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: 3bfbd85c4cd6c5dc3d15239e180c99764a19ca88
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>

diff --git a/src/math/all_test.go b/src/math/all_test.go
index 8d5e0ad4391c1e0e215315997aff1f0dfbb03cd6..886267bc1756c108fc87f1551eeda06b60574b08 100644 (file)
--- a/src/math/all_test.go
+++ b/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{

diff --git a/src/math/pow.go b/src/math/pow.go
index 3af8c8b649b9b8ea6fa51d06a006af9cdb42c3ca..3f42945376d3fac2888d27cfe848ffecdf3abfdc 100644 (file)
--- a/src/math/pow.go
+++ b/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