linux-kernel - Re: from where comes "_

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Date:	Sat, 21 Jul 2007 15:34:21 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: from where comes "__moddi3"?

On Sat, 21 Jul 2007, Peter Zijlstra wrote:

> On Sat, 2007-07-21 at 15:21 -0400, Robert P. J. Day wrote:
> > again, probably displaying my abject ignorance, but i wrote a
> > trivial module that tries to "var % 15", and i get:
> >
> >   WARNING: "__moddi3" undefined!
> >
> > and, not surprisingly, when i try to insmod:
> >
> >   insmod: error inserting 'seq.ko': -1 Unknown symbol in module
> >
> > (using 16 rather than 15 works fine, as i assume that the modulus
> > call is simply replaced by an optimized  bitwise comparison.)
> >
> >   so ... from where comes __moddi3?  i know there are places in the
> > kernel source tree that do non-power-of-2 moduli, and they work fine,
> > no?  thanks.
>
> let me guess, 32 bit kernel, and var is 64 bit?

um ... yeah.  duh.  var type is actually loff_t.  i should have
realized that.

> gcc translates that into a libgcc call, which we _explicitly_ do not
> have because 64bit divisions are expensive!
>
> use do_div().

gotcha.  thanks.

rday
-- 
========================================================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

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