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cmd/cgo: exporting functions breaks if CGO_FLAGS enables LTO #45275

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mwhudson opened this issue Mar 28, 2021 · 3 comments
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cmd/cgo: exporting functions breaks if CGO_FLAGS enables LTO #45275

mwhudson opened this issue Mar 28, 2021 · 3 comments
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@mwhudson
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What version of Go are you using (go version)?

$ go version
go version devel +23ffb5b9ae Sun Mar 28 03:27:04 2021 +0000 linux/amd64

(but also everything else I've tried)

Does this issue reproduce with the latest release?

Yes.

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

Ubuntu 20.04 amd64 -- but fine details are not important.

go env Output
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/mwhudson/.cache/go-build"
GOENV="/home/mwhudson/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/mwhudson/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/mwhudson/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/snap/go/7221"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/snap/go/7221/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.16.2"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/mwhudson/src/go/src/go.mod"
CGO_CFLAGS="-flto"
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-build2567708190=/tmp/go-build -gno-record-gcc-switches"

What did you do?

(master)mwhudson@anduril:~/src/go/src$ cat ~/src/go-test-cases/cgocallback.go
package main

//
import "C"

//export MyFunction
func MyFunction(arg1, arg2 int) int {
	return arg1 + arg2
}

func main() {

}
(master)mwhudson@anduril:~/src/go/src$ CGO_CFLAGS=-flto ~/src/go/bin/go build ~/src/go-test-cases/cgocallback.go

What did you expect to see?

No output and a binary built.

What did you see instead?

# command-line-arguments
_cgo_export.c:21:13: error: variable ‘_cgoexp_604c78453144_MyFunction’ redeclared as function
cgo-generated-wrappers:1:5: note: previously declared here
lto1: fatal error: errors during merging of translation units
compilation terminated.
lto-wrapper: fatal error: /usr/bin/gcc returned 1 exit status
compilation terminated.
/usr/bin/ld: error: lto-wrapper failed
collect2: error: ld returned 1 exit status
@gopherbot
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Change https://golang.org/cl/305290 mentions this issue: cmd/cgo: make dummy definition of wrapper for exported function match declaration

@ianlancetaylor
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Dup of #43830? Does https://golang.org/cl/293290 fix the problem?

@mwhudson
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Yes, and that fixes my problem and one I'd missed and has tests so I'd say definitely prefer that one over mine :)

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@mwhudson @ianlancetaylor @gopherbot