Larry Finger writes:
> On 08/12/2010 03:26 PM, Linus Torvalds wrote:
> > On Thu, Aug 12, 2010 at 1:18 PM, Larry Finger <Larry.Finger@lwfinger.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.
The loops-turns-into-div is a known bug in early gcc-4.3 releases,
possibly only with -Os. It's fixed in 4.4 and 4.3.latest. I can
probably dig up the gcc PR number if needed.
/Mikael
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