On 6/10/24 16:31, Ryan Libby wrote:
On Sat, Jun 8, 2024 at 8:56 AM Ed Maste <ema...@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 <ema...@freebsd.org>
AuthorDate: 2024-03-19 14:31:39 +0000
Commit: Ed Maste <ema...@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
