Re: Who cares about performance (or Intel's CPU errata)?

[David Brown <david.brown@hesbynett.no>]

On 28/05/2023 01:30, Andrew Pinski via Gcc wrote:
> On Sat, May 27, 2023 at 3:54 PM Stefan Kanthak <stefan.kanthak@nexgo.de> wrote:
>>

>>          sete    al
>>          movzx   eax, al                  # superfluous
> 
> No it is not superfluous, well ok it is because of the context of eax
> (besides the lower 8 bits) are already zero'd but keeping that track
> is a hard problem and is turning problem really. And I suspect it
> would cause another false dependency later on too.
> 
> For -Os -march=skylake (and -Oz instead of -Os) we get:
>          popcnt  rdi, rdi
>          popcnt  rsi, rsi
>          add     esi, edi
>          xor     eax, eax
>          dec     esi
>          sete    al
> 
> Which is exactly what you want right?
> 
> Thanks,
> Andrew
> 
There is also the option of using "bool" as the return type for boolean 
functions, rather than "int".  When returning a "bool", gcc does not add 
the "movzx eax, al" instruction.  (There are some circumstances where 
returning "int" for a boolean value is a better choice, but usually 
"bool" makes more sense, and it can often be a touch more efficient.)

David