Andrew Morton wrote:
On Wed, 25 Apr 2012 15:26:23 +0400
Konstantin Khlebnikov<khlebnikov@xxxxxxxxxx> wrote:
Sometimes we want to check some expressions correctness in compile-time without
generating extra code. "(void)(e)" does not work if expression has side-effects.
This patch introduces macro unused_expression() which helps in this situation.
Cast to "long" required because sizeof does not work for bit-fields.
Signed-off-by: Konstantin Khlebnikov<khlebnikov@xxxxxxxxxx>
---
include/linux/compiler.h | 2 ++
1 file changed, 2 insertions(+)
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 923d093..46fbda3 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -310,4 +310,6 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
*/
#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+#define unused_expression(e) ((void)(sizeof((__force long)(e))))
+
btw, please let's not add undocumented stuff, particularly obscure
undocumented stuff.
I typed this in:
Thanks!
--- a/include/linux/compiler.h~compilerh-introduce-unused_expression-macro-fix
+++ a/include/linux/compiler.h
@@ -310,6 +310,11 @@ void ftrace_likely_update(struct ftrace_
*/
#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))
+/*
+ * unused_expression() permits the compiler to check the validity of the
+ * expression but avoids the generation of any code, even if that expression
+ * has side-effects.
+ */
#define unused_expression(e) ((void)(sizeof((__force long)(e))))
#endif /* __LINUX_COMPILER_H */
but then decided I didn't want to apply the patches (yet?).
I'm OK with that. But probably Hugh Dickins (now in CC) aggressively wants to
remove side-effects of VM_BUG_ON's in case CONFIG_DEBUG_VM=n
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