Date: Mon, 6 Sep 2021 10:53:18 -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: <20210906175318.GA87474@troutmask.apl.washington.edu> In-Reply-To: <01D81389-45D1-4278-8DC1-7995D34D0276@FreeBSD.org> References: <20210906152836.GA86615@troutmask.apl.washington.edu> <01D81389-45D1-4278-8DC1-7995D34D0276@FreeBSD.org>
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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
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