linux-kernel - [PATCH 2/2] lib/string.c: Optimize memchr()

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Date:   Sun, 10 Jul 2022 22:28:22 +0800
From:   Yu-Jen Chang <arthurchang09@...il.com>
To:     andy@...nel.org, akinobu.mita@...il.com
Cc:     jserv@...s.ncku.edu.tw, linux-kernel@...r.kernel.org,
        Yu-Jen Chang <arthurchang09@...il.com>
Subject: [PATCH 2/2] lib/string.c: Optimize memchr()

The original version of memchr() is implemented with the byte-wise
comparing technique, which does not fully use 64-bits or 32-bits
registers in CPU. We use word-wide comparing so that 8 characters
can be compared at the same time on CPU. This code is base on
David Laight's implementation.

We create two files to measure the performance. The first file
contains on average 10 characters ahead the target character.
The second file contains at least 1000 characters ahead the
target character. Our implementation of “memchr()” is slightly
better in the first test and nearly 4x faster than the orginal
implementation in the second test.

Signed-off-by: Yu-Jen Chang <arthurchang09@...il.com>
Signed-off-by: Ching-Chun (Jim) Huang <jserv@...s.ncku.edu.tw>
---
 lib/string.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/lib/string.c b/lib/string.c
index 80469e6c3..8ca965431 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -905,21 +905,35 @@ EXPORT_SYMBOL(strnstr);
 #ifndef __HAVE_ARCH_MEMCHR
 /**
  * memchr - Find a character in an area of memory.
- * @s: The memory area
+ * @p: The memory area
  * @c: The byte to search for
- * @n: The size of the area.
+ * @length: The size of the area.
  *
  * returns the address of the first occurrence of @c, or %NULL
  * if @c is not found
  */
-void *memchr(const void *s, int c, size_t n)
+void *memchr(const void *p, int c, unsigned long length)
 {
-	const unsigned char *p = s;
-	while (n-- != 0) {
-        	if ((unsigned char)c == *p++) {
-			return (void *)(p - 1);
+	u64 mask, val;
+	const void *end = p + length;
+
+	c &= 0xff;
+	if (p <= end - 8) {
+		mask = c;
+		MEMCHR_MASK_GEN(mask);
+
+		for (; p <= end - 8; p += 8) {
+			val = *(u64 *)p ^ mask;
+			if ((val + 0xfefefefefefefeffu) &
+			    (~val & 0x8080808080808080u))
+				break;
 		}
 	}
+
+	for (; p < end; p++)
+		if (*(unsigned char *)p == c)
+			return (void *)p;
+
 	return NULL;
 }
 EXPORT_SYMBOL(memchr);
-- 
2.25.1