[cfe-dev] [RFC] Delayed target-specific diagnostic when compiling for the devices.

[Finkel, Hal J. via cfe-dev]

On 1/17/19 9:52 AM, Alexey Bataev wrote:
> Because the type is not compatible with the target device.


But it's not that simple. The situation is that the programming
environment supports the type, but *operations* on that type are not
supported in certain contexts (e.g., when compiled for a certain
device). As you point out, we already need to move in this explicit
direction by, for example, allowing typedefs for types that are not
supported in all contexts, function declarations, and so on. In the end,
we should allow our users to design their classes and abstractions using
good software-engineering practice without worrying about access-context
partitioning.

Also, the other problem here is that the function I used as an example
is a very common C++ idiom. There are a lot of classes with function
that return a reference to themselves. Classes can have lots of data
members, and those members might not be accessed on the device (even if
the class itself might be accessed on the device). We're moving to a
world in which unified memory is common - the promise of this technology
is that configuration data and complex data structures, which might be
occasionally accessed (but for which explicitly managing data movement
is not performance relevant) are handled transparently. If use of these
data structures is transitively poisoned by use of any type not
supported on the device (including by pointers to types that use those
types), then we'll force unhelpful and technically-unnecessary
refactoring, thus reducing the value of the feature.

In the current implementation we pre-process the source twice, and so we
can:

 1. Use ifdefs to change the data memebers when compiling for different
targets. This is hard to get right because, in order to keep the data
layout otherwise the same, the user needs to understand the layout rules
in order to put something in the structure that is supported on the
target and keeps the layout the same (this is very error prone). Also,
if we move to a single-preprocessing-stage model, this no longer works.

 2. Replace all pointers to relevant types with void*, or similar, and
use a lot of casts. This is also bad.

We shouldn't be forcing users to play these games. The compiler knows
the layout on the host and it can use it on the target. The fact that
some operations on some types might not be supported on the target is
not relevant to handling pointers/references to containing types.

Thanks again,

Hal


>
> -------------
> Best regards,
> Alexey Bataev
>
> 17.01.2019 10:50, Finkel, Hal J. пишет:
>> On 1/17/19 9:27 AM, Alexey Bataev wrote:
>>> It should be compilable for the device only iff function foo is not used
>>> on the device.
>> Says whom? I disagree. This function should work on the device. Why
>> should it not?
>>
>>  -Hal
>>
>>
>>> -------------
>>> Best regards,
>>> Alexey Bataev
>>>
>>> 17.01.2019 10:24, Finkel, Hal J. пишет:
>>>> On 1/17/19 4:05 AM, Alexey Bataev wrote:
>>>>> Best regards,
>>>>> Alexey Bataev
>>>>>
>>>>>> 17 янв. 2019 г., в 0:46, Finkel, Hal J. <hfinkel at anl.gov> написал(а):
>>>>>>
>>>>>>
>>>>>>> On 1/16/19 8:45 AM, Alexey Bataev wrote:
>>>>>>>
>>>>>>> Yes, I thought about this. But we need to delay the diagnostic until
>>>>>>> the Codegen phase. What I need is the way to associate the diagnostic
>>>>>>> with the function so that this diagnostic is available in CodeGen.
>>>>>>>
>>>>>>> Also, we need to postpone the diagnotics not only for functions,
>>>>>>> but,for example, for some types. For example, __float128 type is not
>>>>>>> supported by CUDA. We can get error messages when we ran into
>>>>>>> something like `typedef __float128 SomeOtherType` (say, in some system
>>>>>>> header files) and get the error diagnostic when we compile for the
>>>>>>> device. Though, actually, this type is not used in the device code,
>>>>>>> the diagnostic is still emitted and we need to delay too and emit it
>>>>>>> only iff the type is used in the device code.
>>>>>>>
>>>>>> This should be fixed for CUDA too, right?
>>>>>>
>>>>>> Also, we still get to have pointers to aggregates containing those types
>>>>>> on the device, right?
>>>>>>
>>>>> No, why? This is not allowed and should be diagnosed too. If somebody tries somehow to use not allowed type for the device variables/functions - it should be diagnosed.
>>>> Because this should be allowed. If I have:
>>>>
>>>> struct X {
>>>>   int a;
>>>>   __float128 b;
>>>> };
>>>>
>>>> and we have some function which does this:
>>>>
>>>> X *foo(X *x) {
>>>>   return x;
>>>> }
>>>>
>>>> We'll certainly want this function to compile for all targets, even if
>>>> there's no __float128 support on some accelerator. The whole model only
>>>> really makes sense if the accelerator shares the aggregate-layout rules
>>>> of the host, and this is a needless hassle for users if this causes an
>>>> error (especially in a unified-memory environment where configuration
>>>> data structures, etc. are shared between devices).
>>>>
>>>> Thanks again,
>>>>
>>>> Hal
>>>>
>>>>
>>>>>> Thanks again,
>>>>>>
>>>>>> Hal
>>>>>>
>>>>>>
>>>>>>> -------------
>>>>>>> Best regards,
>>>>>>> Alexey Bataev
>>>>>>> 15.01.2019 17:33, John McCall пишет:
>>>>>>>>> On 15 Jan 2019, at 17:20, Alexey Bataev wrote:
>>>>>>>>> This is not only for asm, we need to delay all target-specific
>>>>>>>>> diagnostics.
>>>>>>>>> I'm not saying that we need to move the host diagnostic, only the
>>>>>>>>> diagnostic for the device compilation.
>>>>>>>>> As for Cuda, it is a little but different. In Cuda the programmer
>>>>>>>>> must explicitly mark the device functions,  while in OpenMP it must
>>>>>>>>> be done implicitly. Thus, we cannot reuse the solution used for Cuda.
>>>>>>>> All it means is that you can't just use the solution used for CUDA
>>>>>>>> "off the shelf".  The basic idea of associating diagnostics with the
>>>>>>>> current function and then emitting those diagnostics later when you
>>>>>>>> realize that you have to emit that function is still completely
>>>>>>>> applicable.
>>>>>>>>
>>>>>>>> John.
>>>>>> -- 
>>>>>> Hal Finkel
>>>>>> Lead, Compiler Technology and Programming Languages
>>>>>> Leadership Computing Facility
>>>>>> Argonne National Laboratory
>>>>>>
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory