1 //===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===//
3 // The LLVM Compiler Infrastructure
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.
8 //===----------------------------------------------------------------------===//
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.
14 //===----------------------------------------------------------------------===//
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"
27 struct PNE : public MachineFunctionPass {
28 bool runOnMachineFunction(MachineFunction &Fn) {
31 // 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);
36 //std::cerr << "AFTER PHI NODE ELIM:\n";
41 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
42 AU.addPreserved<LiveVariables>();
43 MachineFunctionPass::getAnalysisUsage(AU);
47 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
48 /// in predecessor basic blocks.
50 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
53 RegisterPass<PNE> X("phi-node-elimination",
54 "Eliminate PHI nodes for register allocation");
58 const PassInfo *llvm::PHIEliminationID = X.getPassInfo();
60 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
61 /// predecessor basic blocks.
63 bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
64 if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI)
65 return false; // Quick exit for normal case...
67 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
68 const TargetInstrInfo &MII = MF.getTarget().getInstrInfo();
69 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
71 // VRegPHIUseCount - Keep track of the number of times each virtual register
72 // is used by PHI nodes in this block.
73 std::map<unsigned, unsigned> VRegPHIUseCount;
75 // Get an iterator to the first instruction after the last PHI node (this may
76 // allso be the end of the basic block). While we are scanning the PHIs,
77 // populate the VRegPHIUseCount map.
78 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
79 while (AfterPHIsIt != MBB.end() &&
80 AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI) {
81 MachineInstr *PHI = AfterPHIsIt;
82 for (unsigned i = 1, e = PHI->getNumOperands(); i < e; i += 2)
83 VRegPHIUseCount[PHI->getOperand(i).getReg()]++;
84 ++AfterPHIsIt; // Skip over all of the PHI nodes...
87 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) {
88 // Unlink the PHI node from the basic block... but don't delete the PHI yet
89 MachineInstr *MI = MBB.remove(MBB.begin());
91 assert(MRegisterInfo::isVirtualRegister(MI->getOperand(0).getReg()) &&
92 "PHI node doesn't write virt reg?");
94 unsigned DestReg = MI->getOperand(0).getReg();
96 // Create a new register for the incoming PHI arguments
97 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
98 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
100 // Insert a register to register copy in the top of the current block (but
101 // after any remaining phi nodes) which copies the new incoming register
102 // into the phi node destination.
104 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
106 // Update live variable information if there is any...
108 MachineInstr *PHICopy = prior(AfterPHIsIt);
110 // Add information to LiveVariables to know that the incoming value is
111 // killed. Note that because the value is defined in several places (once
112 // each for each incoming block), the "def" block and instruction fields
113 // for the VarInfo is not filled in.
115 LV->addVirtualRegisterKilled(IncomingReg, &MBB, PHICopy);
117 // Since we are going to be deleting the PHI node, if it is the last use
118 // of any registers, or if the value itself is dead, we need to move this
119 // information over to the new copy we just inserted...
121 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
122 RKs = LV->killed_range(MI);
123 std::vector<std::pair<MachineInstr*, unsigned> > Range;
124 if (RKs.first != RKs.second) {
125 // Copy the range into a vector...
126 Range.assign(RKs.first, RKs.second);
128 // Delete the range...
129 LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
131 // Add all of the kills back, which will update the appropriate info...
132 for (unsigned i = 0, e = Range.size(); i != e; ++i)
133 LV->addVirtualRegisterKilled(Range[i].second, &MBB, PHICopy);
136 RKs = LV->dead_range(MI);
137 if (RKs.first != RKs.second) {
139 Range.assign(RKs.first, RKs.second);
140 LV->removeVirtualRegistersDead(RKs.first, RKs.second);
141 for (unsigned i = 0, e = Range.size(); i != e; ++i)
142 LV->addVirtualRegisterDead(Range[i].second, &MBB, PHICopy);
146 // Adjust the VRegPHIUseCount map to account for the removal of this PHI
148 for (unsigned i = 1; i != MI->getNumOperands(); i += 2)
149 VRegPHIUseCount[MI->getOperand(i).getReg()]--;
151 // Now loop over all of the incoming arguments, changing them to copy into
152 // the IncomingReg register in the corresponding predecessor basic block.
154 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
155 MachineOperand &opVal = MI->getOperand(i-1);
157 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
158 // source path the PHI.
159 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
161 MachineBasicBlock::iterator I = opBlock.getFirstTerminator();
163 // Check to make sure we haven't already emitted the copy for this block.
164 // This can happen because PHI nodes may have multiple entries for the
165 // same basic block. It doesn't matter which entry we use though, because
166 // all incoming values are guaranteed to be the same for a particular bb.
168 // If we emitted a copy for this basic block already, it will be right
169 // where we want to insert one now. Just check for a definition of the
170 // register we are interested in!
172 bool HaveNotEmitted = true;
174 if (I != opBlock.begin()) {
175 MachineBasicBlock::iterator PrevInst = prior(I);
176 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
177 MachineOperand &MO = PrevInst->getOperand(i);
178 if (MO.isRegister() && MO.getReg() == IncomingReg)
180 HaveNotEmitted = false;
186 if (HaveNotEmitted) { // If the copy has not already been emitted, do it.
187 assert(MRegisterInfo::isVirtualRegister(opVal.getReg()) &&
188 "Machine PHI Operands must all be virtual registers!");
189 unsigned SrcReg = opVal.getReg();
190 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);
192 // Now update live variable information if we have it.
194 // We want to be able to insert a kill of the register if this PHI
195 // (aka, the copy we just inserted) is the last use of the source
196 // value. Live variable analysis conservatively handles this by
197 // saying that the value is live until the end of the block the PHI
198 // entry lives in. If the value really is dead at the PHI copy, there
199 // will be no successor blocks which have the value live-in.
201 // Check to see if the copy is the last use, and if so, update the
202 // live variables information so that it knows the copy source
203 // instruction kills the incoming value.
205 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
207 // Loop over all of the successors of the basic block, checking to see
208 // if the value is either live in the block, or if it is killed in the
209 // block. Also check to see if this register is in use by another PHI
210 // node which has not yet been eliminated. If so, it will be killed
211 // at an appropriate point later.
213 bool ValueIsLive = false;
214 for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(),
215 E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) {
216 MachineBasicBlock *MBB = *SI;
218 // Is it alive in this successor?
219 unsigned SuccIdx = LV->getMachineBasicBlockIndex(MBB);
220 if (SuccIdx < InRegVI.AliveBlocks.size() &&
221 InRegVI.AliveBlocks[SuccIdx]) {
226 // Is it killed in this successor?
227 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
228 if (InRegVI.Kills[i].first == MBB) {
233 // Is it used by any PHI instructions in this block?
235 std::map<unsigned,unsigned>::iterator I =
236 VRegPHIUseCount.find(SrcReg);
237 ValueIsLive = I != VRegPHIUseCount.end() && I->second;
241 // Okay, if we now know that the value is not live out of the block,
242 // we can add a kill marker to the copy we inserted saying that it
243 // kills the incoming value!
246 MachineBasicBlock::iterator Prev = prior(I);
247 LV->addVirtualRegisterKilled(SrcReg, &opBlock, Prev);
253 // really delete the PHI instruction now!