[llvm-dev] trunc nsw/nuw?

Bruce Hoult via llvm-dev
Thu, 06 Jul 2017 15:41:53 -0000

According to 6.3.1.3/3 of the C standard (I didn't check C++):

"3   Otherwise, the new type is signed and the value cannot be represented
in it; either the result is implementation-defined or an
implementation-defined signal is raised."

I *think* that means that IF a signal is raised then the signal raised
could be one that you can't guarantee to be able to return from
("SIGFPE,
SIGILL, SIGSEGV, or any other implementation-defined value corresponding to
a computational exception") and thus it is implementation defined whether
the program will terminate.

That provides pretty big scope to optimize around :-)

Note also that while unsigned variables require the implementation to act
AS IF running on a binary machine, signed variables have no such
requirement. Most implementations do in fact truncate by taking the
remainder modulus the number of values that can be represented in the
destination type, but that might not be a power of two.

I would guess there is very little code in the wild that conforms to a
strict interpretation of all this.

On Thu, Jul 6, 2017 at 5:24 PM, Alexandre Isoard via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi Hal,
>
> I just want to check I understand correctly the subtlety, in C++ an
> overflowing signed addition is an undefined behavior (so Clang adds the nsw
> keyword to the sign add), while a signed cast that overflow is
"only"
> implementation defined (so Clang cant risk to produce poison on trunc). Is
> that right?
>
> If trunc produced undef instead of poison, clang could use it, but then it
> is less useful and/or unwanted?
>
> On Wed, Jul 5, 2017 at 10:30 PM, Hal Finkel <hfinkel at anl.gov>
wrote:
>
>>
>> On 07/05/2017 03:10 PM, Alexandre Isoard wrote:
>>
>> Ah, ok. I read it wrong. In *neither* case it is UB.
>>
>> Hum, can an implementation define it as UB? :-)
>>
>>
>> Nope :-)
>>
>> The only case I've thought of where we could add these for C++
would be
>> on conversions to (most) enums (because they used signed underlying
types
>> and the out-of-bounds mapping won't generally be one of the allowed
values
>> (or maybe we can define this to be the case?)).
>>
>>  -Hal
>>
>>
>>
>> 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*
>>
>>
>> --
>> Hal Finkel
>> Lead, Compiler Technology and Programming Languages
>> Leadership Computing Facility
>> Argonne National Laboratory
>>
>>
>
>
> --
> *Alexandre Isoard*
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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