Date: Sat, 18 Dec 2010 15:48:45 +0000 From: Alexander Best <arundel@freebsd.org> To: Dimitry Andric <dim@FreeBSD.org> Cc: freebsd-toolchain@freebsd.org Subject: Re: clang and -mfpmath=387 on ARCH=amd64 Message-ID: <20101218154845.GA36640@freebsd.org> In-Reply-To: <4D0B8178.9010609@FreeBSD.org> References: <20101211114707.GA60390@freebsd.org> <4D051FD1.7070100@FreeBSD.org> <20101214204006.GA81772@freebsd.org> <4D0B8178.9010609@FreeBSD.org>
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On Fri Dec 17 10, Dimitry Andric wrote:
> On 2010-12-14 21:40, Alexander Best wrote:
> >On Sun Dec 12 10, Dimitry Andric wrote:
> ...
> >>I have compiled two GENERIC kernels on amd64 with and without the
> >>-mfpmath=386 option, and those resulted in exactly the same binaries
> >>(apart from the compilation timestamp).
> >>
> >>So I think the whole -mfpmath=387 option is nonsensical anyway. The
> >>comment just above those CFLAGS in sys/conf/kern.mk says:
> >
> >i think for i386 the case is clear: -mfpmath=387 *is* the default. for
> >amd64 it
> >depends.
>
> On amd64 -mfpmath=sse is the default. However, floating point is not
> allowed at all in the kernel, and even if it were used, it would result
> in a compile error, because -mno-sse is also used. You can verify this
> by compiling a small program on amd64 that uses floating point, and
> passing the -mno-sse option.
>
>
> >if we can be sure that the -mno-sse[2-3]? options will set
> >-mfpmath=387 there is no need to set -mfpmath=387. it seems from your
> >tests and
> >also from a logical sense of point that this is the case. however the gcc
> >manual doesn't really state this matter. so it could in fact be possible
> >that
> >even with -mno-sse[2-3]? set, -mfpmath=sse remains the default setting for
> >amd64.
>
> Yes it is the default; but as I said, it makes no difference. Neither
> i387 nor sse hardware FP instructions should ever be generated in the
> kernel.
so this patch should keep clang from complaing while compiling the kernel and
in addition to that won't have any impact on gcc.
cheers.
alex
>
> It would be nice if gcc/clang had an option "-mdie-on-any-fp", but they
> don't, unfortunately. :)
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diff --git a/sys/conf/kern.mk b/sys/conf/kern.mk
index 4d8636c..e277b74 100644
--- a/sys/conf/kern.mk
+++ b/sys/conf/kern.mk
@@ -66,7 +66,7 @@ INLINE_LIMIT?= 15000
#
.if ${MACHINE_CPUARCH} == "amd64"
CFLAGS+= -mcmodel=kernel -mno-red-zone \
- -mfpmath=387 -mno-sse -mno-sse2 -mno-sse3 -mno-mmx -mno-3dnow \
+ -mno-sse -mno-sse2 -mno-sse3 -mno-mmx -mno-3dnow \
-msoft-float -fno-asynchronous-unwind-tables
INLINE_LIMIT?= 8000
.endif
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