Compiler Options

Joerg Pernfuss elessar at
Sat Nov 11 01:31:00 UTC 2006

On Sat, 11 Nov 2006 10:19:42 +1030
"Daniel O'Connor" <doconnor at> wrote:

> On Saturday 11 November 2006 08:54, Jeremy Chadwick wrote:
> > The kernel itself _will not_ use any SSE or MMX operations when
> > built. This is because these optimisations are known to break the
> > FreeBSD kernel.  This applies to all i386 architectures, and
> > probably 64-bit architectures too (not sure).
> I think this is mainly because the kernel has no FPU context so you
> can't actually use any FPU operation (including SSE & MMX) without
> potentially trashing userland data.

If I remember correctly, this was the reason why some improvements
from DragonFlyBSD were not ported to FreeBSD (or were ported but
not committed). There was no conclusive evidence that the speed
improvement for some operations would result in a net gain, due to
register saving/restoring overhead during context switches.

> (I could be wrong but the above is what I remember reading on various
> lists over the years :)

I think you remember right. Or I remember wrong too :)

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