[PATCH] D59035: [X86] Promote i8 CMOV's (PR40965)

[Andrea Di Biagio via Phabricator via llvm-commits]

andreadb added a comment.

> why can't we simply widen the hands of select, like
> 
> define i16 @new(i1 %c) {
> 
>   %ret = select i1 %c, i16 117, i16 -19
>   ret i16 %ret
> 
> }
> 
> https://rise4fun.com/Alive/cs8
>  ?

I never said that we cannot fix it in a different way. I just wanted to point out a poor codegen issue, and show one possible way to fix it (if you had to do it manually).

If by widening the select operands we fix the issue then great.

To answer to the question:
"Why do you think we'd need to flip the CMOV condition?"

I don't *think* that it is needed if you use a different mapping for registers.
The reason why your solution doesn't require to flip the CMOV condition is only because regalloc swapped the mappings for the moves.

With your approach you get:

movl    $117, %ecx
movl    $65517, %eax            # imm = 0xFFED

In my case, I didn't touch register mappings:

movl    $117, %eax
movl    $-19, %ecx

Since the result has to be moved to EAX, in your case you can simply write

cmov


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