Messages in this thread Patch in this message | | | Date | Tue, 16 Jun 2015 17:51:40 -0500 | From | Orestes Leal Rodriguez <> | Subject | [PATCH 1/1] lib: small update for strlen, strnlen, use less cpu instructions |
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> Use the force^Wcheckpatch.pl. This is the output of checkpatch.pl: output of checkpatch: total: 0 errors, 0 warnings, 42 lines checked /root/string.c.patch has no obvious style problems and is ready for submission
> You need to explain that in the commit message, my young padawan. Very small update to strlen and strnlen that now use less cpu instructions by using a counter to avoid memory address arithmetic, which cause that the compiler adds more machine instructions for computing the length of the string just before returning from the functions, the old machine code is like the following:
mov -0x4(%ebp),%edx mov 0x8(%ebp),%eax sub %eax,%edx mov %edx,%eax leave ret
now in the new versions the value is not calculated anymore, instead he value of the counter is put on eax after the condition inside the loop no longer holds, and then return:
mov -0x4(%ebp),%eax leave ret
With this a few cpu instructions are saved.
Signed-off-by: Orestes Leal Rodriguez <lukes357@gmail.com> ---
Signed-off-by: Orestes Leal Rodriguez <lukes357@gmail.com> diff --git a/lib/string.c b/lib/string.c index 992bf30..c873436 100644 --- a/lib/string.c +++ b/lib/string.c @@ -17,6 +17,10 @@ * * Sat Feb 09 2002, Jason Thomas <jason@topic.com.au>, * Matthew Hawkins <matt@mh.dropbear.id.au> * - Kissed strtok() goodbye + * + * * Tuesday June 16 2015, Orestes Leal Rodriguez <lukes357@gmail.com> + * - strlen, strnlen: by using a single counter we use less cpu instructions + * by avoiding substracting the memory addresses before return */
#include <linux/types.h> @@ -401,11 +405,11 @@ EXPORT_SYMBOL(strim); */ size_t strlen(const char *s) { - const char *sc; + size_t sz = 0;
- for (sc = s; *sc != '\0'; ++sc) - /* nothing */; - return sc - s; + for (; *s++ != '\0'; sz++) + /* empty */; + return sz; } EXPORT_SYMBOL(strlen); #endif @@ -418,12 +422,13 @@ EXPORT_SYMBOL(strlen); */ size_t strnlen(const char *s, size_t count) { - const char *sc; + size_t sz = 0;
- for (sc = s; count-- && *sc != '\0'; ++sc) - /* nothing */; - return sc - s; + for (; count-- && *s++ != '\0'; sz++) + /* empty */; + return sz; } + EXPORT_SYMBOL(strnlen); #endif
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