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cmd/compile: casting a negative float to uint64 has different behavior on arm64 and amd64 #52943

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e11jah opened this issue May 17, 2022 · 1 comment

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@e11jah
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@e11jah e11jah commented May 17, 2022

What version of Go are you using (go version)?

$ go version
go1.18.2 linux/arm64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOENV="/root/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/root/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/root/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_arm64"
GOVCS=""
GOVERSION="go1.18.2"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3141436172=/tmp/go-build -gno-record-gcc-switches"

What did you do?

package main

import "runtime"

func main() {
	var f64 float64 = -2.99

	println(runtime.GOARCH)
	println("casting float64 to uint64 given negative num", f64, uint64(f64))
}

amd64 result:

amd64
casting float64 to uint64 given negative num -2.990000e+000 18446744073709551614

arm64 result:

arm64
casting float64 to uint64 given negative num -2.990000e+000 0

What did you expect to see?

The behavior of casting a negative float to unsigned int should be the same on different platforms, just like rust.

What did you see instead?

var x float64 = -2.99
uint64(x) == 0
@cherrymui
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@cherrymui cherrymui commented May 17, 2022

The spec says (https://go.dev/ref/spec#Conversions)

In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value the conversion succeeds but the result value is implementation-dependent.

This works as intended. Thanks.

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