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

Finkel, Hal J. via cfe-dev cfe-dev at lists.llvm.org
Thu Jan 17 09:40:21 PST 2019

On 1/17/19 11:11 AM, Alexey Bataev wrote:
> The compiler does not know anything about the layout on the host when
> it compiles for the device.

No, the compiler does know about the host layout (e.g., can't we
construct this by calling getContext().getAuxTargetInfo(), or similar?).

> We cannot do anything with the types that are not supported by the
> target device and we cannot use the layout from the host. And it is
> user responsibility to write and use the code that is compatible with
> the the target devices.
> He/she does not need to use macros/void* types, there are templates.
No. This doesn't solve the problem (because you still need to share the
instantiations between the devices). Also, even if it did, does not
address the legacy-code problem that the feature is intended to address.
The user already has classes and data on the host and wishes to access
*parts* of that data on the device. We should make as much of that work
as possible.

> You cannot use classes, which use types incompatible with the device.
> There is a problem with the data layout on the device and we just
> don't know how to represent such classes on the device.

There's no reason for this to be true. To be clear, the model of a
shared address space only makes sense, from a user perspective, if the
data layout is the same between the host and the target. Not mostly
similar, but the same. Otherwise, users will constantly be tracking down
subtle data-layout incompatibilities.

Thanks again,


> -------------
> Best regards,
> Alexey Bataev
> 17.01.2019 11:47, Finkel, Hal J. пишет:
>> 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

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