cmd/compile: pack structs containing anonymous fields more tightly

[josharian]

I attempted to refactor some common fields (E) out of a struct (T1), yielding (T2). Unfortunately, this changed the packing of the fields, which makes this refactoring infeasible in this case.

type E struct {
	X int
	B uint8
}

type T1 struct {
	X int
	B uint8
	C uint8
}

type T2 struct {
	E
	C uint8
}

Observe the Offset of C in this code: https://play.golang.com/p/ac5CCIWIlZS

On a first pass, I don't see anything in the spec that forbids T1 and T2 being laid out identically in memory.

[tmthrgd]

Would that not break the following?

var t T2
e := &t.E
*e = E{}

[josharian]

I don’t follow. Why would it?

[randall77]

Right now E.Size() is 16 (on 64-bit archs). We would need E.Size() to be 9 for this proposal to work.
But an array of E would still need to be strided by 16, so we'd need to introduce the concept of two different kinds of size for each type.

[tmthrgd]

Consider a slightly more involved example:

func main() {
    var t T2
    t.C = 42
    modifyE(&t.E)
    println(t.C)
}

//go:noinline
func modifyE(e *E) {
    *e = E{}
}

modifyE would zero 8/16 bytes and override the C field. Keep in mind that function could exist in a different package or *E could be passed through as an interface.