PHIElimination.cpp revision 927ce5dfaea00453889080cd1c6daf3eb197ad74 
1//===-- PhiElimination.cpp - Eliminate PHI nodes by inserting copies ------===//
2//
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.
6//
7//===----------------------------------------------------------------------===//
8
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
16namespace {
17  struct PNE : public MachineFunctionPass {
18    bool runOnMachineFunction(MachineFunction &Fn) {
19      bool Changed = false;
20
21      // Eliminate PHI instructions by inserting copies into predecessor blocks.
22      //
23      for (MachineFunction::iterator I = Fn.begin(), E = Fn.end(); I != E; ++I)
24	Changed |= EliminatePHINodes(Fn, *I);
25
26      //std::cerr << "AFTER PHI NODE ELIM:\n";
27      //Fn.dump();
28      return Changed;
29    }
30
31    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
32      AU.addPreserved<LiveVariables>();
33      MachineFunctionPass::getAnalysisUsage(AU);
34    }
35
36  private:
37    /// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions
38    /// in predecessor basic blocks.
39    ///
40    bool EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB);
41  };
42
43  RegisterPass<PNE> X("phi-node-elimination",
44		      "Eliminate PHI nodes for register allocation");
45}
46
47const PassInfo *PHIEliminationID = X.getPassInfo();
48
49/// EliminatePHINodes - Eliminate phi nodes by inserting copy instructions in
50/// predecessor basic blocks.
51///
52bool PNE::EliminatePHINodes(MachineFunction &MF, MachineBasicBlock &MBB) {
53  if (MBB.empty() || MBB.front()->getOpcode() != TargetInstrInfo::PHI)
54    return false;   // Quick exit for normal case...
55
56  LiveVariables *LV = getAnalysisToUpdate<LiveVariables>();
57  const TargetInstrInfo &MII = MF.getTarget().getInstrInfo();
58  const MRegisterInfo *RegInfo = MF.getTarget().getRegisterInfo();
59
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());
64
65    assert(MI->getOperand(0).isVirtualRegister() &&
66           "PHI node doesn't write virt reg?");
67
68    unsigned DestReg = MI->getOperand(0).getAllocatedRegNum();
69
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);
73
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.
77    //
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);
82
83    // Update live variable information if there is any...
84    if (LV) {
85      MachineInstr *PHICopy = *(AfterPHIsIt-1);
86
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.
92      //
93      LV->addVirtualRegisterDead(IncomingReg, PHICopy);
94
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...
98      //
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);
105      }
106
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);
112      }
113    }
114
115    // Now loop over all of the incoming arguments, changing them to copy into
116    // the IncomingReg register in the corresponding predecessor basic block.
117    //
118    for (int i = MI->getNumOperands() - 1; i >= 2; i-=2) {
119      MachineOperand &opVal = MI->getOperand(i-1);
120
121      // Get the MachineBasicBlock equivalent of the BasicBlock that is the
122      // source path the PHI.
123      MachineBasicBlock &opBlock = *MI->getOperand(i).getMachineBasicBlock();
124
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.
129      //
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
133      // this basic block!
134      //
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;
139          break;
140        }
141
142      if (HaveNotEmitted) {
143        MachineBasicBlock::iterator I = opBlock.end();
144        if (I != opBlock.begin()) {  // Handle empty blocks
145          --I;
146          // must backtrack over ALL the branches in the previous block
147          while (MII.isTerminatorInstr((*I)->getOpcode()) &&
148                 I != opBlock.begin())
149            --I;
150
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()))
154            ++I;
155        }
156
157	assert(opVal.isVirtualRegister() &&
158	       "Machine PHI Operands must all be virtual registers!");
159	RegInfo->copyRegToReg(opBlock, I, IncomingReg, opVal.getReg(), RC);
160      }
161    }
162
163    // really delete the PHI instruction now!
164    delete MI;
165  }
166
167  return true;
168}
169