[arch-dev-public] x86_64: improving default CFLAGS

[Alexander Baldeck]

Andreas Radke wrote:
> http://bbs.archlinux.org/viewtopic.php?pid=243370#p243370
>
> "-mtune=cpu-type
>     Tune to cpu-type everything applicable about the generated code,
> except for the ABI and the set of available instructions."
>
> can somebody explain that a bit more? so mtune=nocona would not enable
> SSE3 instructions like i expected?
>
> Andy
>
>   
What -mtune will do is to optimize runtime code to run better in a
specific flavor of -march.
Meaning, that it will use generic x86_64 instruction set with no SSE3
enabled but more extensively
use advantages and work around disadvantages in yourr CPU variant. Such
as multi-scalarity and
things. -mtune normally doesn't optimize enough to notice any
difference, if I were you I wouldn't go
as far as to rebuild most of the repo just for the sake of a highly
questionable speedup.

Generally the -m<instruction set> or -m<arch specifics> give more
optimization because they also
will go and use extra instructions. Then again, they make the variety of
possible CPUs smaller. :)

Cheers,

-L