string: memchr_inv() speed improvements

- Generate a 64-bit pattern more efficiently

memchr_inv needs to generate a 64-bit pattern filled with a target
character. The operation can be done by more efficient way.

- Don't call the slow check_bytes() if the memory area is 64-bit aligned

memchr_inv compares contiguous 64-bit words with the 64-bit pattern as
much as possible. The outside of the region is checked by check_bytes()
that scans for each byte. Unfortunately, the first 64-bit word is
unexpectedly scanned by check_bytes() even if the memory area is aligned
to a 64-bit boundary.

Both changes were originally suggested by Eric Dumazet.

Signed-off-by: Akinobu Mita <[e-mail]>
Suggested-by: Eric Dumazet <[e-mail]>
Cc: Brian Norris <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

lib/string.c

···
 	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;
+	value64 = value;
+#if defined(ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
+	value64 *= 0x0101010101010101;
+#elif defined(ARCH_HAS_FAST_MULTIPLIER)
+	value64 *= 0x01010101;
+	value64 |= value64 << 32;
+#else
+	value64 |= value64 << 8;
+	value64 |= value64 << 16;
+	value64 |= value64 << 32;
+#endif
 
+	prefix = (unsigned long)start % 8;
 	if (prefix) {
-		u8 *r = check_bytes8(start, value, prefix);
+		u8 *r;
+
+		prefix = 8 - prefix;
+		r = check_bytes8(start, value, prefix);
 		if (r)
 			return r;
 		start += prefix;