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"
17 struct PNE : public MachineFunctionPass {
18 bool runOnMachineFunction(MachineFunction &Fn) {
21 // Eliminate PHI instructions by inserting copies into predecessor blocks.
23 for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
24 Changed |= EliminatePHINodes(Fn, *I);
26 //std::cerr << "AFTER PHI NODE ELIM:\n";
31 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
32 AU.addPreserved<LiveVariables>();
33 MachineFunctionPass::getAnalysisUsage(AU);
37 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
38 /// in predecessor basic blocks.
40 bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
43 RegisterPass<PNE> X("phi-node-elimination",
44 "Eliminate PHI nodes for register allocation");
47 const PassInfo *PHIEliminationID = X.getPassInfo();
49 /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
50 /// predecessor basic blocks.
52 bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
53 if (MBB.empty() || MBB.front()->getOpcode() != TargetInstrInfo::PHI)
54 return false; // Quick exit for normal case...
56 LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
57 const TargetInstrInfo &MII = MF.getTarget().getInstrInfo();
58 const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
60 while (MBB.front()->getOpcode() == TargetInstrInfo::PHI) {
61 MachineInstr *MI = MBB.front();
62 // Unlink the PHI node from the basic block... but don't delete the PHI yet
63 MBB.erase(MBB.begin());
65 assert(MI->getOperand(0).isVirtualRegister() &&
66 "PHI node doesn't write virt reg?");
68 unsigned DestReg = MI->getOperand(0).getAllocatedRegNum();
70 // Create a new register for the incoming PHI arguments
71 const TargetRegisterClass *RC = MF.getSSARegMap()->getRegClass(DestReg);
72 unsigned IncomingReg = MF.getSSARegMap()->createVirtualRegister(RC);
74 // Insert a register to register copy in the top of the current block (but
75 // after any remaining phi nodes) which copies the new incoming register
76 // into the phi node destination.
78 MachineBasicBlock::iterator AfterPHIsIt = MBB.begin();
79 while (AfterPHIsIt != MBB.end() &&
80 (*AfterPHIsIt)->getOpcode() == TargetInstrInfo::PHI)
81 ++AfterPHIsIt; // Skip over all of the PHI nodes...
82 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
84 // Update live variable information if there is any...
86 MachineInstr *PHICopy = *(AfterPHIsIt-1);
88 // Add information to LiveVariables to know that the incoming value is
89 // dead. This says that the register is dead, not killed, because we
90 // cannot use the live variable information to indicate that the variable
91 // is defined in multiple entry blocks. Instead, we pretend that this
92 // instruction defined it and killed it at the same time.
94 LV->addVirtualRegisterDead(IncomingReg, 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 if (RKs.first != RKs.second) {
103 for (LiveVariables::killed_iterator I = RKs.first; I != RKs.second; ++I)
104 LV->addVirtualRegisterKilled(I->second, PHICopy);
105 LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
108 RKs = LV->dead_range(MI);
109 if (RKs.first != RKs.second) {
110 for (LiveVariables::killed_iterator I = RKs.first; I != RKs.second; ++I)
111 LV->addVirtualRegisterDead(I->second, PHICopy);
112 LV->removeVirtualRegistersDead(RKs.first, RKs.second);
116 // Now loop over all of the incoming arguments, changing them to copy into
117 // the IncomingReg register in the corresponding predecessor basic block.
119 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
120 MachineOperand &opVal = MI->getOperand(i-1);
122 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
123 // source path the PHI.
124 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
126 // Figure out where to insert the copy, which is at the end of the
127 // predecessor basic block, but before any terminator/branch
129 MachineBasicBlock::iterator I = opBlock.end();
130 if (I != opBlock.begin()) { // Handle empty blocks
132 // must backtrack over ALL the branches in the previous block
133 while (MII.isTerminatorInstr((*I)->getOpcode()) &&
134 I != opBlock.begin())
137 // move back to the first branch instruction so new instructions
138 // are inserted right in front of it and not in front of a non-branch
139 if (!MII.isTerminatorInstr((*I)->getOpcode()))
143 // Check to make sure we haven't already emitted the copy for this block.
144 // This can happen because PHI nodes may have multiple entries for the
145 // same basic block. It doesn't matter which entry we use though, because
146 // all incoming values are guaranteed to be the same for a particular bb.
148 // If we emitted a copy for this basic block already, it will be right
149 // where we want to insert one now. Just check for a definition of the
150 // register we are interested in!
152 bool HaveNotEmitted = true;
154 if (I != opBlock.begin()) {
155 MachineInstr *PrevInst = *(I-1);
156 for (unsigned i = 0, e = PrevInst->getNumOperands(); i != e; ++i) {
157 MachineOperand &MO = PrevInst->getOperand(i);
158 if (MO.isVirtualRegister() && MO.getReg() == IncomingReg)
159 if (MO.opIsDef() || MO.opIsDefAndUse()) {
160 HaveNotEmitted = false;
166 if (HaveNotEmitted) {
167 assert(opVal.isVirtualRegister() &&
168 "Machine PHI Operands must all be virtual registers!");
169 RegInfo->copyRegToReg(opBlock, I, IncomingReg, opVal.getReg(), RC);
173 // really delete the PHI instruction now!