Date: Mon, 6 Sep 2021 18:45:11 +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: <01D81389-45D1-4278-8DC1-7995D34D0276@FreeBSD.org> In-Reply-To: <20210906152836.GA86615@troutmask.apl.washington.edu> References: <20210906152836.GA86615@troutmask.apl.washington.edu>
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[-- Attachment #1 --] 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 [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- Version: GnuPG/MacGPG2 v2.2 Comment: GPGTools - http://gpgtools.org iQEzBAEBCgAdFiEEyzPHvybPbOpU9MCxQlsJDh9CUqAFAmE2U6cACgkQQlsJDh9C UqDiPgf/TFqUqY43Quu2WJD1I51lnxkFi2QywwPENaIxMv3RvrhjD/3D59oeLTrA RO8HW0v4qDnRxGAFkHRHJnxcRgi2KI7WEEYVZ0IZ2OQLb1RamgkSUeoMoHCJcgTF gPFF4JAANDAnldP/0lVZrOGttENvNo0wJqbWCaWtG6Z8lex6k5XDNej8pNxqo8/z FTe5LRucg6IuPpmrhf/3XQkNnFf1yEaDAnsrbtqaelkz2aRzVTuhUajk4c5FRFsM F4kqRWsvz/urjrkEbfwCxQKO7lFt6LJGsWDOZ7BB96zAAX04r9Lb0FZbNrWT8cWn sQYIXhBMVbYKi/T7i55QTS3FlLXGxg== =Bs0m -----END PGP SIGNATURE-----
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