From: Steve Kargl <kargls@comcast.net> To: Ryan Libby <rlibby@gmail.com>, Ed Maste <emaste@freebsd.org>, kargl@freebsd.org Cc: src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 888796ade284 - main - libm: fma: correct zero sign with small inputs In-Reply-To: <CAHgpiFyTM-cWgyStKa4Ln7NNv84DtxjJw0sAP_-VynPvCe-=Ag@mail.gmail.com> References: <202406081556.458FuJYN022360@gitrepo.freebsd.org> <CAHgpiFyTM-cWgyStKa4Ln7NNv84DtxjJw0sAP_-VynPvCe-=Ag@mail.gmail.com>
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On 6/10/24 16:31, Ryan Libby wrote: > On Sat, Jun 8, 2024 at 8:56 AM Ed Maste <emaste@freebsd.org> wrote: >> The branch main has been updated by emaste: >> >> URL: https://cgit.FreeBSD.org/src/commit/?id=888796ade2842486d3167067e8034254c38aadd3 >> >> commit 888796ade2842486d3167067e8034254c38aadd3 >> Author: Ed Maste <emaste@FreeBSD.org> >> AuthorDate: 2024-03-19 14:31:39 +0000 >> Commit: Ed Maste <emaste@FreeBSD.org> >> CommitDate: 2024-06-08 15:55:36 +0000 >> >> libm: fma: correct zero sign with small inputs >> >> PR: 277783 >> Reported by: Victor Stinner >> Submitted by: kargl >> MFC after: 1 week >> Differential Revision: https://reviews.freebsd.org/D44433 >> --- >> lib/msun/src/s_fma.c | 4 +++- >> lib/msun/src/s_fmal.c | 4 +++- >> 2 files changed, 6 insertions(+), 2 deletions(-) >> >> diff --git a/lib/msun/src/s_fma.c b/lib/msun/src/s_fma.c >> index b8a342646d85..4d08b40cc71a 100644 >> --- a/lib/msun/src/s_fma.c >> +++ b/lib/msun/src/s_fma.c >> @@ -267,7 +267,9 @@ fma(double x, double y, double z) >> */ >> fesetround(oround); >> volatile double vzs = zs; /* XXX gcc CSE bug workaround */ >> - return (xy.hi + vzs + ldexp(xy.lo, spread)); >> + xs = ldexp(xy.lo, spread); >> + xy.hi += vzs; >> + return (xy.hi == 0 ? xs : xy.hi + xs); >> } >> >> if (oround != FE_TONEAREST) { >> diff --git a/lib/msun/src/s_fmal.c b/lib/msun/src/s_fmal.c >> index 3d333632127c..12f9c364670b 100644 >> --- a/lib/msun/src/s_fmal.c >> +++ b/lib/msun/src/s_fmal.c >> @@ -248,7 +248,9 @@ fmal(long double x, long double y, long double z) >> */ >> fesetround(oround); >> volatile long double vzs = zs; /* XXX gcc CSE bug workaround */ >> - return (xy.hi + vzs + ldexpl(xy.lo, spread)); >> + xs = ldexpl(xy.lo, spread); >> + xy.hi += vzs; >> + return (xy.hi == 0 ? xs : xy.hi + xs); >> } >> >> if (oround != FE_TONEAREST) { > This seems to have caused the lib/msun/fma_tests:zeroes test to fail in > the FE_ROUNDDOWN mode on amd64, now finding 0 while expecting -0. I > don't know if the test is wrong or too strict, or if the new result is > wrong. > > https://ci.freebsd.org/job/FreeBSD-main-amd64-test/25249/testReport/junit/lib.msun/fma_test/zeroes/ > > Reproduces with > kyua debug -k /usr/tests/Kyuafile lib/msun/fma_test:zeroes The test is unreadable except by the original author. I'll note that there is a comment in fma_test.c /* * This is needed because clang constant-folds fma in ways that are incorrect * in rounding modes other than FE_TONEAREST. */ static volatile double one = 1.0; and the failing tests are testall(-one, one, one, rd ? -0.0 : 0.0, ALL_STD_EXCEPT, 0); testall(one, -one, one, rd ? -0.0 : 0.0, ALL_STD_EXCEPT, 0); testall(-one, -one, -one, rd ? -0.0 : 0.0, ALL_STD_EXCEPT, 0); If one compares against MPFR, fesetround(FE_TONEAREST); dx = -1; dy = dz = 1; mpfr_fma(a, x, y, z, MPFR_RNDN); mpfr_printf("% Re % e\n", a, fma(dx, dy, dz)); fesetround(FE_DOWNWARD); mpfr_fma(a, x, y, z, MPFR_RNDD); mpfr_printf("% Re % e\n", a, fma(dx, dy, dz)); fesetround(FE_UPWARD); mpfr_fma(a, x, y, z, MPFR_RNDU); mpfr_printf("% Re % e\n", a, fma(dx, dy, dz)); fesetround(FE_TOWARDZERO); mpfr_fma(a, x, y, z, MPFR_RNDZ); mpfr_printf("% Re % e\n", a, fma(dx, dy, dz)); % gcc13 -fno-builtin -o z a.c -I/usr/local/include -L/usr/local/lib -lmpfr -lgmp -lm % ./z 0.0000000000000000e+00 0.000000e+00 -0.0000000000000000e+00 -0.000000e+00 0.0000000000000000e+00 0.000000e+00 0.0000000000000000e+00 0.000000e+00 0.0000000000000000e+00 0.000000e+00 -0.0000000000000000e+00 -0.000000e+00 0.0000000000000000e+00 0.000000e+00 0.0000000000000000e+00 0.000000e+00 0.0000000000000000e+00 0.000000e+00 -0.0000000000000000e+00 -0.000000e+00 0.0000000000000000e+00 0.000000e+00 0.0000000000000000e+00 0.000000e+00 cc gives a similar result. -- steve
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