Date: Mon, 6 Sep 2021 18:55:07 +0100 From: Mark Murray <markm@FreeBSD.org> To: Steve Kargl <sgk@troutmask.apl.washington.edu> Cc: freebsd-current@freebsd.org Subject: Re: BUG in libm's powf Message-ID: <AAA24AAD-86C5-4736-AD0C-EFBCA3FA59B2@FreeBSD.org> In-Reply-To: <20210906175318.GA87474@troutmask.apl.washington.edu> References: <20210906152836.GA86615@troutmask.apl.washington.edu> <01D81389-45D1-4278-8DC1-7995D34D0276@FreeBSD.org> <20210906175318.GA87474@troutmask.apl.washington.edu>
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[-- Attachment #1 --] 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 [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG/MacGPG2 v2.2 Comment: GPGTools - http://gpgtools.org iQEzBAEBCgAdFiEEyzPHvybPbOpU9MCxQlsJDh9CUqAFAmE2VfsACgkQQlsJDh9C UqAjVwf+OyV1HRj+qGQyQvnneKNd+FIznK82ZjKfMppRgSrHdPndUqJiMgn5VYGn SbUSQhwL0iqVUz6apIqZ7o9jQMBJE4YyemWgbcLl/l3KsgkKL6tn5n07iKFgO3EM cRBPmNGmoTaGeZLO/KhO7DXLA5OxQ3SifDK52AkODAMHwxIv//oPwq3O3tYSFnl+ C1Zx4QVZ8546UUJp5FV/MGyGZ8OU1Kf/wJPG7Tf3SHpHbAlpZQFM3XY4h70KPHme C0cdV5FK4qLtcaaERaR+dwpN7PZma4HttHNnB6deyTzQ3UKHWcoaaht8OsvUAky/ KNEx/b/5jUauEtcfjERaXmyKtLHunA== =4mRH -----END PGP SIGNATURE-----help
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