math: Expm1 returns -1 with large negative argument.

Fixes #11442

Change-Id: I2053fe752c6a122924d28565f1338f73e00ed417
Reviewed-on: https://go-review.googlesource.com/11791
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/math/all_test.go

@@ -946,16 +946,20 @@ var expSC = []float64{
 
 var vfexpm1SC = []float64{
 	Inf(-1),
+	-710,
 	Copysign(0, -1),
 	0,
+	710,
 	Inf(1),
 	NaN(),
 }
 var expm1SC = []float64{
+	-1,
 	-1,
 	Copysign(0, -1),
 	0,
 	Inf(1),
+	Inf(1),
 	NaN(),
 }
 

src/math/expm1.go

@@ -158,11 +158,11 @@ func expm1(x float64) float64 {
 
 	// filter out huge argument
 	if absx >= Ln2X56 { // if |x| >= 56 * ln2
-		if absx >= Othreshold { // if |x| >= 709.78...
-			return Inf(1) // overflow
-		}
 		if sign {
-			return -1 // x < -56*ln2, return -1.0
+			return -1 // x < -56*ln2, return -1
+		}
+		if absx >= Othreshold { // if |x| >= 709.78...
+			return Inf(1)
 		}
 	}