[llvm] r219542 - Return undef on FP <-> Int conversions that overflow (PR21330).

[Sanjay Patel]

Hi all -

Can we do more than just generate an undef to inform the customer that
their program will go off the rails here?
A warning? An illegal instruction?
Is this better handled in the front-end or in the optimizers?

I filed http://llvm.org/bugs/show_bug.cgi?id=21093 for the related case of
a broken libcall.


On Fri, Oct 17, 2014 at 8:11 PM, Chandler Carruth <chandlerc at google.com>
wrote:

>
> On Fri, Oct 17, 2014 at 6:49 PM, Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> On Fri, Oct 17, 2014 at 5:17 PM, Quentin Colombet <qcolombet at apple.com>
>> wrote:
>>
>>> Hi Sanjay,
>>>
>>> I do not think this change is correct.
>>> I have attached a test case with (before.ll) and without (after.ll) that
>>> change and the related C file (test.c).
>>>
>>
>> signed short a = 8*8*8*548.54 - 3.14; // <-- the value of a is undef.
>>
>> This line has undefined behavior; both C and C++ are very clear about
>> this. The transformation is correct at the language semantics level. And it
>> appears correct at the IR level too, whether you interpret "the results are
>> undefined" as meaning undefined behavior or (strictly weaker) an undef
>> value.
>
>
> I think it would be good to make the LangRef much more clear about this. I
> think "produces an undef value" is more concrete?
>
>
>> It would be correct to treat this as poison, not just as undef, from a
>> language semantics perspective.
>
>
> I don't think there is any utility in thinking about poison here.... undef
> seems a good way to model this. I'm increasingly of the opinion we can all
> the optimizations we would ever want from undef alone.
>
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