Date: Fri, 17 Dec 2010 16:27:52 +0100 From: Dimitry Andric <dim@FreeBSD.org> To: Alexander Best <arundel@freebsd.org> Cc: freebsd-toolchain@freebsd.org Subject: Re: clang and -mfpmath=387 on ARCH=amd64 Message-ID: <4D0B8178.9010609@FreeBSD.org> In-Reply-To: <20101214204006.GA81772@freebsd.org> References: <20101211114707.GA60390@freebsd.org> <4D051FD1.7070100@FreeBSD.org> <20101214204006.GA81772@freebsd.org>
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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. It would be nice if gcc/clang had an option "-mdie-on-any-fp", but they don't, unfortunately. :)
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