1 //===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===//
3 // This pass eliminates machine instruction PHI nodes by inserting copy
4 // instructions. This destroys SSA information, but is the desired input for
5 // some register allocators.
7 //===----------------------------------------------------------------------===//
9 #include "llvm/CodeGen/MachineFunctionPass.h"
10 #include "llvm/CodeGen/MachineInstr.h"
11 #include "llvm/CodeGen/SSARegMap.h"
12 #include "llvm/CodeGen/LiveVariables.h"
13 #include "llvm/Target/TargetInstrInfo.h"
14 #include "llvm/Target/TargetMachine.h"
15 #include "llvm/Support/CFG.h"
18 struct PNE : public MachineFunctionPass {
19 bool runOnMachineFunction(MachineFunction &Fn) {
22 // Eliminate PHI instructions by inserting copies into predecessor blocks.
24 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
25 Changed |= EliminatePHINodes(Fn, *I);
27 //std::cerr << "AFTER PHI NODE ELIM:\n";
32 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
33 AU.addPreserved<LiveVariables>();
34 MachineFunctionPass::getAnalysisUsage(AU);
38 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
39 /// in predecessor basic blocks.
41 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
44 RegisterPass<PNE> X("phi-node-elimination",
45 "Eliminate PHI nodes for register allocation");
48 const PassInfo *PHIEliminationID = X.getPassInfo();
50 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
51 /// predecessor basic blocks.
53 bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
54 if (MBB.empty() || MBB.front()->getOpcode() != TargetInstrInfo::PHI)
55 return false; // Quick exit for normal case...
57 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
58 const TargetInstrInfo &MII = MF.getTarget().getInstrInfo();
59 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
61 while (MBB.front()->getOpcode() == TargetInstrInfo::PHI) {
62 MachineInstr *MI = MBB.front();
63 // Unlink the PHI node from the basic block... but don't delete the PHI yet
64 MBB.erase(MBB.begin());
66 assert(MI->getOperand(0).isVirtualRegister() &&
67 "PHI node doesn't write virt reg?");
69 unsigned DestReg = MI->getOperand(0).getAllocatedRegNum();
71 // Create a new register for the incoming PHI arguments
72 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
73 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
75 // Insert a register to register copy in the top of the current block (but
76 // after any remaining phi nodes) which copies the new incoming register
77 // into the phi node destination.
79 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
80 while (AfterPHIsIt != MBB.end() &&
81 (*AfterPHIsIt)->getOpcode() == TargetInstrInfo::PHI)
82 ++AfterPHIsIt; // Skip over all of the PHI nodes...
83 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
85 // Update live variable information if there is any...
87 MachineInstr *PHICopy = *(AfterPHIsIt-1);
89 // Add information to LiveVariables to know that the incoming value is
90 // killed. Note that because the value is defined in several places (once
91 // each for each incoming block), the "def" block and instruction fields
92 // for the VarInfo is not filled in.
94 LV->addVirtualRegisterKilled(IncomingReg, &MBB, PHICopy);
96 // Since we are going to be deleting the PHI node, if it is the last use
97 // of any registers, or if the value itself is dead, we need to move this
98 // information over to the new copy we just inserted...
100 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
101 RKs = LV->killed_range(MI);
102 std::vector<std::pair<MachineInstr*, unsigned> > Range;
103 if (RKs.first != RKs.second) {
104 // Copy the range into a vector...
105 Range.assign(RKs.first, RKs.second);
107 // Delete the range...
108 LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
110 // Add all of the kills back, which will update the appropriate info...
111 for (unsigned i = 0, e = Range.size(); i != e; ++i)
112 LV->addVirtualRegisterKilled(Range[i].second, &MBB, PHICopy);
115 RKs = LV->dead_range(MI);
116 if (RKs.first != RKs.second) {
118 Range.assign(RKs.first, RKs.second);
119 LV->removeVirtualRegistersDead(RKs.first, RKs.second);
120 for (unsigned i = 0, e = Range.size(); i != e; ++i)
121 LV->addVirtualRegisterDead(Range[i].second, &MBB, PHICopy);
125 // Now loop over all of the incoming arguments, changing them to copy into
126 // the IncomingReg register in the corresponding predecessor basic block.
