[PATCH v2 3/3] lib/find_bit: optimize find_next_bit() functions
From: Yury Norov
Date: Tue Aug 23 2022 - 21:26:47 EST
Over the past couple years, the function _find_next_bit() was extended
with parameters that modify its behavior to implement and- zero- and le-
flavors. The parameters are passed at compile time, but current design
prevents a compiler from optimizing out the conditionals.
As find_next_bit() API grows, I expect that more parameterss will be added.
Current designs would require more conditional code in _find_next_bit(),
which would bloat the helper even more and make it barely readable.
This patch replaces _find_next_bit() with a macro FIND_NEXT_BIT, and adds
a set of wrappers, so that the compile-time optimization becomes possible.
The common logic is moved to the new macro, and all flavors may be
generated by providing an EXPRESSION macro parameter, like in this example:
#define FIND_NEXT_BIT(EXPRESSION, size, start) ...
find_next_xornot_and_bit(addr1, addr2, addr3, size, start)
{
return FIND_NEXT_BIT(addr1[idx] ^ ~addr2[idx] & addr3[idx], size, start);
}
The EXPRESSION may be of any complexity, as soon as it only refers
the bitmap(s) and an iterator idx.
I ran find_bit_benchmark 16 times on top of 6.0-rc2 and 16 times on top
of this series. The results for kvm/x86_64 are:
v6.0-rc2 Optimized Difference Z-score
Random dense bitmap ns ns ns %
find_next_bit: 787735 670546 117189 14.9 3.97
find_next_zero_bit: 777492 664208 113284 14.6 10.51
find_last_bit: 830925 687573 143352 17.3 2.35
find_first_bit: 3874366 3306635 567731 14.7 1.84
find_first_and_bit: 40677125 37739887 2937238 7.2 1.36
find_next_and_bit: 347865 304456 43409 12.5 1.35
Random sparse bitmap
find_next_bit: 19816 14021 5795 29.2 6.10
find_next_zero_bit: 1318901 1223794 95107 7.2 1.41
find_last_bit: 14573 13514 1059 7.3 6.92
find_first_bit: 1313321 1249024 64297 4.9 1.53
find_first_and_bit: 8921 8098 823 9.2 4.56
find_next_and_bit: 9796 7176 2620 26.7 5.39
Where the statistics is significant (z-score > 3), the improvement
is ~15%.
According to bloat-o-meter, the Image size is 10-11K less:
x86_64/defconfig:
add/remove: 32/14 grow/shrink: 61/782 up/down: 6344/-16521 (-10177)
arm64/defconfig:
add/remove: 3/2 grow/shrink: 50/714 up/down: 608/-11556 (-10948)
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
---
Checkpatch warns that the FIND_NEXT_BIT() is a "Macros with flow
control statements", but in this case I think it's false positive
because the label is defined in the same code block as the 'goto'
statement, and control can't flow out of the block.
