math: handle int64 overflows for odd integer exponents in Pow(-0, y)

The existing implementation does a float64 to int64 conversion in order to check whether the number is odd, however it does not check for overflows. If an overflow occurs, the result is implementation-defined and while it happens to work on amd64 and i386, it produces an incorrect result on arm64 and possibly other architectures.

This change fixes that and also avoids calling isOddInt altogether if the base is +0, because it's unnecessary.

(I was considering avoiding the extra check if runtime.GOARCH is "amd64" or "i386", but I can't see this pattern being used anywhere outside the tests. And having separate files with build tags just for isOddInt() seems like an overkill)

Fixes #57465

Change-Id: Ieb243796194412aa6b98fac05fd19766ca2413ef
GitHub-Last-Rev: 3bfbd85c4c
GitHub-Pull-Request: golang/go#57494
Reviewed-on: https://go-review.googlesource.com/c/go/+/459815
Auto-Submit: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Bypass: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

src/math/all_test.go

@@ -1687,6 +1687,12 @@ var vfpowSC = [][2]float64{
 	{Nextafter(1, -2), float64(1 << 63)},
 	{Nextafter(-1, 2), float64(1 << 63)},
 	{Nextafter(-1, -2), float64(1 << 63)},
+
+	// Issue #57465
+	{Copysign(0, -1), 1e19},
+	{Copysign(0, -1), -1e19},
+	{Copysign(0, -1), 1<<53 - 1},
+	{Copysign(0, -1), -(1<<53 - 1)},
 }
 var powSC = []float64{
 	0,               // pow(-Inf, -Pi)
@@ -1762,6 +1768,12 @@ var powSC = []float64{
 	0,       // pow(Nextafter(1, -2), float64(1 << 63))
 	0,       // pow(Nextafter(-1, 2), float64(1 << 63))
 	Inf(1),  // pow(Nextafter(-1, -2), float64(1 << 63))
+
+	// Issue #57465
+	0,               // pow(-0, 1e19)
+	Inf(1),          // pow(-0, -1e19)
+	Copysign(0, -1), // pow(-0, 1<<53 -1)
+	Inf(-1),         // pow(-0, -(1<<53 -1))
 }
 
 var vfpow10SC = []int{

src/math/pow.go

@@ -5,6 +5,15 @@
 package math
 
 func isOddInt(x float64) bool {
+	if Abs(x) >= (1 << 53) {
+		// 1 << 53 is the largest exact integer in the float64 format.
+		// Any number outside this range will be truncated before the decimal point and therefore will always be
+		// an even integer.
+		// Without this check and if x overflows int64 the int64(xi) conversion below may produce incorrect results
+		// on some architectures (and does so on arm64). See issue #57465.
+		return false
+	}
+
 	xi, xf := Modf(x)
 	return xf == 0 && int64(xi)&1 == 1
 }
@@ -54,12 +63,12 @@ func pow(x, y float64) float64 {
 	case x == 0:
 		switch {
 		case y < 0:
-			if isOddInt(y) {
-				return Copysign(Inf(1), x)
+			if Signbit(x) && isOddInt(y) {
+				return Inf(-1)
 			}
 			return Inf(1)
 		case y > 0:
-			if isOddInt(y) {
+			if Signbit(x) && isOddInt(y) {
 				return x
 			}
 			return 0