From 7f9ee2ba5b3b60a4e71650abe5d94e0ef85720d1 Mon Sep 17 00:00:00 2001 From: Dmitry Panov Date: Wed, 28 Dec 2022 23:05:26 +0000 Subject: [PATCH] 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 Run-TryBot: Keith Randall Reviewed-by: Matthew Dempsky TryBot-Result: Gopher Robot Reviewed-by: Keith Randall TryBot-Bypass: Keith Randall Reviewed-by: Keith Randall --- src/math/all_test.go | 12 ++++++++++++ src/math/pow.go | 15 ++++++++++++--- 2 files changed, 24 insertions(+), 3 deletions(-) diff --git a/src/math/all_test.go b/src/math/all_test.go index 8d5e0ad439..886267bc17 100644 --- 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 3af8c8b649..3f42945376 100644 --- 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 -- 2.48.1