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"
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 *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 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) {
72 // Unlink the PHI node from the basic block... but don't delete the PHI yet
73 MachineBasicBlock::iterator begin = MBB.begin();
74 MachineInstr *MI = MBB.remove(begin);
76 assert(MRegisterInfo::isVirtualRegister(MI->getOperand(0).getReg()) &&
77 "PHI node doesn't write virt reg?");
79 unsigned DestReg = MI->getOperand(0).getAllocatedRegNum();
81 // Create a new register for the incoming PHI arguments
82 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
83 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
85 // Insert a register to register copy in the top of the current block (but
86 // after any remaining phi nodes) which copies the new incoming register
87 // into the phi node destination.
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 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
95 // Update live variable information if there is any...
97 MachineInstr *PHICopy = --AfterPHIsIt;
99 // Add information to LiveVariables to know that the incoming value is
100 // killed. Note that because the value is defined in several places (once
101 // each for each incoming block), the "def" block and instruction fields
102 // for the VarInfo is not filled in.
104 LV->addVirtualRegisterKilled(IncomingReg, &MBB, PHICopy);
106 // Since we are going to be deleting the PHI node, if it is the last use
107 // of any registers, or if the value itself is dead, we need to move this
108 // information over to the new copy we just inserted...
110 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
111 RKs = LV->killed_range(MI);
112 std::vector<std::pair<MachineInstr*, unsigned> > Range;
113 if (RKs.first != RKs.second) {
114 // Copy the range into a vector...
115 Range.assign(RKs.first, RKs.second);
117 // Delete the range...
118 LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
120 // Add all of the kills back, which will update the appropriate info...
121 for (unsigned i = 0, e = Range.size(); i != e; ++i)
122 LV->addVirtualRegisterKilled(Range[i].second, &MBB, PHICopy);
125 RKs = LV->dead_range(MI);
126 if (RKs.first != RKs.second) {
128 Range.assign(RKs.first, RKs.second);
129 LV->removeVirtualRegistersDead(RKs.first, RKs.second);
130 for (unsigned i = 0, e = Range.size(); i != e; ++i)
131 LV->addVirtualRegisterDead(Range[i].second, &MBB, PHICopy);
135 // Now loop over all of the incoming arguments, changing them to copy into
136 // the IncomingReg register in the corresponding predecessor basic block.
138 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
139 MachineOperand &opVal = MI->getOperand(i-1);
141 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
142 // source path the PHI.
143 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
145 // Figure out where to insert the copy, which is at the end of the
146 // predecessor basic block, but before any terminator/branch
148 MachineBasicBlock::iterator I = opBlock.end();
149 if (I != opBlock.begin()) { // Handle empty blocks
151 // must backtrack over ALL the branches in the previous block
152 while (MII.isTerminatorInstr(I->getOpcode()) &&
153 I != opBlock.begin())
156 // move back to the first branch instruction so new instructions
157 // are inserted right in front of it and not in front of a non-branch
158 if (!MII.isTerminatorInstr(I->getOpcode()))
162 // Check to make sure we haven't already emitted the copy for this block.
163 // This can happen because PHI nodes may have multiple entries for the
164 // same basic block. It doesn't matter which entry we use though, because
165 // all incoming values are guaranteed to be the same for a particular bb.
167 // If we emitted a copy for this basic block already, it will be right
168 // where we want to insert one now. Just check for a definition of the
169 // register we are interested in!
171 bool HaveNotEmitted = true;
173 if (I != opBlock.begin()) {
174 MachineBasicBlock::iterator PrevInst = 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 const BasicBlock *BB = opBlock.getBasicBlock();
215 for (succ_const_iterator SI = succ_begin(BB), E = succ_end(BB);
216 SI != E && !ValueIsLive; ++SI) {
217 const std::pair<MachineBasicBlock*, unsigned> &
218 SuccInfo = LV->getBasicBlockInfo(*SI);
220 // Is it alive in this successor?
221 unsigned SuccIdx = SuccInfo.second;
222 if (SuccIdx < InRegVI.AliveBlocks.size() &&
223 InRegVI.AliveBlocks[SuccIdx]) {
228 // Is it killed in this successor?
229 MachineBasicBlock *MBB = SuccInfo.first;
230 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
231 if (InRegVI.Kills[i].first == MBB) {
236 // Is it used by any PHI instructions in this block?
237 if (ValueIsLive) break;
239 // Loop over all of the PHIs in this successor, checking to see if
240 // the register is being used...
241 for (MachineBasicBlock::iterator BBI = MBB->begin(), E=MBB->end();
242 BBI != E && BBI->getOpcode() == TargetInstrInfo::PHI;
244 for (unsigned i = 1, e = BBI->getNumOperands(); i < e; i += 2)
245 if (BBI->getOperand(i).getReg() == SrcReg) {
251 // Okay, if we now know that the value is not live out of the block,
252 // we can add a kill marker to the copy we inserted saying that it
253 // kills the incoming value!
256 MachineBasicBlock::iterator Prev = I;
258 LV->addVirtualRegisterKilled(SrcReg, &opBlock, Prev);
264 // really delete the PHI instruction now!
270 } // End llvm namespace