cmd/compile: fix devirtualization of promoted interface methods

A method selector expression can pick out a method or promoted method
(represented by ODOTMETH), but it can also pick out an interface
method from an embedded interface-typed field (represented by
ODOTINTER).

In the case that we're picking out an interface method, we're not able
to fully devirtualize the method call. However, we're still able to
improve escape analysis somewhat. E.g., the included test case
demonstrates that we can optimize "i.M()" to "i.(T).I.M()", which
means the T literal can be stack allocated instead of heap allocated.

Fixes #42279.

Change-Id: Ifa21d19011e2f008d84f9624b7055b4676b6d188
Reviewed-on: https://go-review.googlesource.com/c/go/+/266300
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

Author: mdempsky

src/cmd/compile/internal/gc/inl.go

@@ -1446,19 +1446,27 @@ func devirtualizeCall(call *Node) {
 	x.Type = typ
 	x = nodlSym(call.Left.Pos, OXDOT, x, call.Left.Sym)
 	x = typecheck(x, ctxExpr|ctxCallee)
-	if x.Op != ODOTMETH {
-		// TODO(mdempsky): Figure out how to avoid this and
-		// turn back into a Fatalf.
+	switch x.Op {
+	case ODOTMETH:
 		if Debug.m != 0 {
-			Warnl(call.Pos, "failed to devirtualize %v", x)
+			Warnl(call.Pos, "devirtualizing %v to %v", call.Left, typ)
+		}
+		call.Op = OCALLMETH
+		call.Left = x
+	case ODOTINTER:
+		// Promoted method from embedded interface-typed field (#42279).
+		if Debug.m != 0 {
+			Warnl(call.Pos, "partially devirtualizing %v to %v", call.Left, typ)
+		}
+		call.Op = OCALLINTER
+		call.Left = x
+	default:
+		// TODO(mdempsky): Turn back into Fatalf after more testing.
+		if Debug.m != 0 {
+			Warnl(call.Pos, "failed to devirtualize %v (%v)", x, x.Op)
 		}
 		return
 	}
-	if Debug.m != 0 {
-		Warnl(call.Pos, "devirtualizing %v to %v", call.Left, typ)
-	}
-	call.Op = OCALLMETH
-	call.Left = x
 
 	// Duplicated logic from typecheck for function call return
 	// value types.

test/escape_iface.go

@@ -255,3 +255,11 @@ func dotTypeEscape2() { // #13805, #15796
 		sink, *(&ok) = y.(*int)
 	}
 }
+
+func issue42279() {
+	type I interface{ M() }
+	type T struct{ I }
+
+	var i I = T{} // ERROR "T\{\} does not escape"
+	i.M()         // ERROR "partially devirtualizing i.M to T"
+}