Re: [gentoo-amd64] CFLAGS question from a AMD64 newbie

[Volker Armin Hemmann]

On Dienstag 09 Dezember 2008, Sami Näätänen wrote:
> So hi from a amd64 newbie. Not so newbie with Gentoo though. :)
>
> My system is an Intel quad core core2 with a 2.4 GHz clock speed coupled
> with a 4GB of memory. No overclocking etc. Want this to be stable. :)
>
> I'm just curious what people use as their stable CFLAGS in amd64 Gentoo?
> (Sorry if this has been up lately, but I just switched to 64bit env so...)
>
>
> Here is mine and some explanation of why (And I use ~arch system with gcc
> 4.3)
>
> The flags are in order they are used in my CFLAGS and CXXFLAGS.
>
> Gives stable base
> -O2

yes

>
> Want to optimize for my system, but don't want "native"
> -march=core2

ok

>
> If some ebuilds filter march this will still cache optimize etc for my
> system -mtune=core2

I would scrap that.

>
> Faster floating point math and better chance of vectorization
> -mfpmath=sse

superfluos. March with amd64 sse is used by default.

>
> These because of the march might get filtered
> -mmmx -msse -msse2 -msse3 -mssse3

if march get filtered, these might one of the reasons, I would remove them.
>
> For loop vectorization
> -ftree-vectorize

scrap that.


> -pipe
>
once upon a time I used this flags:

#CFLAGS="-march=k8 -O2 -pipe -fweb -ftracer -fpeel-loops -msse3"
and even
#CFLAGS="-march=k8 -O2 -fweb -ftracer -fpeel-loops -ftree-vectorize -frename-
registers -floop-optimize2 -msse3 -pipe"

to hunt down a java bug, I recompiled the whole system with:

CFLAGS="-march=k8 -O2 -msse3 -pipe"

and surprise - it was as fast as before - and compiling was faster too!