Re: BUG in libm's powf

From: Mark Murray <markm_at_FreeBSD.org>
Date: Mon, 6 Sep 2021 18:45:11 +0100
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


Received on Mon Sep 06 2021 - 17:45:11 UTC

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