linux-kernel - Re: llist code relies on undefined behaviour, upsets llvm/clang

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Message-ID: <20170116224229.17d1f6fc@kryten>
Date:   Mon, 16 Jan 2017 22:42:29 +1100
From:   Anton Blanchard <anton@...ba.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     behanw@...verseincode.com, ying.huang@...el.com,
        akpm@...ux-foundation.org, oleg@...hat.com,
        Segher Boessenkool <segher@...nel.crashing.org>, mingo@...e.hu,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: llist code relies on undefined behaviour, upsets llvm/clang

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