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

Commit Message

Yury Norov March 21, 2021, 9:54 p.m. UTC

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

Signed-off-by: Yury Norov <[e-mail]>
Suggested-by: Rasmus Villemoes <[e-mail]>
---
 include/asm-generic/bitsperlong.h | 12 ++++++++++++
 include/linux/bitmap.h            |  8 --------
 2 files changed, 12 insertions(+), 8 deletions(-)

Patch

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 70a932470b2d..8c4adfa5cceb 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -222,14 +222,6 @@  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);