lkml.org 
[lkml]   [2019]   [Mar]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[RFCv2] string: Use faster alternatives when constant arguments are used
I messed up the return value for strcat in the first patch. Here's a fixed
version, ready for some scathing reviews.

From: Sultan Alsawaf <sultan@kerneltoast.com>

When strcpy, strcat, and strcmp are used with a literal string, they can
be optimized to memcpy or memcmp calls. These alternatives are faster
since knowing the length of a string argument beforehand allows
traversal through the string word at a time without being concerned
about looking for the terminating zero character. In some cases, the
replaced calls to memcpy or memcmp can even be optimized out completely
for a significant speed up.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
---
include/linux/string.h | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)

diff --git a/include/linux/string.h b/include/linux/string.h
index 7927b875f..59c301c0e 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -476,4 +476,34 @@ static __always_inline size_t str_has_prefix(const char *str, const char *prefix
return strncmp(str, prefix, len) == 0 ? len : 0;
}

+/*
+ * Replace some common string helpers with faster alternatives when one of the
+ * arguments is a constant (i.e., literal string). This uses strlen instead of
+ * sizeof for calculating the string length in order to silence compiler
+ * warnings that may arise due to what the compiler thinks is incorrect sizeof
+ * usage. The strlen calls on constants are folded into scalar values at compile
+ * time, so performance is not reduced by using strlen.
+ */
+#define strcpy(dest, src) \
+ __builtin_choose_expr(__builtin_constant_p(src), \
+ memcpy((dest), (src), strlen(src) + 1), \
+ (strcpy)((dest), (src)))
+
+#define strcat(dest, src) \
+ __builtin_choose_expr(__builtin_constant_p(src), \
+ ({ \
+ memcpy(strchr((dest), '\0'), (src), strlen(src) + 1); \
+ (dest); \
+ }), \
+ (strcat)((dest), (src)))
+
+#define strcmp(dest, src) \
+ __builtin_choose_expr(__builtin_constant_p(dest), \
+ __builtin_choose_expr(__builtin_constant_p(src), \
+ (strcmp)((dest), (src)), \
+ memcmp((dest), (src), strlen(dest) + 1)), \
+ __builtin_choose_expr(__builtin_constant_p(src), \
+ memcmp((dest), (src), strlen(src) + 1), \
+ (strcmp)((dest), (src))))
+
#endif /* _LINUX_STRING_H_ */
--
2.21.0
\
 
 \ /
  Last update: 2019-03-24 03:30    [W:0.055 / U:33.936 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site