[PATCH] D144451: [X86] Optimize umax(X,1)

[Kazu Hirata via Phabricator via llvm-commits]

kazu added a comment.

In D144451#4142080 <https://reviews.llvm.org/D144451#4142080>, @goldstein.w.n wrote:

> Slightly unrelated, is it possible to get the codegen to be:
>
>   	xorl	%eax, %eax
>   	cmp	$0x1, %edi
>   	adc	%edi, %eax
>
> ?
>
> If so that would be preferable (except in register pressure corner case b.c `eax` and `edi` now have slight live-range overlap).
> On target like ICX it saves a full instruction b.c no move-elim.

I've never thought of that.  Very interesting.  The comparison of the two approaches is such a close call.  I have a //slight// preference to the mov-cmp-adc route.  If we assume that users typically do not care about the value of `x` past `umax(x,1)`, which I haven't surveyed in real-world applications, we shouldn't have to make a copy of the source operand with a `mov`, especially in large enough functions where the calling conventions (edi->eax) aren't much of a constraint.  Friendliness under high register pressure along with the 6-byte encoding (without a `mov`) is a plus.


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