Re: BUG in libm's powf

From: Steve Kargl <sgk_at_troutmask.apl.washington.edu>
Date: Mon, 6 Sep 2021 11:04:59 -0700
No, thank you for the quick response.

Of course, a one character diff might be easier to review. :-)

-- 
steve 

On Mon, Sep 06, 2021 at 06:55:07PM +0100, Mark Murray wrote:
> Thanks!
> 
> And it's committed!
> 
> M
> 
> > On 6 Sep 2021, at 18:53, Steve Kargl <sgk_at_troutmask.apl.washington.edu> wrote:
> > 
> > Fine with me.  I don't have a phabricator account and
> > bugzilla reports seems to get lost in the ether.
> > 
> > --
> > steve
> > 
> > On Mon, Sep 06, 2021 at 06:45:11PM +0100, Mark Murray wrote:
> >> Hi
> >> 
> >> I've opened a Phab ticket for this. I hope that's OK?
> >> 
> >> https://reviews.freebsd.org/D31865
> >> 
> >> M
> >> 
> >>> On 6 Sep 2021, at 16:28, Steve Kargl <sgk_at_troutmask.apl.washington.edu> wrote:
> >>> 
> >>> Paul Zimmermann has identified a bug in Openlibm's powf(),
> >>> which is identical to FreeBSD's libm.  Both derived from
> >>> fdlibm. https://github.com/JuliaMath/openlibm/issues/212.
> >>> 
> >>> Consider
> >>> 
> >>> % cat h.c
> >>> #include <math.h>
> >>> #include <stdio.h>
> >>> int
> >>> main(void)
> >>> {
> >>>  float x, y, z;
> >>>  x =  0x1.ffffecp-1F;
> >>>  y = -0x1.000002p+27F;
> >>>  z =  0x1.557a86p115F;
> >>>  printf("%e %e %e <-- should be %e\n", x, y, powf(x,y), z);
> >>>  return 0;
> >>> }
> >>> 
> >>> % cc -o h -fno-builtin h.c -lm && ./h
> >>> 9.999994e-01 -1.342177e+08 inf <-- should be 5.540807e+34
> >>> 
> >>> Note, clang seems to have a builtin for powf(), but one cannot
> >>> count of clang being the only consumer of libm.  With the patch
> >>> at the end of this email, I get
> >>> 
> >>> % cc -o h -fno-builtin h.c -L/home/kargl/trunk/math/libm/msun -lmath && ./h
> >>> 9.999994e-01 -1.342177e+08 5.540807e+34 <-- should be 5.540807e+34
> >>> 
> >>> Watch for copy and paste whitespace corruption.
> >>> 
> >>> --- /usr/src/lib/msun/src/e_powf.c	2021-02-21 03:29:00.956878000 -0800
> >>> +++ src/e_powf.c	2021-09-06 08:17:09.800008000 -0700
> >>> _at__at_ -136,7 +136,7 _at__at_
> >>>    /* |y| is huge */
> >>> 	if(iy>0x4d000000) { /* if |y| > 2**27 */
> >>> 	/* over/underflow if x is not close to one */
> >>> -	    if(ix<0x3f7ffff7) return (hy<0)? sn*huge*huge:sn*tiny*tiny;
> >>> +	    if(ix<0x3f7ffff6) return (hy<0)? sn*huge*huge:sn*tiny*tiny;
> >>> 	    if(ix>0x3f800007) return (hy>0)? sn*huge*huge:sn*tiny*tiny;
> >>> 	/* now |1-x| is tiny <= 2**-20, suffice to compute
> >>> 	   log(x) by x-x^2/2+x^3/3-x^4/4 */
> >>> 
> >>> 
> >>> --
> >>> Steve
> >>> 
> >> 
> >> --
> >> Mark R V Murray
> >> 
> > 
> > 
> > 
> > --
> > Steve
> > 
> 
> --
> Mark R V Murray
> 



-- 
Steve
Received on Mon Sep 06 2021 - 18:04:59 UTC

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