5g: fix -f()

R=ken2
CC=golang-dev
https://golang.org/cl/5161041

src/cmd/5g/cgen.c

@@ -213,11 +213,11 @@ cgen(Node *n, Node *res)
 		goto ret;
 
 	case OMINUS:
+		regalloc(&n1, nl->type, N);
+		cgen(nl, &n1);
 		nodconst(&n3, nl->type, 0);
 		regalloc(&n2, nl->type, res);
-		regalloc(&n1, nl->type, N);
 		gmove(&n3, &n2);
-		cgen(nl, &n1);
 		gins(optoas(OSUB, nl->type), &n1, &n2);
 		gmove(&n2, res);
 		regfree(&n1);

test/fixedbugs/bug368.go

+// $G $D/$F.go && $L $F.$A && ./$A.out
+
+// Copyright 2011 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
+
+// 5g bug used to set up the 0 for -f() before calling f,
+// and the call to f smashed the register.
+
+func f(n int) int {
+	s := 0
+	for i := 0; i < n; i++ {
+		s += i>>1
+	}
+	return s
+}
+
+func main() {
+	x := -f(100)
+	if x != -2450 {
+		println(x)
+		panic("broken")
+	}
+}