6l doesn't really use 64-bit addresses. If you try to use a 64-bit address
it will silently create incorrect code. This patch will at least warn you
that you're trying to do something that 6l can not do.

Reference: /n/sources/patch/sorry/6l.64-bit-addr.warn
Date: Sat Jun 18 02:02:53 CES 2011
Signed-off-by: rminnich@gmail.com

--- /sys/src/cmd/6l/span.c	Sat Jun 18 02:01:31 2011
+++ /sys/src/cmd/6l/span.c	Sat Jun 18 02:01:27 2011
@@ -668,6 +668,8 @@
 
 	rex &= (0x40 | Rxr);
 	v = a->offset;
+	if ((vlong)v != a->offset) 
+		sysfatal("asmandsz:Trying to emit %#ullx and 32 bits is not sufficient\n", a->offset);
 	t = a->type;
 	if(a->index != D_NONE) {
 		if(t >= D_INDIR) {