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vl: atomic instruction scheduling

make extremely sure that atomic operations are never re-ordered
even if their results are otherwise unused.

--- a/sys/src/cmd/vl/sched.c

+++ b/sys/src/cmd/vl/sched.c

@@ -601,6 +601,15 @@

 		if(regoff(&sa->p.from) == regoff(&sb->p.from))

 			return 1;

+	/*

+	 * special case

+	 * atomic instructions cannot pass.

+	 */

+	if(sa->p.as == ALL || sb->p.as == ALL)

+		return 1;

+	if(sa->p.as == ASC || sb->p.as == ASC)

+		return 1;

+

 	x = (sa->set.cc & (sb->set.cc|sb->used.cc)) |

 		(sb->set.cc & sa->used.cc);

 	if(x) {

-- 

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