Message-ID: <20030710100417.83333.qmail@web11801.mail.yahoo.com>
Date: Thu, 10 Jul 2003 12:04:17 +0200 (CEST)
From: =?iso-8859-1?q?Etienne=20Lorrain?= <etienne_lorrain@yahoo.fr>
Subject: memset (was: Redundant memset in AIO read_events)
To: linux-kernel@vger.kernel.org
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 Note that using memset() is better reserved to initialise variable-size
 structures or buffers. Even if memset() is extremely optimised,
 it is still not as fast as not doing anything.

read_events(...) {
   struct io_event ent;
   memset(&ent, 0, sizeof(ent));
   while (...) {
      aio_read_evt(ctx, &ent);
   }
   ...
}

 Should be written (when "ent" has to be cleared):
read_events(...) {
   struct io_event ent = {};
   while (...) {
      aio_read_evt(ctx, &ent);
   }
   ...
}

  Just compare the code generated by (using GCC):
   struct io_event ent;
   memset(&ent, 0, sizeof(ent));
   ent.data = 0;
   if (ent.obj != 0) printf ("bad");

  And:
   struct io_event ent = {};
   ent.data = 0;
   if (ent.obj != 0) printf ("bad");

  and that is even without speaking of complete variable elimination
 when the structure is not used, unknown pointer alignement when
 memset function is not inlined, or aliasing optimisation.

  Etienne.

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