cmd/compile: s.f aliases itself

The change in 20907 fixed varexpr but broke aliased.  After that change,
a reference to a field in a struct would not be seen as aliasing itself.
Before that change, it would, but only because all fields in a struct
aliased everything.

This CL changes the compiler to consider all references to a field as
aliasing all other fields in that struct.  This is imperfect--a
reference to one field does not alias another field--but is a simple fix
for the immediate problem.  A better fix would require tracking the
specific fields as well.

Fixes #15042.

Change-Id: I5c95c0dd7b0699e53022fce9bae2e8f50d6d1d04
Reviewed-on: https://go-review.googlesource.com/21390
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -2335,6 +2335,12 @@ func aliased(n *Node, all []*Node, i int) bool {
 		return false
 	}
 
+	// Treat all fields of a struct as referring to the whole struct.
+	// We could do better but we would have to keep track of the fields.
+	for n.Op == ODOT {
+		n = n.Left
+	}
+
 	// Look for obvious aliasing: a variable being assigned
 	// during the all list and appearing in n.
 	// Also record whether there are any writes to main memory.
@@ -2346,6 +2352,11 @@ func aliased(n *Node, all []*Node, i int) bool {
 	var a *Node
 	for _, an := range all[:i] {
 		a = outervalue(an.Left)
+
+		for a.Op == ODOT {
+			a = a.Left
+		}
+
 		if a.Op != ONAME {
 			memwrite = 1
 			continue
@@ -2436,7 +2447,8 @@ func varexpr(n *Node) bool {
 		return varexpr(n.Left) && varexpr(n.Right)
 
 	case ODOT: // but not ODOTPTR
-		return varexpr(n.Left)
+		// Should have been handled in aliased.
+		Fatalf("varexpr unexpected ODOT")
 	}
 
 	// Be conservative.

test/fixedbugs/issue15042.go

@@ -0,0 +1,27 @@
+// run
+
+// Copyright 2016 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.
+
+// Exchanging two struct fields was compiled incorrectly.
+
+package main
+
+type S struct {
+	i int
+}
+
+func F(c bool, s1, s2 S) (int, int) {
+	if c {
+		s1.i, s2.i = s2.i, s1.i
+	}
+	return s1.i, s2.i
+}
+
+func main() {
+	i, j := F(true, S{1}, S{20})
+	if i != 20 || j != 1 {
+		panic(i+j)
+	}
+}