From: Peter Zijlstra <peterz@infradead•org>
To: David Laight <David.Laight@ACULAB•COM>
Cc: 'Anton Blanchard' <anton@samba•org>,
"behanw@converseincode•com" <behanw@converseincode•com>,
"ying.huang@intel•com" <ying.huang@intel•com>,
"akpm@linux-foundation•org" <akpm@linux-foundation•org>,
"oleg@redhat•com" <oleg@redhat•com>,
Segher Boessenkool <segher@kernel•crashing.org>,
"mingo@elte•hu" <mingo@elte•hu>,
"linuxppc-dev@lists•ozlabs.org" <linuxppc-dev@lists•ozlabs.org>,
"linux-kernel@vger•kernel.org" <linux-kernel@vger•kernel.org>
Subject: Re: llist code relies on undefined behaviour, upsets llvm/clang
Date: Mon, 16 Jan 2017 17:25:04 +0100 [thread overview]
Message-ID: <20170116162504.GA6500@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <063D6719AE5E284EB5DD2968C1650D6DB0264AA5@AcuExch.aculab.com>
On Mon, Jan 16, 2017 at 02:34:43PM +0000, David Laight wrote:
> From: Anton Blanchard
> > Sent: 15 January 2017 21:36
> > I was debugging a hang on a ppc64le kernel built with clang, and it
> > looks to be undefined behaviour with pointer wrapping in the llist code.
> >
> > A test case is below. llist_for_each_entry() does container_of() on a
> > NULL pointer, which wraps our pointer negative, then adds the same
> > offset back in and expects to get back to NULL. Unfortunately clang
> > decides that this can never be NULL and optimises it into an infinite
> > loop.
> ...
> > #define llist_for_each_entry(pos, node, member) \
> > for ((pos) = llist_entry((node), typeof(*(pos)), member); \
> > &(pos)->member != NULL; \
> > (pos) = llist_entry((pos)->member.next, typeof(*(pos)), member))
>
> Maybe the above could be rewritten as (untested):
> for ((pos) = NULL; (!(pos) ? (node) : ((pos)->member.next) || (pos) = 0) && \
> (((pos) = !(pos) ? llist_entry((node), typeof(*(pos)), member) \
> : llist_entry((pos)->member.next, typeof(*(pos)), member)),1); )
> Provided the compiler assumes that the loop body is never executed with 'pos == 0'
> it should generate the same code.
That's far uglier code and to what point? The compiler should simply not
assume silly things.
prev parent reply other threads:[~2017-01-16 16:25 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-01-15 21:36 llist code relies on undefined behaviour, upsets llvm/clang Anton Blanchard
2017-01-16 9:05 ` Peter Zijlstra
2017-01-16 11:42 ` Anton Blanchard
2017-01-16 12:53 ` Peter Zijlstra
2017-01-16 13:09 ` Andrey Ryabinin
2017-01-16 14:34 ` David Laight
2017-01-16 16:25 ` Peter Zijlstra [this message]
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