include/linux/find.h | 23 ++++++++-----
lib/find_bit.c | 78 +++++++++++++++-----------------------------
lib/find_bit.h | 21 ++++++++++++
lib/find_bit_be.c | 19 +++++++++++
4 files changed, 81 insertions(+), 60 deletions(-)
diff --git a/include/linux/find.h b/include/linux/find.h
index 2464bff5de04..dead6f53a97b 100644
--- a/include/linux/find.h
+++ b/include/linux/find.h
@@ -8,9 +8,12 @@
#include <linux/bitops.h>
-extern unsigned long _find_next_bit(const unsigned long *addr1,
- const unsigned long *addr2, unsigned long nbits,
- unsigned long start, unsigned long invert, unsigned long le);
+unsigned long _find_next_bit(const unsigned long *addr1, unsigned long nbits,
+ unsigned long start);
+unsigned long _find_next_and_bit(const unsigned long *addr1, const unsigned long *addr2,
+ unsigned long nbits, unsigned long start);
+unsigned long _find_next_zero_bit(const unsigned long *addr, unsigned long nbits,
+ unsigned long start);
extern unsigned long _find_first_bit(const unsigned long *addr, unsigned long size);
extern unsigned long _find_first_and_bit(const unsigned long *addr1,
const unsigned long *addr2, unsigned long size);
@@ -19,6 +22,10 @@ extern unsigned long _find_last_bit(const unsigned long *addr, unsigned long siz
#ifdef __BIG_ENDIAN
unsigned long _find_first_zero_bit_le(const unsigned long *addr, unsigned long size);
+unsigned long _find_next_zero_bit_le(const unsigned long *addr, unsigned
+ long size, unsigned long offset);
+unsigned long _find_next_bit_le(const unsigned long *addr, unsigned
+ long size, unsigned long offset);
#endif
#ifndef find_next_bit
@@ -45,7 +52,7 @@ unsigned long find_next_bit(const unsigned long *addr, unsigned long size,
return val ? __ffs(val) : size;
}
- return _find_next_bit(addr, NULL, size, offset, 0UL, 0);
+ return _find_next_bit(addr, size, offset);
}
#endif
@@ -75,7 +82,7 @@ unsigned long find_next_and_bit(const unsigned long *addr1,
return val ? __ffs(val) : size;
}
- return _find_next_bit(addr1, addr2, size, offset, 0UL, 0);
+ return _find_next_and_bit(addr1, addr2, size, offset);
}
#endif
@@ -103,7 +110,7 @@ unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size,
return val == ~0UL ? size : ffz(val);
}
- return _find_next_bit(addr, NULL, size, offset, ~0UL, 0);
+ return _find_next_zero_bit(addr, size, offset);
}
#endif
@@ -251,7 +258,7 @@ unsigned long find_next_zero_bit_le(const void *addr, unsigned
return val == ~0UL ? size : ffz(val);
}
- return _find_next_bit(addr, NULL, size, offset, ~0UL, 1);
+ return _find_next_zero_bit_le(addr, size, offset);
}
#endif
@@ -284,7 +291,7 @@ unsigned long find_next_bit_le(const void *addr, unsigned
return val ? __ffs(val) : size;
}
- return _find_next_bit(addr, NULL, size, offset, 0UL, 1);
+ return _find_next_bit_le(addr, size, offset);
}
#endif
diff --git a/lib/find_bit.c b/lib/find_bit.c
index ccc4fb1dfc71..357750d25ff9 100644
--- a/lib/find_bit.c
+++ b/lib/find_bit.c
@@ -21,58 +21,6 @@
#include "find_bit.h"
-#if !defined(find_next_bit) || !defined(find_next_zero_bit) || \
- !defined(find_next_bit_le) || !defined(find_next_zero_bit_le) || \
- !defined(find_next_and_bit)
-/*
- * This is a common helper function for find_next_bit, find_next_zero_bit, and
- * find_next_and_bit. The differences are:
- * - The "invert" argument, which is XORed with each fetched word before
- * searching it for one bits.
- * - The optional "addr2", which is anded with "addr1" if present.
- */
-unsigned long _find_next_bit(const unsigned long *addr1,
- const unsigned long *addr2, unsigned long nbits,
- unsigned long start, unsigned long invert, unsigned long le)
-{
- unsigned long tmp, mask;
-
- if (unlikely(start >= nbits))
- return nbits;
-
- tmp = addr1[start / BITS_PER_LONG];
- if (addr2)
- tmp &= addr2[start / BITS_PER_LONG];
- tmp ^= invert;
-
- /* Handle 1st word. */
- mask = BITMAP_FIRST_WORD_MASK(start);
- if (le)
- mask = swab(mask);
-
- tmp &= mask;
-
- start = round_down(start, BITS_PER_LONG);
-
- while (!tmp) {
- start += BITS_PER_LONG;
- if (start >= nbits)
- return nbits;
-
- tmp = addr1[start / BITS_PER_LONG];
- if (addr2)
- tmp &= addr2[start / BITS_PER_LONG];
- tmp ^= invert;
- }
-
- if (le)
- tmp = swab(tmp);
-
- return min(start + __ffs(tmp), nbits);
-}
-EXPORT_SYMBOL(_find_next_bit);
-#endif
-
#ifndef find_first_bit
/*
* Find the first set bit in a memory region.
