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 if (AfterPHIsIt != MBB.end())
80 while ((*AfterPHIsIt)->getOpcode() == TargetInstrInfo::PHI) ++AfterPHIsIt;
81 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
83 // Update live variable information if there is any...
85 MachineInstr *PHICopy = *(AfterPHIsIt-1);
87 // Add information to LiveVariables to know that the incoming value is
88 // dead. This says that the register is dead, not killed, because we
89 // cannot use the live variable information to indicate that the variable
90 // is defined in multiple entry blocks. Instead, we pretend that this
91 // instruction defined it and killed it at the same time.
93 LV->addVirtualRegisterDead(IncomingReg, PHICopy);
95 // Since we are going to be deleting the PHI node, if it is the last use
96 // of any registers, or if the value itself is dead, we need to move this
97 // information over to the new copy we just inserted...
99 std::pair<LiveVariables::killed_iterator, LiveVariables::killed_iterator>
100 RKs = LV->killed_range(MI);
101 if (RKs.first != RKs.second) {
102 for (LiveVariables::killed_iterator I = RKs.first; I != RKs.second; ++I)
103 LV->addVirtualRegisterKilled(I->second, PHICopy);
104 LV->removeVirtualRegistersKilled(RKs.first, RKs.second);
107 RKs = LV->dead_range(MI);
108 if (RKs.first != RKs.second) {
109 for (LiveVariables::killed_iterator I = RKs.first; I != RKs.second; ++I)
110 LV->addVirtualRegisterDead(I->second, PHICopy);
111 LV->removeVirtualRegistersDead(RKs.first, RKs.second);
115 // Now loop over all of the incoming arguments, changing them to copy into
116 // the IncomingReg register in the corresponding predecessor basic block.
118 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
119 MachineOperand &opVal = MI->getOperand(i-1);
121 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
122 // source path the PHI.
123 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
125 // Check to make sure we haven't already emitted the copy for this block.
126 // This can happen because PHI nodes may have multiple entries for the
127 // same basic block. It doesn't matter which entry we use though, because
128 // all incoming values are guaranteed to be the same for a particular bb.
130 // Note that this is N^2 in the number of phi node entries, but since the
131 // # of entries is usually small, this is not a problem. FIXME: this
132 // should just check to see if there is already a copy in the bottom of
135 bool HaveNotEmitted = true;
136 for (int op = MI->getNumOperands() - 1; op != i; op -= 2)
137 if (&opBlock == MI->getOperand(op).getMachineBasicBlock()) {
138 HaveNotEmitted = false;
142 if (HaveNotEmitted) {
143 MachineBasicBlock::iterator I = opBlock.end();
144 if (I != opBlock.begin()) { // Handle empty blocks
146 // must backtrack over ALL the branches in the previous block
147 while (MII.isTerminatorInstr((*I)->getOpcode()) &&
148 I != opBlock.begin())
151 // move back to the first branch instruction so new instructions
152 // are inserted right in front of it and not in front of a non-branch
153 if (!MII.isTerminatorInstr((*I)->getOpcode()))
157 assert(opVal.isVirtualRegister() &&
158 "Machine PHI Operands must all be virtual registers!");
159 RegInfo->copyRegToReg(opBlock, I, IncomingReg, opVal.getReg(), RC);
163 // really delete the PHI instruction now!