LLVM API Documentation
00001 //===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===// 00002 // 00003 // The LLVM Compiler Infrastructure 00004 // 00005 // This file was developed by the LLVM research group and is distributed under 00006 // the University of Illinois Open Source License. See LICENSE.TXT for details. 00007 // 00008 //===----------------------------------------------------------------------===// 00009 // 00010 // This pass eliminates machine instruction PHI nodes by inserting copy 00011 // instructions. This destroys SSA information, but is the desired input for 00012 // some register allocators. 00013 // 00014 //===----------------------------------------------------------------------===// 00015 00016 #include "llvm/CodeGen/Passes.h" 00017 #include "llvm/CodeGen/MachineFunctionPass.h" 00018 #include "llvm/CodeGen/MachineInstr.h" 00019 #include "llvm/CodeGen/SSARegMap.h" 00020 #include "llvm/CodeGen/LiveVariables.h" 00021 #include "llvm/Target/TargetInstrInfo.h" 00022 #include "llvm/Target/TargetMachine.h" 00023 #include "llvm/ADT/DenseMap.h" 00024 #include "llvm/ADT/STLExtras.h" 00025 using namespace llvm; 00026 00027 namespace { 00028 struct PNE : public MachineFunctionPass { 00029 bool runOnMachineFunction(MachineFunction &Fn) { 00030 bool Changed = false; 00031 00032 // Eliminate PHI instructions by inserting copies into predecessor blocks. 00033 // 00034 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I) 00035 Changed |= EliminatePHINodes(Fn, *I); 00036 00037 //std::cerr << "AFTER PHI NODE ELIM:\n"; 00038 //Fn.dump(); 00039 return Changed; 00040 } 00041 00042 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 00043 AU.addPreserved<LiveVariables>(); 00044 MachineFunctionPass::getAnalysisUsage(AU); 00045 } 00046 00047 private: 00048 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions 00049 /// in predecessor basic blocks. 00050 /// 00051 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB); 00052 }; 00053 00054 RegisterPass<PNE> X("phi-node-elimination", 00055 "Eliminate PHI nodes for register allocation"); 00056 } 00057 00058 00059 const PassInfo *llvm::PHIEliminationID = X.getPassInfo(); 00060 00061 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in 00062 /// predecessor basic blocks. 00063 /// 00064 bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) { 00065 if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI) 00066 return false; // Quick exit for normal case... 00067 00068 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>(); 00069 const TargetInstrInfo &MII = *MF.getTarget().getInstrInfo(); 00070 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo(); 00071 00072 // VRegPHIUseCount - Keep track of the number of times each virtual register 00073 // is used by PHI nodes in successors of this block. 00074 DenseMap<unsigned, VirtReg2IndexFunctor> VRegPHIUseCount; 00075 VRegPHIUseCount.grow(MF.getSSARegMap()->getLastVirtReg()); 00076 00077 unsigned BBIsSuccOfPreds = 0; // Number of times MBB is a succ of preds 00078 for (MachineBasicBlock::pred_iterator PI = MBB.pred_begin(), 00079 E = MBB.pred_end(); PI != E; ++PI) 00080 for (MachineBasicBlock::succ_iterator SI = (*PI)->succ_begin(), 00081 E = (*PI)->succ_end(); SI != E; ++SI) { 00082 BBIsSuccOfPreds += *SI == &MBB; 00083 for (MachineBasicBlock::iterator BBI = (*SI)->begin(); BBI !=(*SI)->end() && 00084 BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI) 00085 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2) 00086 VRegPHIUseCount[BBI->getOperand(i).getReg()]++; 00087 } 00088 00089 // Get an iterator to the first instruction after the last PHI node (this may 00090 // also be the end of the basic block). While we are scanning the PHIs, 00091 // populate the VRegPHIUseCount map. 00092 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin(); 00093 while (AfterPHIsIt != MBB.end() && 00094 AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI) 00095 ++AfterPHIsIt; // Skip over all of the PHI nodes... 00096 00097 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) { 00098 // Unlink the PHI node from the basic block, but don't delete the PHI yet. 00099 MachineInstr *MPhi = MBB.remove(MBB.begin()); 00100 00101 assert(MRegisterInfo::isVirtualRegister(MPhi->getOperand(0).getReg()) && 00102 "PHI node doesn't write virt reg?"); 00103 00104 unsigned DestReg = MPhi->getOperand(0).getReg(); 00105 00106 // Create a new register for the incoming PHI arguments 00107 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg); 00108 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC); 00109 00110 // Insert a register to register copy in the top of the current block (but 00111 // after any remaining phi nodes) which copies the new incoming register 00112 // into the phi node destination. 00113 // 00114 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC); 00115 00116 // Update live variable information if there is any... 00117 if (LV) { 00118 MachineInstr *PHICopy = prior(AfterPHIsIt); 00119 00120 // Add information to LiveVariables to know that the incoming value is 00121 // killed. Note that because the value is defined in several places (once 00122 // each for each incoming block), the "def" block and instruction fields 00123 // for the VarInfo is not filled in. 00124 // 00125 LV->addVirtualRegisterKilled(IncomingReg, PHICopy); 00126 00127 // Since we are going to be deleting the PHI node, if it is the last use 00128 // of any registers, or if the value itself is dead, we need to move this 00129 // information over to the new copy we just inserted. 