how a pointer to a struct member keeps the struct alive in Go

Question

Given the following golang code:

type Pointer struct { x, y int }

func foo(p *Pointer) *int {
    return &p.y
}

CompilerExplorer shows that return &p.y compiles to

TESTB   AL, (AX)
ADDQ    $8, AX
RET

It's easy to understand. TESTB is a null check, then ADDQ produce a pointer to py by adding offset of Pointer::y to p .

What I don't understand is, given a pointer to py , how does the garbage collector knows it's not just an arbitrary *int , but a pointer to a Pointer::y , so p must remain alive as long as a pointer to py is still alive?

Answer 1

After reading the source code I found the answer.

1. Go use arena / span based allocation.
2. Arenas and spans are aligned with pages, so it's trivial to calculate which arena is the pointer pointing to.
3. There's a global vector stores metadata of all arenas and spans.
4. Each span has a fixed "element size". All objects in the same span have the same size.
5. Given the pointer p , base address of span b and element size of span s , we know the pointer is pointing n-th element in the span, where n = (p - b) / s .
6. So the address of n-th object in the span b + s * n , which needs to be marked as alive.