[LLVMdev] MachineOperand SubReg

[dag at cray.com]

<dag at cray.com> writes:

> For reaching definitions I think simply tracking definitions via super
> registers should be sufficient.  Even if a definition defines a
> subregister I can consider it to define the "most super" register.  This
> will be pessimistic for cases like x86 AH/AL, but I hardly care about
> those at the moment.  :)

I did a big *facepalm* immediately after hitting "Send."  If AH and AL
come in defined via separate predecessors and the block only defined AH,
then AL should remain live through the block.  It would be wrong to kill
it by considering AH to define RAX.

So I do have to track sub/super register explicitly.  Not difficult,
just a bit more work.

                          -David