[llvm-dev] [X86] Is it possible to implicitly promote a virtual subregister to a super?

[Tim Northover via llvm-dev]

On Wed, 20 Jan 2021 at 16:30, Constable, Scott D
<scott.d.constable at intel.com> wrote:
> The actual implementation I'm doing is a bit more complex than the example I provided for illustrative purposes. In my case, I don't need to care about the upper bits at all.

OK, in that case I think SUBREG_TO_REG is what you want. Something like

BuildMI(MBB, InsertPt, Loc, TII->get(X86::SUBREG_TO_REG), ExtendedReg)
    .addImm(0) // Never been entirely sure what this was here for.
It's always 0.
    .addReg(NarrowReg)
    .addImm(X86::sub_16bit);

then use ExtendedReg (a GR32 vreg) in the compare.

Cheers.

Tim.