[PATCH net-next v5 12/21] bitmap: introduce generic optimized bitmap_size()

Alexander Lobakin
2024-02-01T12:24:33Z

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>
---
include/linux/bitmap.h | 8 +++++---
include/linux/cpumask.h | 2 +-
tools/include/linux/bitmap.h | 7 ++++---
drivers/md/dm-clone-metadata.c | 5 -----
drivers/s390/cio/idset.c | 2 +-
lib/math/prime_numbers.c | 2 --
6 files changed, 11 insertions(+), 15 deletions(-)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 7ca0379be8c1..9a6a27a7f675 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -218,9 +218,11 @@ void bitmap_fold(unsigned long *dst, const unsigned long *orig,
#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;
@@ -230,7 +232,7 @@ static inline void bitmap_zero(unsigned long *dst, unsigned int nbits)

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;
@@ -241,7 +243,7 @@ static inline void bitmap_fill(unsigned long *dst, unsigned int nbits)
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;
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index cfb545841a2c..a2064c2a9441 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -839,7 +839,7 @@ static inline int cpulist_parse(const char *buf, struct cpumask *dstp)
*/
static inline unsigned int cpumask_size(void)
{
- return BITS_TO_LONGS(large_cpumask_bits) * sizeof(long);
+ return bitmap_size(large_cpumask_bits);
}

/*
diff --git a/tools/include/linux/bitmap.h b/tools/include/linux/bitmap.h
index 8c6852dba04f..210c13b1b857 100644
--- a/tools/include/linux/bitmap.h
+++ b/tools/include/linux/bitmap.h
@@ -26,13 +26,14 @@ bool __bitmap_intersects(const unsigned long *bitmap1,
#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));
}
}

@@ -84,7 +85,7 @@ static inline void bitmap_or(unsigned long *dst, const unsigned long *src1,
*/
static inline unsigned long *bitmap_zalloc(int nbits)
{
- return calloc(1, BITS_TO_LONGS(nbits) * sizeof(unsigned long));
+ return calloc(1, bitmap_size(nbits));
}

/*
diff --git a/drivers/md/dm-clone-metadata.c b/drivers/md/dm-clone-metadata.c
index c43d55672bce..47c1fa7aad8b 100644
--- a/drivers/md/dm-clone-metadata.c
+++ b/drivers/md/dm-clone-metadata.c
@@ -465,11 +465,6 @@ static void __destroy_persistent_data_structures(struct dm_clone_metadata *cmd)

/*---------------------------------------------------------------------------*/

-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)
{
diff --git a/drivers/s390/cio/idset.c b/drivers/s390/cio/idset.c
index 0a1105a483bf..e5f28370a903 100644
--- a/drivers/s390/cio/idset.c
+++ b/drivers/s390/cio/idset.c
@@ -18,7 +18,7 @@ struct idset {

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)
diff --git a/lib/math/prime_numbers.c b/lib/math/prime_numbers.c
index d42cebf7407f..d3b64b10da1c 100644
--- a/lib/math/prime_numbers.c
+++ b/lib/math/prime_numbers.c
@@ -6,8 +6,6 @@
#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;
--
2.43.0