[PATCH] D81632: Fix undefined behavior in PeepholeOptimizer.

[Bill Wendling via Phabricator via llvm-commits]

void added a comment.

In D81632#2094672 <https://reviews.llvm.org/D81632#2094672>, @linzj wrote:

> > Which platform did you run valgrind on? The x86 platform should set SubIdx on all paths through it...
>
> ARM. armv8-linux-android.


Strange. SubIdx is initialized on all paths that return `true` in that function. So I'm not sure why valgrind complained...

My one worry with the change in this patch is that `0` is ostensibly a valid index, right? So how do we distinguish it from an invalid one if `isCoalescableExtInstr` returns `true` without properly initializing SubIdx?

  bool AArch64InstrInfo::isCoalescableExtInstr(const MachineInstr &MI,
                                               Register &SrcReg, Register &DstReg,
                                               unsigned &SubIdx) const {
    switch (MI.getOpcode()) {
    default:
      return false;
    case AArch64::SBFMXri: // aka sxtw
    case AArch64::UBFMXri: // aka uxtw
      // Check for the 32 -> 64 bit extension case, these instructions can do
      // much more.
      if (MI.getOperand(2).getImm() != 0 || MI.getOperand(3).getImm() != 31)
        return false;
      // This is a signed or unsigned 32 -> 64 bit extension.
      SrcReg = MI.getOperand(1).getReg();
      DstReg = MI.getOperand(0).getReg();
      SubIdx = AArch64::sub_32;
      return true;
    }
  }


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  https://reviews.llvm.org/D81632/new/

https://reviews.llvm.org/D81632