[PATCH v3 1/5] lib/find: introduce find_nth_and_andnot_bit

[Yury Norov]

The function is used to implement in-place bitmaps traversing without
storing intermediate result in temporary bitmaps, in the following patches.

Signed-off-by: Yury Norov <yury.norov@xxxxxxxxx>
---
 include/linux/find.h | 33 +++++++++++++++++++++++++++++++++
 lib/find_bit.c       |  9 +++++++++
 2 files changed, 42 insertions(+)

diff --git a/include/linux/find.h b/include/linux/find.h
index 3f59c2fbe438..b594207a0010 100644
--- a/include/linux/find.h
+++ b/include/linux/find.h
@@ -23,6 +23,9 @@ unsigned long __find_nth_and_bit(const unsigned long *addr1, const unsigned long
 				unsigned long size, unsigned long n);
 unsigned long __find_nth_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
 					unsigned long size, unsigned long n);
+unsigned long __find_nth_and_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
+					const unsigned long *addr3, unsigned long size,
+					unsigned long n);
 extern unsigned long _find_first_and_bit(const unsigned long *addr1,
 					 const unsigned long *addr2, unsigned long size);
 extern unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size);
@@ -244,6 +247,36 @@ unsigned long find_nth_andnot_bit(const unsigned long *addr1, const unsigned lon
 	return __find_nth_andnot_bit(addr1, addr2, size, n);
 }
 
+/**
+ * find_nth_and_andnot_bit - find N'th set bit in 2 memory regions,
+ *			     excluding those set in 3rd region
+ * @addr1: The 1st address to start the search at
+ * @addr2: The 2nd address to start the search at
+ * @addr3: The 3rd address to start the search at
+ * @size: The maximum number of bits to search
+ * @n: The number of set bit, which position is needed, counting from 0
+ *
+ * Returns the bit number of the N'th set bit.
+ * If no such, returns @size.
+ */
+static __always_inline
+unsigned long find_nth_and_andnot_bit(const unsigned long *addr1,
+					const unsigned long *addr2,
+					const unsigned long *addr3,
+					unsigned long size, unsigned long n)
+{
+	if (n >= size)
+		return size;
+
+	if (small_const_nbits(size)) {
+		unsigned long val =  *addr1 & *addr2 & (~*addr3) & GENMASK(size - 1, 0);
+
+		return val ? fns(val, n) : size;
+	}
+
+	return __find_nth_and_andnot_bit(addr1, addr2, addr3, size, n);
+}
+
 #ifndef find_first_and_bit
 /**
  * find_first_and_bit - find the first set bit in both memory regions
diff --git a/lib/find_bit.c b/lib/find_bit.c
index 18bc0a7ac8ee..c10920e66788 100644
--- a/lib/find_bit.c
+++ b/lib/find_bit.c
@@ -155,6 +155,15 @@ unsigned long __find_nth_andnot_bit(const unsigned long *addr1, const unsigned l
 }
 EXPORT_SYMBOL(__find_nth_andnot_bit);
 
+unsigned long __find_nth_and_andnot_bit(const unsigned long *addr1,
+					const unsigned long *addr2,
+					const unsigned long *addr3,
+					unsigned long size, unsigned long n)
+{
+	return FIND_NTH_BIT(addr1[idx] & addr2[idx] & ~addr3[idx], size, n);
+}
+EXPORT_SYMBOL(__find_nth_and_andnot_bit);
+
 #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)
-- 
2.34.1