[RFCv2] string: Use faster alternatives when constant arguments are used

[Sultan Alsawaf <sultan@kerneltoast.com>]

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