[PATCH] D75014: [InstrEmitter, SystemZ] Copy Access registers with the correct register class.

Fri Feb 28 12:38:53 PST 2020

jonpa added a comment.

In D75014#1897768 <https://reviews.llvm.org/D75014#1897768>, @uweigand wrote:

> > It seems safest to build the target instructions compared to just constrain the virtual register class of the register of the COPY.
>
> I'm not sure I understand this: can you explain what problem you see with constraining the register class?

I remember seeing that the register allocator would create a new virtual register and give it the register class based on calling MI->getRegClassConstraint() (or TII->getRegClass() directly). So in theory, it seems that if there is no MCInstrDesc anywhere that demands a particular register class, regalloc might feel free to take the optimal one (GRX32). I am not sure this is needed, but there is no mechanism that I know of that would constrain a *COPY* register regclass, although it may be that a COPY of a physreg into a virtreg is left alone.

Maybe someone could instead confirm that physreg copies do not get their virtreg regclasses changed ever. Maybe that is obvious and I just wasn't aware. Or, if there is no guarantee for this, perhaps a target hook like I suggested (getPhysRegCopyRegClass()) would be a better solution after all, since that would also then be an error if regalloc broke that.

> Also, if you do directly emit the EAR etc., I'm wondering why you still keep the COPY in as well?

I figured that the coalescer should typically remove it. And I wasn't sure if there could ever be a problem with any other connected virtregs involved having to use a high-register. In that case there would have to be a copy to/from a GRH32.

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