cmd/gc: when expanding append inline, preserve arguments

Fixes #3369.

R=golang-dev, gri, lvd, r
CC=golang-dev
https://golang.org/cl/5876044

diff --git a/src/cmd/gc/walk.c b/src/cmd/gc/walk.c
index 5c8282b52e5e930a9aa62455581b0448bf414794..ff6f1d28bd7040c086d157071893b62041b13bb2 100644 (file)
--- a/src/cmd/gc/walk.c
+++ b/src/cmd/gc/walk.c
@@ -2358,6 +2358,12 @@ append(Node *n, NodeList **init)
 
        walkexprlistsafe(n->list, init);
 
+       // walkexprlistsafe will leave OINDEX (s[n]) along if both s
+       // and n are name or literal, but those may index the slice we're
+       // modifying here.  Fix explicitly.
+       for(l = n->list; l; l=l->next)
+               l->n = cheapexpr(l->n, init);
+
        nsrc = n->list->n;
        argc = count(n->list) - 1;
        if (argc < 1) {

diff --git a/test/fixedbugs/bug428.go b/test/fixedbugs/bug428.go
new file mode 100644 (file)
index 0000000..298c455
--- /dev/null
+++ b/test/fixedbugs/bug428.go
@@ -0,0 +1,19 @@
+// run
+
+// Copyright 2012 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 when the compiler expands append inline it does not
+// overwrite a value before it needs it (issue 3369).
+
+package main
+
+func main() {
+       s := make([]byte, 5, 6)
+       copy(s, "12346")
+       s = append(s[:len(s)-1], '5', s[len(s)-1])
+       if string(s) != "123456" {
+               panic(s)
+       }
+}