libgo: update to Go 1.8 release candidate 1

Compiler changes:
      * Change map assignment to use mapassign and assign value directly.
      * Change string iteration to use decoderune, faster for ASCII strings.
      * Change makeslice to take int, and use makeslice64 for larger values.
      * Add new noverflow field to hmap struct used for maps.
    
    Unresolved problems, to be fixed later:
      * Commented out test in go/types/sizes_test.go that doesn't compile.
      * Commented out reflect.TestStructOf test for padding after zero-sized field.
    
    Reviewed-on: https://go-review.googlesource.com/35231

gotools/:
	Updates for Go 1.8rc1.
	* Makefile.am (go_cmd_go_files): Add bug.go.
	(s-zdefaultcc): Write defaultPkgConfig.
	* Makefile.in: Rebuild.

From-SVN: r244456

libgo/go/math/gamma.go

@@ -91,23 +91,31 @@ var _gamS = [...]float64{
 }
 
 // Gamma function computed by Stirling's formula.
-// The polynomial is valid for 33 <= x <= 172.
-func stirling(x float64) float64 {
+// The pair of results must be multiplied together to get the actual answer.
+// The multiplication is left to the caller so that, if careful, the caller can avoid
+// infinity for 172 <= x <= 180.
+// The polynomial is valid for 33 <= x <= 172; larger values are only used
+// in reciprocal and produce denormalized floats. The lower precision there
+// masks any imprecision in the polynomial.
+func stirling(x float64) (float64, float64) {
+	if x > 200 {
+		return Inf(1), 1
+	}
 	const (
 		SqrtTwoPi   = 2.506628274631000502417
 		MaxStirling = 143.01608
 	)
 	w := 1 / x
 	w = 1 + w*((((_gamS[0]*w+_gamS[1])*w+_gamS[2])*w+_gamS[3])*w+_gamS[4])
-	y := Exp(x)
+	y1 := Exp(x)
+	y2 := 1.0
 	if x > MaxStirling { // avoid Pow() overflow
 		v := Pow(x, 0.5*x-0.25)
-		y = v * (v / y)
+		y1, y2 = v, v/y1
 	} else {
-		y = Pow(x, x-0.5) / y
+		y1 = Pow(x, x-0.5) / y1
 	}
-	y = SqrtTwoPi * y * w
-	return y
+	return y1, SqrtTwoPi * w * y2
 }
 
 // Gamma returns the Gamma function of x.
@@ -125,22 +133,26 @@ func Gamma(x float64) float64 {
 	switch {
 	case isNegInt(x) || IsInf(x, -1) || IsNaN(x):
 		return NaN()
+	case IsInf(x, 1):
+		return Inf(1)
 	case x == 0:
 		if Signbit(x) {
 			return Inf(-1)
 		}
 		return Inf(1)
-	case x < -170.5674972726612 || x > 171.61447887182298:
-		return Inf(1)
 	}
 	q := Abs(x)
 	p := Floor(q)
 	if q > 33 {
 		if x >= 0 {
-			return stirling(x)
+			y1, y2 := stirling(x)
+			return y1 * y2
 		}
+		// Note: x is negative but (checked above) not a negative integer,
+		// so x must be small enough to be in range for conversion to int64.
+		// If |x| were >= 2⁶³ it would have to be an integer.
 		signgam := 1
-		if ip := int(p); ip&1 == 0 {
+		if ip := int64(p); ip&1 == 0 {
 			signgam = -1
 		}
 		z := q - p
@@ -152,7 +164,14 @@ func Gamma(x float64) float64 {
 		if z == 0 {
 			return Inf(signgam)
 		}
-		z = Pi / (Abs(z) * stirling(q))
+		sq1, sq2 := stirling(q)
+		absz := Abs(z)
+		d := absz * sq1 * sq2
+		if IsInf(d, 0) {
+			z = Pi / absz / sq1 / sq2
+		} else {
+			z = Pi / d
+		}
 		return float64(signgam) * z
 	}