llvm dev - Dec 2017 - MachineInstr - different treatment of subregs for checking mods and reads

MachineInstr::modifiesRegister() and MachineInstr::readsRegister() seem to
disagree on whether to check only the super-reg (reads) or any overlapping
register (modifies).

I suppose this makes some sense, but I find it confusing and kind of
limiting. I would like to implement something along the lines of
`MachineInstr::readsAnyOverlappingRegister()` that would have the same
semantics other than it would check whether the instruction reads the
register, it's super-register or some sub-register.
Basically, I would just replace `TRI->isSubRegister(MOReg, Reg)` in a
version of `findRegisterUseOperandIdx()`, with `TRI->regsOverlap(MOReg,
Reg)`. I think this would accomplish what I'm after.

The reason I would find this useful is that I have a pre-emit pass that
finds the instruction that defines a register and under certain conditions,
I'd be able to remove the defining instruction. But I can't remove it if
there's another use of any portion of the register between the def and the
use I am considering.
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> On Dec 15, 2017, at 6:51 AM, Nemanja Ivanovic via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> MachineInstr::modifiesRegister() and MachineInstr::readsRegister() seem to
disagree on whether to check only the super-reg (reads) or any overlapping
register (modifies).
> 
> I suppose this makes some sense, but I find it confusing and kind of
limiting. I would like to implement something along the lines of
`MachineInstr::readsAnyOverlappingRegister()` that would have the same semantics
other than it would check whether the instruction reads the register, it's
super-register or some sub-register.
> Basically, I would just replace `TRI->isSubRegister(MOReg, Reg)` in a
version of `findRegisterUseOperandIdx()`, with `TRI->regsOverlap(MOReg,
Reg)`. I think this would accomplish what I'm after.
> 
> The reason I would find this useful is that I have a pre-emit pass that
finds the instruction that defines a register and under certain conditions,
I'd be able to remove the defining instruction. But I can't remove it if
there's another use of any portion of the register between the def and the
use I am considering.
1) Just write that function as part of your pass. If it turns out to be useful
for more than 1 pass, we can move it to MachineInstr.
2) I'm not 100% sure I understand your motivation, but to me it sounds a bit
like you are re-implementing some form of liveness check so why not use
LiveRegUnits/LivePhysRegs instead?

- Matthias