OpenCL-Header versioning

Currently OpenCL headers are versioned based on the OpenCL spec (1.2, 2.0, 2.2 etc)

Apart from the core and optional core features that we change only with OpenCL version, we often add types and APIs required by KHR extensions in some of the standard headers (e.g. clext.h). Sometimes, the extension might be written on top of 1.2 or 2.0.

Current versioning doesn't allow differentiating between the OpenCL headers before and after such changes. Having finer versioning may help recommend right version of headers users need to use and potentially clarify this dependency in the extension specs.

Filing this issue to discuss pros and cons of adding the finer versioning.

Start by reviewing how the current OpenCL spec-based header versions are represented and how KHR extension types and APIs are added to the standard headers. Compare the tradeoffs of finer versioning, and consider the issue complete when a versioning approach and its implications for extension dependencies are clearly documented.

Written by the indexing model from the issue text.