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cmd/compile: -1.0 * 0.0 doesn't yield -0.0 #38396

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perillo opened this issue Apr 12, 2020 · 2 comments
Closed

cmd/compile: -1.0 * 0.0 doesn't yield -0.0 #38396

perillo opened this issue Apr 12, 2020 · 2 comments
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@perillo
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perillo commented Apr 12, 2020

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

$ go version
go version go1.14.2 linux/amd64

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="on"
GOARCH="amd64"
GOBIN="/home/manlio/.local/bin"
GOCACHE="/home/manlio/.cache/go-build"
GOENV="/home/manlio/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE="*.local"
GONOPROXY=""
GONOSUMDB="*.local"
GOOS="linux"
GOPATH="/home/manlio/.local/lib/go:/home/manlio/src/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build291401117=/tmp/go-build -gno-record-gcc-switches"
GOROOT/bin/go version: go version go1.14.2 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.14.2
uname -sr: Linux 5.6.3-arch1-1
/usr/lib/libc.so.6: GNU C Library (GNU libc) stable release version 2.31.
gdb --version: GNU gdb (GDB) 9.1

What did you do?

https://play.golang.org/p/F9vVfqkyonb

What did you expect to see?

-0

This is the result returned by C and Python.

What did you see instead?

0

Is the only way to get a negative zero in Go math.Copysign(0.0, -1.0) ?

Thanks.
@perillo
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perillo commented Apr 12, 2020

Dividing zero by the negative infinite correctly yields a negative zero.
https://play.golang.org/p/Vby8qc8gEUK

@randall77
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You're running afoul of the ideal constant arithmetic in Go. This code generates a -0:

func main() {
     var x float64 = -1.0
     var y float64 = 0.0
     z := x*y
     println(z)
}

Constant arithmetic has its own rules. It is exact(-ish) and doesn't behave like IEEE754.

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