Compiler Options

Daniel O'Connor doconnor at
Fri Nov 10 23:49:48 UTC 2006

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.

Also, the cost of saving/restoring the context is quite high so potential 
benefits are largely negated.

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

Daniel O'Connor software and network engineer
for Genesis Software -
"The nice thing about standards is that there
are so many of them to choose from."
  -- Andrew Tanenbaum
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