bitmap: introduce generic optimized bitmap_size()

The number of times yet another open coded
`BITS_TO_LONGS(nbits) * sizeof(long)` can be spotted is huge.
Some generic helper is long overdue.

Add one, bitmap_size(), but with one detail.
BITS_TO_LONGS() uses DIV_ROUND_UP(). The latter works well when both
divident and divisor are compile-time constants or when the divisor
is not a pow-of-2. When it is however, the compilers sometimes tend
to generate suboptimal code (GCC 13):

48 83 c0 3f add $0x3f,%rax
48 c1 e8 06 shr $0x6,%rax
48 8d 14 c5 00 00 00 00 lea 0x0(,%rax,8),%rdx

%BITS_PER_LONG is always a pow-2 (either 32 or 64), but GCC still does
full division of `nbits + 63` by it and then multiplication by 8.
Instead of BITS_TO_LONGS(), use ALIGN() and then divide by 8. GCC:

8d 50 3f lea 0x3f(%rax),%edx
c1 ea 03 shr $0x3,%edx
81 e2 f8 ff ff 1f and $0x1ffffff8,%edx

Now it shifts `nbits + 63` by 3 positions (IOW performs fast division
by 8) and then masks bits[2:0]. bloat-o-meter:

add/remove: 0/0 grow/shrink: 20/133 up/down: 156/-773 (-617)

Clang does it better and generates the same code before/after starting
from -O1, except that with the ALIGN() approach it uses %edx and thus
still saves some bytes:

add/remove: 0/0 grow/shrink: 9/133 up/down: 18/-538 (-520)

Note that we can't expand DIV_ROUND_UP() by adding a check and using
this approach there, as it's used in array declarations where
expressions are not allowed.
Add this helper to tools/ as well.

Reviewed-by: Przemek Kitszel <[e-mail]>
Acked-by: Yury Norov <[e-mail]>
Signed-off-by: Alexander Lobakin <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/md/dm-clone-metadata.c

···
 
 /*---------------------------------------------------------------------------*/
 
-static size_t bitmap_size(unsigned long nr_bits)
-{
-	return BITS_TO_LONGS(nr_bits) * sizeof(long);
-}
-
 static int __dirty_map_init(struct dirty_map *dmap, unsigned long nr_words,
 			    unsigned long nr_regions)
 {

drivers/s390/cio/idset.c

···
 
 static inline unsigned long idset_bitmap_size(int num_ssid, int num_id)
 {
-	return BITS_TO_LONGS(num_ssid * num_id) * sizeof(unsigned long);
+	return bitmap_size(size_mul(num_ssid, num_id));
 }
 
 static struct idset *idset_new(int num_ssid, int num_id)

include/linux/bitmap.h

···
 #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 bitmap_size(nbits)	(ALIGN(nbits, BITS_PER_LONG) / BITS_PER_BYTE)
+
 static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
 {
-	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
+	unsigned int len = bitmap_size(nbits);
 
 	if (small_const_nbits(nbits))
 		*dst = 0;
···
 
 static inline void bitmap_fill(unsigned long *dst, unsigned int nbits)
 {
-	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
+	unsigned int len = bitmap_size(nbits);
 
 	if (small_const_nbits(nbits))
 		*dst = ~0UL;
···
 static inline void bitmap_copy(unsigned long *dst, const unsigned long *src,
 			unsigned int nbits)
 {
-	unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
+	unsigned int len = bitmap_size(nbits);
 
 	if (small_const_nbits(nbits))
 		*dst = *src;

include/linux/cpumask.h

···
  */
 static inline unsigned int cpumask_size(void)
 {
-	return BITS_TO_LONGS(large_cpumask_bits) * sizeof(long);
+	return bitmap_size(large_cpumask_bits);
 }
 
 /*

lib/math/prime_numbers.c

···
 #include <linux/prime_numbers.h>
 #include <linux/slab.h>
 
-#define bitmap_size(nbits) (BITS_TO_LONGS(nbits) * sizeof(unsigned long))
-
 struct primes {
 	struct rcu_head rcu;
 	unsigned long last, sz;

tools/include/linux/bitmap.h

···
 #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 bitmap_size(nbits)	(ALIGN(nbits, BITS_PER_LONG) / BITS_PER_BYTE)
+
 static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)
 {
 	if (small_const_nbits(nbits))
 		*dst = 0UL;
 	else {
-		int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
-		memset(dst, 0, len);
+		memset(dst, 0, bitmap_size(nbits));
 	}
 }
 
···
  */
 static inline unsigned long *bitmap_zalloc(int nbits)
 {
-	return calloc(1, BITS_TO_LONGS(nbits) * sizeof(unsigned long));
+	return calloc(1, bitmap_size(nbits));
 }
 
 /*