critical floating point incompatibility

[Bakul Shah]

On Mon, 26 Jan 2009 16:51:28 EST John Baldwin <jhb at 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 at 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

PS:
<tangent>

On a mac, cc -m64 builds 64 bit binaries and cc -m32 builds
32 bit binaries.  The following script makes it as easy to do
so on a 64 bit FreeBSD -- at least on the few programs I
tried.  Ideally the right magic needs to be folded in gcc's
builtin "specs".

#!/bin/sh
args=/usr/bin/cc
while [ ".$1" != . ]
do
    a=$1; shift
    case $a in
    -m32) args="$args -B/usr/lib32 -I/usr/include32 -m32";;
    *) args="$args $a";;
    esac
done
$args

Ideally x86_64 platforms run *all* i386 programs (that don't
depend on a 32 bit kernel).

</tangent>