00130 // 00131 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator> 00132 RKs = LV->killed_range(MPhi); 00133 std::vector<std::pair<MachineInstr*, unsigned> > Range; 00134 if (RKs.first != RKs.second) // Delete the range. 00135 LV->removeVirtualRegistersKilled(RKs.first, RKs.second); 00136 00137 RKs = LV->dead_range(MPhi); 00138 if (RKs.first != RKs.second) { 00139 // Works as above... 00140 Range.assign(RKs.first, RKs.second); 00141 LV->removeVirtualRegistersDead(RKs.first, RKs.second); 00142 for (unsigned i = 0, e = Range.size(); i != e; ++i) 00143 LV->addVirtualRegisterDead(Range[i].second, PHICopy); 00144 } 00145 } 00146 00147 // Adjust the VRegPHIUseCount map to account for the removal of this PHI 00148 // node. 00149 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2) 00150 VRegPHIUseCount[MPhi->getOperand(i).getReg()] -= BBIsSuccOfPreds; 00151 00152 // Now loop over all of the incoming arguments, changing them to copy into 00153 // the IncomingReg register in the corresponding predecessor basic block. 00154 // 00155 for (int i = MPhi->getNumOperands() - 1; i >= 2; i-=2) { 00156 MachineOperand &opVal = MPhi->getOperand(i-1); 00157 00158 // Get the MachineBasicBlock equivalent of the BasicBlock that is the 00159 // source path the PHI. 00160 MachineBasicBlock &opBlock = *MPhi->getOperand(i).getMachineBasicBlock(); 00161 00162 MachineBasicBlock::iterator I = opBlock.getFirstTerminator(); 00163 00164 // Check to make sure we haven't already emitted the copy for this block. 00165 // This can happen because PHI nodes may have multiple entries for the 00166 // same basic block. It doesn't matter which entry we use though, because 00167 // all incoming values are guaranteed to be the same for a particular bb. 00168 // 00169 // If we emitted a copy for this basic block already, it will be right 00170 // where we want to insert one now. Just check for a definition of the 00171 // register we are interested in! 00172 // 00173 bool HaveNotEmitted = true; 00174 00175 if (I != opBlock.begin()) { 00176 MachineBasicBlock::iterator PrevInst = prior(I); 00177 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) { 00178 MachineOperand &MO = PrevInst->getOperand(i); 00179 if (MO.isRegister() && MO.getReg() == IncomingReg) 00180 if (MO.isDef()) { 00181 HaveNotEmitted = false; 00182 break; 00183 } 00184 } 00185 } 00186 00187 if (HaveNotEmitted) { // If the copy has not already been emitted, do it. 00188 assert(MRegisterInfo::isVirtualRegister(opVal.getReg()) && 00189 "Machine PHI Operands must all be virtual registers!"); 00190 unsigned SrcReg = opVal.getReg(); 00191 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC); 00192 00193 // Now update live variable information if we have it. 00194 if (LV) { 00195 // We want to be able to insert a kill of the register if this PHI 00196 // (aka, the copy we just inserted) is the last use of the source 00197 // value. Live variable analysis conservatively handles this by 00198 // saying that the value is live until the end of the block the PHI 00199 // entry lives in. If the value really is dead at the PHI copy, there 00200 // will be no successor blocks which have the value live-in. 00201 // 00202 // Check to see if the copy is the last use, and if so, update the 00203 // live variables information so that it knows the copy source 00204 // instruction kills the incoming value. 00205 // 00206 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg); 00207 00208 // Loop over all of the successors of the basic block, checking to see 00209 // if the value is either live in the block, or if it is killed in the 00210 // block. Also check to see if this register is in use by another PHI 00211 // node which has not yet been eliminated. If so, it will be killed 00212 // at an appropriate point later. 00213 // 00214 bool ValueIsLive = false; 00215 for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(), 00216 E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) { 00217 MachineBasicBlock *SuccMBB = *SI; 00218 00219 // Is it alive in this successor? 00220 unsigned SuccIdx = SuccMBB->getNumber(); 00221 if (SuccIdx < InRegVI.AliveBlocks.size() && 00222 InRegVI.AliveBlocks[SuccIdx]) { 00223 ValueIsLive = true; 00224 break; 00225 } 00226 00227 // Is it killed in this successor? 00228 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i) 00229 if (InRegVI.Kills[i]->getParent() == SuccMBB) { 00230 ValueIsLive = true; 00231 break; 00232 } 00233 00234 // Is it used by any PHI instructions in this block? 00235 if (!ValueIsLive) 00236 ValueIsLive = VRegPHIUseCount[SrcReg] != 0; 00237 } 00238 00239 // Okay, if we now know that the value is not live out of the block, 00240 // we can add a kill marker to the copy we inserted saying that it 00241 // kills the incoming value! 00242 // 00243 if (!ValueIsLive) { 00244 MachineBasicBlock::iterator Prev = prior(I); 00245 LV->addVirtualRegisterKilled(SrcReg, Prev); 00246 00247 // This vreg no longer lives all of the way through opBlock. 00248 unsigned opBlockNum = opBlock.getNumber(); 00249 if (opBlockNum < InRegVI.AliveBlocks.size()) 00250 InRegVI.AliveBlocks[opBlockNum] = false; 00251 } 00252 } 00253 } 00254 } 00255 00256 // Really delete the PHI instruction now! 00257 delete MPhi; 00258 } 00259 return true; 00260 }