purpose of unsupported_extension class and context extensions

[f-koehler]

The unsupported_extension exception is currently only thrown by opengl_create_shared_context(). Maybe one could also throw it when the constructor of a general context fails, if there is an unsupported extension specified by const cl_context_properites *properties.

Maybe there also should be a way to construct contexts with extensions without having to create 0-terminated lists by hand (maybe a vector class where the user can add extensions).

[kylelutz]

The idea was to provide certain functions/classes (like the OpenGL interop APIs) but throw an unsupported_extension_error exception if the underyling compute device didn't support the extension (which is only detectable at run-time and thus we use exceptions). For reference, you can see the full list of current OpenCL extensions here: http://www.khronos.org/registry/cl/ (at the bottom of the page).

Currently clCreateContext() will throw an opencl_error exception on failure (which will contain the error code/string). It could be interesting to throw an unsupported_extension_error instead if we can detect that clCreateContext() failed due to the device lacking support for an extension. Can you give me some examples of cl_context_properties values and return codes from clCreateContext() you would use to check if you should throw the unsupported_extension_error exception instead of the regular opencl_error exception?

And i really like the idea of supporting a context constructor which takes a std::vector<cl_context_properties> argument! That would make it much more natural to use from c++. and with c++11 they could use intializer lists and do something like this:

boost::compute::context ctx(device, {CL_GL_CONTEXT_KHR, glXGetCurrentContext()});

It would also be nice to offer the same API for creating command queues with properties:

boost::compute::command_queue queue(ctx, dev, {CL_QUEUE_PROFILING_ENABLE});

[jszuppe]

After looking at clCreateContext I think in general we can't throw an unsupported_extension_error from boost::compute::context, because there is no special error code for unsupported extension error in OpenCL. CL_INVALID_PROPERTY that is returned by clCreateContext when extension is not supported is also returned for other errors.

CL_INVALID_PROPERTY - if context property name in properties is not a supported property name, if the value specified for a supported property name is not valid, or if the same property name is specified more than once. However if the extension cl_khr_gl_sharing is enabled, then CL_INVALID_PROPERTY is returned if an attribute name other than those listed in the table for properties above or if CL_CONTEXT_INTEROP_USER_SYNC is specified in properties.

CL_INVALID_PROPERTY - if the cl_khr_gl_sharing extension is enabled and an attribute name other than those specified in the table above (table 4.5) or if CL_CONTEXT_INTEROP_USER_SYNC is specified in properties.