[dev.link] cmd/compile, cmd/link: remove dead methods if type is not used in interface

Currently, a method of a reachable type is live if it matches a
method of a reachable interface. In fact, we only need to retain
the method if the type is actually converted to an interface. If
the type is never converted to an interface, there is no way to
call the method through an interface method call (but the type
descriptor could still be used, e.g. in calling
runtime.newobject).

A type can be used in an interface in two ways:
- directly converted to interface. (Any interface counts, as it
  is possible to convert one interface to another.)
- obtained by reflection from a related type (e.g. obtaining an
  interface of T from []T).

For the former, we let the compiler emit a marker on the type
descriptor symbol when it is converted to an interface. In the
linker, we only need to check methods of marked types.

For the latter, when the linker visits a marked type, it needs to
visit all its "child" types as marked (i.e. potentially could be
converted to interface).

This reduces binary size:
cmd/compile	18792016	18706096 (-0.5%)
cmd/go		14120572	13398948 (-5.1%)

Change-Id: I4465c7eeabf575f4dc84017214c610fa05ae31fd
Reviewed-on: https://go-review.googlesource.com/c/go/+/237298
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>

2 files changed, 45 insertions, 0 deletions

diff --git a/src/cmd/link/internal/ld/testdata/deadcode/ifacemethod.go b/src/cmd/link/internal/ld/testdata/deadcode/ifacemethod.go
new file mode 100644
index 0000000000..b62f18c342
--- /dev/null
+++ b/src/cmd/link/internal/ld/testdata/deadcode/ifacemethod.go
@@ -0,0 +1,23 @@
+// 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.
+
+// Test that a method of a reachable type is not necessarily
+// live even if it matches an interface method, as long as
+// the type is never converted to an interface.
+
+package main
+
+type I interface{ M() }
+
+type T int
+
+func (T) M() { println("XXX") }
+
+var p *T
+var e interface{}
+
+func main() {
+	p = new(T) // used T, but never converted to interface
+	e.(I).M()  // used I and I.M
+}
diff --git a/src/cmd/link/internal/ld/testdata/deadcode/ifacemethod2.go b/src/cmd/link/internal/ld/testdata/deadcode/ifacemethod2.go
new file mode 100644
index 0000000000..48ba55d35e
--- /dev/null
+++ b/src/cmd/link/internal/ld/testdata/deadcode/ifacemethod2.go
@@ -0,0 +1,22 @@
+// 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.
+
+// Test that a method *is* live if it matches an interface
+// method and the type is "indirectly" converted to an
+// interface through reflection.
+
+package main
+
+import "reflect"
+
+type I interface{ M() }
+
+type T int
+
+func (T) M() { println("XXX") }
+
+func main() {
+	e := reflect.ValueOf([]T{1}).Index(0).Interface()
+	e.(I).M()
+}