Mailing List Archive: Re: svn commit: r1893004 - /httpd/httpd/trunk/modules/metadata/mod_unique

On 9/7/21 3:52 AM, ylavic@apache.org wrote:
> Author: ylavic
> Date: Tue Sep 7 01:52:48 2021
> New Revision: 1893004
>
> URL: http://svn.apache.org/viewvc?rev=1893004&view=rev
> Log:
> mod_unique_id: Follow up to r1892915 and r1893002: Atomic counter.
>
> * modules/metadata/mod_unique_id.c(gen_unique_id):
> Use an atomic 32bit counter to close the race condition, using the lower
> 16 bits for uuencoding still.
>
>
> Modified:
> httpd/httpd/trunk/modules/metadata/mod_unique_id.c
>
> Modified: httpd/httpd/trunk/modules/metadata/mod_unique_id.c
> URL: http://svn.apache.org/viewvc/httpd/httpd/trunk/modules/metadata/mod_unique_id.c?rev=1893004&r1=1893003&r2=1893004&view=diff
> ==============================================================================
> --- httpd/httpd/trunk/modules/metadata/mod_unique_id.c (original)
> +++ httpd/httpd/trunk/modules/metadata/mod_unique_id.c Tue Sep 7 01:52:48 2021

> @@ -194,16 +212,24 @@ static const char *gen_unique_id(const r
> unique_id_rec foo;
> unsigned char pad[2];
> } paddedbuf;
> + apr_uint32_t counter;
> unsigned char *x,*y;
> int i,j,k;
>
> memcpy(&new_unique_id.root, &cur_unique_id.root, ROOT_SIZE);
> new_unique_id.stamp = htonl((unsigned int)apr_time_sec(r->request_time));
> new_unique_id.thread_index = htonl((unsigned int)r->connection->id);
> - new_unique_id.counter = cur_unique_id.counter++;
> +#ifdef APR_HAS_THREADS
> + if (is_threaded_mpm)
> + counter = apr_atomic_inc32(&cur_unique_counter);
> + else
> +#endif
> + counter = cur_unique_counter++;
>
> - /* Set counter in network byte order for the uuencoded unique id. */
> - new_unique_id.counter = htons(new_unique_id.counter);
> + /* The counter is two bytes for the uuencoded unique id, in network
> + * byte order.
> + */
> + new_unique_id.counter = htons(counter % APR_UINT16_MAX);

Isn't '&' doing the same as '%' here and is more performant?

Regards

Rüdiger

On Tue, Sep 7, 2021 at 8:07 AM Ruediger Pluem <rpluem@apache.org> wrote:
>
> On 9/7/21 3:52 AM, ylavic@apache.org wrote:
> >
> > - /* Set counter in network byte order for the uuencoded unique id. */
> > - new_unique_id.counter = htons(new_unique_id.counter);
> > + /* The counter is two bytes for the uuencoded unique id, in network
> > + * byte order.
> > + */
> > + new_unique_id.counter = htons(counter % APR_UINT16_MAX);
>
> Isn't '&' doing the same as '%' here and is more performant?

With a *literal* ^2-1 modulus, the compiler generates the exact same
code (a '&'). The modulo is possibly more readable..
But yeah, if the modulus were a variable, the '&' is clearly more performant.

Regards;
Yann.