8g: make a[byte(x)] truncate x

R=ken2
CC=golang-dev
https://golang.org/cl/223069

src/cmd/8g/cgen.c

@@ -215,7 +215,14 @@ cgen(Node *n, Node *res)
 			break;
 		}
 		mgen(nl, &n1, res);
-		gmove(&n1, res);
+		if(n->type->width > widthptr)
+			tempname(&n2, n->type);
+		else
+			regalloc(&n2, n->type, res);
+		gmove(&n1, &n2);
+		gmove(&n2, res);
+		if(n2.op == OREGISTER)
+			regfree(&n2);
 		mfree(&n1);
 		break;
 

test/fixedbugs/bug259.go

@@ -0,0 +1,16 @@
+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2010 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
+
+import "fmt"
+
+var x = uint32(0x01020304)
+var y = [...]uint32{1,2,3,4,5}
+
+func main() {
+	fmt.Sprint(y[byte(x)])
+}