lib: add fast path for find_next_*_bit()

Like for bitmap functions, find_next_*_bit() users will benefit if
we'll handle a case of small-size bitmaps that fit into a single
word. In the best case, compiler will be able to replace a function
call with a single ffs or ffz instruction.

include/asm-generic/bitops/find.h

@@ -16,6 +16,18 @@ static __always_inline
 unsigned long find_next_bit(const unsigned long *addr, unsigned long size,
 			    unsigned long offset)
 {
+	if (small_const_nbits(size)) {
+		unsigned long val;
+
+		if (unlikely(offset >= size))
+			return size;
+
+		val = *addr & BITMAP_FIRST_WORD_MASK(offset)
+				& BITMAP_LAST_WORD_MASK(size);
+
+		return val ? __ffs(val) : size;
+	}
+
 	return _find_next_bit(addr, NULL, size, offset, 0UL, 0);
 }
 #endif
@@ -36,6 +48,18 @@ unsigned long find_next_and_bit(const unsigned long *addr1,
 		const unsigned long *addr2, unsigned long size,
 		unsigned long offset)
 {
+	if (small_const_nbits(size)) {
+		unsigned long val;
+
+		if (unlikely(offset >= size))
+			return size;
+
+		val = *addr1 & *addr2 & BITMAP_FIRST_WORD_MASK(offset)
+				      & BITMAP_LAST_WORD_MASK(size);
+
+		return val ? __ffs(val) : size;
+	}
+
 	return _find_next_bit(addr1, addr2, size, offset, 0UL, 0);
 }
 #endif
@@ -54,6 +78,18 @@ static __always_inline
 unsigned long find_next_zero_bit(const unsigned long *addr, unsigned long size,
 				 unsigned long offset)
 {
+	if (small_const_nbits(size)) {
+		unsigned long val;
+
+		if (unlikely(offset >= size))
+			return size;
+
+		val = *addr | !BITMAP_FIRST_WORD_MASK(offset)
+			    | !BITMAP_LAST_WORD_MASK(size);
+
+		return val == ~0UL ? size : ffz(val);
+	}
+
 	return _find_next_bit(addr, NULL, size, offset, ~0UL, 0);
 }
 #endif

include/asm-generic/bitops/le.h

@@ -36,6 +36,16 @@ static __always_inline
 unsigned long find_next_zero_bit_le(const void *addr, unsigned
 		long size, unsigned long offset)
 {
+	if (small_const_nbits(size)) {
+		unsigned long fz;
+
+		if (unlikely(offset >= size))
+			return size;
+
+		fz = ffz(swab(*addr & BITMAP_FIRST_WORD_MASK(offset)));
+		return fz < size ? fz : size;
+	}
+
 	return _find_next_bit(addr, NULL, size, offset, ~0UL, 1);
 }
 #endif
@@ -45,6 +55,16 @@ static __always_inline
 unsigned long find_next_bit_le(const void *addr, unsigned
 		long size, unsigned long offset)
 {
+	if (small_const_nbits(size)) {
+		unsigned long fs;
+
+		if (unlikely(offset >= size))
+			return size;
+
+		fs = __ffs(swab(*addr & BITMAP_FIRST_WORD_MASK(offset)));
+		return fs < size ? fs : size;
+	}
+
 	return _find_next_bit(addr, NULL, size, offset, 0UL, 1);
 }
 #endif