Add a way to see all possible values for -fsycl-targets flag

[AlexeySachkov]

Is your feature request related to a problem? Please describe

This is split from #1191 where folks were confused about which targets are supported by our SYCL implementation and which spelling should be used for them, i.e. what are the possible values for -fsycl-target compiler flag?

Describe the solution you would like

There is no proposal for this yet and the solution/interface should be developed as part of this issue. Original issue mentioned -fsycl-targets=list as an example of how it could be used. Displaying the full list under --help is also an option

Describe alternatives you have considered

No response

Additional context

No response

[ayylol]

Hey, just wondering: Would this list all possible -fsycl-targets? or just the ones supported by the build of the compiler? (For example, if we build the compiler without cuda support it wouldn't list the nvptx64-nvidia-cuda target)

[christgau]

For example, if we build the compiler without cuda support it wouldn't list the nvptx64-nvidia-cuda target

From a user's perspective, this would be bad. It would be best if only those targets supported by the compiler (or its plugins 😬) will be listed

[AlexeySachkov]

Hey, just wondering: Would this list all possible -fsycl-targets? or just the ones supported by the build of the compiler? (For example, if we build the compiler without cuda support it wouldn't list the nvptx64-nvidia-cuda target)

Theoretically, yes. Our build scripts have a knowledge about which targets/backends are being built. I'm not entirely sure if those have a direct mapping to SYCL targets, but that is definitely something that we can propagate ourselves through our own CMake options from our configure.py script if that would be needed.

[AlexeySachkov]

For example, if we build the compiler without cuda support it wouldn't list the nvptx64-nvidia-cuda target

From a user's perspective, this would be bad. It would be best if only those targets supported by the compiler (or its plugins 😬) will be listed

Plugins are dynamically loaded and compiler can't know which plugins will be available on a system that is going to execute an application. There maybe zero plugins available during build and that's fine. Therefore, I don't think that we will be able to be precise (or even reasonable) with our assumptions about available plugins at compile-time.

[christgau]

Sorry, my quote was wrong. What would be bad is:

Would this list all possible @-fsycl-targets@?

Plugins are dynamically loaded and compiler can't know which plugins will be available on a system that is going to execute an application.

I see. Makes sense.