Re: [Bug 8501] udivdi3 absence with gcc-4.3.0 on kernels 2.6.20.11 & 2.6.22.-rc1

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gcc-4.3 appears to have cunningly converted this:

static inline void timespec_add_ns(struct timespec *a, u64 ns)
{
	ns += a->tv_nsec;
	while(unlikely(ns >= NSEC_PER_SEC)) {
		ns -= NSEC_PER_SEC;
		a->tv_sec++;
	}
	a->tv_nsec = ns;
}

into a divide-by-1000000000 operation, so it emits a call to udivdi3 and we
don't link.

Exactly.  It obviously is a bug in the kernel that it depends
on certain compiler optimisations that it doesn't have direct
control over to happen or not.  OTOH, GCC's behaviour here is
probably a non-optimal code issue; it doesn't seem to take the
unlikely() into account when doing the loop transform.

I expect that this optimisation will remain in gcc-4.3

If someone files a *useable* problem report it most likely
will be taken care of, actually.

and we'll end up
having major kernel releases which don't build on i386 with major gcc
releases, which isn't altogether desirable.

Yeah, like 4.2.0 with powerpc.  Seems like no one tested it :-(

I suspect we'll need to fix this
fairly urgently, and to backport the fix into a number of kernel releases.

If it is 4.3 only, you could instead try to work *with* the GCC
people.  It _is_ very fragile code of course, it wouldn't hurt
to replace it with something better.

We use the above idiom in several places. A suitable fix might be to hunt down those various sites and then make them call a helper function which
does

	if (unlikely(ns >= NSEC_PER_SEC)) {
		do_div(...)
	}

(Better would be to inline the comparison and to uninline the do_div(),
if it's a 32-bit arch.  Doing all this in a backportable fashion may
prove tricky)

Perhaps putting a compiler barrier in there would be enough -- like
an empty asm() that takes the loop variable as input.


Segher

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