@@ -108,6 +56,32 @@ unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size
EXPORT_SYMBOL(_find_first_zero_bit);
#endif
+#ifndef find_next_bit
+unsigned long _find_next_bit(const unsigned long *addr, unsigned long nbits, unsigned long start)
+{
+ return FIND_NEXT_BIT(addr[idx], nbits, start);
+}
+EXPORT_SYMBOL(_find_next_bit);
+#endif
+
+#ifndef find_next_and_bit
+unsigned long _find_next_and_bit(const unsigned long *addr1, const unsigned long *addr2,
+ unsigned long nbits, unsigned long start)
+{
+ return FIND_NEXT_BIT(addr1[idx] & addr2[idx], nbits, start);
+}
+EXPORT_SYMBOL(_find_next_and_bit);
+#endif
+
+#ifndef find_next_zero_bit
+unsigned long _find_next_zero_bit(const unsigned long *addr, unsigned long nbits,
+ unsigned long start)
+{
+ return FIND_NEXT_BIT(~addr[idx], nbits, start);
+}
+EXPORT_SYMBOL(_find_next_zero_bit);
+#endif
+
#ifndef find_last_bit
unsigned long _find_last_bit(const unsigned long *addr, unsigned long size)
{
diff --git a/lib/find_bit.h b/lib/find_bit.h
index b4b6245ddbf6..6b6312f93301 100644
--- a/lib/find_bit.h
+++ b/lib/find_bit.h
@@ -21,4 +21,25 @@
sz; \
})
+#define FIND_NEXT_BIT(EXPRESSION, size, start) \
+({ \
+ unsigned long mask, idx, tmp, sz = (size), __start = (start); \
+ \
+ if (unlikely(__start >= sz)) \
+ goto out; \
+ \
+ mask = word_op(BITMAP_FIRST_WORD_MASK(__start)); \
+ idx = __start / BITS_PER_LONG; \
+ \
+ for (tmp = (EXPRESSION) & mask; !tmp; tmp = (EXPRESSION)) { \
+ if (idx > sz / BITS_PER_LONG) \
+ goto out; \
+ idx++; \
+ } \
+ \
+ sz = min(idx * BITS_PER_LONG + __ffs(word_op(tmp)), sz); \
+out: \
+ sz; \
+})
+
#endif /* _LIB_FIND_BIT_H */
diff --git a/lib/find_bit_be.c b/lib/find_bit_be.c
index 36173cb7e012..cbf669aaf3cb 100644
--- a/lib/find_bit_be.c
+++ b/lib/find_bit_be.c
@@ -21,3 +21,22 @@ unsigned long _find_first_zero_bit_le(const unsigned long *addr, unsigned long s
}
EXPORT_SYMBOL(_find_first_zero_bit_le);
#endif
+
+#ifndef find_next_zero_bit_le
+unsigned long _find_next_zero_bit_le(const unsigned long *addr, unsigned
+ long size, unsigned long offset)
+{
+ return FIND_NEXT_BIT(~addr[idx], size, offset);
+}
+EXPORT_SYMBOL(_find_next_zero_bit_le);
+#endif
+
+#ifndef find_next_bit_le
+unsigned long _find_next_bit_le(const unsigned long *addr, unsigned
+ long size, unsigned long offset)
+{
+ return FIND_NEXT_BIT(addr[idx], size, offset);
+}
+EXPORT_SYMBOL(_find_next_bit_le);
+
+#endif
--
2.34.1