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/LiveVariables.h"
17 #include "llvm/CodeGen/Passes.h"
18 #include "llvm/CodeGen/MachineFunctionPass.h"
19 #include "llvm/CodeGen/MachineInstr.h"
20 #include "llvm/CodeGen/SSARegMap.h"
21 #include "llvm/Target/TargetInstrInfo.h"
22 #include "llvm/Target/TargetMachine.h"
23 #include "llvm/ADT/DenseMap.h"
24 #include "llvm/ADT/STLExtras.h"
25 #include "llvm/ADT/Statistic.h"
31 Statistic<> NumAtomic("phielim", "Number of atomic phis lowered");
32 Statistic<> NumSimple("phielim", "Number of simple phis lowered");
34 struct PNE : public MachineFunctionPass {
35 bool runOnMachineFunction(MachineFunction &Fn) {
38 // Eliminate PHI instructions by inserting copies into predecessor blocks.
39 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
40 Changed |= EliminatePHINodes(Fn, *I);
45 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
46 AU.addPreserved<LiveVariables>();
47 MachineFunctionPass::getAnalysisUsage(AU);
51 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
52 /// in predecessor basic blocks.
54 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
55 void LowerAtomicPHINode(MachineBasicBlock &MBB,
56 MachineBasicBlock::iterator AfterPHIsIt,
57 DenseMap<unsigned, VirtReg2IndexFunctor> &VUC);
60 RegisterPass<PNE> X("phi-node-elimination",
61 "Eliminate PHI nodes for register allocation");
65 const PassInfo *llvm::PHIEliminationID = X.getPassInfo();
67 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
68 /// predecessor basic blocks.
70 bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
71 if (MBB.empty() || MBB.front().getOpcode() != TargetInstrInfo::PHI)
72 return false; // Quick exit for basic blocks without PHIs.
74 // VRegPHIUseCount - Keep track of the number of times each virtual register
75 // is used by PHI nodes in successors of this block.
76 DenseMap<unsigned, VirtReg2IndexFunctor> VRegPHIUseCount;
77 VRegPHIUseCount.grow(MF.getSSARegMap()->getLastVirtReg());
79 for (MachineBasicBlock::pred_iterator PI = MBB.pred_begin(),
80 E = MBB.pred_end(); PI != E; ++PI)
81 for (MachineBasicBlock::succ_iterator SI = (*PI)->succ_begin(),
82 E = (*PI)->succ_end(); SI != E; ++SI)
83 for (MachineBasicBlock::iterator BBI = (*SI)->begin(), E = (*SI)->end();
84 BBI != E && BBI->getOpcode() == TargetInstrInfo::PHI; ++BBI)
85 for (unsigned i = 1, e = BBI->getNumOperands(); i != e; i += 2)
86 VRegPHIUseCount[BBI->getOperand(i).getReg()]++;
88 // Get an iterator to the first instruction after the last PHI node (this may
89 // also be the end of the basic block).
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...
95 while (MBB.front().getOpcode() == TargetInstrInfo::PHI) {
96 LowerAtomicPHINode(MBB, AfterPHIsIt, VRegPHIUseCount);
101 /// InstructionUsesRegister - Return true if the specified machine instr has a
102 /// use of the specified register.
103 static bool InstructionUsesRegister(MachineInstr *MI, unsigned SrcReg) {
104 for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i)
105 if (MI->getOperand(0).isRegister() &&
106 MI->getOperand(0).getReg() == SrcReg &&
107 MI->getOperand(0).isUse())
112 /// LowerAtomicPHINode - Lower the PHI node at the top of the specified block,
113 /// under the assuption that it needs to be lowered in a way that supports
114 /// atomic execution of PHIs. This lowering method is always correct all of the
116 void PNE::LowerAtomicPHINode(MachineBasicBlock &MBB,
117 MachineBasicBlock::iterator AfterPHIsIt,
118 DenseMap<unsigned, VirtReg2IndexFunctor> &VRegPHIUseCount) {
119 // Unlink the PHI node from the basic block, but don't delete the PHI yet.
120 MachineInstr *MPhi = MBB.remove(MBB.begin());
122 unsigned DestReg = MPhi->getOperand(0).getReg();
124 // Create a new register for the incoming PHI arguments/
125 MachineFunction &MF = *MBB.getParent();
126 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
127 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
129 // Insert a register to register copy in the top of the current block (but
130 // after any remaining phi nodes) which copies the new incoming register
131 // into the phi node destination.
133 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
134 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
136 // Update live variable information if there is any...
137 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
139 MachineInstr *PHICopy = prior(AfterPHIsIt);
141 // Add information to LiveVariables to know that the incoming value is
142 // killed. Note that because the value is defined in several places (once
143 // each for each incoming block), the "def" block and instruction fields
144 // for the VarInfo is not filled in.
146 LV->addVirtualRegisterKilled(IncomingReg, PHICopy);
148 // Since we are going to be deleting the PHI node, if it is the last use
149 // of any registers, or if the value itself is dead, we need to move this
150 // information over to the new copy we just inserted.
152 LV->removeVirtualRegistersKilled(MPhi);
154 // If the result is dead, update LV.
