Re: BUG in libm's powf

From: Steve Kargl <sgk_at_troutmask.apl.washington.edu>
Date: Mon, 6 Sep 2021 10:53:18 -0700
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
Received on Mon Sep 06 2021 - 17:53:18 UTC

Original text of this message