[src/trunk]: src/sys/sys void invading the users' namespace with __launder_ty...

[mrg <mrg%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/137c2b84609f
branches:  trunk
changeset: 791637:137c2b84609f
user:      mrg <mrg%NetBSD.org@localhost>
date:      Wed Nov 27 04:24:15 2013 +0000

description:
void invading the users' namespace with __launder_type().
adjust the comment a little.

diffstat:

 sys/sys/queue.h |  29 +++++++++++++++++------------
 1 files changed, 17 insertions(+), 12 deletions(-)

diffs (47 lines):

diff -r 101708165c1d -r 137c2b84609f sys/sys/queue.h
--- a/sys/sys/queue.h   Wed Nov 27 02:04:48 2013 +0000
+++ b/sys/sys/queue.h   Wed Nov 27 04:24:15 2013 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: queue.h,v 1.62 2013/11/26 16:57:04 christos Exp $      */
+/*     $NetBSD: queue.h,v 1.63 2013/11/27 04:24:15 mrg Exp $   */
 
 /*
  * Copyright (c) 1991, 1993
@@ -640,21 +640,26 @@
  */
 
 /*
- * We use this ugly hack to work around the fact that the CIRCLEQ
- * macros violate the C aliasing rules by comparing 'struct name *'
- * and 'struct type *' (see CIRCLEQ_HEAD() below).  Modern compilers
- * (such as GCC 4.8) declare this to be always false and this makes
- * most of the macros below not function correctly.
+ * __launder_type():  We use this ugly hack to work around the the compiler
+ * noticing that two types may not alias each other and elide tests in code.
+ * We hit this in the CIRCLEQ macros when comparing 'struct name *' and
+ * 'struct type *' (see CIRCLEQ_HEAD()).  Modern compilers (such as GCC
+ * 4.8) declare these comparisons as always false, causing the code to
+ * not run as designed.
+ *
+ * This hack is only to be used for comparisons and thus can be fully const.
+ * Do not use for assignment.
  *
- * This hack is only to be used for comparisons and thus can be
- * fully const.  Do not use for assignment.
+ * If we ever choose to change the ABI of the CIRCLEQ macros, we could fix
+ * this by changing the head/tail sentinal values, but see the note above
+ * this one.
  */
-static inline const void * __launder_type(const void *x);
+static inline const void * __launder_type(const void *);
 static inline const void *
-__launder_type(const void *x)
+__launder_type(const void *__x)
 {
-       __asm volatile("" : "+r" (x));
-       return x;
+       __asm __volatile("" : "+r" (__x));
+       return __x;
 }
 
 #if defined(_KERNEL) && defined(QUEUEDEBUG)