[SYCL] Add documentation and change default val for min range x to enable range rounding

[hdelan]

Some users found that 1024 was too conservative a min X dim to enable range rounding. This lowers that val.

It also adds some documentation for SYCL range rounding, which had been missing. Perhaps this should be in its own file. Let me know where to put it and I will do so.

Ping @DuncanMcBain

[hdelan]

@bader any further comments? Do you think I should put this documentation in a different place or is EnvironmentVariables.md OK?

[bader]

Do you think I should put this documentation in a different place or is EnvironmentVariables.md OK?

I would move the first section to a separate document in design directory and put a link to it in EnvironmentVariables.md. The second section describing environment variables belongs to EnvironmentVariables.md 👍.

Is SYCL_PARALLEL_FOR_RANGE_ROUNDING_PARAMS supported by the implementation? If no, we probably should state that explicitly. EnvironmentVariables.md is part of the "User documentation" and we might create wrong expectations from our users.

[hdelan]

I would move the first section to a separate document in design directory and put a link to it in EnvironmentVariables.md. The second section describing environment variables belongs to EnvironmentVariables.md 👍.

Done.

Is SYCL_PARALLEL_FOR_RANGE_ROUNDING_PARAMS supported by the implementation? If no, we probably should state that explicitly. EnvironmentVariables.md is part of the "User documentation" and we might create wrong expectations from our users.

Yes this is supported and works in DPC++ at the moment.

[bader]

@hdelan, please, add new document to https://github.com/intel/llvm/blob/sycl/sycl/doc/index.rst to fix the Doxygen documentation build.

[hdelan]

Thanks, change made

[hdelan]

@bader checks now passing so I think this is good to go

[bader]

@cperkinsintel, @intel/llvm-reviewers-runtime, please, approve to unblock the merge.

[bso-intel]

LGTM