128 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
129 MachineOperand &opVal = MI->getOperand(i-1);
131 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
132 // source path the PHI.
133 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
135 // Figure out where to insert the copy, which is at the end of the
136 // predecessor basic block, but before any terminator/branch
138 MachineBasicBlock::iterator I = opBlock.end();
139 if (I != opBlock.begin()) { // Handle empty blocks
141 // must backtrack over ALL the branches in the previous block
142 while (MII.isTerminatorInstr((*I)->getOpcode()) &&
143 I != opBlock.begin())
146 // move back to the first branch instruction so new instructions
147 // are inserted right in front of it and not in front of a non-branch
148 if (!MII.isTerminatorInstr((*I)->getOpcode()))
152 // Check to make sure we haven't already emitted the copy for this block.
153 // This can happen because PHI nodes may have multiple entries for the
154 // same basic block. It doesn't matter which entry we use though, because
155 // all incoming values are guaranteed to be the same for a particular bb.
157 // If we emitted a copy for this basic block already, it will be right
158 // where we want to insert one now. Just check for a definition of the
159 // register we are interested in!
161 bool HaveNotEmitted = true;
163 if (I != opBlock.begin()) {
164 MachineInstr *PrevInst = *(I-1);
165 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
166 MachineOperand &MO = PrevInst->getOperand(i);
167 if (MO.isVirtualRegister() && MO.getReg() == IncomingReg)
168 if (MO.opIsDefOnly() || MO.opIsDefAndUse()) {
169 HaveNotEmitted = false;
175 if (HaveNotEmitted) { // If the copy has not already been emitted, do it.
176 assert(opVal.isVirtualRegister() &&
177 "Machine PHI Operands must all be virtual registers!");
178 unsigned SrcReg = opVal.getReg();
179 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);
181 // Now update live variable information if we have it.
183 // We want to be able to insert a kill of the register if this PHI
184 // (aka, the copy we just inserted) is the last use of the source
185 // value. Live variable analysis conservatively handles this by
186 // saying that the value is live until the end of the block the PHI
187 // entry lives in. If the value really is dead at the PHI copy, there
188 // will be no successor blocks which have the value live-in.
190 // Check to see if the copy is the last use, and if so, update the
191 // live variables information so that it knows the copy source
192 // instruction kills the incoming value.
194 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
196 // Loop over all of the successors of the basic block, checking to see
197 // if the value is either live in the block, or if it is killed in the
198 // block. Also check to see if this register is in use by another PHI
199 // node which has not yet been eliminated. If so, it will be killed
200 // at an appropriate point later.
202 bool ValueIsLive = false;
203 BasicBlock *BB = opBlock.getBasicBlock();
204 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB);
205 SI != E && !ValueIsLive; ++SI) {
206 const std::pair<MachineBasicBlock*, unsigned> &
207 SuccInfo = LV->getBasicBlockInfo(*SI);
209 // Is it alive in this successor?
210 unsigned SuccIdx = SuccInfo.second;
211 if (SuccIdx < InRegVI.AliveBlocks.size() &&
212 InRegVI.AliveBlocks[SuccIdx]) {
217 // Is it killed in this successor?
218 MachineBasicBlock *MBB = SuccInfo.first;
219 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
220 if (InRegVI.Kills[i].first == MBB) {
225 // Is it used by any PHI instructions in this block?
226 if (ValueIsLive) break;
228 // Loop over all of the PHIs in this successor, checking to see if
229 // the register is being used...
230 for (MachineBasicBlock::iterator BBI = MBB->begin(), E=MBB->end();
231 BBI != E && (*BBI)->getOpcode() == TargetInstrInfo::PHI;
233 for (unsigned i = 1, e = (*BBI)->getNumOperands(); i < e; i += 2)
234 if ((*BBI)->getOperand(i).getReg() == SrcReg) {
240 // Okay, if we now know that the value is not live out of the block,
241 // we can add a kill marker to the copy we inserted saying that it
242 // kills the incoming value!
245 LV->addVirtualRegisterKilled(SrcReg, &opBlock, *(I-1));
250 // really delete the PHI instruction now!