[llvm-dev] trunc nsw/nuw?

[Alexandre Isoard via llvm-dev]

Ah, ok. I read it wrong. In *neither* case it is UB.

Hum, can an implementation define it as UB? :-)

On Wed, Jul 5, 2017 at 9:09 PM, Alexandre Isoard <alexandre.isoard at gmail.com
> wrote:

>
>
> On Wed, Jul 5, 2017 at 3:59 PM, Hal Finkel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>>
>> On 07/04/2017 01:41 AM, Dr.-Ing. Christoph Cullmann via llvm-dev wrote:
>>
>>> Hi,
>>>
>>> Hi Alexandre,
>>>>
>>>> LLVM currently doesn't have trunc nsw/nuw, no.
>>>> Which frontend would emit such instructions? Any application in mind?
>>>> Just asking because if no frontend could emit those, then the
>>>> motivation to
>>>> add nsw/nuw support to trunc would be very low I guess.
>>>>
>>> I think the clang frontend could use that to allow better static
>>> analysis of integer overflows
>>> on the LLVM IR.
>>>
>>> It might be interesting for static analysis tools that want to know if
>>> you truncate something
>>> that is too large for the target range but you need the sign to
>>> determine, e.g. the C/C++ frontend
>>> could use that flag for trunc of signed/unsigned integers, then you
>>> could e.g. check if
>>>
>>> (uint8_t)x
>>>
>>> changes the values if x has stuff out of the 0..255 range.
>>>
>>> e.g. x as int with -100 => trunc nuw to 8 => bad
>>>
>>> (int8_t)x
>>>
>>>    => trunc nsw to 8 => ok
>>>
>>
>> I'm not sure that a C/C++ frontend could add this flag. In C++, the
>> conversion for unsigned types is specified and for signed types, it's
>> implementation defined, but in neither case is it UB.
>>
>
> Hmm, I don't get it.
>
> Two questions:
> - if the conversion for unsigned types is specified, how is it undefined
> behavior?
> - if the conversion for signed types is UB, then this is a direct
> application of those flags, isn't it?
>
>  -Hal
>>
>>
>>
>>> Greetings
>>> Christoph
>>>
>>> Thanks,
>>>> Nuno
>>>>
>>>> -----Original Message-----
>>>> From: Alexandre Isoard via llvm-dev
>>>> Sent: Monday, July 3, 2017 8:38 PM
>>>> To: llvm-dev
>>>> Subject: [llvm-dev] trunc nsw/nuw?
>>>>
>>>>
>>>> Hello,
>>>>
>>>>  From [1], trunc does not seems to have a nsw/nuw attribute.
>>>> Is it possible to have that? Or do we have that and it is not
>>>> up-to-date?
>>>>
>>>> The definition would be:
>>>>
>>>> If the nuw keyword is present, the result value of the trunc is a poison
>>>> value if the truncated high order bits are non-zero. If the nsw keyword
>>>> is
>>>> present, the result value of the trunc is a poison value if the
>>>> truncated
>>>> high order bits are not all equal to the non-truncated bit of the
>>>> highest
>>>> order.
>>>>
>>>> This allow to cancel out:
>>>> - sext with trunc nsw
>>>> - zext with trunc nuw
>>>>
>>>> And probably to commute with add/sub/mul/lshr/ashr/shl/urem/udiv/udiv
>>>> (with
>>>> the correct flags).
>>>>
>>>> [1]: http://llvm.org/docs/LangRef.html#trunc-to-instruction
>>>>
>>>>
>>>> --
>>>>
>>>> Alexandre Isoard
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
>
> --
> *Alexandre Isoard*
>



-- 
*Alexandre Isoard*
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