linux-kernel - Re: from where comes "_

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Message-Id: <1185046274.5652.15.camel@lappy>
Date:	Sat, 21 Jul 2007 21:31:13 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Robert P. J. Day" <rpjday@...dspring.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: from where comes "__moddi3"?

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?

gcc translates that into a libgcc call, which we _explicitly_ do not
have because 64bit divisions are expensive!

use do_div().

16 works because gcc translates that into a right shift.

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