cmd/link: propagate UsedInIface through method descriptor

The linker prunes methods that are not directly reachable if the
receiver type is never converted to interface. A type can be
converted to interface using reflection through other types.
The linker already takes this into consideration but it missed
the case that the intermediate is a method descriptor. Handle
this case.

Change-Id: I590efc5da163c326db8d43583908a2ef67f65d9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/255858
Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

src/cmd/link/internal/ld/deadcode.go

@@ -130,6 +130,19 @@ func (d *deadcodePass) flood() {
 				}
 				if usedInIface {
 					methods = append(methods, methodref{src: symIdx, r: i})
+					// The method descriptor is itself a type descriptor, and
+					// it can be used to reach other types, e.g. by using
+					// reflect.Type.Method(i).Type.In(j). We need to traverse
+					// its child types with UsedInIface set. (See also the
+					// comment below.)
+					rs := r.Sym()
+					if !d.ldr.AttrUsedInIface(rs) {
+						d.ldr.SetAttrUsedInIface(rs, true)
+						if d.ldr.AttrReachable(rs) {
+							d.ldr.SetAttrReachable(rs, false)
+							d.mark(rs, symIdx)
+						}
+					}
 				}
 				i += 2
 				continue
@@ -215,9 +228,15 @@ func (d *deadcodePass) mark(symIdx, parent loader.Sym) {
 		if *flagDumpDep {
 			to := d.ldr.SymName(symIdx)
 			if to != "" {
+				if d.ldr.AttrUsedInIface(symIdx) {
+					to += " <UsedInIface>"
+				}
 				from := "_"
 				if parent != 0 {
 					from = d.ldr.SymName(parent)
+					if d.ldr.AttrUsedInIface(parent) {
+						from += " <UsedInIface>"
+					}
 				}
 				fmt.Printf("%s -> %s\n", from, to)
 			}

src/cmd/link/internal/ld/deadcode_test.go

@@ -32,6 +32,7 @@ func TestDeadcode(t *testing.T) {
 		{"typedesc", "", "type.main.T"},
 		{"ifacemethod", "", "main.T.M"},
 		{"ifacemethod2", "main.T.M", ""},
+		{"ifacemethod3", "main.S.M", ""},
 	}
 	for _, test := range tests {
 		test := test

src/cmd/link/internal/ld/testdata/deadcode/ifacemethod3.go

@@ -0,0 +1,29 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Like ifacemethod2.go, this tests that a method *is* live
+// if the type is "indirectly" converted to an interface
+// using reflection with a method descriptor as intermediate.
+
+package main
+
+import "reflect"
+
+type S int
+
+func (s S) M() { println("S.M") }
+
+type I interface { M() }
+
+type T float64
+
+func (t T) F(s S) {}
+
+func main() {
+	var t T
+	ft := reflect.TypeOf(t).Method(0).Type
+	at := ft.In(1)
+	v := reflect.New(at).Elem()
+	v.Interface().(I).M()
+}