Messages in this thread Patch in this message | | | From | Sultan Alsawaf <> | Subject | [RFC] string: Use faster alternatives when constant arguments are used | Date | Sat, 23 Mar 2019 18:44:45 -0700 |
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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 | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+)
diff --git a/include/linux/string.h b/include/linux/string.h index 7927b875f..65db58342 100644 --- a/include/linux/string.h +++ b/include/linux/string.h @@ -476,4 +476,31 @@ 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), \ + (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
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