[PATCH] linux/bitmap.h: Improve BITMAP_{LAST,FIRST}_WORD_MASK

[PATCH] linux/bitmap.h: Improve BITMAP_{LAST,FIRST}_WORD_MASK

[Rasmus Villemoes]

The macro BITMAP_LAST_WORD_MASK can be implemented without a
conditional, which will generally lead to slightly better generated
code (221 bytes saved for allmodconfig-GCOV_KERNEL, ~2k with
GCOV_KERNEL). As a small bonus, this also ensures that the nbits
parameter is expanded exactly once.

In BITMAP_FIRST_WORD_MASK, if start is signed gcc is technically
allowed to assume it is positive (or divisible by BITS_PER_LONG), and
hence just do the simple mask. It doesn't seem to use this, and even
on an architecture like x86 where the shift only depends on the lower
5 or 6 bits, and these bits are not affected by the signedness of the
expression, gcc still generates code to compute the C99 mandated value
of start % BITS_PER_LONG. So just use a mask explicitly, also for
consistency with BITMAP_LAST_WORD_MASK.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 include/linux/bitmap.h | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index dbfbf4990005..be4fa5ddf36c 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -172,12 +172,8 @@ extern unsigned int bitmap_ord_to_pos(const unsigned long *bitmap, unsigned int
 extern int bitmap_print_to_pagebuf(bool list, char *buf,
 				   const unsigned long *maskp, int nmaskbits);
 
-#define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) % BITS_PER_LONG))
-#define BITMAP_LAST_WORD_MASK(nbits)					\
-(									\
-	((nbits) % BITS_PER_LONG) ?					\
-		(1UL<<((nbits) % BITS_PER_LONG))-1 : ~0UL		\
-)
+#define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
+#define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
 
 #define small_const_nbits(nbits) \
 	(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG)
-- 
2.1.3

Re: [PATCH] linux/bitmap.h: Improve BITMAP_{LAST,FIRST}_WORD_MASK

[George Spelvin]

Looks good.

Reviewed-by: George Spelvin <linux@horizon.com>