[PATCH v2 3/3] lib/find_bit: optimize find_next_bit() functions

[Yury Norov <yury.norov@gmail.com>]

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@linux-foundation.org>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
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