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 // dead. This says that the register is dead, not killed, because we
91 // cannot use the live variable information to indicate that the variable
92 // is defined in multiple entry blocks. Instead, we pretend that this
93 // instruction defined it and killed it at the same time.
95 LV->addVirtualRegisterDead(IncomingReg, &MBB, PHICopy);
97 // Since we are going to be deleting the PHI node, if it is the last use
98 // of any registers, or if the value itself is dead, we need to move this
99 // information over to the new copy we just inserted...
101 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
102 RKs = LV->killed_range(MI);
103 std::vector<std::pair<MachineInstr*, unsigned> > Range;
104 if (RKs.first != RKs.second) {
105 // Copy the range into a vector...
106 Range.assign(RKs.first, RKs.second);
108 // Delete the range...
109 LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
111 // Add all of the kills back, which will update the appropriate info...
112 for (unsigned i = 0, e = Range.size(); i != e; ++i)
113 LV->addVirtualRegisterKilled(Range[i].second, &MBB, PHICopy);
116 RKs = LV->dead_range(MI);
117 if (RKs.first != RKs.second) {
119 Range.assign(RKs.first, RKs.second);
120 LV->removeVirtualRegistersDead(RKs.first, RKs.second);
121 for (unsigned i = 0, e = Range.size(); i != e; ++i)
122 LV->addVirtualRegisterDead(Range[i].second, &MBB, PHICopy);
126 // Now loop over all of the incoming arguments, changing them to copy into
127 // the IncomingReg register in the corresponding predecessor basic block.
129 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
130 MachineOperand &opVal = MI->getOperand(i-1);
132 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
133 // source path the PHI.
134 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
136 // Figure out where to insert the copy, which is at the end of the
137 // predecessor basic block, but before any terminator/branch
139 MachineBasicBlock::iterator I = opBlock.end();
140 if (I != opBlock.begin()) { // Handle empty blocks
142 // must backtrack over ALL the branches in the previous block
143 while (MII.isTerminatorInstr((*I)->getOpcode()) &&
144 I != opBlock.begin())
147 // move back to the first branch instruction so new instructions
148 // are inserted right in front of it and not in front of a non-branch
149 if (!MII.isTerminatorInstr((*I)->getOpcode()))
153 // Check to make sure we haven't already emitted the copy for this block.
154 // This can happen because PHI nodes may have multiple entries for the
155 // same basic block. It doesn't matter which entry we use though, because
156 // all incoming values are guaranteed to be the same for a particular bb.
158 // If we emitted a copy for this basic block already, it will be right
159 // where we want to insert one now. Just check for a definition of the
160 // register we are interested in!
162 bool HaveNotEmitted = true;
164 if (I != opBlock.begin()) {
165 MachineInstr *PrevInst = *(I-1);
166 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
167 MachineOperand &MO = PrevInst->getOperand(i);
168 if (MO.isVirtualRegister() && MO.getReg() == IncomingReg)
169 if (MO.opIsDef() || MO.opIsDefAndUse()) {
170 HaveNotEmitted = false;
176 if (HaveNotEmitted) { // If the copy has not already been emitted, do it.
177 assert(opVal.isVirtualRegister() &&
178 "Machine PHI Operands must all be virtual registers!");
179 unsigned SrcReg = opVal.getReg();
180 RegInfo->copyRegToReg(opBlock, I, IncomingReg, SrcReg, RC);
182 // Now update live variable information if we have it.
184 // We want to be able to insert a kill of the register if this PHI
185 // (aka, the copy we just inserted) is the last use of the source
186 // value. Live variable analysis conservatively handles this by
187 // saying that the value is live until the end of the block the PHI
188 // entry lives in. If the value really is dead at the PHI copy, there
189 // will be no successor blocks which have the value live-in.
191 // Check to see if the copy is the last use, and if so, update the
192 // live variables information so that it knows the copy source
193 // instruction kills the incoming value.
195 LiveVariables::VarInfo &InRegVI = LV->getVarInfo(SrcReg);
197 // Loop over all of the successors of the basic block, checking to
198 // see if the value is either live in the block, or if it is killed
201 bool ValueIsLive = false;
202 BasicBlock *BB = opBlock.getBasicBlock();
203 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB);
205 const std::pair<MachineBasicBlock*, unsigned> &
206 SuccInfo = LV->getBasicBlockInfo(*SI);
208 // Is it alive in this successor?
209 unsigned SuccIdx = SuccInfo.second;
210 if (SuccIdx < InRegVI.AliveBlocks.size() &&
211 InRegVI.AliveBlocks[SuccIdx]) {
216 // Is it killed in this successor?
217 MachineBasicBlock *MBB = SuccInfo.first;
218 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
219 if (InRegVI.Kills[i].first == MBB) {
225 // Okay, if we now know that the value is not live out of the block,
226 // we can add a kill marker to the copy we inserted saying that it
227 // kills the incoming value!
230 // One more complication to worry about. There may actually be
231 // multiple PHI nodes using this value on this branch. If we aren't
232 // careful, the first PHI node will end up killing the value, not
233 // letting it get the to the copy for the final PHI node in the
234 // block. Therefore we have to check to see if there is already a
235 // kill in this block, and if so, extend the lifetime to our new
238 for (unsigned i = 0, e = InRegVI.Kills.size(); i != e; ++i)
239 if (InRegVI.Kills[i].first == &opBlock) {
240 std::pair<LiveVariables::killed_iterator,
241 LiveVariables::killed_iterator> Range
242 = LV->killed_range(InRegVI.Kills[i].second);
243 LV->removeVirtualRegistersKilled(Range.first, Range.second);
247 LV->addVirtualRegisterKilled(SrcReg, &opBlock, *(I-1));
253 // really delete the PHI instruction now!