diff options
| author | ICHINOSE Shogo <shogo82148@gmail.com> | 2025-05-18 17:28:09 +0000 |
|---|---|---|
| committer | Gopher Robot <gobot@golang.org> | 2025-05-19 14:45:48 -0700 |
| commit | 498899e20549a9e30f7641fc3a3592f89a933a95 (patch) | |
| tree | 1942e42c5c6b0d4b91aa535c8e0c66047c230cef /src/math | |
| parent | 2cde950049dd46cae3ae91cde3e270825793ba0f (diff) | |
| download | go-498899e20549a9e30f7641fc3a3592f89a933a95.tar.xz | |
math: fix portable FMA implementation when x*y ~ 0, x*y < 0 and z = 0
Adding zero usually does not change the original value.
However, there is an exception with negative zero. (e.g. (-0) + (+0) = (+0))
This applies when x * y is negative and underflows.
Fixes #73757
Change-Id: Ib7b54bdacd1dcfe3d392802ea35cdb4e989f9371
GitHub-Last-Rev: 30d74883b21667fc9439d9d14932b7edb3e72cd5
GitHub-Pull-Request: golang/go#73759
Reviewed-on: https://go-review.googlesource.com/c/go/+/673856
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Diffstat (limited to 'src/math')
| -rw-r--r-- | src/math/all_test.go | 5 | ||||
| -rw-r--r-- | src/math/fma.go | 9 |
2 files changed, 13 insertions, 1 deletions
diff --git a/src/math/all_test.go b/src/math/all_test.go index c253b7bc02..4e5f451762 100644 --- a/src/math/all_test.go +++ b/src/math/all_test.go @@ -2126,6 +2126,11 @@ var fmaC = []struct{ x, y, z, want float64 }{ // Issue #61130 {-1, 1, 1, 0}, {1, 1, -1, 0}, + + // Issue #73757 + {0x1p-1022, -0x1p-1022, 0, Copysign(0, -1)}, + {Copysign(0, -1), 1, 0, 0}, + {1, Copysign(0, -1), 0, 0}, } var sqrt32 = []float32{ diff --git a/src/math/fma.go b/src/math/fma.go index ba03fbe8a9..c806b914da 100644 --- a/src/math/fma.go +++ b/src/math/fma.go @@ -96,9 +96,16 @@ func FMA(x, y, z float64) float64 { bx, by, bz := Float64bits(x), Float64bits(y), Float64bits(z) // Inf or NaN or zero involved. At most one rounding will occur. - if x == 0.0 || y == 0.0 || z == 0.0 || bx&uvinf == uvinf || by&uvinf == uvinf { + if x == 0.0 || y == 0.0 || bx&uvinf == uvinf || by&uvinf == uvinf { return x*y + z } + // Handle z == 0.0 separately. + // Adding zero usually does not change the original value. + // However, there is an exception with negative zero. (e.g. (-0) + (+0) = (+0)) + // This applies when x * y is negative and underflows. + if z == 0.0 { + return x * y + } // Handle non-finite z separately. Evaluating x*y+z where // x and y are finite, but z is infinite, should always result in z. if bz&uvinf == uvinf { |