Alexander Lobakin a37fbe666c 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 <przemyslaw.kitszel@intel.com>
Acked-by: Yury Norov <yury.norov@gmail.com>
Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
2024-04-01 10:49:28 +01:00
..
2024-01-19 11:59:11 -08:00
2024-02-01 13:06:40 +01:00
2018-08-16 12:14:42 -07:00
2023-11-03 07:08:36 -10:00
2021-01-21 14:06:00 -07:00
2022-03-07 12:48:35 -07:00
2021-08-19 09:02:55 +09:00
2023-02-02 22:50:01 -08:00
2023-02-02 22:50:01 -08:00
2021-01-03 20:05:18 -05:00
2023-03-19 10:02:04 -07:00
2022-03-07 12:48:35 -07:00
2022-04-29 14:38:01 -07:00
2022-10-03 14:03:21 -07:00
2024-03-11 09:38:17 -07:00
2021-08-19 09:02:55 +09:00
2024-02-12 10:35:40 -06:00
2024-03-23 08:43:21 -07:00
2021-07-08 11:48:20 -07:00
2023-08-21 13:46:25 -07:00
2023-04-17 18:01:23 +02:00
2021-07-08 11:48:20 -07:00
2023-10-16 12:44:06 -04:00
2023-08-24 16:20:18 -07:00
2023-10-16 12:44:06 -04:00
2024-03-14 10:58:27 -07:00
2018-10-16 13:45:44 +02:00
2022-10-03 17:34:32 -07:00
2021-07-08 11:48:20 -07:00
2024-02-15 12:17:28 -05:00
2022-06-03 10:34:34 -07:00
2024-02-20 20:47:32 -08:00
2021-06-18 11:43:09 +02:00
2021-07-08 11:48:20 -07:00
2022-01-20 08:52:54 +02:00
2024-02-22 10:24:48 -08:00