cmd/8g: Make clearfat non-interleaved with pointer calculations.

clearfat (used to zero initialize structures) will use AX for x86 block ops. If we write to AX while calculating the dest pointer, we will fill the structure with incorrect values.
Since 64-bit arithmetic uses AX to synthesize a 64-bit register, getting an adress by indexing with 64-bit ops can clobber the register.

Fixes #5820.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/11383043

src/cmd/8g/ggen.c

@@ -78,9 +78,9 @@ clearfat(Node *nl)
 	c = w % 4;	// bytes
 	q = w / 4;	// quads
 
-	gconreg(AMOVL, 0, D_AX);
 	nodreg(&n1, types[tptr], D_DI);
 	agen(nl, &n1);
+	gconreg(AMOVL, 0, D_AX);
 
 	if(q >= 4) {
 		gconreg(AMOVL, q, D_CX);

test/fixedbugs/issue5820.go

+// run
+
+// Copyright 2013 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.
+
+// issue 5820: register clobber when clearfat and 64 bit arithmetic is interleaved.
+
+package main
+
+func main() {
+	array := make([][]int, 2)
+	index := uint64(1)
+	array[index] = nil
+	if array[1] != nil {
+		panic("array[1] != nil")
+	}
+}