linux-kernel - [PATCH] un-improve strrchr()

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Message-ID: <20150628164403.GA7169@p183.telecom.by>
Date:	Sun, 28 Jun 2015 19:44:03 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	akpm@...ux-foundation.org
Cc:	linux-kernel@...r.kernel.org, linux@...musvillemoes.dk
Subject: [PATCH] un-improve strrchr()

Commit 8da53d4595a53fb9a3380dd4d1c9bc24c7c9aab8
("lib/string.c: improve strrchr()") changed strrchr() implementation
from "rewind to the end and search backwards" to "search forward"
optimizing for characher not found case. However, common case is exactly
the opposite: string is absolute pathname, c is '/' always to be found.

Previous code did 1 branch per character + 1 branch for every character
in the last path component. Current code does 2 branches per characher
regardless.

Patch reverts to previous implementation (sans fixed coding style).

Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
---

	Cc linux-kernel

 lib/string.c |   11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

--- a/lib/string.c
+++ b/lib/string.c
@@ -313,12 +313,13 @@ EXPORT_SYMBOL(strchrnul);
  */
 char *strrchr(const char *s, int c)
 {
-	const char *last = NULL;
+	const char *p = s + strlen(s);
+
 	do {
-		if (*s == (char)c)
-			last = s;
-	} while (*s++);
-	return (char *)last;
+		if (*p == (char)c)
+			return (char *)p;
+	} while (--p >= s);
+	return NULL;
 }
 EXPORT_SYMBOL(strrchr);
 #endif
--
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