linux-kernel - Re: Problem with commit deda2e81961e96be4f2c09328baca4710a2fd1a0

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Message-ID: <4C645C7D.1050903@lwfinger.net>
Date:	Thu, 12 Aug 2010 15:41:33 -0500
From:	Larry Finger <Larry.Finger@...inger.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Jason Wessel <jason.wessel@...driver.com>,
	John Stultz <johnstul@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Problem with commit deda2e81961e96be4f2c09328baca4710a2fd1a0

On 08/12/2010 03:26 PM, Linus Torvalds wrote:
> On Thu, Aug 12, 2010 at 1:18 PM, Larry Finger <Larry.Finger@...inger.net> wrote:
>>
>> With the above commit, building an i386 version of the kernel results in the
>> following from the build:
>>
>> kernel/built-in.o: In function `logarithmic_accumulation':
>> /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to
>> `__umoddi3'
>> /home/finger/linux-realtek/kernel/time/timekeeping.c:715: undefined reference to
>> `__udivdi3'
>> make: *** [.tmp_vmlinux1] Error 1
>>
>> Reverting the patch allows the system to build correctly.
> 
> Damn. It's your compiler turning a while-loop into a divide. Which
> likely isn't even an optimization, but whatever.
> 
> John: I think that while-loop needs to be something like
> 
>    if (raw_nsecs >= NSEC_PER_SEC) {
>       u64 raw_secs = raw_nsecs;
>       raw_nsecs = do_div(raw_secs, NSEC_PER_SEC);
>       raw_time.tv_sec += taw_secs;
>   }
>   raw_time.tc_nsec = raw_nsecs;
> 
> which is sad and overly complicated, but the simple thing seems to get
> messed up by the compiler.
> 
> Untested. Maybe I got the complex do_div() semantics wrong. Somebody
> needs to check.

I'll try it. The system with the problem has gcc (SUSE Linux) 4.3.2
[gcc-4_3-branch revision 141291]. That is on a 64-bit system cross compiling
with ARCH=i386.

The real 32-bit system with gcc (SUSE Linux) 4.5.0 20100604 [gcc-4_5-branch
revision 160292] builds OK. It just wasn't finished when I wrote the first message.

Larry

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