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.

[akpm@linux-foundation.org: rename `w' to `ret']
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Anders Larsen <al@alarsen.net>
Cc: Alasdair Kergon <agk@redhat.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Matthew Wilcox <matthew@wil.cx>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: Tony Luck <tony.luck@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 38 insertions, 0 deletions

diff --git a/lib/memweight.c b/lib/memweight.c
new file mode 100644
index 000000000000..e35fc8771893
--- /dev/null
+++ b/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);