Provide a way to get the plan name at runtime

[chrisseaton]

Ruby has a way to print various configuration options (which JIT it's using etc.) We'd like to be able to print out which GC is being used, so the plan name. We think we'd like a method get_name on Plan for example, but we're not very experienced with the architecture of MMTk.

[qinsoon]

That should be possible with the current public API. Plan is an option:

mmtk-core/src/util/options.rs

Lines 308 to 309 in ce82a7d

	// The plan to use. This needs to be initialized before creating an MMTk instance (currently by setting env vars)
	plan: PlanSelector [env_var: true, command_line: false] [always_valid] = PlanSelector::NoGC,

You can store it as a string somewhere, and create a C function to get it.

lazy_static!{
    // Get the plan option from from the MMTk instance. Dereference it to get a PlanSelector.
    // Debug print it into a String, and then into a CString
    static ref MMTK_PLAN_STR: CString = CString::new(format!("{:?}", *SINGLETON.get_options().plan)).unwrap();
}

#[no_mangle]
pub extern "C" fn get_plan() -> *const c_char {
    MMTK_PLAN_STR.as_ptr()
}

[chrisseaton]

@wks could you please add a extern char *mmtk_plan_name(void) function that I can use from Ruby?

[wks]

Sure. I'll work on that.

[wks]

@chrisseaton Instead of adding it to mmtk-core, I added a C API in the mmtk-ruby repository, because mmtk-core doesn't expose C API. All public Rust functions are callable from the binding, and the binding can make wrappers in whatever language relevant for that VM. The commit is: mmtk/mmtk-ruby@7c08f3d

I also added a method in the Ruby VM so that we can query the plan name in Ruby. Please have a look and see if my code has any problems. mmtk/ruby#6