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bitmap: introduce generic optimized bitmap_size()

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commit a37fbe666c016fd89e4460d0ebfcea05baba46dc upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Alexander Lobakin
2024-03-27 16:23:49 +01:00
committed by Greg Kroah-Hartman
parent 90a6b797e9
commit 188729977a
4 changed files with 11 additions and 8 deletions
Download Patch File Download Diff File Expand all files Collapse all files

8
include/linux/bitmap.h
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@@ -212,12 +212,14 @@ extern int bitmap_print_to_pagebuf(bool list, char *buf,
#define small_const_nbits(nbits) \
(__builtin_constant_p(nbits) && (nbits) <= BITS_PER_LONG && (nbits) > 0)
#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 {
unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
unsigned int len = bitmap_size(nbits);
memset(dst, 0, len);
}
}
@@ -227,7 +229,7 @@ static inline void bitmap_fill(unsigned long *dst, unsigned int nbits)
if (small_const_nbits(nbits))
*dst = ~0UL;
else {
unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
unsigned int len = bitmap_size(nbits);
memset(dst, 0xff, len);
}
}
@@ -238,7 +240,7 @@ static inline void bitmap_copy(unsigned long *dst, const unsigned long *src,
if (small_const_nbits(nbits))
*dst = *src;
else {
unsigned int len = BITS_TO_LONGS(nbits) * sizeof(unsigned long);
unsigned int len = bitmap_size(nbits);
memcpy(dst, src, len);
}
}