[LLVMdev] deleting or replacing a MachineInst

[Chris Sears]

This seems a very natural approach but I probably am having a trouble with
the iterator invalidation. However, looking at other peephole optimizers
passes, I couldn't see how to do this:

#define BUILD_INS(opcode, new_reg, i)                               \
  BuildMI(*MBB, MBBI, MBBI->getDebugLoc(), TII->get(X86::opcode))   \
    .addReg(X86::new_reg, kill).addImm(i)

  for (MachineFunction::iterator MFI = MF.begin(), MFE = MF.end();
       MFI != MFE; ++MFI) {
    MachineBasicBlock* MBB = MFI;

    for (MachineBasicBlock::iterator MBBI = MBB->begin();
        MBBI != MBB->end(); ++MBBI) {
      MachineInstr *NewMI = NULL;
      OldMI = MBBI;

      // %EFLAGS<imp-def>   is getting copied
      // %RDX<imp-use,kill> is not getting copied (when it appears)
      switch (OldMI->getOpcode()) {
      default:                continue;
      // ....

      case X86::BT64ri8:
      case X86::BT32ri8:
      case X86::BT16ri8: {
          assert(OldMI->getNumOperands() >= 2);
          MachineOperand &Reg = OldMI->getOperand(0);
          MachineOperand &Imm = OldMI->getOperand(1);
          assert(Reg.isReg());
          assert(Imm.isImm());
          imm = Imm.getImm();

          if (imm >= 32)
            continue;

          kill = getKillRegState(Reg.isKill());

          switch (Reg.getReg()) {
          default:              assert(false);
          case X86::RAX:
          case X86::EAX:
          case X86::AX:
            if (imm < 8)        NewMI = BUILD_INS(TEST8i8, AL, 1 << imm);
            else if (imm < 16)  NewMI = BUILD_INS(TEST8ri, AH, 1 << (imm -
8));
            else                NewMI = BUILD_INS(BT32ri8, EAX, imm);
            break;

          // ...
          }
        }
        break;
      }

      // NewMI has been inserted before OldMI
      if (NewMI != NULL) {
//        MBB->remove_instr(OldMI);      // I've tried these (and others)
//        OldMI->removeFromParent();
//        MBB.erase(OldMI);
        NumX86Peepholes++;
      }
    }
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