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

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>

src/math/all_test.go

@@ -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{

src/math/ldexp.go

@@ -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