linux-kernel - Re: undefined reference to _

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Date:	Wed, 07 May 2008 10:29:24 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Segher Boessenkool <segher@...nel.crashing.org>
CC:	Robert Hancock <hancockr@...w.ca>,
	Christian Kujau <lists@...dbynature.de>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: undefined reference to __udivdi3 (gcc-4.3)

Segher Boessenkool wrote:
>> I assume it's one or both of these loops in arch/x86/xen/time.c 
>> do_stolen_accounting() that are being optimized into a divide which 
>> generates a libgcc call:
>>
>>     while (stolen >= NS_PER_TICK) {
>>         ticks++;
>>         stolen -= NS_PER_TICK;
>>     }
>>
>> or
>>
>>     while (blocked >= NS_PER_TICK) {
>>         ticks++;
>>         blocked -= NS_PER_TICK;
>>     }
>
> That looks plausible.

Yep.  Probably both.

>> Not sure if that is a sustainable fix, though..
>
> It should be.  The asm() arg tells GCC that the asm() could modify
> "ns" in some way, so GCC cannot optimise away the loop, since it
> doesn't have the required info about the induction variable to do
> that. 

Yep, it's guaranteed to work.  But it's an ugly hack to work around an 
over-enthusiastic compiler, and so is an inherent maintainability burden.

I think the correct fix here is to introduce an iter_div_rem() function 
which contains this hack, so we can avoid scattering it all over the 
place.  I'll cook up a patch.

    J
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