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

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Message-Id: <20070518152935.b5a4b3bf.akpm@linux-foundation.org>
Date:	Fri, 18 May 2007 15:29:35 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>
Cc:	"bugme-daemon@...nel-bugs.osdl.org" 
	<bugme-daemon@...zilla.kernel.org>, linux-kernel@...r.kernel.org,
	malitzke@...ronets.com,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [Bug 8501] udivdi3  absence with gcc-4.3.0 on kernels 2.6.20.11
 & 2.6.22.-rc1

On Fri, 18 May 2007 12:41:24 -0700
bugme-daemon@...zilla.kernel.org wrote:

> http://bugzilla.kernel.org/show_bug.cgi?id=8501

Problem.

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.

I expect that this optimisation will remain in gcc-4.3 and we'll end up
having major kernel releases which don't build on i386 with major gcc
releases, which isn't altogether desirable.  I suspect we'll need to fix this
fairly urgently, and to backport the fix into a number of kernel releases.

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)
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