string: introduce memweight()

memweight() is the function that counts the total number of bits set in
memory area.  Unlike bitmap_weight(), memweight() takes pointer and size
in bytes to specify a memory area which does not need to be aligned to
long-word boundary.

[[email protected]: rename `w' to `ret']
Signed-off-by: Akinobu Mita <[email protected]>
Cc: Anders Larsen <[email protected]>
Cc: Alasdair Kergon <[email protected]>
Cc: Laurent Pinchart <[email protected]>
Cc: Mark Fasheh <[email protected]>
Cc: Joel Becker <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Andreas Dilger <[email protected]>
Cc: "Theodore Ts'o" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mauro Carvalho Chehab <[email protected]>
Cc: Tony Luck <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

include/linux/string.h

@@ -145,4 +145,7 @@ static inline bool strstarts(const char *str, const char *prefix)
 	return strncmp(str, prefix, strlen(prefix)) == 0;
 }
 #endif
+
+extern size_t memweight(const void *ptr, size_t bytes);
+
 #endif /* _LINUX_STRING_H_ */

lib/Makefile

@@ -22,7 +22,7 @@ lib-y	+= kobject.o klist.o
 obj-y += bcd.o div64.o sort.o parser.o halfmd4.o debug_locks.o random32.o \
 	 bust_spinlocks.o hexdump.o kasprintf.o bitmap.o scatterlist.o \
 	 string_helpers.o gcd.o lcm.o list_sort.o uuid.o flex_array.o \
-	 bsearch.o find_last_bit.o find_next_bit.o llist.o
+	 bsearch.o find_last_bit.o find_next_bit.o llist.o memweight.o
 obj-y += kstrtox.o
 obj-$(CONFIG_TEST_KSTRTOX) += test-kstrtox.o
 

lib/memweight.c

@@ -0,0 +1,38 @@
+#include <linux/export.h>
+#include <linux/bug.h>
+#include <linux/bitmap.h>
+
+/**
+ * memweight - count the total number of bits set in memory area
+ * @ptr: pointer to the start of the area
+ * @bytes: the size of the area
+ */
+size_t memweight(const void *ptr, size_t bytes)
+{
+	size_t ret = 0;
+	size_t longs;
+	const unsigned char *bitmap = ptr;
+
+	for (; bytes > 0 && ((unsigned long)bitmap) % sizeof(long);
+			bytes--, bitmap++)
+		ret += hweight8(*bitmap);
+
+	longs = bytes / sizeof(long);
+	if (longs) {
+		BUG_ON(longs >= INT_MAX / BITS_PER_LONG);
+		ret += bitmap_weight((unsigned long *)bitmap,
+				longs * BITS_PER_LONG);
+		bytes -= longs * sizeof(long);
+		bitmap += longs * sizeof(long);
+	}
+	/*
+	 * The reason that this last loop is distinct from the preceding
+	 * bitmap_weight() call is to compute 1-bits in the last region smaller
+	 * than sizeof(long) properly on big-endian systems.
+	 */
+	for (; bytes > 0; bytes--, bitmap++)
+		ret += hweight8(*bitmap);
+
+	return ret;
+}
+EXPORT_SYMBOL(memweight);