lib/find_bit: optimize find_next_bit() functions

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 parameters will be added.
Current design 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 optimizations become possible.

The common logic is moved to the new macro, and all flavors may be
generated by providing a FETCH macro parameter, like in this example:

#define FIND_NEXT_BIT(FETCH, MUNGE, size, start) ...

find_next_xornot_and_bit(addr1, addr2, addr3, size, start)
{
return FIND_NEXT_BIT(addr1[idx] ^ ~addr2[idx] & addr3[idx],
/* nop */, size, start);
}

The FETCH may be of any complexity, as soon as it only refers the bitmap(s)
and an iterator idx.

MUNGE is here to support _le code generation for BE builds. May be
empty.

I ran find_bit_benchmark 16 times on top of 6.0-rc2 and 16 times on top
of 6.0-rc2 + 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 the 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 <[e-mail]>
Signed-off-by: Yury Norov <[e-mail]>

include/linux/find.h

···
 
 #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);
···
 
 #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
···
 		return val ? __ffs(val) : size;
 	}
 
-	return _find_next_bit(addr, NULL, size, offset, 0UL, 0);
+	return _find_next_bit(addr, size, offset);
 }
 #endif
 
···
 		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
 
···
 		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
 
···
 		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
 
···
 		return val ? __ffs(val) : size;
 	}
 
-	return _find_next_bit(addr, NULL, size, offset, 0UL, 1);
+	return _find_next_bit_le(addr, size, offset);
 }
 #endif
 

lib/find_bit.c

···
 	sz;									\
 })
 
-#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.
+ * Common helper for find_next_bit() function family
+ * @FETCH: The expression that fetches and pre-processes each word of bitmap(s)
+ * @MUNGE: The expression that post-processes a word containing found bit (may be empty)
+ * @size: The bitmap size in bits
+ * @start: The bitnumber to start searching at
  */
-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
+#define FIND_NEXT_BIT(FETCH, MUNGE, size, start)				\
+({										\
+	unsigned long mask, idx, tmp, sz = (size), __start = (start);		\
+										\
+	if (unlikely(__start >= sz))						\
+		goto out;							\
+										\
+	mask = MUNGE(BITMAP_FIRST_WORD_MASK(__start));				\
+	idx = __start / BITS_PER_LONG;						\
+										\
+	for (tmp = (FETCH) & mask; !tmp; tmp = (FETCH)) {			\
+		if ((idx + 1) * BITS_PER_LONG >= sz)				\
+			goto out;						\
+		idx++;								\
+	}									\
+										\
+	sz = min(idx * BITS_PER_LONG + __ffs(MUNGE(tmp)), sz);			\
+out:										\
+	sz;									\
+})
 
 #ifndef find_first_bit
 /*
···
 	return FIND_FIRST_BIT(~addr[idx], /* nop */, 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], /* nop */, 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], /* nop */, 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], /* nop */, nbits, start);
+}
+EXPORT_SYMBOL(_find_next_zero_bit);
 #endif
 
 #ifndef find_last_bit
···
 	return FIND_FIRST_BIT(~addr[idx], swab, size);
 }
 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], swab, 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], swab, size, offset);
+}
+EXPORT_SYMBOL(_find_next_bit_le);
 
 #endif