32 33 // Eliminate PHI instructions by inserting copies into predecessor blocks. 34 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) 35 Changed |= EliminatePHINodes(Fn, *I); 36 37 return Changed; 38 } 39 40 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 41 AU.addPreserved<LiveVariables>(); 42 MachineFunctionPass::getAnalysisUsage(AU); 43 } 44 45 private: 46 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions 47 /// in predecessor basic blocks. 48 /// 49 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB); 50 void LowerAtomicPHINode(MachineBasicBlock &MBB, 51 MachineBasicBlock::iterator AfterPHIsIt, 52 DenseMap<unsigned, VirtReg2IndexFunctor> &VUC, 53 unsigned BBIsSuccOfPreds); 54 }; 55 56 RegisterPass<PNE> X("phi-node-elimination", 57 "Eliminate PHI nodes for register allocation"); 58} 59 60 61const PassInfo *llvm::PHIEliminationID = X.getPassInfo(); 62 63/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in 64/// predecessor basic blocks. 65/// 66bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) { 67 if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI) 68 return false; // Quick exit for basic blocks without PHIs. 69 70 // VRegPHIUseCount - Keep track of the number of times each virtual register 71 // is used by PHI nodes in successors of this block. 72 DenseMap<unsigned, VirtReg2IndexFunctor> VRegPHIUseCount; 73 VRegPHIUseCount.grow(MF.getSSARegMap()->getLastVirtReg()); 74 75 unsigned BBIsSuccOfPreds = 0; // Number of times MBB is a succ of preds 76 for (MachineBasicBlock::pred_iterator PI = MBB.pred_begin(), 77 E = MBB.pred_end(); PI != E; ++PI) 78 for (MachineBasicBlock::succ_iterator SI = (*PI)->succ_begin(), 79 E = (*PI)->succ_end(); SI != E; ++SI) { 80 BBIsSuccOfPreds += *SI == &MBB; 81 for (MachineBasicBlock::iterator BBI = (*SI)->begin(); BBI !=(*SI)->end() && 82 BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI) 83 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) 84 VRegPHIUseCount[BBI->getOperand(i).getReg()]++; 85 } 86 87 // Get an iterator to the first instruction after the last PHI node (this may 88 // also be the end of the basic block). 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 LowerAtomicPHINode(MBB, AfterPHIsIt, VRegPHIUseCount, BBIsSuccOfPreds); 96 } 97 return true; 98} 99 100/// LowerAtomicPHINode - Lower the PHI node at the top of the specified block, 101/// under the assuption that it needs to be lowered in a way that supports 102/// atomic execution of PHIs. This lowering method is always correct all of the 103/// time. 104void PNE::LowerAtomicPHINode(MachineBasicBlock &MBB, 105 MachineBasicBlock::iterator AfterPHIsIt, 106 DenseMap<unsigned, VirtReg2IndexFunctor> &VRegPHIUseCount, 107 unsigned BBIsSuccOfPreds) { 108 // Unlink the PHI node from the basic block, but don't delete the PHI yet. 109 MachineInstr *MPhi = MBB.remove(MBB.begin()); 110 111 unsigned DestReg = MPhi->getOperand(0).getReg(); 112 113 // Create a new register for the incoming PHI arguments/ 114 MachineFunction &MF = *MBB.getParent(); 115 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg); 116 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC); 117 118 // Insert a register to register copy in the top of the current block (but 119 // after any remaining phi nodes) which copies the new incoming register 120 // into the phi node destination. 121 // 122 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo(); 123 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC); 124 125 // Update live variable information if there is any... 126 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>(); 127 if (LV) { 128 MachineInstr *PHICopy = prior(AfterPHIsIt); 129 130 // Add information to LiveVariables to know that the incoming value is 131 // killed. Note that because the value is defined in several places (once 132 // each for each incoming block), the "def" block and instruction fields 133 // for the VarInfo is not filled in. 134 // 135 LV->addVirtualRegisterKilled(IncomingReg, PHICopy); 136 137 // Since we are going to be deleting the PHI node, if it is the last use 138 // of any registers, or if the value itself is dead, we need to move this 139 // information over to the new copy we just inserted. 