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SubjectRe: [RFCv2] string: Use faster alternatives when constant arguments are used
On Sat, Mar 23, 2019 at 07:24:06PM -0700, Sultan Alsawaf wrote:
> 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
>

Explicitly cc'ing some folks who have touched include/linux/string.h in
the past and might want to take a look at this.

Nathan

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