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.

Rasmus Villemoes committed 11 years ago

Commit 89c1e791 parent 8a72ed61 file · +2 −6Browse files

Unified · Split

include/linux/bitmap.h+2−6

@@ -172,12 +172,8 @@ extern unsigned int bitmap_ord_to_pos(const unsigned long *bitmap, unsigned int · 171 unmodified lines
172	172	extern int bitmap_print_to_pagebuf(bool list, char *buf,
173	173	const unsigned long *maskp, int nmaskbits);
174	174
175		−#define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) % BITS_PER_LONG))
176		−#define BITMAP_LAST_WORD_MASK(nbits) \
177		−( \
178		− ((nbits) % BITS_PER_LONG) ? \
179		− (1UL<<((nbits) % BITS_PER_LONG))-1 : ~0UL \
180		−)
	175	+#define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
	176	+#define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
181	177
182	178	#define small_const_nbits(nbits) \
183	179	(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG)