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