Sorry for the wrong mailing list, i mailed to gcc@xxxxxxxxxxx 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@xxxxxxxxxx>: > Luca Zorzo <lucazorzo@xxxxxxxxx> 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@xxxxxxxxxxx, > not the mailing list gcc@xxxxxxxxxxxx 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 >
