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SubjectRe: [RFC v3] string: Use faster alternatives when constant arguments are used
On Sat, Mar 23, 2019 at 08:31:53PM -0700, Nathan Chancellor wrote:
> 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

Thanks. One last revision with some nitpicks fixed is attached, though I doubt
it'll influence any comments on the concept of this anyway.

---
include/linux/string.h | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)

diff --git a/include/linux/string.h b/include/linux/string.h
index 7927b875f..40a8a9436 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((dest) + strlen(dest), (src), strlen(src) + 1); \
+ (dest); \
+ }), \
+ (strcat)((dest), (src)))
+
+#define strcmp(left, right) \
+ __builtin_choose_expr(__builtin_constant_p(left), \
+ __builtin_choose_expr(__builtin_constant_p(right), \
+ (strcmp)((left), (right)), \
+ memcmp((left), (right), strlen(left) + 1)), \
+ __builtin_choose_expr(__builtin_constant_p(right), \
+ memcmp((left), (right), strlen(right) + 1), \
+ (strcmp)((left), (right))))
+
#endif /* _LINUX_STRING_H_ */
--
2.21.0
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