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Re: March=native with a main 64bit system and 32bit chroot


Sorry for the wrong mailing list, i mailed to gcc@gcc.gnu.org because
i thought it was half a bug, but as i can understand it isn't.
So why gentoo team suggest nocona for 64 bit and prescott for 32 bit?
(http://en.gentoo-wiki.com/wiki/Safe_Cflags/Intel#Pentium_4_.28Prescott.29_.2F_Celeron_D)
Is there a way to test if it is better prescott or nocona?
Thanks.


2009/5/26 Ian Lance Taylor <iant@google.com>:
> Luca Zorzo <lucazorzo@gmail.com> writes:
>
>> I've a main Gentoo 64bit system with CHOST="x86_64-pc-linux-gnu" and
>> CFLAGS="-march=native -mtune=native -O2 -pipe -fomit-frame-pointer".
>> My cpu is a Pentium 4 Prescott and i'm using gcc-4.3.2.
>>
>> With this little script:
>> "echo 'float x(float x){return x < 0 ? -x : x;}' > x.c && gcc
>> -fverbose-asm -mtune=native -march=native -S x.c && grep
>> '\(-march\|-mtune\)' x.s && rm -fr x.c && rm -fr x.s "
>> i've found that gcc uses march=nocona and mtune=nocona, and this is ok.
>>
>> In this main system i've some 32bit chroots with
>> CHOST="i686-pc-linux-gnu" and CFLAGS="-march=native -mtune=native -O2
>> -pipe -fomit-frame-pointer".
>> But this time gcc is using march=nocona and mtune=nocona again, that
>> is wrong i think.
>>
>> Should i use march=native or march=prescott (that is what i was using
>> with old gcc versions) for my chroots?
>>
>> If march=prescott is the right choice i think that the gcc "native"
>> detection should consider also $CHOST.
>
> This question is appropriate for the mailing list gcc-help@gcc.gnu.org,
> not the mailing list gcc@gcc.gnu.org. ?Please take any followups to
> gcc-help. ?Thanks.
>
> Your CPU is the same whether you have a chroot or not, and whether you
> are running in 32-bit mode or in 64-bit mode, so it is correct for gcc
> to handle -mtune=native -march=native the same way in both cases.
> Naturally the -march option will be affected by whether you are in
> 32-bit mode or not. ?As far as I know the -mtune option doesn't make any
> difference.
>
> It's reasonable to ask whether -mtune=nocona gives the best results when
> using a Prescott. ?Right now -mtune=prescott and -mtune=nocona are
> handled identically in any case. ?I don't know if there is room for
> improvement there or not.
>
> Ian
>


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