Replace sweeping_heaps map with a counter

We implemented some bit twiddling logic with an unsigned int to have a
neat way of tracking which heaps were currently sweeping, but we
actually don't need to care which heap is sweeping right now, just
whether some are or not, so we can replace this with a counter.

Author: eightbitraptor

gc/default/default.c

@@ -680,7 +680,7 @@ typedef struct rb_objspace {
 
     unsigned long live_ractor_cache_count;
 
-    unsigned int sweeping_heaps; /* bitfield: bit i set while heap i is sweeping */
+    int sweeping_heap_count;
 
     int fork_vm_lock_lev;
 } rb_objspace_t;
@@ -1018,7 +1018,7 @@ gc_mode_verify(enum gc_mode mode)
 static inline bool
 has_sweeping_pages(rb_objspace_t *objspace)
 {
-    return objspace->sweeping_heaps != 0;
+    return objspace->sweeping_heap_count != 0;
 }
 
 static inline size_t
@@ -3017,7 +3017,7 @@ gc_abort(void *objspace_ptr)
     }
 
     if (is_lazy_sweeping(objspace)) {
-        objspace->sweeping_heaps = 0;
+        objspace->sweeping_heap_count = 0;
         for (int i = 0; i < HEAP_COUNT; i++) {
             rb_heap_t *heap = &heaps[i];
 
@@ -3753,7 +3753,7 @@ gc_sweep_start_heap(rb_objspace_t *objspace, rb_heap_t *heap)
 {
     heap->sweeping_page = ccan_list_top(&heap->pages, struct heap_page, page_node);
     if (heap->sweeping_page) {
-        objspace->sweeping_heaps |= (1u << (heap - heaps));
+        objspace->sweeping_heap_count++;
     }
     heap->free_pages = NULL;
     heap->pooled_pages = NULL;
@@ -3982,7 +3982,8 @@ gc_sweep_step(rb_objspace_t *objspace, rb_heap_t *heap)
     } while ((sweep_page = heap->sweeping_page));
 
     if (!heap->sweeping_page) {
-        objspace->sweeping_heaps &= ~(1u << (heap - heaps));
+        objspace->sweeping_heap_count--;
+        GC_ASSERT(objspace->sweeping_heap_count >= 0);
         gc_sweep_finish_heap(objspace, heap);
 
         if (!has_sweeping_pages(objspace)) {