[ruby/mmtk] Add mmtk_plan to GC.config

https://github.com/ruby/mmtk/commit/67da9ea5b8

gc/mmtk/mmtk.c

@@ -633,6 +633,7 @@ rb_gc_impl_config_get(void *objspace_ptr)
     VALUE hash = rb_hash_new();
 
     rb_hash_aset(hash, ID2SYM(rb_intern_const("mmtk_worker_count")), RB_ULONG2NUM(mmtk_worker_count()));
+    rb_hash_aset(hash, ID2SYM(rb_intern_const("mmtk_plan")), rb_str_new_cstr((const char *)mmtk_plan()));
 
     return hash;
 }

gc/mmtk/mmtk.h

@@ -147,6 +147,8 @@ MMTk_Address mmtk_last_heap_address(void);
 
 size_t mmtk_worker_count(void);
 
+const uint8_t *mmtk_plan(void);
+
 bool mmtk_is_mmtk_object(MMTk_Address addr);
 
 #endif  /* MMTK_H */

gc/mmtk/src/api.rs

@@ -333,6 +333,20 @@ pub extern "C" fn mmtk_worker_count() -> usize {
     memory_manager::num_of_workers(mmtk())
 }
 
+#[no_mangle]
+pub extern "C" fn mmtk_plan() -> *const u8 {
+    static NO_GC: &[u8] = b"NoGC\0";
+    static MARK_SWEEP: &[u8] = b"MarkSweep\0";
+    static IMMIX: &[u8] = b"Immix\0";
+
+    match *crate::BINDING.get().unwrap().mmtk.get_options().plan {
+        PlanSelector::NoGC => NO_GC.as_ptr(),
+        PlanSelector::MarkSweep => MARK_SWEEP.as_ptr(),
+        PlanSelector::Immix => IMMIX.as_ptr(),
+        _ => panic!("Unknown plan")
+    }
+}
+
 // =============== Miscellaneous ===============
 
 #[no_mangle]

test/mmtk/test_configuration.rb

@@ -12,6 +12,15 @@ module MMTk
       RUBY
     end
 
+    # TODO: Add NoGC once this is fixed: https://github.com/mmtk/mmtk-core/pull/1263
+    %w(MarkSweep Immix).each do |plan|
+      define_method(:"test_MMTK_PLAN_#{plan}") do
+        assert_separately([{ "MMTK_PLAN" => plan }], <<~RUBY)
+          assert_equal("#{plan}", GC.config[:mmtk_plan])
+        RUBY
+      end
+    end
+
     %w(MMTK_THREADS MMTK_HEAP_MIN MMTK_HEAP_MAX MMTK_HEAP_MODE MMTK_PLAN).each do |var|
       define_method(:"test_invalid_#{var}") do
         exit_code = assert_in_out_err(