linux-kernel - [PATCH] make isdigit() table lookupless

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Message-ID: <20160826190047.GA12536@p183.telecom.by>
Date:   Fri, 26 Aug 2016 22:00:47 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     akpm@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org
Subject: [PATCH] make isdigit() table lookupless

Make isdigit into a simple range checking inline function:

	return '0' <= c && c <= '9';

This code is 1 branch, not 2 because any reasonable compiler can
optimize this code into SUB+CMP, so the code

	while (isdigit((c = *s++)))
		...

remains 1 branch per iteration HOWEVER it suddenly doesn't do table
lookup priming cacheline nobody cares about.

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

 include/linux/ctype.h |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/include/linux/ctype.h
+++ b/include/linux/ctype.h
@@ -22,7 +22,10 @@ extern const unsigned char _ctype[];
 #define isalnum(c)	((__ismask(c)&(_U|_L|_D)) != 0)
 #define isalpha(c)	((__ismask(c)&(_U|_L)) != 0)
 #define iscntrl(c)	((__ismask(c)&(_C)) != 0)
-#define isdigit(c)	((__ismask(c)&(_D)) != 0)
+static inline int isdigit(int c)
+{
+	return '0' <= c && c <= '9';
+}
 #define isgraph(c)	((__ismask(c)&(_P|_U|_L|_D)) != 0)
 #define islower(c)	((__ismask(c)&(_L)) != 0)
 #define isprint(c)	((__ismask(c)&(_P|_U|_L|_D|_SP)) != 0)