lib: extend the scope of small_const_nbits() macro

find_bit would also benefit from small_const_nbits() optimizations.  The
detailed comment is provided by Rasmus Villemoes.

Link: https://lkml.kernel.org/r/[email protected]
Suggested-by: Rasmus Villemoes <[email protected]>
Signed-off-by: Yury Norov <[email protected]>
Acked-by: Rasmus Villemoes <[email protected]>
Acked-by: Andy Shevchenko <[email protected]>
Cc: Alexey Klimov <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: David Sterba <[email protected]>
Cc: Dennis Zhou <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Cc: Jianpeng Ma <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: John Paul Adrian Glaubitz <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Rich Felker <[email protected]>
Cc: Stefano Brivio <[email protected]>
Cc: Wei Yang <[email protected]>
Cc: Wolfram Sang <[email protected]>
Cc: Yoshinori Sato <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/asm-generic/bitsperlong.h

@@ -23,4 +23,16 @@
 #define BITS_PER_LONG_LONG 64
 #endif
 
+/*
+ * small_const_nbits(n) is true precisely when it is known at compile-time
+ * that BITMAP_SIZE(n) is 1, i.e. 1 <= n <= BITS_PER_LONG. This allows
+ * various bit/bitmap APIs to provide a fast inline implementation. Bitmaps
+ * of size 0 are very rare, and a compile-time-known-size 0 is most likely
+ * a sign of error. They will be handled correctly by the bit/bitmap APIs,
+ * but using the out-of-line functions, so that the inline implementations
+ * can unconditionally dereference the pointer(s).
+ */
+#define small_const_nbits(nbits) \
+	(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
+
 #endif /* __ASM_GENERIC_BITS_PER_LONG */

include/linux/bitmap.h

@@ -223,14 +223,6 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
 #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
 #define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
 
-/*
- * The static inlines below do not handle constant nbits==0 correctly,
- * so make such users (should any ever turn up) call the out-of-line
- * versions.
- */
-#define small_const_nbits(nbits) \
-	(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
-
 static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
 {
 	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);