array index bug

x[i]
x not addressable, i >= UINF
double evaluated i
second eval killed live registers

manifested as gob instability

R=ken
OCL=34097
CL=34099

src/cmd/6g/cgen.c

@@ -470,9 +470,9 @@ agen(Node *n, Node *res)
 			agen(nl, &n3);
 			goto index;
 		}
-		cgen(nr, res);
 		tempname(&tmp, nr->type);
-		gmove(res, &tmp);
+		cgen(nr, &tmp);
+		nr = &tmp;
 
 	irad:
 		regalloc(&n3, types[tptr], res);

test/fixedbugs/bug199.go

@@ -0,0 +1,25 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2009 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.
+
+package main
+
+type S struct {
+	a []int;
+}
+var s = &S{make([]int, 10)}
+
+func main() {
+	s.a[f()] = 1	// 6g used to call f twice here
+}
+
+var n int
+func f() int{
+	if n++; n > 1 {
+		panicln("f twice");
+	}
+	return 0
+}
+