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

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Date:   Mon, 16 Jan 2017 16:09:45 +0300
From:   Andrey Ryabinin <ryabinin.a.a@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Anton Blanchard <anton@...ba.org>, behanw@...verseincode.com,
        kernel test robot <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Segher Boessenkool <segher@...nel.crashing.org>,
        Ingo Molnar <mingo@...e.hu>,
        LKML <linux-kernel@...r.kernel.org>,
        linuxppc-dev@...ts.ozlabs.org
Subject: Re: llist code relies on undefined behaviour, upsets llvm/clang

2017-01-16 15:53 GMT+03:00 Peter Zijlstra <peterz@...radead.org>:
> On Mon, Jan 16, 2017 at 10:42:29PM +1100, Anton Blanchard wrote:
>> 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.
>
> Our comment there states:
>
> # disable invalid "can't wrap" optimizations for signed / pointers
> KBUILD_CFLAGS   += $(call cc-option,-fno-strict-overflow)
>
> So this option should apply to pointer arithmetic, therefore I would
> expect -fno-strict-overflow to actually apply here, or am I missing
> something?

That case is null pointer check optimization.  '->member' has non-zero
offset in struct, so LLVM assumes that pos->member != NULL
and optimize away this check.
LLVM/clang currently doesn't have -fno-delete-null-pointer-checks