natives/math: Use Go implementation for extreme Ldexp inputs.

dmitshur · dmitshur · commit f6627b0e1b5a · 2018-08-21T00:07:06.000-04:00
In Go 1.11, there has been an improvement made to Ldexp for some extreme values. The previous Math.pow-using algorithm doesn't produce correct results. Use the Go implementation of ldexp for those values. Continue to use the current Math.pow approach for other inputs because it's much faster: $ gopherjs test -v --run=none --bench=Ldexp math BenchmarkLdexp 2000000000 0.44 ns/op Compared to using the Go implementation for all inputs, without the Math.pow fast path: $ gopherjs test -v --run=none --bench=Ldexp math BenchmarkLdexp 5000000 203 ns/op Follows golang/go@4b265fb. Updates golang/go#23407.
diff --git a/compiler/natives/src/math/math.go b/compiler/natives/src/math/math.go
@@ -119,16 +119,13 @@ func IsNaN(f float64) (is bool) {
 }
 
 func Ldexp(frac float64, exp int) float64 {
-	if frac == 0 {
-		return frac
+	if -1024 < exp && exp < 1024 { // Use Math.pow for small exp values where it's viable. For performance.
+		if frac == 0 {
+			return frac
+		}
+		return frac * math.Call("pow", 2, exp).Float()
 	}
-	if exp >= 1024 {
-		return frac * math.Call("pow", 2, 1023).Float() * math.Call("pow", 2, exp-1023).Float()
-	}
-	if exp <= -1024 {
-		return frac * math.Call("pow", 2, -1023).Float() * math.Call("pow", 2, exp+1023).Float()
-	}
-	return frac * math.Call("pow", 2, exp).Float()
+	return ldexp(frac, exp)
 }
 
 func Log(x float64) float64 {
