Date: Fri, 17 Sep 2004 12:32:07 +0200
From: Stelian Pop <stelian@popies.net>
To: Peter Chubb <peterc@gelato.unsw.edu.au>
Cc: Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org
Subject: Re: [RFC, 2.6] a simple FIFO implementation
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On Fri, Sep 17, 2004 at 11:00:22AM +1000, Peter Chubb wrote:

> Andrew> All the struct needs is `head', `tail' and
> Andrew> `number_of_bytes_at_buf', all unsigned.
> 
> Andrew> add(char c) {
> Andrew>       p-> buf[p->head++ % p->number_of_bytes_at_buf] = c;
> Andrew> }
> 
> This depends on how expensive % is.  On IA64, something like this:
> 
>      add(char c) {
> 	     int i = p->head == p->len ? p->head++ : 0;
> 	     p->buf[i] = c;
>      }
> 
> is cheaper, as % generates a subroutine call to __modsi3.  It also is
> shorter =-- 12 bundles as opposed to 15.

Yup, but this is when doing char-by-char operations.

When using one or two memcpy() we have only two '%' intead of 
(perhaps) many 'if's.

Of course the memcpy() has its own while() loop but since it's 
optimized in assembly (and I guess we're not going to implement
kfifo_get/put in assembly) I will stay with the memcpy + '%' version.

Stelian.
-- 
Stelian Pop <stelian@popies.net>    
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