Date: Wed, 28 Jan 2009 15:35:30 -0500 From: John Baldwin <jhb@freebsd.org> To: Bakul Shah <bakul@bitblocks.com> Cc: Kostik Belousov <kostikbel@gmail.com>, freebsd-hackers@freebsd.org, Carl Shapiro <carl.shapiro@gmail.com> Subject: Re: critical floating point incompatibility Message-ID: <200901281535.31501.jhb@freebsd.org> In-Reply-To: <20090128192421.C9E3D5B13@mail.bitblocks.com> References: <20071220093950.GA79196@server.vk2pj.dyndns.org> <200901261651.29116.jhb@freebsd.org> <20090128192421.C9E3D5B13@mail.bitblocks.com>
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On Wednesday 28 January 2009 2:24:21 pm Bakul Shah wrote: > On Mon, 26 Jan 2009 16:51:28 EST John Baldwin <jhb@freebsd.org> wrote: > > On Friday 21 December 2007 3:16:33 pm Kostik Belousov wrote: > > > On Fri, Dec 21, 2007 at 10:11:24AM -0800, Bakul Shah wrote: > > > > Peter Jeremy <peterjeremy@optushome.com.au> wrote: > > > > > On Wed, Dec 19, 2007 at 09:40:34PM -0800, Carl Shapiro wrote: > > > > > >The default setting of the x87 floating point control word on the i386 > > > > > >port is 0x127F. Among other things, this value sets the precision > > > > > >control to double precision. The default setting of the x87 floating > > > > > >point control word on the AMD64 is 0x37F. > > > > > ... > > > > > >It seems clear that the right thing to do is to set the floating point > > > > > >environment to the i386 default for i386 binaries. Is the current > > > > > >behavior intended? > > > > > > > > > > I believe this is an oversight. See the thread beginning > > > > > > > http://lists.freebsd.org/pipermail/freebsd-stable/2007-November/037947.html > > > > > > > > >From reading Bruce's last message in that thread, seems to me > > > > may be default for 64bit binaries should be the same as on > > > > i386. Anyone wanting different behavior can always call > > > > fpsetprec() etc. > > > > > > > > I think the fix is to change __INITIAL_FPUCW__ in > > > > /sys/amd64/include/fpu.h to 0x127F like on i386. > > > I think this shall be done for 32-bit processes only, or we get into > > > another ABI breaking nightmare. > > > > How about something like this: (Carl, can you please test this?) > > Your patch works fine on a recent -current. Here is a > program Carl had sent me more than a year ago for testing > this. May be some varition of it can be added to > compatibility tests. > > #include <stdio.h> > int main(void) > { > unsigned short cw; > __asm__ __volatile__ ("fnstcw %0":"=m"(*&cw)); > printf("cw=%#x\n", cw); > return 0; > } > > -- bakul Cool, thanks for testing! -- John Baldwin
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