cmd/compile: using CONV instead of CONVNOP for interface conversions

We shouldn't use CONVNOP for conversions between two different
nonempty interface types, because we want to update the itab
in those situations.

Fixes #18595

After this CL, we are guaranteed that itabs are unique, that is
there is only one itab per compile-time-type/concrete type pair.
See also the tests in CL 35115 and 35116 which make sure this
invariant holds even for shared libraries and plugins.

Unique itabs are required for CL 34810 (faster type switch code).

R=go1.9

Change-Id: Id27d2e01ded706680965e4cb69d7c7a24ac2161b
Reviewed-on: https://go-review.googlesource.com/35119
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -743,9 +743,21 @@ func assignop(src *Type, dst *Type, why *string) Op {
 	// and either src or dst is not a named type or
 	// both are empty interface types.
 	// For assignable but different non-empty interface types,
-	// we want to recompute the itab.
-	if eqtype(src.Orig, dst.Orig) && (src.Sym == nil || dst.Sym == nil || src.IsEmptyInterface()) {
-		return OCONVNOP
+	// we want to recompute the itab. Recomputing the itab ensures
+	// that itabs are unique (thus an interface with a compile-time
+	// type I has an itab with interface type I).
+	if eqtype(src.Orig, dst.Orig) {
+		if src.IsEmptyInterface() {
+			// Conversion between two empty interfaces
+			// requires no code.
+			return OCONVNOP
+		}
+		if (src.Sym == nil || dst.Sym == nil) && !src.IsInterface() {
+			// Conversion between two types, at least one unnamed,
+			// needs no conversion. The exception is nonempty interfaces
+			// which need to have their itab updated.
+			return OCONVNOP
+		}
 	}
 
 	// 3. dst is an interface type and src implements dst.

test/fixedbugs/issue18595.go

+// run
+
+// Copyright 2017 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.
+
+// This test makes sure that itabs are unique.
+// More explicitly, we require that only one itab structure exists for the pair of
+// a given compile-time interface type and underlying concrete type.
+// Ensuring this invariant enables fixes for 18492 (improve type switch code).
+
+package main
+
+type I interface {
+	M()
+}
+type J interface {
+	M()
+}
+
+type T struct{}
+
+func (*T) M() {}
+
+func main() {
+	test1()
+	test2()
+}
+
+func test1() {
+	t := new(T)
+	var i1, i2 I
+	var j interface {
+		M()
+	}
+	i1 = t
+	j = t
+	i2 = j
+	if i1 != i2 {
+		panic("interfaces not equal")
+	}
+}
+
+func test2() {
+	t := new(T)
+	i1 := (I)(t)
+	i2 := (I)((interface {
+		M()
+	})((J)(t)))
+	if i1 != i2 {
+		panic("interfaces not equal")
+	}
+}