[PATCH v2 1/2] bug: introduce BUILD_BUG_ON_INVALID() macro

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Sometimes we want to check some expressions correctness in compile-time.
"(void)(e);" or "if (e);" can be dangerous if expression has side-effects, and
gcc sometimes generates a lot of code, even if the expression has no effect.

This patch introduces macro BUILD_BUG_ON_INVALID() for such checks,
it forces a compilation error if expression is invalid without any extra code.

[Cast to "long" required because sizeof does not work for bit-fields.]

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
---
 include/linux/bug.h |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/include/linux/bug.h b/include/linux/bug.h
index 72961c3..aaac4bb 100644
--- a/include/linux/bug.h
+++ b/include/linux/bug.h
@@ -30,6 +30,13 @@ struct pt_regs;
 #define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
 #define BUILD_BUG_ON_NULL(e) ((void *)sizeof(struct { int:-!!(e); }))
 
+/*
+ * BUILD_BUG_ON_INVALID() 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 BUILD_BUG_ON_INVALID(e) ((void)(sizeof((__force long)(e))))
+
 /**
  * BUILD_BUG_ON - break compile if a condition is true.
  * @condition: the condition which the compiler should know is false.

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