[PATCH 05/12] lib: extend the scope of small_const_nbits() macro

Yury Norov
2021-04-01T00:33:08Z

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

Suggested-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
Acked-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
include/asm-generic/bitsperlong.h | 12 ++++++++++++
include/linux/bitmap.h | 8 --------
2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/include/asm-generic/bitsperlong.h b/include/asm-generic/bitsperlong.h
index 3905c1c93dc2..1023e2a4bd37 100644
--- a/include/asm-generic/bitsperlong.h
+++ b/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 */
diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 2cb1d7cfe8f9..a36cfcec4e77 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -230,14 +230,6 @@ 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);
--
2.25.1