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

[Andrew Pinski <pinskia@gmail.com>]

On Sat, May 27, 2023 at 2:25 PM Stefan Kanthak <stefan.kanthak@nexgo.de> wrote:
>
> Just to show how SLOPPY, INCONSEQUENTIAL and INCOMPETENT GCC's developers are:
>
> --- dontcare.c ---
> int ispowerof2(unsigned __int128 argument) {
>     return __builtin_popcountll(argument) + __builtin_popcountll(argument >> 64) == 1;
> }
> --- EOF ---
>
> GCC 13.3    gcc -march=haswell -O3
>
> https://gcc.godbolt.org/z/PPzYsPzMc
> ispowerof2(unsigned __int128):
>         popcnt  rdi, rdi
>         popcnt  rsi, rsi
>         add     esi, edi
>         xor     eax, eax
>         cmp     esi, 1
>         sete    al
>         ret
>
> OOPS: what about Intel's CPU errata regarding the false dependency on POPCNTs output?

Because the popcount is going to the same register, there is no false
dependency ....
The false dependency errata only applies if the result of the popcnt
is going to a different register, the processor thinks it depends on
the result in that register from a previous instruction but it does
not (which is why it is called a false dependency). In this case it
actually does depend on the previous result since the input is the
same as the input.

Thanks,
Andrew

>
> See https://gcc.godbolt.org/z/jdjTc3EET for comparison!
>
> FIX YOUR BUGS, KIDS!
>
> Stefan