[patch 102/147] lib/string: optimized memmove

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From: Matteo Croce <mcroce@xxxxxxxxxxxxx>
Subject: lib/string: optimized memmove

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.

Link: https://lkml.kernel.org/r/20210702123153.14093-3-mcroce@xxxxxxxxxxxxxxxxxxx
Signed-off-by: Matteo Croce <mcroce@xxxxxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: David Laight <David.Laight@xxxxxxxxxx>
Cc: Drew Fustini <drew@xxxxxxxxxxxxxxx>
Cc: Emil Renner Berthing <kernel@xxxxxxxx>
Cc: Guo Ren <guoren@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: Nick Kossifidis <mick@xxxxxxxxxxxx>
Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 lib/string.c |   18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

--- a/lib/string.c~lib-string-optimized-memmove
+++ a/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;
 	}
_




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