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 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) {
72 // Unlink the PHI node from the basic block... but don't delete the PHI yet
73 MachineInstr *MI = MBB.remove(MBB.begin());
75 assert(MRegisterInfo::isVirtualRegister(MI->getOperand(0).getReg()) &&
76 "PHI node doesn't write virt reg?");
78 unsigned DestReg = MI->getOperand(0).getReg();
80 // Create a new register for the incoming PHI arguments
81 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
82 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
84 // Insert a register to register copy in the top of the current block (but
85 // after any remaining phi nodes) which copies the new incoming register
86 // into the phi node destination.
88 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
89 while (AfterPHIsIt != MBB.end() &&
90 AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI)
91 ++AfterPHIsIt; // Skip over all of the PHI nodes...
92 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
94 // Update live variable information if there is any...
96 MachineInstr *PHICopy = --AfterPHIsIt;
98 // Add information to LiveVariables to know that the incoming value is
99 // killed. Note that because the value is defined in several places (once
100 // each for each incoming block), the "def" block and instruction fields
101 // for the VarInfo is not filled in.
103 LV->addVirtualRegisterKilled(IncomingReg, &MBB, PHICopy);
105 // Since we are going to be deleting the PHI node, if it is the last use
106 // of any registers, or if the value itself is dead, we need to move this
107 // information over to the new copy we just inserted...
109 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
110 RKs = LV->killed_range(MI);
111 std::vector<std::pair<MachineInstr*, unsigned> > Range;
112 if (RKs.first != RKs.second) {
113 // Copy the range into a vector...
114 Range.assign(RKs.first, RKs.second);
116 // Delete the range...
117 LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
119 // Add all of the kills back, which will update the appropriate info...
120 for (unsigned i = 0, e = Range.size(); i != e; ++i)
121 LV->addVirtualRegisterKilled(Range[i].second, &MBB, PHICopy);
124 RKs = LV->dead_range(MI);
125 if (RKs.first != RKs.second) {
127 Range.assign(RKs.first, RKs.second);
128 LV->removeVirtualRegistersDead(RKs.first, RKs.second);
129 for (unsigned i = 0, e = Range.size(); i != e; ++i)
130 LV->addVirtualRegisterDead(Range[i].second, &MBB, PHICopy);
134 // Now loop over all of the incoming arguments, changing them to copy into
135 // the IncomingReg register in the corresponding predecessor basic block.
137 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
138 MachineOperand &opVal = MI->getOperand(i-1);
140 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
141 // source path the PHI.
142 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
144 MachineBasicBlock::iterator I = opBlock.getFirstTerminator();
146 // Check to make sure we haven't already emitted the copy for this block.
147 // This can happen because PHI nodes may have multiple entries for the
148 // same basic block. It doesn't matter which entry we use though, because
149 // all incoming values are guaranteed to be the same for a particular bb.
151 // If we emitted a copy for this basic block already, it will be right
152 // where we want to insert one now. Just check for a definition of the
153 // register we are interested in!
155 bool HaveNotEmitted = true;
157 if (I != opBlock.begin()) {
158 MachineBasicBlock::iterator PrevInst = prior(I);
159 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
160 MachineOperand &MO = PrevInst->getOperand(i);
161 if (MO.isRegister() && MO.getReg() == IncomingReg)
163 HaveNotEmitted = false;
169 if (HaveNotEmitted) { // If the copy has not already been emitted, do it.
170 assert(MRegisterInfo::isVirtualRegister(opVal.getReg()) &&
171 "Machine PHI Operands must all be virtual registers!");
172 unsigned SrcReg = opVal.getReg();
173 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);
175 // Now update live variable information if we have it.
177 // We want to be able to insert a kill of the register if this PHI
178 // (aka, the copy we just inserted) is the last use of the source
179 // value. Live variable analysis conservatively handles this by
180 // saying that the value is live until the end of the block the PHI
181 // entry lives in. If the value really is dead at the PHI copy, there
182 // will be no successor blocks which have the value live-in.
184 // Check to see if the copy is the last use, and if so, update the
185 // live variables information so that it knows the copy source
186 // instruction kills the incoming value.
188 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
190 // Loop over all of the successors of the basic block, checking to see
191 // if the value is either live in the block, or if it is killed in the
192 // block. Also check to see if this register is in use by another PHI
193 // node which has not yet been eliminated. If so, it will be killed
194 // at an appropriate point later.
196 bool ValueIsLive = false;
197 for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(),
198 E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) {
199 MachineBasicBlock *MBB = *SI;
201 // Is it alive in this successor?
202 unsigned SuccIdx = LV->getMachineBasicBlockIndex(MBB);
203 if (SuccIdx < InRegVI.AliveBlocks.size() &&
204 InRegVI.AliveBlocks[SuccIdx]) {
209 // Is it killed in this successor?
210 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
211 if (InRegVI.Kills[i].first == MBB) {
216 // Is it used by any PHI instructions in this block?
217 if (ValueIsLive) break;
219 // Loop over all of the PHIs in this successor, checking to see if
220 // the register is being used...
221 for (MachineBasicBlock::iterator BBI = MBB->begin(), E=MBB->end();
222 BBI != E && BBI->getOpcode() == TargetInstrInfo::PHI;
224 for (unsigned i = 1, e = BBI->getNumOperands(); i < e; i += 2)
225 if (BBI->getOperand(i).getReg() == SrcReg) {
231 // Okay, if we now know that the value is not live out of the block,
232 // we can add a kill marker to the copy we inserted saying that it
233 // kills the incoming value!
236 MachineBasicBlock::iterator Prev = prior(I);
237 LV->addVirtualRegisterKilled(SrcReg, &opBlock, Prev);
243 // really delete the PHI instruction now!