cmd/cgo: Conflicting types for generated symbols prevent LTO-enabled compilation #43830
Description
What version of Go are you using (go version)?
$ go version go version go1.15.7 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 GOARCH="amd64" GOHOSTOS="linux"
What did you do?
Enable link time optimization CFLAGS when building a Go program containing an //export directive.
package main
import "fmt"
import "C"
//export add
func add(x int32, y int32) int32 {
return x + y
}
func main() {
fmt.Println(add(1,1))
}
The above program builds normally using go build without additional CFLAGS. However, building with CGO_CFLAGS='-flto -ffat-lto-objects' go build produces the following error.
_cgo_export.c:21:13: error: variable ‘_cgoexp_c166a99bca60_add’ 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
This is due to the same symbol name being used in separate contexts as both a function and an integer.
What did you expect to see?
Program builds when enabling link time optimization via CFLAGs.
What did you see instead?
Generated intermediate C code for //export is incompatible with link time optimization.
Notes
Additional conflicting symbols may be created if the program binds the value of an exported function's C handle to a Go variable. The following program demonstrates the behavior.
package main
import "fmt"
// extern int add(int, int);
import "C"
//export add
func add(a int32, b int32) int32 {
return a + b
}
func main() {
fn := C.add
fmt.Printf("%v\n", fn)
}
When built with CGO_CFLAGS='-flto -ffat-lto-objects' go build, the linker yields the following error.
cgo-generated-wrappers:1:13: error: function ‘add’ redeclared as variable
_cgo_export.c:24:9: note: previously declared here
_cgo_export.c:21:13: error: variable ‘_cgoexp_2526879f8d2a_add’ redeclared as function
cgo-generated-wrappers:4:5: 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
The interesting aspect to note is the redeclaration of the add symbol in addition to the previously encountered _cgoexp*_add symbol. This comes from introducing the assignment statement of C.add to the variable fn, and causes cgo to emit an additional pair of matching symbols with conflicting types via another code path.