Commit 89c1e79e authored Apr 15, 2015 by Rasmus Villemoes Committed by Linus Torvalds Apr 15, 2015

linux/bitmap.h: improve BITMAP_{LAST,FIRST}_WORD_MASK



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>
Cc: Tejun Heo <tj@kernel.org>
Reviewed-by: George Spelvin <linux@horizon.com>
Cc: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 8a72ed6f

include/linux/bitmap.h

+2 −6

Original line number	Diff line number	Diff line
		@@ -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)

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