Date: Sat, 14 Jul 2001 21:08:56 -0700 From: Jordan Hubbard <jkh@freebsd.org> To: Jim.Pirzyk@disney.com Cc: jmcoopr@webmail.bmi.net, stephen@math.missouri.edu, freebsd-hackers@freebsd.org Subject: Re: math library difference between linux emulation and native freebsd (and native linux) Message-ID: <20010714210856G.jkh@freebsd.org> In-Reply-To: <01071421031403.00729@snoopy> References: <200107141907.MAA07030@smtp.bmi.net> <01071421031403.00729@snoopy>
next in thread | previous in thread | raw e-mail | index | archive | help
Seems a reasonable thing to do - have you found where in the linux
compatability code one might set it?
From: Jim Pirzyk <Jim.Pirzyk@disney.com>
Subject: Re: math library difference between linux emulation and native freebsd (and native linux)
Date: Sat, 14 Jul 2001 21:03:14 -0700
> So the solution to my problem was to set the __INITIAL_NPXCW__ to
> 0x37F. What I can think of is that the freebsd binary sets
> the Control Word to this before running but the linux binary
> does not (because it is assumed to already be set by the kernel
> at boot time). So I would think the linux kernel module would need
> to set it also.
>
> - JimP
>
> On Saturday 14 July 2001 12:00 pm, jmcoopr@webmail.bmi.net wrote:
> > In <3B50966E.1D6E5DCC@math.missouri.edu>, on 07/14/2001
> > at 01:58 PM, Stephen Montgomery-Smith <stephen@math.missouri.edu>
> >
> > said:
> > >Yes, I tried out the program
> > >
> > >#include <stdio.h>
> > >#include <math.h>
> > >main() {
> > > double x,y;
> > > int i;
> > >
> > > x = 53.278500;
> > > y = exp(x);
> > > printf("%8lf\n",x);
> > > for(i=0;i<sizeof(double);i++)
> > > printf("%x ",((unsigned char*)(&x))[i]);
> > > printf("\n");
> > > printf("%8lf\n",y);
> > > for(i=0;i<sizeof(double);i++)
> > > printf("%x ",((unsigned char*)(&y))[i]);
> > > printf("\n");
> > >}
> > >
> > >On FreeBSD and Linux I get
> > >53.278500
> > >cf f7 53 e3 a5 a3 4a 40
> > >137581029243568449912832.000000
> > >e7 7d 89 54 48 22 bd 44
> > >
> > >and on Linux emulation under FreeBSD I get
> > >53.278500
> > >cf f7 53 e3 a5 a3 4a 40
> > >137581029243567812378624.000000
> > >c1 7d 89 54 48 22 bd 44
> > >
> > >I don't really know the format of IEEE very well, but it looks like
> > >some low order bits are different - the answers are really different.
> > >
> > >I also tried the same experiment with sin and gamma - then the problem
> > >does not occur. Well except that the answer for gamma(53.278500) is
> > >reported as 157.464664 which is way wrong.
> > >
> > >When I tried it for x=52 they gave almost the same answer, only
> > >seperated by the last bit (the decimal versions were reported as the
> > >same). For x=54 they both gave the same wrong answer 160.331128.
> > >
> > >I don't know a whole lot about IEEE. What is the largest number it is
> > >supposed to handle? Looking at man math it says it should handle
> > >numbers as large as 1.8e308 - we certainly are not in that range!!!
> >
> > But remember, floating point is DIFFERENT--nothing is exact. You can
> > create a number with a magnitude about 1.8e308, but you sure don't get
> > 308 significant digits. Very big and very small floating point
> > quantities need special treatment to avoid all sorts of underflow,
> > overflow, singularity, degraded precision, etc. Naive, calculator-like
> > assumptions about floating point will get one into trouble very, very
> > quickly.
> >
> > The real question should be how many significant digits are the Linux /
> > FreeBSD implementations alike.
> >
> > For:
> >
> > 4-byte floats--6 digits is all you're likely to get, the rest is noise
> > 8-byte doubles--12-13 digits is all that is reasonable
> > 10-byte long doubles--15 digits, and then the world falls in
> >
> > Expectations beyond this are beyond the capability of i386 floating
> > point hardware. They're also way beyond what you usually need.
> >
> > jmc
> >
> >
> >
> > ------------------------------------------------------
> > jmcoopr@webmail.bmi.net
> >
> > Using OS/2 since 1.0
> > ------------------------------------------------------
>
> --
> --- @(#) $Id: dot.signature,v 1.10 2001/05/17 23:38:49 Jim.Pirzyk Exp $
> __o Jim.Pirzyk@disney.com ------------- pirzyk@freebsd.org
> _'\<,_ Senior Systems Engineer, Walt Disney Feature Animation
> (*)/ (*)
>
> To Unsubscribe: send mail to majordomo@FreeBSD.org
> with "unsubscribe freebsd-hackers" in the body of the message
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20010714210856G.jkh>