-mno-sse -mno-sse2 -mno-mmx -mno-3dnow when compiling kernel
peter at wemm.org
Thu Feb 17 13:49:31 PST 2005
On Sunday 13 February 2005 10:29 am, Thomas Krause -CI- wrote:
> why are the Switches "-mno-sse -mno-sse2 -mno-mmx -mno-3dnow" are set
> when compiling an amd64 kernel? I think, the CPU supports all thease
> I have no CPUTYPE specified in /etc/make.conf, as I could not
> find a matching option in /usr/src/share/examples/etc/make.conf.
For speed reasons, we restrict the kernel to using integer instructions
only. This means we can avoid the considerable expense of
saving/restoring floating point context unnecessarily for syscalls,
On FreeBSD/i386, the compiler generally doesn't generate floating point
instructions unless there is a reason to. But on FreeBSD/amd64, the
compiler will do it readily because things like MMX, SSE and SSE2 are
*known* to exist on this family and are part of the function calling
conventions, especially for things like varargs functions.
The upshot of this is that we are not in a lot of danger of the i386
compiler accidently generating floating point code for the kernel, but
on FreeBSD/amd64, it can and does if you give it the chance.
How the code for the kernel is generated has no bearing on the user code
environment at all. The floating point/mmx/sse/sse2 context is
reserved entirely for the user.
Note that we *could* concievably set aside a few xmm registers for
faster data copies inside the kernel, but we don't do it yet. I
believe linux makes %xmm0 and %xmm1 available in the kernel and saves
them with every trap into kernel mode.
Peter Wemm - peter at wemm.org; peter at FreeBSD.org; peter at yahoo-inc.com
"All of this is for nothing if we don't go to the stars" - JMS/B5
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