mm/slab.c: Fix build breakage when compiling -O0

[Daniel Santos]

BUILD_BUG_ON is more robust, detecting when we're not optimizing and,
thus, when __builtin_constant_p will always return zero and produce a
false-positive.  Further, this distinguishes between the errors of size
not being a compile-time constant and size just not being an accepted
value.
---
 mm/slab.c |   10 ++++------
 1 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index e901a36..1a618b2 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -308,10 +308,9 @@ static void cache_reap(struct work_struct *unused);
  */
 static __always_inline int index_of(const size_t size)
 {
-	extern void __bad_size(void);
+	int i = 0;
 
-	if (__builtin_constant_p(size)) {
-		int i = 0;
+	BUILD_BUG_ON(!__builtin_constant_p(size));
 
 #define CACHE(x) \
 	if (size <=x) \
@@ -320,9 +319,8 @@ static __always_inline int index_of(const size_t size)
 		i++;
 #include <linux/kmalloc_sizes.h>
 #undef CACHE
-		__bad_size();
-	} else
-		__bad_size();
+
+	BUILD_BUG(); /* bad size */
 	return 0;
 }
 
-- 
1.7.3.4


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