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 {