Messages in this thread | | | Date | Sat, 23 Mar 2019 20:31:53 -0700 | From | Nathan Chancellor <> | Subject | Re: [RFCv2] string: Use faster alternatives when constant arguments are used |
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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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