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

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2017-01-16 15:53 GMT+03:00 Peter Zijlstra <peterz@infradead.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

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llist code relies on undefined behaviour, upsets llvm/clang Anton Blanchard <anton@samba.org> - 2017-01-15 22:40 +0100
  Re: llist code relies on undefined behaviour, upsets llvm/clang Peter Zijlstra <peterz@infradead.org> - 2017-01-16 10:10 +0100
    Re: llist code relies on undefined behaviour, upsets llvm/clang Anton Blanchard <anton@samba.org> - 2017-01-16 12:50 +0100
      Re: llist code relies on undefined behaviour, upsets llvm/clang Peter Zijlstra <peterz@infradead.org> - 2017-01-16 14:00 +0100
        Re: llist code relies on undefined behaviour, upsets llvm/clang Andrey Ryabinin <ryabinin.a.a@gmail.com> - 2017-01-16 14:10 +0100
  RE: llist code relies on undefined behaviour, upsets llvm/clang David Laight <David.Laight@ACULAB.COM> - 2017-01-16 15:40 +0100
    Re: llist code relies on undefined behaviour, upsets llvm/clang Peter Zijlstra <peterz@infradead.org> - 2017-01-16 17:30 +0100

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