lib/string.c: introduce memchr_inv()

memchr_inv() is mainly used to check whether the whole buffer is filled
with just a specified byte.

The function name and prototype are stolen from logfs and the
implementation is from SLUB.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Christoph Lameter <cl@linux-foundation.org>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Matt Mackall <mpm@selenic.com>
Acked-by: Joern Engel <joern@logfs.org>
Cc: Marcin Slusarz <marcin.slusarz@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 54 insertions, 0 deletions

diff --git a/lib/string.c b/lib/string.c
index 01fad9b203e1..11df54325fb8 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -756,3 +756,57 @@ void *memchr(const void *s, int c, size_t n)
 }
 EXPORT_SYMBOL(memchr);
 #endif
+
+static void *check_bytes8(const u8 *start, u8 value, unsigned int bytes)
+{
+        while (bytes) {
+                if (*start != value)
+                        return (void *)start;
+                start++;
+                bytes--;
+        }
+        return NULL;
+}
+
+/**
+ * memchr_inv - Find an unmatching character in an area of memory.
+ * @start: The memory area
+ * @c: Find a character other than c
+ * @bytes: The size of the area.
+ *
+ * returns the address of the first character other than @c, or %NULL
+ * if the whole buffer contains just @c.
+ */
+void *memchr_inv(const void *start, int c, size_t bytes)
+{
+        u8 value = c;
+        u64 value64;
+        unsigned int words, prefix;
+
+        if (bytes <= 16)
+                return check_bytes8(start, value, bytes);
+
+        value64 = value | value << 8 | value << 16 | value << 24;
+        value64 = (value64 & 0xffffffff) | value64 << 32;
+        prefix = 8 - ((unsigned long)start) % 8;
+
+        if (prefix) {
+                u8 *r = check_bytes8(start, value, prefix);
+                if (r)
+                        return r;
+                start += prefix;
+                bytes -= prefix;
+        }
+
+        words = bytes / 8;
+
+        while (words) {
+                if (*(u64 *)start != value64)
+                        return check_bytes8(start, value, 8);
+                start += 8;
+                words--;
+        }
+
+        return check_bytes8(start, value, bytes % 8);
+}
+EXPORT_SYMBOL(memchr_inv);