Re: BUG in libm's powf
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Date: Tue, 07 Sep 2021 08:00:24 UTC
> On 6 Sep 2021, at 21:22, Gordon Bergling <gbergling@gmail.com> wrote: > > could you turn to test program into an AFT test to prevent further regressions? Certainly! M > > —Gordon > >> Am 06.09.2021 um 20:04 schrieb Steve Kargl <sgk@troutmask.apl.washington.edu>: >> >> 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@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@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 >>>>>> @@ -136,7 +136,7 @@ >>>>>> /* |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 >> > -- Mark R V Murray