cmd/cgo: unaligned access warning with zig #62480
Description
What version of Go are you using (go version)?
$ go version go version go1.21.0 windows/amd64
Does this issue reproduce with the latest release?
Yes.
What did you do?
This is the smallest repro
package main
/*
#include <stdint.h>
typedef struct Data {
int32_t a;
int64_t x;
} Data;
*/
import "C"
//export call
func call(v *C.int, p C.Data) {}
func main() {}
Then using zig 0.11 for CC and compiling for linux/arm:
$ CGO_ENABLED=1 \
CC="/path/to/zig cc -target arm-linux-gnueabihf" \
GOARCH=arm \
GOOS=linux \
go build -ldflags="-s -w" -buildmode c-shared -o out.so .
_cgo_export.c:29:8: warning: field p1 within '_cgo_argtype' is less aligned than 'Data' (aka 'struct Data')
and is usually due to '_cgo_argtype' being packed, which can lead to unaligned accesses [-Wunaligned-access]
Data p1;
^
1 warning generated.
Ideally there wouldn't be a warning.
I would guess the same warning will be shown with clang, however the setup for cross-compile is more annoying and I didn't get it to work at the moment.
Debugging
Using go tool cgo it shows that it ends up generating:
CGO_NO_SANITIZE_THREAD
void call(int* v, Data p)
{
size_t _cgo_ctxt = _cgo_wait_runtime_init_done();
typedef struct {
int* p0;
Data p1;
} __attribute__((__packed__)) _cgo_argtype;
static _cgo_argtype _cgo_zero;
_cgo_argtype _cgo_a = _cgo_zero;
_cgo_a.p0 = v;
_cgo_a.p1 = p;
_cgo_tsan_release();
crosscall2(_cgoexp_bd29973577f7_call, &_cgo_a, 20, _cgo_ctxt);
_cgo_tsan_acquire();
_cgo_release_context(_cgo_ctxt);
}
It seems this warning was added in https://reviews.llvm.org/D116221
As far as I understand the warning is happening due to having __attribute__((__packed__)) and typedef struct Data not having it. After adding __attribute__((__packed__)) to the Data struct the warning disappears.