Make rb_gc_obj_optimal_size always return allocatable size

It may return sizes that aren't allocatable for arrays and strings.

Author: peterzhu2118

gc.c

@@ -3317,7 +3317,15 @@ rb_gc_obj_optimal_size(VALUE obj)
 {
     switch (BUILTIN_TYPE(obj)) {
       case T_ARRAY:
-        return rb_ary_size_as_embedded(obj);
+        {
+            size_t size = rb_ary_size_as_embedded(obj);
+            if (rb_gc_size_allocatable_p(size)) {
+                return size;
+            }
+            else {
+                return sizeof(struct RArray);
+            }
+        }
 
       case T_OBJECT:
         if (rb_shape_obj_too_complex_p(obj)) {
@@ -3328,7 +3336,15 @@ rb_gc_obj_optimal_size(VALUE obj)
         }
 
       case T_STRING:
-        return rb_str_size_as_embedded(obj);
+        {
+            size_t size = rb_str_size_as_embedded(obj);
+            if (rb_gc_size_allocatable_p(size)) {
+                return size;
+            }
+            else {
+                return sizeof(struct RString);
+            }
+        }
 
       case T_HASH:
         return sizeof(struct RHash) + (RHASH_ST_TABLE_P(obj) ? sizeof(st_table) : sizeof(ar_table));

gc/default/default.c

@@ -5484,10 +5484,9 @@ gc_compact_destination_pool(rb_objspace_t *objspace, rb_heap_t *src_pool, VALUE
         return src_pool;
     }
 
-    size_t idx = 0;
-    if (rb_gc_impl_size_allocatable_p(obj_size)) {
-        idx = heap_idx_for_size(obj_size);
-    }
+    GC_ASSERT(rb_gc_impl_size_allocatable_p(obj_size));
+
+    size_t idx = heap_idx_for_size(obj_size);
 
     return &heaps[idx];
 }