Re: [gentoo-user] common flags for 2 cpu?

[Florian Philipp]

Am 17.08.2012 10:58, schrieb Jorge Almeida:
> I'm in need of some expert advise about CFLAGS. I'm going to install
> Gentoo in two systems, an Atom 330 and a P4 prescott. Since compiling
> lots of stuff in an Atom is less than joyful, I intend to do almost
> all compilation on the prescott and produce binary packages to use on
> the Atom. So, something like
> CFLAGS="-march=native ..." is out. I know I could set
> CFLAGS="-march=i686 -O2 -pipe -fomit-frame-pointer", but I would like
> to extract a bit more juice from the hw, while still keeping stable
> systems. So, I thought of setting CFLAGS for both systems as
> -march=i686, plus the flags that are pulled by -march=native in both
> systems, plus some other flags:
>
[...]
>
> This is what is pulled by -march=native, using gcc -march=native -E -v
> - </dev/null 2>&1 | grep cc1
>
> Prescott:
> -march=prescott -mno-cx16 -mno-sahf -mno-movbe -mno-aes -mno-pclmul
> -mno-popcnt -mno-abm -mno-lwp -mno-fma -mno-fma4 -mno-xop -mno-bmi
> -mno-bmi2 -mno-tbm -mno-avx -mno-avx2 -mno-sse4.2 -mno-sse4.1
> -mno-lzcnt -mno-rdrnd -mno-f16c -mno-fsgsbase --param l1-cache-size=16
> --param l1-cache-line-size=64 --param l2-cache-size=1024
> -mtune=prescott
>
> Atom:
> -march=atom -mcx16 -msahf -mmovbe -mno-aes -mno-pclmul -mno-popcnt
> -mno-abm -mno-lwp -mno-fma -mno-fma4 -mno-xop -mno-bmi -mno-bmi2
> -mno-tbm -mno-avx -mno-avx2 -mno-sse4.2 -mno-sse4.1 -mno-lzcnt
> -mno-rdrnd -mno-f16c -mno-fsgsbase --param l1-cache-size=24 --param
> l1-cache-line-size=64 --param l2-cache-size=512 -mtune=atom
>
[...]
>
> Some questions:
>
> 1) Is this strategy right? If so, any other flags to add? (or any
> flags to remove from the list?)
>
> 2) The --param flags are the ones of the computer that will do the
> compiling. I'm guessing the produced binaries are compatible with cpu
> with different --param flags. Is this right?
>
> TIA
>
> Jorge Almeida
>

1) Yes. But as you can see, -march=prescott is basically a subset of
atom. In fact, before there was a -march=atom option, prescott was the
best flag for atoms. I think you can avoid some hassle by simply
enabling "-march=prescott --param l1-cache-line-size=64 --param
l2-cache-size=512".

2) Yes, the param flags do not affect compatibility. Using the lower
value will probably be better but this is just an educated guess.

Regards,
Florian Philipp