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Message-ID: <ZSQykJtkemZTiYHP@yury-ThinkPad>
Date: Mon, 9 Oct 2023 10:04:16 -0700
From: Yury Norov <yury.norov@...il.com>
To: Alexander Lobakin <aleksander.lobakin@...el.com>
Cc: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
Rasmus Villemoes <linux@...musvillemoes.dk>,
Alexander Potapenko <glider@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Eric Dumazet <edumazet@...gle.com>,
David Ahern <dsahern@...nel.org>,
Przemek Kitszel <przemyslaw.kitszel@...el.com>,
Simon Horman <simon.horman@...igine.com>,
netdev@...r.kernel.org, linux-btrfs@...r.kernel.org,
dm-devel@...hat.com, ntfs3@...ts.linux.dev,
linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 08/14] bitmap: introduce generic optimized bitmap_size()
On Mon, Oct 09, 2023 at 05:10:20PM +0200, Alexander Lobakin wrote:
> 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.
OK, I see your point. Indeed, opencoding this again and again may be
annoying.
Acked-by: Yury Norov <yury.norov@...il.com>
> 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 divides `nbits + 63` 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@...el.com>
> Signed-off-by: Alexander Lobakin <aleksander.lobakin@...el.com>
> ---
> drivers/md/dm-clone-metadata.c | 5 -----
> include/linux/bitmap.h | 8 +++++---
> include/linux/cpumask.h | 2 +-
> lib/math/prime_numbers.c | 2 --
> tools/include/linux/bitmap.h | 8 +++++---
> 5 files changed, 11 insertions(+), 14 deletions(-)
>
> 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/include/linux/bitmap.h b/include/linux/bitmap.h
> index 03644237e1ef..63e422f8ba3d 100644
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -237,9 +237,11 @@ extern int bitmap_print_list_to_buf(char *buf, const unsigned long *maskp,
> #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;
> @@ -249,7 +251,7 @@ 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;
> @@ -260,7 +262,7 @@ 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 f10fb87d49db..dbdbf1451cad 100644
> --- a/include/linux/cpumask.h
> +++ b/include/linux/cpumask.h
> @@ -821,7 +821,7 @@ 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/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;
> diff --git a/tools/include/linux/bitmap.h b/tools/include/linux/bitmap.h
> index f3566ea0f932..81a2299ace15 100644
> --- a/tools/include/linux/bitmap.h
> +++ b/tools/include/linux/bitmap.h
> @@ -2,6 +2,7 @@
> #ifndef _TOOLS_LINUX_BITMAP_H
> #define _TOOLS_LINUX_BITMAP_H
>
> +#include <linux/align.h>
> #include <string.h>
> #include <linux/bitops.h>
> #include <linux/find.h>
> @@ -25,13 +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));
> }
> }
>
> @@ -83,7 +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));
> }
>
> /*
> --
> 2.41.0
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