155 if (LV->RegisterDefIsDead(MPhi, DestReg)) {
156 LV->addVirtualRegisterDead(DestReg, PHICopy);
157 LV->removeVirtualRegistersDead(MPhi);
160 // Realize that the destination register is defined by the PHI copy now, not
162 LV->getVarInfo(DestReg).DefInst = PHICopy;
165 // Adjust the VRegPHIUseCount map to account for the removal of this PHI
167 unsigned NumPreds = (MPhi->getNumOperands()-1)/2;
168 for (unsigned i = 1; i != MPhi->getNumOperands(); i += 2)
169 VRegPHIUseCount[MPhi->getOperand(i).getReg()] -= NumPreds;
171 // Now loop over all of the incoming arguments, changing them to copy into
172 // the IncomingReg register in the corresponding predecessor basic block.
174 std::set<MachineBasicBlock*> MBBsInsertedInto;
175 for (int i = MPhi->getNumOperands() - 1; i >= 2; i-=2) {
176 unsigned SrcReg = MPhi->getOperand(i-1).getReg();
177 assert(MRegisterInfo::isVirtualRegister(SrcReg) &&
178 "Machine PHI Operands must all be virtual registers!");
180 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
181 // source path the PHI.
182 MachineBasicBlock &opBlock = *MPhi->getOperand(i).getMachineBasicBlock();
184 // Check to make sure we haven't already emitted the copy for this block.
185 // This can happen because PHI nodes may have multiple entries for the
187 if (!MBBsInsertedInto.insert(&opBlock).second)
188 continue; // If the copy has already been emitted, we're done.
190 // Get an iterator pointing to the first terminator in the block (or end()).
191 // This is the point where we can insert a copy if we'd like to.
192 MachineBasicBlock::iterator I = opBlock.getFirstTerminator();
195 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);
197 // Now update live variable information if we have it. Otherwise we're done
200 // We want to be able to insert a kill of the register if this PHI
201 // (aka, the copy we just inserted) is the last use of the source
202 // value. Live variable analysis conservatively handles this by
203 // saying that the value is live until the end of the block the PHI
204 // entry lives in. If the value really is dead at the PHI copy, there
205 // will be no successor blocks which have the value live-in.
207 // Check to see if the copy is the last use, and if so, update the
208 // live variables information so that it knows the copy source
209 // instruction kills the incoming value.
211 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
213 // Loop over all of the successors of the basic block, checking to see
214 // if the value is either live in the block, or if it is killed in the
215 // block. Also check to see if this register is in use by another PHI
216 // node which has not yet been eliminated. If so, it will be killed
217 // at an appropriate point later.
220 // Is it used by any PHI instructions in this block?
221 bool ValueIsLive = VRegPHIUseCount[SrcReg] != 0;
223 std::vector<MachineBasicBlock*> OpSuccBlocks;
225 // Otherwise, scan successors, including the BB the PHI node lives in.
226 for (MachineBasicBlock::succ_iterator SI = opBlock.succ_begin(),
227 E = opBlock.succ_end(); SI != E && !ValueIsLive; ++SI) {
228 MachineBasicBlock *SuccMBB = *SI;
230 // Is it alive in this successor?
231 unsigned SuccIdx = SuccMBB->getNumber();
232 if (SuccIdx < InRegVI.AliveBlocks.size() &&
233 InRegVI.AliveBlocks[SuccIdx]) {
238 OpSuccBlocks.push_back(SuccMBB);
241 // Check to see if this value is live because there is a use in a successor
244 switch (OpSuccBlocks.size()) {
246 MachineBasicBlock *MBB = OpSuccBlocks[0];
247 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
248 if (InRegVI.Kills[i]->getParent() == MBB) {
255 MachineBasicBlock *MBB1 = OpSuccBlocks[0], *MBB2 = OpSuccBlocks[1];
256 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
257 if (InRegVI.Kills[i]->getParent() == MBB1 ||
258 InRegVI.Kills[i]->getParent() == MBB2) {
265 std::sort(OpSuccBlocks.begin(), OpSuccBlocks.end());
266 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
267 if (std::binary_search(OpSuccBlocks.begin(), OpSuccBlocks.end(),
268 InRegVI.Kills[i]->getParent())) {
275 // Okay, if we now know that the value is not live out of the block,
276 // we can add a kill marker in this block saying that it kills the incoming
279 // In our final twist, we have to decide which instruction kills the
280 // register. In most cases this is the copy, however, the first
281 // terminator instruction at the end of the block may also use the value.
282 // In this case, we should mark *it* as being the killing block, not the
284 bool FirstTerminatorUsesValue = false;
285 if (I != opBlock.end()) {
286 FirstTerminatorUsesValue = InstructionUsesRegister(I, SrcReg);
288 // Check that no other terminators use values.
290 for (MachineBasicBlock::iterator TI = next(I); TI != opBlock.end();
292 assert(!InstructionUsesRegister(TI, SrcReg) &&
293 "Terminator instructions cannot use virtual registers unless"
294 "they are the first terminator in a block!");
299 MachineBasicBlock::iterator KillInst;
300 if (!FirstTerminatorUsesValue)
305 // Finally, mark it killed.
306 LV->addVirtualRegisterKilled(SrcReg, KillInst);
308 // This vreg no longer lives all of the way through opBlock.
309 unsigned opBlockNum = opBlock.getNumber();
310 if (opBlockNum < InRegVI.AliveBlocks.size())
311 InRegVI.AliveBlocks[opBlockNum] = false;
315 // Really delete the PHI instruction now!