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.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 (by
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)->getOpcode() == TargetInstrInfo::PHI) ++AfterPHIsIt;
80 RegInfo->copyRegToReg(MBB, AfterPHIsIt, DestReg, IncomingReg, RC);
82 // Add information to LiveVariables to know that the incoming value is dead
83 if (LV) LV->addVirtualRegisterKill(IncomingReg, *(AfterPHIsIt-1));
85 // Now loop over all of the incoming arguments turning them into copies into
86 // the IncomingReg register in the corresponding predecessor basic block.
88 for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
89 MachineOperand &opVal = MI->getOperand(i-1);
91 // Get the MachineBasicBlock equivalent of the BasicBlock that is the
92 // source path the phi
93 MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
95 // Check to make sure we haven't already emitted the copy for this block.
96 // This can happen because PHI nodes may have multiple entries for the
97 // same basic block. It doesn't matter which entry we use though, because
98 // all incoming values are guaranteed to be the same for a particular bb.
100 // Note that this is N^2 in the number of phi node entries, but since the
101 // # of entries is tiny, this is not a problem.
103 bool HaveNotEmitted = true;
104 for (int op = MI->getNumOperands() - 1; op != i; op -= 2)
105 if (&opBlock == MI->getOperand(op).getMachineBasicBlock()) {
106 HaveNotEmitted = false;
110 if (HaveNotEmitted) {
111 MachineBasicBlock::iterator I = opBlock.end()-1;
113 // must backtrack over ALL the branches in the previous block
114 while (MII.isTerminatorInstr((*I)->getOpcode()) && I != opBlock.begin())
117 // move back to the first branch instruction so new instructions
118 // are inserted right in front of it and not in front of a non-branch
119 if (!MII.isTerminatorInstr((*I)->getOpcode()))
122 assert(opVal.isVirtualRegister() &&
123 "Machine PHI Operands must all be virtual registers!");
124 RegInfo->copyRegToReg(opBlock, I, IncomingReg, opVal.getReg(), RC);
128 // really delete the PHI instruction now!