Re: do_div64 generic

From: Bernardo Innocenti <bernie@develer.com>
To: george anzinger <george@mvista.com>, Andrew Morton <akpm@osdl.org>
Cc: linux-kernel@vger.kernel.org, rmk@arm.linux.org.uk, torvalds@osdl.org
Subject: Re: do_div64 generic
Date: Wed, 16 Jul 2003 20:33:34 +0200	[thread overview]
Message-ID: <200307162033.34242.bernie@develer.com> (raw)
In-Reply-To: <3F149747.3090107@mvista.com>

On Wednesday 16 July 2003 02:07, george anzinger wrote:

> > It is only used in nanosleep(), and then only in the case where the sleep
> > terminated early.
> >
> > If someone is calling nanosleep() so frequently for this to matter, the
> > time spent in divide is the least of their problems.  Unless you have
> > some real-worldish benchmarks to demonstrate otherwise?
>
> It is also used in the jiffies to timespec and jiffies to timeval code
> in timer.h, if memory serves.

 It's true that it's being used in more places than Andrew initially thought,
but a quick scan for jiffies_to_timeval() and jiffies_to_timespec() indicates
that it's not being used in any place where performance matters too much.

 I still consider do_div() a big kludge to workaround extremely poor code
generated by GCC. If we allow this, I see no reason for not allowing a very
similar kludge which provides even better performance.

 I agree with Andrew that we've run out of time for breaking things again,
so I won't be suggesting to implement a whole new set of 64bit operations
and use them all over the kernel. However, the div_long_long_rem() function
has been _already_ in use on i386 for some time, therefore the safest fix
we can do at this point is adding generic support for all other archs.

> > You know what they say about premtur optmstns, and having to propagate
> > funky new divide primitives across N architectures is indeed evil.
>
> Hm.  I only want the simple div.  64-bit/32-bit in two 32-bit results.
>   Is this funky?  And the "evil" #ifdef allows archs to not do it.

The div_long_long_rem() interface is even somewhat cleaner than do_div() and
can be implemented easily on architectures providing a 64/32 -> 32q/32r operation
(most 32bit processors do have it).

> > Bernardo, can you do the patch please?

 I would be glad to do it once the discussion has settled, whatever
the final decision will be. Just don't make me do it twice, please ;-)

-- 
  // Bernardo Innocenti - Develer S.r.l., R&D dept.
\X/  http://www.develer.com/

Please don't send Word attachments - http://www.gnu.org/philosophy/no-word-attachments.html