cmd/compile: optimize convT2I as a two-word copy when T is pointer-shaped

See #14874

This change adds a compiler optimization for pointer shaped convT2I.
Since itab symbols are now emitted by the compiler, the itab address can
be directly moved into the iface structure.

Change-Id: I311483af544519ca682c5f872960717ead772f26
Reviewed-on: https://go-review.googlesource.com/20901Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

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

@@ -947,7 +947,7 @@ func typename(t *Type) *Node {
 	return n
 }
 
-func itabnamesym(t, itype *Type) *Sym {
+func itabname(t, itype *Type) *Node {
 	if t == nil || (Isptr[t.Etype] && t.Type == nil) || isideal(t) {
 		Fatalf("itabname %v", t)
 	}
@@ -961,7 +961,13 @@ func itabnamesym(t, itype *Type) *Sym {
 
 		itabs = append(itabs, itabEntry{t: t, itype: itype, sym: s})
 	}
-	return s.Def.Sym
+
+	n := Nod(OADDR, s.Def, nil)
+	n.Type = Ptrto(s.Def.Type)
+	n.Addable = true
+	n.Ullman = 2
+	n.Typecheck = 1
+	return n
 }
 
 // isreflexive reports whether t has a reflexive equality operator.

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

@@ -987,19 +987,21 @@ opswitch:
 	case OCONVIFACE:
 		n.Left = walkexpr(n.Left, init)
 
-		// Optimize convT2E as a two-word copy when T is pointer-shaped.
-		if isnilinter(n.Type) && isdirectiface(n.Left.Type) {
-			l := Nod(OEFACE, typename(n.Left.Type), n.Left)
+		// Optimize convT2E or convT2I as a two-word copy when T is pointer-shaped.
+		if isdirectiface(n.Left.Type) {
+			var t *Node
+			if isnilinter(n.Type) {
+				t = typename(n.Left.Type)
+			} else {
+				t = itabname(n.Left.Type, n.Type)
+			}
+			l := Nod(OEFACE, t, n.Left)
 			l.Type = n.Type
 			l.Typecheck = n.Typecheck
 			n = l
 			break
 		}
 
-		if isdirectiface(n.Left.Type) {
-			itabnamesym(n.Left.Type, n.Type)
-		}
-
 		var ll []*Node
 		if !Isinter(n.Left.Type) {
 			ll = append(ll, typename(n.Left.Type))