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>
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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
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