Date: Mon, 6 Sep 2021 11:04:59 -0700 From: Steve Kargl <sgk@troutmask.apl.washington.edu> To: Mark Murray <markm@freebsd.org> Cc: freebsd-current@freebsd.org Subject: Re: BUG in libm's powf Message-ID: <20210906180459.GA87487@troutmask.apl.washington.edu> In-Reply-To: <AAA24AAD-86C5-4736-AD0C-EFBCA3FA59B2@FreeBSD.org> References: <20210906152836.GA86615@troutmask.apl.washington.edu> <01D81389-45D1-4278-8DC1-7995D34D0276@FreeBSD.org> <20210906175318.GA87474@troutmask.apl.washington.edu> <AAA24AAD-86C5-4736-AD0C-EFBCA3FA59B2@FreeBSD.org>
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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
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