llvm dev - May 2016 - Replacing an instruction in a post-RA pass

I'm writing a pass that looks at the operands of certain non-commutable
instructions and swaps a couple of them if certain conditions exist (a
register bank conflict in the instruction). If the conflict exists, I build
a new instruction which has the 2nd and 3rd operands swapped (using
BuildMI).  Then I want to get rid of the original instruction. I had done
some searching and found that eraseFromParent is used to do this so that's
what I'm using in the code below:

     for (MachineFunction::iterator MFI = MF.begin(), E = MF.end();
       MFI != E; ++MFI) {
       // for each instr
       MachineBasicBlock::instr_iterator II = MFI->instr_begin();
       while( II != MFI->instr_end()) {
          opcode = II->getOpcode();
          if(II->isCommutable() || isLoadRR(opcode)){
            MachineBasicBlock& MBB = *MFI;
            MachineInstr& MI = *II;
            DebugLoc DL = MI.getDebugLoc();

            MachineOperand& reg1 = MI.getOperand(0);
            MachineOperand& reg2 = MI.getOperand(1);
            MachineOperand& reg3 = MI.getOperand(2);
            if(reg1.isReg() && reg2.isReg() && reg3.isReg()){
              if((reg1.getReg()-8)%4 == (reg3.getReg()-8)%4){
                MachineBasicBlock::instr_iterator NII = std::next(II);
                //conflict if reg1 and reg3 are in same bank
                errs() <<  "Conflict: ";
                printOp(opcode);
                errs() <<  " has " << num_operands
<< " register
operands:\n";
                errs() <<  " r1: " << (reg1.getReg()-8)
<< " r2: " <<
(reg2.getReg()-8)
                       <<  " r3: " << (reg3.getReg()-8)
<< "\n";
                //build the swapped version
                BuildMI(MBB, II, DL,
TII.get(opcode),reg1.getReg()).addReg(reg3.getReg()).addReg(reg2.getReg());
                MI.eraseFromParent();
                II = NII;
              }
            }
          }
          ++II;
       }
     }

Unfortunately, this leads to a segfault. Is this the proper way to do this
or is there another suggested way of doing it?

Phil
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On 5/9/2016 2:43 PM, Phil Tomson via llvm-dev wrote:
>
>         MachineBasicBlock::instr_iterator II = MFI->instr_begin();
>         while( II != MFI->instr_end()) {
>           [...]
>                  MachineBasicBlock::instr_iterator NII = std::next(II);
>[...]
>                  MI.eraseFromParent();
>                  II = NII;
>                }
>              }
>            }
>            ++II;
>
> Unfortunately, this leads to a segfault. Is this the proper way to do
> this or is there another suggested way of doing it?
What if II is the last instruction?

-Krzysztof


-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, 
hosted by The Linux Foundation

Ah, yes, thank you very much for that hint.

Phil

On Mon, May 9, 2016 at 12:47 PM, Krzysztof Parzyszek via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> On 5/9/2016 2:43 PM, Phil Tomson via llvm-dev wrote:
>
>>
>>         MachineBasicBlock::instr_iterator II = MFI->instr_begin();
>>         while( II != MFI->instr_end()) {
>>           [...]
>>                  MachineBasicBlock::instr_iterator NII = std::next(II);
>> [...]
>>                  MI.eraseFromParent();
>>                  II = NII;
>>                }
>>              }
>>            }
>>            ++II;
>>
>> Unfortunately, this leads to a segfault. Is this the proper way to do
>> this or is there another suggested way of doing it?
>>
>
> What if II is the last instruction?
>
> -Krzysztof
>
>
> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted
> by The Linux Foundation
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
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>
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