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/DenseMap.h"
24 #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 *llvm::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 // VRegPHIUseCount - Keep track of the number of times each virtual register
73 // is used by PHI nodes in successors of this block.
74 DenseMap<unsigned, VirtReg2IndexFunctor> VRegPHIUseCount;
75 VRegPHIUseCount.grow(MF.getSSARegMap()->getLastVirtReg());
77 unsigned BBIsSuccOfPreds = 0; // Number of times MBB is a succ of preds
78 for (MachineBasicBlock::pred_iterator PI = MBB.pred_begin(),
79 E = MBB.pred_end(); PI != E; ++PI)
80 for (MachineBasicBlock::succ_iterator SI = (*PI)->succ_begin(),
81 E = (*PI)->succ_end(); SI != E; ++SI) {
82 BBIsSuccOfPreds += *SI == &MBB;
83 for (MachineBasicBlock::iterator BBI = (*SI)->begin(); BBI !=(*SI)->end() &&
84 BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI)
85 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
86 VRegPHIUseCount[BBI->getOperand(i).getReg()]++;
89 // Get an iterator to the first instruction after the last PHI node (this may
90 // also be the end of the basic block). While we are scanning the PHIs,
91 // populate the VRegPHIUseCount map.
92 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
93 while (AfterPHIsIt != MBB.end() &&
94 AfterPHIsIt->getOpcode() == TargetInstrInfo::PHI)
95 ++AfterPHIsIt; // Skip over all of the PHI nodes...
97 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) {
98 // Unlink the PHI node from the basic block... but don't delete the PHI yet
99 MachineInstr *MI = MBB.remove(MBB.begin());
101 assert(MRegisterInfo::isVirtualRegister(MI->getOperand(0).getReg()) &&
102 "PHI node doesn't write virt reg?");
104 unsigned DestReg = MI->getOperand(0).getReg();
106 // Create a new register for the incoming PHI arguments
107 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
108 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
110 // Insert a register to register copy in the top of the current block (but
111 // after any remaining phi nodes) which copies the new incoming register
112 // into the phi node destination.
114 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
116 // Update live variable information if there is any...
118 MachineInstr *PHICopy = prior(AfterPHIsIt);
120 // Add information to LiveVariables to know that the incoming value is
121 // killed. Note that because the value is defined in several places (once
122 // each for each incoming block), the "def" block and instruction fields
123 // for the VarInfo is not filled in.
125 LV->addVirtualRegisterKilled(IncomingReg, PHICopy);
127 // Since we are going to be deleting the PHI node, if it is the last use
128 // of any registers, or if the value itself is dead, we need to move this
129 // information over to the new copy we just inserted...
131 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
132 RKs = LV->killed_range(MI);
133 std::vector<std::pair<MachineInstr*, unsigned> > Range;
134 if (RKs.first != RKs.second) {
135 // Copy the range into a vector...
136 Range.assign(RKs.first, RKs.second);
138 // Delete the range...
139 LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
141 // Add all of the kills back, which will update the appropriate info...
142 for (unsigned i = 0, e = Range.size(); i != e; ++i)
143 LV->addVirtualRegisterKilled(Range[i].second, PHICopy);
146 RKs = LV->dead_range(MI);
147 if (RKs.first != RKs.second) {
149 Range.assign(RKs.first, RKs.second);
150 LV->removeVirtualRegistersDead(RKs.first, RKs.second);
151 for (unsigned i = 0, e = Range.size(); i != e; ++i)
152 LV->addVirtualRegisterDead(Range[i].second, PHICopy);
156 // Adjust the VRegPHIUseCount map to account for the removal of this PHI
158 for (unsigned i = 1; i != MI->getNumOperands(); i += 2)
159 VRegPHIUseCount[MI->getOperand(i).getReg()] -= BBIsSuccOfPreds;
161 // Now loop over all of the incoming arguments, changing them to copy into
162 // the IncomingReg register in the corresponding predecessor basic block.
164 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
165 MachineOperand &opVal = MI->getOperand(i-1);
167 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
168 // source path the PHI.
169 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
171 MachineBasicBlock::iterator I = opBlock.getFirstTerminator();
173 // Check to make sure we haven't already emitted the copy for this block.
174 // This can happen because PHI nodes may have multiple entries for the
175 // same basic block. It doesn't matter which entry we use though, because
176 // all incoming values are guaranteed to be the same for a particular bb.
178 // If we emitted a copy for this basic block already, it will be right
179 // where we want to insert one now. Just check for a definition of the
180 // register we are interested in!
182 bool HaveNotEmitted = true;
184 if (I != opBlock.begin()) {
185 MachineBasicBlock::iterator PrevInst = prior(I);
186 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
187 MachineOperand &MO = PrevInst->getOperand(i);
188 if (MO.isRegister() && MO.getReg() == IncomingReg)
190 HaveNotEmitted = false;
196 if (HaveNotEmitted) { // If the copy has not already been emitted, do it.
197 assert(MRegisterInfo::isVirtualRegister(opVal.getReg()) &&
198 "Machine PHI Operands must all be virtual registers!");
199 unsigned SrcReg = opVal.getReg();
200 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);
202 // Now update live variable information if we have it.
204 // We want to be able to insert a kill of the register if this PHI
205 // (aka, the copy we just inserted) is the last use of the source
206 // value. Live variable analysis conservatively handles this by
207 // saying that the value is live until the end of the block the PHI
208 // entry lives in. If the value really is dead at the PHI copy, there
209 // will be no successor blocks which have the value live-in.
211 // Check to see if the copy is the last use, and if so, update the
212 // live variables information so that it knows the copy source
213 // instruction kills the incoming value.
215 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
217 // Loop over all of the successors of the basic block, checking to see
218 // if the value is either live in the block, or if it is killed in the
219 // block. Also check to see if this register is in use by another PHI
220 // node which has not yet been eliminated. If so, it will be killed
221 // at an appropriate point later.
223 bool ValueIsLive = false;
224 for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(),
225 E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) {
226 MachineBasicBlock *SuccMBB = *SI;
228 // Is it alive in this successor?
229 unsigned SuccIdx = SuccMBB->getNumber();
230 if (SuccIdx < InRegVI.AliveBlocks.size() &&
231 InRegVI.AliveBlocks[SuccIdx]) {
236 // Is it killed in this successor?
237 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
238 if (InRegVI.Kills[i].first == SuccMBB) {
243 // Is it used by any PHI instructions in this block?
245 ValueIsLive = VRegPHIUseCount[SrcReg] != 0;
248 // Okay, if we now know that the value is not live out of the block,
249 // we can add a kill marker to the copy we inserted saying that it
250 // kills the incoming value!
253 MachineBasicBlock::iterator Prev = prior(I);
254 LV->addVirtualRegisterKilled(SrcReg, Prev);
260 // really delete the PHI instruction now!