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/MachineFunctionPass.h"
17 #include "llvm/CodeGen/MachineInstr.h"
18 #include "llvm/CodeGen/SSARegMap.h"
19 #include "llvm/CodeGen/LiveVariables.h"
20 #include "llvm/Target/TargetInstrInfo.h"
21 #include "llvm/Target/TargetMachine.h"
22 #include "llvm/Support/CFG.h"
23 #include "Support/STLExtras.h"
28 struct PNE : public MachineFunctionPass {
29 bool runOnMachineFunction(MachineFunction &Fn) {
32 // 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);
37 //std::cerr << "AFTER PHI NODE ELIM:\n";
42 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
43 AU.addPreserved<LiveVariables>();
44 MachineFunctionPass::getAnalysisUsage(AU);
48 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
49 /// in predecessor basic blocks.
51 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
54 RegisterPass<PNE> X("phi-node-elimination",
55 "Eliminate PHI nodes for register allocation");
59 const PassInfo *PHIEliminationID = X.getPassInfo();
61 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
62 /// predecessor basic blocks.
64 bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
65 if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI)
66 return false; // Quick exit for normal case...
68 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
69 const TargetInstrInfo &MII = MF.getTarget().getInstrInfo();
70 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
72 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) {
73 // Unlink the PHI node from the basic block... but don't delete the PHI yet
74 MachineBasicBlock::iterator begin = MBB.begin();
75 MachineInstr *MI = MBB.remove(begin);
77 assert(MRegisterInfo::isVirtualRegister(MI->getOperand(0).getReg()) &&
78 "PHI node doesn't write virt reg?");
80 unsigned DestReg = MI->getOperand(0).getReg();
82 // Create a new register for the incoming PHI arguments
83 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
84 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
86 // Insert a register to register copy in the top of the current block (but
87 // after any remaining phi nodes) which copies the new incoming register
88 // into the phi node destination.
90 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
91 while (AfterPHIsIt != MBB.end() &&
92 AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI)
93 ++AfterPHIsIt; // Skip over all of the PHI nodes...
94 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
96 // Update live variable information if there is any...
98 MachineInstr *PHICopy = --AfterPHIsIt;
100 // Add information to LiveVariables to know that the incoming value is
101 // killed. Note that because the value is defined in several places (once
102 // each for each incoming block), the "def" block and instruction fields
103 // for the VarInfo is not filled in.
105 LV->addVirtualRegisterKilled(IncomingReg, &MBB, PHICopy);
107 // Since we are going to be deleting the PHI node, if it is the last use
108 // of any registers, or if the value itself is dead, we need to move this
109 // information over to the new copy we just inserted...
111 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
112 RKs = LV->killed_range(MI);
113 std::vector<std::pair<MachineInstr*, unsigned> > Range;
114 if (RKs.first != RKs.second) {
115 // Copy the range into a vector...
116 Range.assign(RKs.first, RKs.second);
118 // Delete the range...
119 LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
121 // Add all of the kills back, which will update the appropriate info...
122 for (unsigned i = 0, e = Range.size(); i != e; ++i)
123 LV->addVirtualRegisterKilled(Range[i].second, &MBB, PHICopy);
126 RKs = LV->dead_range(MI);
127 if (RKs.first != RKs.second) {
129 Range.assign(RKs.first, RKs.second);
130 LV->removeVirtualRegistersDead(RKs.first, RKs.second);
131 for (unsigned i = 0, e = Range.size(); i != e; ++i)
132 LV->addVirtualRegisterDead(Range[i].second, &MBB, PHICopy);
136 // Now loop over all of the incoming arguments, changing them to copy into
137 // the IncomingReg register in the corresponding predecessor basic block.
139 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
140 MachineOperand &opVal = MI->getOperand(i-1);
142 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
143 // source path the PHI.
144 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
146 MachineBasicBlock::iterator I = opBlock.getFirstTerminator();
148 // Check to make sure we haven't already emitted the copy for this block.
149 // This can happen because PHI nodes may have multiple entries for the
150 // same basic block. It doesn't matter which entry we use though, because
151 // all incoming values are guaranteed to be the same for a particular bb.
153 // If we emitted a copy for this basic block already, it will be right
154 // where we want to insert one now. Just check for a definition of the
155 // register we are interested in!
157 bool HaveNotEmitted = true;
159 if (I != opBlock.begin()) {
160 MachineBasicBlock::iterator PrevInst = prior(I);
161 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
162 MachineOperand &MO = PrevInst->getOperand(i);
163 if (MO.isRegister() && MO.getReg() == IncomingReg)
165 HaveNotEmitted = false;
171 if (HaveNotEmitted) { // If the copy has not already been emitted, do it.
172 assert(MRegisterInfo::isVirtualRegister(opVal.getReg()) &&
173 "Machine PHI Operands must all be virtual registers!");
174 unsigned SrcReg = opVal.getReg();
175 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);
177 // Now update live variable information if we have it.
179 // We want to be able to insert a kill of the register if this PHI
180 // (aka, the copy we just inserted) is the last use of the source
181 // value. Live variable analysis conservatively handles this by
182 // saying that the value is live until the end of the block the PHI
183 // entry lives in. If the value really is dead at the PHI copy, there
184 // will be no successor blocks which have the value live-in.
186 // Check to see if the copy is the last use, and if so, update the
187 // live variables information so that it knows the copy source
188 // instruction kills the incoming value.
190 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
192 // Loop over all of the successors of the basic block, checking to see
193 // if the value is either live in the block, or if it is killed in the
194 // block. Also check to see if this register is in use by another PHI
195 // node which has not yet been eliminated. If so, it will be killed
196 // at an appropriate point later.
198 bool ValueIsLive = false;
199 const BasicBlock *BB = opBlock.getBasicBlock();
200 for (succ_const_iterator SI = succ_begin(BB), E = succ_end(BB);
201 SI != E && !ValueIsLive; ++SI) {
202 const std::pair<MachineBasicBlock*, unsigned> &
203 SuccInfo = LV->getBasicBlockInfo(*SI);
205 // Is it alive in this successor?
206 unsigned SuccIdx = SuccInfo.second;
207 if (SuccIdx < InRegVI.AliveBlocks.size() &&
208 InRegVI.AliveBlocks[SuccIdx]) {
213 // Is it killed in this successor?
214 MachineBasicBlock *MBB = SuccInfo.first;
215 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
216 if (InRegVI.Kills[i].first == MBB) {
221 // Is it used by any PHI instructions in this block?
222 if (ValueIsLive) break;
224 // Loop over all of the PHIs in this successor, checking to see if
225 // the register is being used...
226 for (MachineBasicBlock::iterator BBI = MBB->begin(), E=MBB->end();
227 BBI != E && BBI->getOpcode() == TargetInstrInfo::PHI;
229 for (unsigned i = 1, e = BBI->getNumOperands(); i < e; i += 2)
230 if (BBI->getOperand(i).getReg() == SrcReg) {
236 // Okay, if we now know that the value is not live out of the block,
237 // we can add a kill marker to the copy we inserted saying that it
238 // kills the incoming value!
241 MachineBasicBlock::iterator Prev = prior(I);
242 LV->addVirtualRegisterKilled(SrcReg, &opBlock, Prev);
248 // really delete the PHI instruction now!
254 } // End llvm namespace