cmd/compile: compute embedded field offset in static initialization

Currently, when static initializes a variable with a struct
literal, the compiler uses the field's offset in its containing
struct. This is usually correct, except that with the newly-
allowed direct access of embedded field, the offset does not
include the offset of the embedded type in its containing type.
We need to include all the implicit field offsets in the offset
calculation.

For #9859.

Change-Id: Ifa4fae77383d912fba2638538248b06a3c286373
Reviewed-on: https://go-review.googlesource.com/c/go/+/780220
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>

src/cmd/compile/internal/staticinit/sched.go

@@ -546,7 +546,7 @@ func (s *Schedule) initplan(n ir.Node) {
 			if a.Sym().IsBlank() {
 				continue
 			}
-			s.addvalue(p, a.Field.Offset, a.Value)
+			s.addvalue(p, typecheck.FieldOffset(n.Type(), a.Field), a.Value)
 		}
 
 	case ir.OMAPLIT:

src/cmd/compile/internal/typecheck/subr.go

@@ -790,3 +790,21 @@ var slist []symlink
 type symlink struct {
 	field *types.Field
 }
+
+// FieldOffset returns the offset of field f in t,
+// including any implicit offsets from embedded fields.
+func FieldOffset(t *types.Type, f *types.Field) int64 {
+	if f.Sym == nil {
+		return f.Offset
+	}
+	path, ambig := dotpath(f.Sym, t, nil, false)
+	if path == nil || ambig {
+		return f.Offset
+	}
+	var offset int64
+	for _, d := range path {
+		offset += d.field.Offset
+	}
+	offset += f.Offset
+	return offset
+}

test/complit2.go

@@ -28,6 +28,7 @@ type C struct {
 func main() {
 	eq(A{1, B{b: "foo"}}, A{a: 1, b: "foo"})
 	eq(A{B: B{C: C{c: "foo"}}}, A{c: "foo"})
+	eq(x, A{B: B{b: "foo"}})
 }
 
 func eq(x, y any) {
@@ -35,3 +36,6 @@ func eq(x, y any) {
 		panic(fmt.Sprintf("%v != %v", x, y))
 	}
 }
+
+// test global initializer
+var x = A{b: "foo"}