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From: Anton Blanchard <anton@samba.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: behanw@converseincode.com, ying.huang@intel.com,
	akpm@linux-foundation.org, oleg@redhat.com,
	Segher Boessenkool <segher@kernel.crashing.org>,
	mingo@elte.hu, linux-kernel@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: llist code relies on undefined behaviour, upsets llvm/clang
Date: Mon, 16 Jan 2017 22:42:29 +1100	[thread overview]
Message-ID: <20170116224229.17d1f6fc@kryten> (raw)
In-Reply-To: <20170116090540.GE3159@twins.programming.kicks-ass.net>

Hi Peter,

> Last I checked I couldn't build a x86_64 kernel with llvm. So no, not
> something I've ever ran into.
> 
> Also, I would argue that this is broken in llvm, the kernel very much
> relies on things like this all over the place. Sure, we're way outside
> of what the C language spec says, but who bloody cares ;-)

True, but is there anything preventing gcc from implementing this
optimisation in the future? If we are relying on undefined behaviour we
should have a -fno-strict-* option to cover it.

> If llvm wants to compile the kernel, it needs to learn the C dialect
> the kernel uses.

LLVM has done that before (eg adding -fno-strict-overflow). I don't
think that option covers this case however.

Anton

  reply	other threads:[~2017-01-16 11:42 UTC|newest]

Thread overview: 8+ 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 [this message]
2017-01-16 12:53     ` Peter Zijlstra
2017-01-16 13:09       ` Andrey Ryabinin
2017-01-16 14:34 ` David Laight
2017-01-16 14:34   ` David Laight
2017-01-16 16:25   ` Peter Zijlstra

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