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math: fix Ldexp when result is below ldexp(2, -1075)

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Before this change, the smallest result Ldexp can handle was
ldexp(2, -1075), which is SmallestNonzeroFloat64.
There are some numbers below it should also be rounded to
SmallestNonzeroFloat64. The change fixes this.

Fixes #23407

Change-Id: I76f4cb005a6e9ccdd95b5e5c734079fd5d29e4aa
Reviewed-on: https://go-review.googlesource.com/87338
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
This commit is contained in:
Wèi Cōngruì 2018-01-11 13:11:12 +08:00 committed by Robert Griesemer
parent 9967582f77
commit 4b265fb747
2 changed files with 7 additions and 3 deletions
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4
src/math/all_test.go
View File

@ -1967,6 +1967,8 @@ var vfldexpBC = []fi{
{-1, -1075},
{1, 1024},
{-1, 1024},
{1.0000000000000002, -1075},
{1, -1075},
}
var ldexpBC = []float64{
SmallestNonzeroFloat64,
@ -1977,6 +1979,8 @@ var ldexpBC = []float64{
Copysign(0, -1),
Inf(1),
Inf(-1),
SmallestNonzeroFloat64,
0,
}
var logbBC = []float64{

6
src/math/ldexp.go
View File

@ -25,7 +25,7 @@ func ldexp(frac float64, exp int) float64 {
exp += e
x := Float64bits(frac)
exp += int(x>>shift)&mask - bias
if exp < -1074 {
if exp < -1075 {
return Copysign(0, frac) // underflow
}
if exp > 1023 { // overflow
@ -36,8 +36,8 @@ func ldexp(frac float64, exp int) float64 {
}
var m float64 = 1
if exp < -1022 { // denormal
exp += 52
m = 1.0 / (1 << 52) // 2**-52
exp += 53
m = 1.0 / (1 << 53) // 2**-53
}
x &^= mask << shift
x |= uint64(exp+bias) << shift