gc.c: add optional GC statistics counters

Add gc_total_count, minor_gc_count, major_gc_count (uint32_t) to
mrb_gc, guarded by MRB_GC_STATS. Zero cost when disabled.

Co-authored-by: Claude <noreply@anthropic.com>

include/mruby/gc.h

@@ -64,6 +64,12 @@ typedef struct mrb_gc {
   int arena_capa;                          /* size of protection array */
 #endif
   int arena_idx;
+
+#ifdef MRB_GC_STATS
+  uint32_t gc_total_count;                 /* total GC invocations */
+  uint32_t minor_gc_count;                 /* minor GC count */
+  uint32_t major_gc_count;                 /* major GC count */
+#endif
 } mrb_gc;
 
 MRB_API mrb_bool mrb_object_dead_p(mrb_state *mrb, struct RBasic *object);

src/gc.c

@@ -1322,9 +1322,19 @@ mrb_incremental_gc(mrb_state *mrb)
   if (gc->disabled || gc->iterating) return;
 
   if (is_minor_gc(gc)) {
+#ifdef MRB_GC_STATS
+    gc->gc_total_count++;
+    gc->minor_gc_count++;
+#endif
     incremental_gc_finish(mrb, gc);
   }
   else {
+#ifdef MRB_GC_STATS
+    if (gc->state == MRB_GC_STATE_ROOT) {
+      gc->gc_total_count++;
+      gc->major_gc_count++;
+    }
+#endif
     incremental_gc_step(mrb, gc);
   }
 
@@ -1364,6 +1374,10 @@ mrb_full_gc(mrb_state *mrb)
   if (!mrb->c) return;
   if (gc->disabled || gc->iterating) return;
 
+#ifdef MRB_GC_STATS
+  gc->gc_total_count++;
+  gc->major_gc_count++;
+#endif
   if (is_generational(gc)) {
     /* clear all the old objects back to young */
     clear_all_old(mrb, gc);