bitops: Ensure the compile time HWEIGHT is only used for such

Avoid accidental misuse by failing to compile things

Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <new-submission>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 22 insertions, 11 deletions

diff --git a/include/linux/bitops.h b/include/linux/bitops.h
index ba0fd1eb4af7..25b8b2f33ae9 100644
--- a/include/linux/bitops.h
+++ b/include/linux/bitops.h
@@ -45,19 +45,30 @@ static inline unsigned long hweight_long(unsigned long w)
         return sizeof(w) == 4 ? hweight32(w) : hweight64(w);
 }
 
-#define HWEIGHT8(w)                     \
-      ( (!!((w) & (1ULL << 0))) +       \
-        (!!((w) & (1ULL << 1))) +       \
-        (!!((w) & (1ULL << 2))) +       \
-        (!!((w) & (1ULL << 3))) +       \
-        (!!((w) & (1ULL << 4))) +       \
-        (!!((w) & (1ULL << 5))) +       \
-        (!!((w) & (1ULL << 6))) +       \
+/*
+ * Clearly slow versions of the hweightN() functions, their benefit is
+ * of course compile time evaluation of constant arguments.
+ */
+#define HWEIGHT8(w)                                     \
+      ( BUILD_BUG_ON_ZERO(!__builtin_constant_p(w)) +   \
+        (!!((w) & (1ULL << 0))) +                       \
+        (!!((w) & (1ULL << 1))) +                       \
+        (!!((w) & (1ULL << 2))) +                       \
+        (!!((w) & (1ULL << 3))) +                       \
+        (!!((w) & (1ULL << 4))) +                       \
+        (!!((w) & (1ULL << 5))) +                       \
+        (!!((w) & (1ULL << 6))) +                       \
         (!!((w) & (1ULL << 7))) )
 
-#define HWEIGHT16(w) (HWEIGHT8(w)  + HWEIGHT8(w >> 8))
-#define HWEIGHT32(w) (HWEIGHT16(w) + HWEIGHT16(w >> 16))
-#define HWEIGHT64(w) (HWEIGHT32(w) + HWEIGHT32(w >> 32))
+#define HWEIGHT16(w) (HWEIGHT8(w)  + HWEIGHT8((w) >> 8))
+#define HWEIGHT32(w) (HWEIGHT16(w) + HWEIGHT16((w) >> 16))
+#define HWEIGHT64(w) (HWEIGHT32(w) + HWEIGHT32((w) >> 32))
+
+/*
+ * Type invariant version that simply casts things to the
+ * largest type.
+ */
+#define HWEIGHT(w)   HWEIGHT64((u64)(w))
 
 /**
  * rol32 - rotate a 32-bit value left