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SubjectRe: [PATCH 1/1] lib: small update for strlen, strnlen, use less cpu instructions
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thank you for your comments, begginer mistakes (I guess),

On 6/17/15, Richard Weinberger <richard@nod.at> wrote:
> Am 17.06.2015 um 00:51 schrieb Orestes Leal Rodriguez:
>>> 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
>
> But it does not apply at all.
> Did you test it? I fear your mail client did some whitespace damage.

yes, I patched my original source tree with it.

>
>>> 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.
>
> x86_32 does not matter here as we have already an optimized strlen() in
> arch/x86/lib/string_32.c.
> Did you check whether the optimization is worth on other archs?
> Hint: grep __HAVE_ARCH_STRLEN

I only have access to x86 cpus, but thanks for the hint

>
>>
>> Signed-off-by: Orestes Leal Rodriguez <lukes357@gmail.com>
>> ---
>>
>> Signed-off-by: Orestes Leal Rodriguez <lukes357@gmail.com>
>
> What does this 2nd SoB here?

Again, confusion about where to put the sob.

>
>> 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
>
> No need to add anything here. These days we have git. :-)

Ups!

>
>> */
>>
>> #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 */;
>
> <nitpick>
> Why suddenly "empty" instead of "nothing"?
> </nitpick>

I was bored.

>
>> + 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 */;
>
> Same here.
>
> Thanks,
> //R2D2
>

Thanks to you richard.


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