[SYCL][COMPAT] First sycl_ext_oneapi_compat implementation

[Alcpz]

This is an implementation of the experimental extension proposed in #9646.

The compat extension has two primary goals:

• Improve the adoption of SYCL. The compat extension is designed to provide a familiar programming interface that resembles other popular heterogeneous programming models. By reducing the learning curve, it enables developers to leverage SYCL's power and features more easily.

• Source-to-Source Translation Support. compat is also designed to facilitate automatic source-to-source translation from other heterogeneous programming models to SYCL and offer a more standardized and consistent programming interface. This feature can significantly streamline the migration and integration of existing codebases into the SYCL ecosystem.

The PR also includes testing for the compat extension.

As we stated in the proposal PR, we are open to any suggestions, concerns, or improvements you may have, so please, let us know if you have any.

[Alcpz]

We think that we should refactor our tests from unittests to e2e tests, the reasoning being that we are targeting device code.

Our tests are built upon gtests, so we are wondering:

• Can we use gtests inside e2e, or should we completely avoid external dependencies there?
• Is there any possibility to have a different path, or a different build mechanism, to include the tests included in this PR?

[cperkinsintel]

Extensions should have docs. ( llvm/sycl/extensions/experimental/* ) , especially one as extensive as this. Also, I'd like to better understand the exact use cases. The code here seems fine, but what with wrapping atomics, kernels, and even dimensions, I'd like to get a clear understanding of what this is and how/when a user would use it.

[aelovikov-intel]

Can we use gtests inside e2e, or should we completely avoid external dependencies there?

I think we shouldn't. Not because it's an external dependency, but because both LIT and GTest are testing frameworks and it's a bad idea to mix usage of two.

I think what you're looking for is sycl/unittest-e2e with some extra infrastructure to enable on-device testing. But I don't have any particular image in mind on how that would work.

Update:
I think we need something like this:

sycl/test-e2e:
  CMakeLists.txt
  Basic/
  ... Other current subfolders ...
  gtest/ # Or maybe call it "unittest/"
  lit.cfg.py  
  ...

But also ignore the entire gtest/ from the LIT infrastructure. Then the CMake should provide an additional check-sycl-e2e-gtest target. The existing check-sycl-e2e can depend on it so that existing CI would automatically include those tests. Or maybe we should move the current part of check-sycl-e2e into check-sycl-e2e-lit and make check-sycl-e2e dependent on both *-lit and *-gtest targets.

[Alcpz]

Extensions should have docs. ( llvm/sycl/extensions/experimental/* )

@cperkinsintel The docs are available in #9646. The extension contains quite a bit of code so we divided it in two. I think you will find there an answer for the topics you have brought up.

[Alcpz]

We were advised to refactor the code and split it up in multiple PRs: #9976 is the first.