Convenience macros to help linting the kernel

["Valeriy E. Ushakov" <uwe%stderr.spb.ru@localhost>]

We have too much ad-hoc code like

#define KERNEL_LOCK(count, lwp)         do {(void)(count); (void)(lwp);} while 
(/* CONSTCOND */ 0) /*NOP*/

or 

#define mowner_init(m, type)            do { } while (/* CONSTCOND */ 0)

and the like.  I propose to add the following convenience macros to
sys/cdefs.h so that macros like above can be written uniformly and in
a lint-safe way.

Thoughts?

SY, Uwe
-- 
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Index: cdefs.h
===================================================================
RCS file: /cvsroot/src/sys/sys/cdefs.h,v
retrieving revision 1.68
diff -u -u -r1.68 cdefs.h
--- cdefs.h     27 Jun 2008 01:24:52 -0000      1.68
+++ cdefs.h     20 Jul 2008 02:35:01 -0000
@@ -253,6 +253,30 @@
 #endif
 #endif /* !(__STDC_VERSION__ >= 199901L) */
 
+/*
+ * Statement that does nothing.  Convenient as an expansion for
+ * conditionally defined macros that do nothing under some combination
+ * of defines (e.g. DPRINTF(...) for !DEBUG).
+ */
+#define __NOP__                do {} while (/* CONSTCOND */ 0)
+
+/*
+ * Expression that does nothing.  Suitable for comma expressions.
+ * Convenient as an expansion for conditionally defined macros that do
+ * nothing under some combination of defines (e.g. for !DEBUG case of
+ * checks inlined into an expression as lhs of comma operator).
+ */
+#define __NOP_EXPR__   ({ /* LINTED: expression has null effect */ (void) 0; })
+
+/*
+ * Statement that safely consumes something so that if "x" is a
+ * variable it is not considered "unused" by the compiler or lint.
+ */
+#define __USED(x)      do {                                    \
+               /* LINTED: expression has null effect */        \
+               (void)(x);                                      \
+       } while (/* CONSTCOND */ 0)
+
 #if defined(_KERNEL)
 #if defined(NO_KERNEL_RCSIDS)
 #undef __KERNEL_RCSID