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| author | Michael Munday <mike.munday@lowrisc.org> | 2023-07-01 00:01:26 +0100 |
|---|---|---|
| committer | Keith Randall <khr@golang.org> | 2023-07-05 22:05:30 +0000 |
| commit | c8dad424bf01df69af729845acc151a66b87d594 (patch) | |
| tree | 0423a6e2afa8c95460eb82c11af8aae841995f9b | |
| parent | cd6676126b7e663e6202e98e2f235fff20d5e858 (diff) | |
| download | go-c8dad424bf01df69af729845acc151a66b87d594.tar.gz go-c8dad424bf01df69af729845acc151a66b87d594.zip | |
math: fix portable FMA when x*y < 0 and x*y == -z
When x*y == -z the portable implementation of FMA copied the sign
bit from x*y into the result. This meant that when x*y == -z and
x*y < 0 the result was -0 which is incorrect.
Fixes #61130.
Change-Id: Ib93a568b7bdb9031e2aedfa1bdfa9bddde90851d
Reviewed-on: https://go-review.googlesource.com/c/go/+/507376
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Michael Munday <mike.munday@lowrisc.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
| -rw-r--r-- | src/math/all_test.go | 7 | ||||
| -rw-r--r-- | src/math/fma.go | 5 |
2 files changed, 12 insertions, 0 deletions
diff --git a/src/math/all_test.go b/src/math/all_test.go index 886267bc17..96a398e9c6 100644 --- a/src/math/all_test.go +++ b/src/math/all_test.go @@ -2059,6 +2059,9 @@ var fmaC = []struct{ x, y, z, want float64 }{ // Special {0, 0, 0, 0}, + {Copysign(0, -1), 0, 0, 0}, + {0, 0, Copysign(0, -1), 0}, + {Copysign(0, -1), 0, Copysign(0, -1), Copysign(0, -1)}, {-1.1754226043408471e-38, NaN(), Inf(0), NaN()}, {0, 0, 2.22507385643494e-308, 2.22507385643494e-308}, {-8.65697792e+09, NaN(), -7.516192799999999e+09, NaN()}, @@ -2077,6 +2080,10 @@ var fmaC = []struct{ x, y, z, want float64 }{ {4.612811918325842e+18, 1.4901161193847641e-08, 2.6077032311277997e-08, 6.873625395187494e+10}, {-9.094947033611148e-13, 4.450691014249257e-308, 2.086006742350485e-308, 2.086006742346437e-308}, {-7.751454006381804e-05, 5.588653777189071e-308, -2.2207280111272877e-308, -2.2211612130544025e-308}, + + // Issue #61130 + {-1, 1, 1, 0}, + {1, 1, -1, 0}, } var sqrt32 = []float32{ diff --git a/src/math/fma.go b/src/math/fma.go index ca0bf99f21..ba03fbe8a9 100644 --- a/src/math/fma.go +++ b/src/math/fma.go @@ -132,6 +132,11 @@ func FMA(x, y, z float64) float64 { ps, pe, pm1, pm2, zs, ze, zm1, zm2 = zs, ze, zm1, zm2, ps, pe, pm1, pm2 } + // Special case: if p == -z the result is always +0 since neither operand is zero. + if ps != zs && pe == ze && pm1 == zm1 && pm2 == zm2 { + return 0 + } + // Align significands zm1, zm2 = shrcompress(zm1, zm2, uint(pe-ze)) |