cmd/cgo: miscalculates C array offset when C struct has __attribute__((__packed__))

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

Does this issue reproduce with the latest release?

Yes.

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

C compiler: gcc (Ubuntu 7.5.0-3ubuntu1~18.04) 7.5.0 on Ubuntu 18.04

What did you do?

I execute go run x.go with the following input file:

package main

/*
#include <inttypes.h>
#include <stdlib.h>
#include <stdio.h>

typedef struct P
{
	uint64_t a;
	struct {
		uint64_t b;
		uint8_t c;
	} __attribute__((__packed__));
} P;

typedef struct Q
{
	P d[3];
} Q;

void show(const Q* q)
{
	printf("sizeof(P)=%zu sizeof(Q)=%zu\n", sizeof(P), sizeof(Q));
	for (int i = 0; i < sizeof(q->d) / sizeof(q->d[0]); ++i) {
		const P* p = &q->d[i];
		printf("[%d] a=%016"PRIx64" b=%016"PRIx64" c=%02"PRIx8"\n", i, p->a, p->b, p->c);
	}
}
*/
import "C"
import (
	"fmt"
	"unsafe"
)

func main() {
	var q C.Q
	q.d[0].a = 0xA0A0A0A0A0A0A0A0
	q.d[1].a = 0xA1A1A1A1A1A1A1A1
	q.d[2].a = 0xA2A2A2A2A2A2A2A2
	fmt.Println(unsafe.Sizeof(C.P{}), unsafe.Sizeof(C.Q{}))
	fmt.Println(q)
	C.show(&q)
}

What did you expect to see?

$ go run x.go 
24 72
{[{11574427654092267680 {0 0} [0 0 0 0 0 0 0]} {11646767826930344353 {0 0} [0 0 0 0 0 0 0]} {11719107999768421026 {0 0} [0 0 0 0 0 0 0]}]}
sizeof(P)=24 sizeof(Q)=72
[0] a=a0a0a0a0a0a0a0a0 b=0000000000000000 c=00
[1] a=a1a1a1a1a1a1a1a1 b=0000000000000000 c=00
[2] a=a2a2a2a2a2a2a2a2 b=0000000000000000 c=00

What did you see instead?

$ go run x.go 
32 96
{[{11574427654092267680 {0 0} [0 0 0 0 0 0 0]} {11646767826930344353 {0 0} [0 0 0 0 0 0 0]} {11719107999768421026 {0 0} [0 0 0 0 0 0 0]}]}
sizeof(P)=24 sizeof(Q)=72
[0] a=a0a0a0a0a0a0a0a0 b=0000000000000000 c=00
[1] a=0000000000000000 b=a1a1a1a1a1a1a1a1 c=00
[2] a=0000000000000000 b=0000000000000000 c=a2

Notice that cgo and gcc reports different size for C structs.

Additional Information

If struct P is changed to:

typedef struct P
{
	uint64_t a;
	uint64_t b;
	uint8_t c;
} __attribute__((__packed__)) P;

It also causes cgo to misbehave:

$ go run x.go 
24 72
{[{11574427654092267680 0 0} {11646767826930344353 0 0} {11719107999768421026 0 0}]}
sizeof(P)=17 sizeof(Q)=51
[0] a=a0a0a0a0a0a0a0a0 b=0000000000000000 c=00
[1] a=a100000000000000 b=00a1a1a1a1a1a1a1 c=00
[2] a=0000000000000000 b=a2a2000000000000 c=a2