linux-kernel - [PATCH v2 2/3] lib/string: optimized memmove

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Message-Id: <20210702123153.14093-3-mcroce@linux.microsoft.com>
Date:   Fri,  2 Jul 2021 14:31:52 +0200
From:   Matteo Croce <mcroce@...ux.microsoft.com>
To:     linux-kernel@...r.kernel.org, Nick Kossifidis <mick@....forth.gr>,
        Guo Ren <guoren@...nel.org>,
        Christoph Hellwig <hch@...radead.org>,
        David Laight <David.Laight@...lab.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Emil Renner Berthing <kernel@...il.dk>,
        Drew Fustini <drew@...gleboard.org>
Cc:     linux-arch@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-riscv@...ts.infradead.org
Subject: [PATCH v2 2/3] lib/string: optimized memmove

From: Matteo Croce <mcroce@...rosoft.com>

When the destination buffer is before the source one, or when the
buffers doesn't overlap, it's safe to use memcpy() instead, which is
optimized to use a bigger data size possible.

This "optimization" only covers a common case. In future, proper code
which does the same thing as memcpy() does but backwards can be done.

Signed-off-by: Matteo Croce <mcroce@...rosoft.com>
---
 lib/string.c | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/lib/string.c b/lib/string.c
index caeef4264c43..108b83c34cec 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -975,19 +975,13 @@ EXPORT_SYMBOL(memcpy);
  */
 void *memmove(void *dest, const void *src, size_t count)
 {
-	char *tmp;
-	const char *s;
+	if (dest < src || src + count <= dest)
+		return memcpy(dest, src, count);
+
+	if (dest > src) {
+		const char *s = src + count;
+		char *tmp = dest + count;
 
-	if (dest <= src) {
-		tmp = dest;
-		s = src;
-		while (count--)
-			*tmp++ = *s++;
-	} else {
-		tmp = dest;
-		tmp += count;
-		s = src;
-		s += count;
 		while (count--)
 			*--tmp = *--s;
 	}
-- 
2.31.1