[SYCL][Test] Track amount of inter-dependencies between headers

[AlexeySachkov]

To improve compile-time, we may one day split sycl/sycl.hpp into separate headers, but we need to make sure that those headers don't include every other header so that there is an actual benefit of using them.

This PR introduced a test to track that certain headers which we already consider isolated are not used from anywhere else except from sycl/sycl.hpp.

Note that for now we allow extension headers to include everything and only focusing on the core SYCL headers.

[bader]

👍

[bader]

Suggested change

	// sycl/sycl.hpp is a very heavy header, because it includes all SYCL feature.
	// sycl/sycl.hpp is a very heavy header, because it includes all SYCL features.

[bader]

Suggested change

	// Note that this test ignores "ext" subfolder for now and concentrates on
	// NOTE: This test ignores "ext" subfolder for now and concentrates on

Following FIXME: or TODO: notation, I find it helpful to use NOTE: to highlight the importance of this section and simplify the search.

[bader]

If I get the idea right, we check that #include <sycl/sub_group.hpp> is used only by sycl/sycl.hpp header. I guess this applies to any header declaring SYCL core class, right? If so, do we need to have separate matchers for each header?

Suggested change

	// RUN: grep -rl "#include <sycl/sub_group.hpp>" %sycl_include/sycl \
	// RUN: --exclude-dir=*ext* | FileCheck %s --check-prefix=SUB-GROUP
	// SUB-GROUP: sycl/sycl.hpp
	// SUB-GROUP-EMPTY:
	//
	// RUN: grep -rl "#include <sycl/stream.hpp>" %sycl_include/sycl \
	// RUN: --exclude-dir=*ext* | FileCheck %s --check-prefix=STREAM
	// STREAM: sycl/sycl.hpp
	// STREAM-EMPTY:
	// RUN: grep -rl "#include <sycl/sub_group.hpp>" %sycl_include/sycl \
	// RUN: --exclude-dir=*ext* | FileCheck %s
	// RUN: grep -rl "#include <sycl/stream.hpp>" %sycl_include/sycl \
	// RUN: --exclude-dir=*ext* | FileCheck %s
	//
	// CHECK: sycl/sycl.hpp
	// CHECK-EMPTY:

I guess we can further simplify the command to match all include patterns at once rather than searching one include per grep command.

[AlexeySachkov]

👍

We actually had an offline talk with @aelovikov-intel here with conclusion that this test is not be a good idea :)

If I get the idea right, we check that #include <sycl/sub_group.hpp> is used only by sycl/sycl.hpp header.

That was the idea, right. Note that I'm specifically not taking extensions into account, they can include whatever they want at this point.

I guess this applies to any header declaring SYCL core class, right?

The thing is, SYCL extensions can add extra methods into core SYCL classes. Depending on what such a method is, it may require a full sub_group definition, thus breaking the test. It is not entirely clear what is the course of actions to fix this test then?

Even if we discard extensions, there are tons of inter-dependencies between core SYCL classes and it will be tricky to resolve them all by forward-declarations (see reduction.hpp and atomic_ref.hpp as an example).

We probably could say that stream should not be included anywhere else directly, but it will be hard to say that in general about every header that we have. For example. even after #16012 kernel_bundle.hpp will still be included from one more place (outside of ext/) besides sycl/sycl.hpp.