[SYCLomatic] Block Load headers core

[abhilash1910]

In Progress PR for Load/Store header functions for Block API (related later to #1305 )
cc @yihanwg @danhoeflinger @mmichel11

[mmichel11]

A similar comment to above with regards to the stride. In this case, I believe instead of ITEMS_PER_THREAD it should be the subgroup size.

[mmichel11]

Thinking about the design in more depth, I do not think these APIs should be implemented as free functions. Rather, I think they should be implemented as classes which overload the function call operator or have a member function that invokes the load operation.

My reasoning for this is that certain applications during migration may need to pass a "load" object to a separate function
/ routine in a kernel. If they are implemented as classes, then an object can be passed to the function where it could then be used:

load_striped<...> obj;
my_func(obj);

However, if they are implemented as free functions, then they would need to be passed as function pointers:

auto func_obj = &load_striped<...>;
my_func(func_obj);

which is not supported in the SYCL programming model. To make this work, load_striped would have to be called directly from my_func which I think may be difficult from the migration perspective.

What I think we should do is have two classes work_group_load and sub_group_load and then have some enum type dictating the strategy (ex: striped, blocked, etc). We could then have have specializations of this class to implement each strategy and maybe have some helpers to reduce code duplication.

[yihwang-nv]

LGTM

[mmichel11]

This PR looks to be in good shape for me. However, I think the outstanding comments in oneapi-src/SYCLomatic-test#619 need to be addressed, so we can have confidence in the implementation's correctness.

[zhimingwang36]

here are quite many item usage, ITEM/Item/iterms/ITERMS_PER_WORK_ITEM, maybe it should give a better name for "typename Item", it is a little confusing. or please add comments.

[abhilash1910]

thanks will add comments to clarify.

[zhimingwang36]

can we rename to nd_item? (what is the diff b/w Item and sycl::nd_item)

[abhilash1910]

Yes it is same thing, but to maintain the same variable name across the file (similar for other apis), Item is used.

[abhilash1910]

This PR is now tested with oneapi-src/SYCLomatic-test#619 and all tests are passing.
Tested on application side as well, working as expected.

[danhoeflinger]

LGTM,
There are some changes needed in the testing PR still, but with some local changes to those tests and passing runs, I'm confident enough in this PR now to approve.

[mmichel11]

LGTM