140 // 141 LV->removeVirtualRegistersKilled(MPhi); 142 143 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator> 144 RKs = LV->dead_range(MPhi); 145 if (RKs.first != RKs.second) { 146 for (LiveVariables::killed_iterator I = RKs.first; I != RKs.second; ++I) 147 LV->addVirtualRegisterDead(*I, PHICopy); 148 LV->removeVirtualRegistersDead(MPhi); 149 } 150 } 151 152 // Adjust the VRegPHIUseCount map to account for the removal of this PHI 153 // node. 154 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) 155 VRegPHIUseCount[MPhi->getOperand(i).getReg()] -= BBIsSuccOfPreds; 156 157 // Now loop over all of the incoming arguments, changing them to copy into 158 // the IncomingReg register in the corresponding predecessor basic block. 159 // 160 for (int i = MPhi->getNumOperands() - 1; i >= 2; i-=2) { 161 MachineOperand &opVal = MPhi->getOperand(i-1); 162 163 // Get the MachineBasicBlock equivalent of the BasicBlock that is the 164 // source path the PHI. 165 MachineBasicBlock &opBlock = *MPhi->getOperand(i).getMachineBasicBlock(); 166 167 MachineBasicBlock::iterator I = opBlock.getFirstTerminator(); 168 169 // Check to make sure we haven't already emitted the copy for this block. 170 // This can happen because PHI nodes may have multiple entries for the 171 // same basic block. It doesn't matter which entry we use though, because 172 // all incoming values are guaranteed to be the same for a particular bb. 173 // 174 // If we emitted a copy for this basic block already, it will be right 175 // where we want to insert one now. Just check for a definition of the 176 // register we are interested in! 177 // 178 bool HaveNotEmitted = true; 179 180 if (I != opBlock.begin()) { 181 MachineBasicBlock::iterator PrevInst = prior(I); 182 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) { 183 MachineOperand &MO = PrevInst->getOperand(i); 184 if (MO.isRegister() && MO.getReg() == IncomingReg) 185 if (MO.isDef()) { 186 HaveNotEmitted = false; 187 break; 188 } 189 } 190 } 191 192 if (HaveNotEmitted) { // If the copy has not already been emitted, do it. 193 assert(MRegisterInfo::isVirtualRegister(opVal.getReg()) && 194 "Machine PHI Operands must all be virtual registers!"); 195 unsigned SrcReg = opVal.getReg(); 196 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC); 197 198 // Now update live variable information if we have it. 199 if (LV) { 200 // We want to be able to insert a kill of the register if this PHI 201 // (aka, the copy we just inserted) is the last use of the source 202 // value. Live variable analysis conservatively handles this by 203 // saying that the value is live until the end of the block the PHI 204 // entry lives in. If the value really is dead at the PHI copy, there 205 // will be no successor blocks which have the value live-in. 206 // 207 // Check to see if the copy is the last use, and if so, update the 208 // live variables information so that it knows the copy source 209 // instruction kills the incoming value. 210 // 211 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg); 212 213 // Loop over all of the successors of the basic block, checking to see 214 // if the value is either live in the block, or if it is killed in the 215 // block. Also check to see if this register is in use by another PHI 216 // node which has not yet been eliminated. If so, it will be killed 217 // at an appropriate point later. 218 // 219 bool ValueIsLive = false; 220 for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(), 221 E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) { 222 MachineBasicBlock *SuccMBB = *SI; 223 224 // Is it alive in this successor? 225 unsigned SuccIdx = SuccMBB->getNumber(); 226 if (SuccIdx < InRegVI.AliveBlocks.size() && 227 InRegVI.AliveBlocks[SuccIdx]) { 228 ValueIsLive = true; 229 break; 230 } 231 232 // Is it killed in this successor? 233 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i) 234 if (InRegVI.Kills[i]->getParent() == SuccMBB) { 235 ValueIsLive = true; 236 break; 237 } 238 239 // Is it used by any PHI instructions in this block? 240 if (!ValueIsLive) 241 ValueIsLive = VRegPHIUseCount[SrcReg] != 0; 242 } 243 244 // Okay, if we now know that the value is not live out of the block, 245 // we can add a kill marker to the copy we inserted saying that it 246 // kills the incoming value! 247 // 248 if (!ValueIsLive) { 249 MachineBasicBlock::iterator Prev = prior(I); 250 LV->addVirtualRegisterKilled(SrcReg, Prev); 251 252 // This vreg no longer lives all of the way through opBlock. 253 unsigned opBlockNum = opBlock.getNumber(); 254 if (opBlockNum < InRegVI.AliveBlocks.size()) 255 InRegVI.AliveBlocks[opBlockNum] = false; 256 } 257 } 258 } 259 } 260 261 // Really delete the PHI instruction now! 262 delete MPhi; 263} 264