Add object packing strategies for compaction

This commit adds an alternative packing strategy for compaction. Instead of packing towards "most pinned" pages, we can pack towards "most empty" pages. The idea is that we can double the heap size, then pack all objects towards the empty side of the heap. This will ensure maximum chaos for testing / verification.
github · tenderlove · May 28, 2019 · Apr 26, 2019 · Jan 18, 2019 · Apr 21, 2019
commit c70ceb59928aa2c37befd6b03d73657ba5ece61c
diff --git a/gc.c b/gc.c
@@ -7492,8 +7492,21 @@ int compare_pinned(const void *left, const void *right, void *dummy)
     return right_count - left_count;
 }
 
+int compare_free_slots(const void *left, const void *right, void *dummy)
+{
+    struct heap_page *left_page;
+    struct heap_page *right_page;
+
+    left_page = *(struct heap_page * const *)left;
+    right_page = *(struct heap_page * const *)right;
+
+    return right_page->free_slots - left_page->free_slots;
+}
+
+typedef int page_compare_func_t(const void *, const void *, void *);
+
 static VALUE
-gc_compact_heap(rb_objspace_t *objspace)
+gc_compact_heap(rb_objspace_t *objspace, page_compare_func_t *comparator)
 {
     struct heap_cursor free_cursor;
     struct heap_cursor scan_cursor;
@@ -7506,7 +7519,7 @@ gc_compact_heap(rb_objspace_t *objspace)
 
     page_list = calloc(heap_allocated_pages, sizeof(struct heap_page *));
     memcpy(page_list, heap_pages_sorted, heap_allocated_pages * sizeof(struct heap_page *));
-    ruby_qsort(page_list, heap_allocated_pages, sizeof(struct heap_page *), compare_pinned, NULL);
+    ruby_qsort(page_list, heap_allocated_pages, sizeof(struct heap_page *), comparator, NULL);
 
     init_cursors(objspace, &free_cursor, &scan_cursor, page_list);
 
@@ -7789,7 +7802,7 @@ rb_gc_new_location(VALUE value)
 
         if (BUILTIN_TYPE(value) == T_MOVED) {
             destination = (VALUE)RMOVED(value)->destination;
-            assert(BUILTIN_TYPE(destination) != T_NONE);
+            GC_ASSERT(BUILTIN_TYPE(destination) != T_NONE);
         }
         else {
             destination = value;
@@ -8042,20 +8055,29 @@ gc_ref_update(void *vstart, void *vend, size_t stride, void * data)
     /* For each object on the page */
     for (; v != (VALUE)vend; v += stride) {
         if (!SPECIAL_CONST_P(v)) {
+            void *poisoned = poisoned_object_p(v);
             unpoison_object(v, false);
 
-            if (BUILTIN_TYPE(v) == T_NONE) {
-                heap_page_add_freeobj(objspace, page, v);
-                free_slots++;
+            switch(BUILTIN_TYPE(v)) {
+                case T_NONE:
+                    heap_page_add_freeobj(objspace, page, v);
+                    free_slots++;
+                    break;
+                case T_MOVED:
+                    break;
+                default:
+                    if (RVALUE_WB_UNPROTECTED(v)) {
+                        page->flags.has_uncollectible_shady_objects = TRUE;
+                    }
+                    if (RVALUE_PAGE_MARKING(page, v)) {
+                        page->flags.has_remembered_objects = TRUE;
+                    }
+                    gc_update_object_references(objspace, v);
             }
-            else {
-                if (RVALUE_WB_UNPROTECTED(v)) {
-                    page->flags.has_uncollectible_shady_objects = TRUE;
-                }
-                if (RVALUE_PAGE_MARKING(page, v)) {
-                    page->flags.has_remembered_objects = TRUE;
-                }
-                gc_update_object_references(objspace, v);
+
+            if (poisoned) {
+                GC_ASSERT(BUILTIN_TYPE(v) == T_NONE);
+                poison_object(v);
             }
         }
     }
@@ -8115,7 +8137,7 @@ rb_gc_compact(VALUE mod)
     /* Ensure objects are pinned */
     rb_gc();
 
-    gc_compact_heap(objspace);
+    gc_compact_heap(objspace, compare_pinned);
 
     heap_eden->freelist = NULL;
     gc_update_references(objspace);
@@ -8199,20 +8221,44 @@ gc_check_references_for_moved(VALUE dummy)
  *  make a SEGV.
  */
 static VALUE
-gc_verify_compaction_references(VALUE mod)
+gc_verify_compaction_references(int argc, VALUE *argv, VALUE mod)
 {
     rb_objspace_t *objspace = &rb_objspace;
     VALUE moved_list;
 
     if (dont_gc) return Qnil;
 
+    VALUE opt = Qnil;
+    static ID keyword_ids[2];
+    VALUE kwvals[2];
+
+    kwvals[1] = Qtrue;
+    page_compare_func_t * comparator = compare_pinned;
+
+    rb_scan_args(argc, argv, "0:", &opt);
+
+    if (!NIL_P(opt)) {
+
+	if (!keyword_ids[0]) {
+	    keyword_ids[0] = rb_intern("toward");
+	    keyword_ids[1] = rb_intern("double_heap");
+	}
+
+	rb_get_kwargs(opt, keyword_ids, 0, 2, kwvals);
+        if (rb_intern("empty") == rb_sym2id(kwvals[0])) {
+            comparator = compare_free_slots;
+        }
+    }
+
     /* Ensure objects are pinned */
     rb_gc();
 
-    /* Double heap size */
-    heap_add_pages(objspace, heap_eden, heap_allocated_pages);
+    if (kwvals[1]) {
+        /* Double heap size */
+        heap_add_pages(objspace, heap_eden, heap_allocated_pages);
+    }
 
-    moved_list = gc_compact_heap(objspace);
+    moved_list = gc_compact_heap(objspace, comparator);
 
     heap_eden->freelist = NULL;
     gc_update_references(objspace);
@@ -11472,7 +11518,7 @@ Init_GC(void)
 
     /* internal methods */
     rb_define_singleton_method(rb_mGC, "verify_internal_consistency", gc_verify_internal_consistency, 0);
-    rb_define_singleton_method(rb_mGC, "verify_compaction_references", gc_verify_compaction_references, 0);
+    rb_define_singleton_method(rb_mGC, "verify_compaction_references", gc_verify_compaction_references, -1);
     rb_define_singleton_method(rb_mGC, "verify_transient_heap_internal_consistency", gc_verify_transient_heap_internal_consistency, 0);
 #if MALLOC_ALLOCATED_SIZE
     rb_define_singleton_method(rb_mGC, "malloc_allocated_size", gc_malloc_allocated_size, 0);

diff --git a/test/ruby/test_gc_compact.rb b/test/ruby/test_gc_compact.rb
@@ -53,7 +53,7 @@ def try_to_move_objects
       # All object ids should be equal
       assert_equal 0, assert_object_ids(list_of_objects) # should be 0
 
-      GC.verify_compaction_references
+      GC.verify_compaction_references(toward: :empty)
 
       # Some should have moved
       id_count = assert_object_ids(list_of_objects)
@@ -111,8 +111,9 @@ def test_many_collisions
     ids       = list_of_objects.map(&:object_id)
     addresses = list_of_objects.map(&self.:memory_location)
 
-    GC.verify_compaction_references
+    GC.verify_compaction_references(toward: :empty)
 
+    return
     new_tenants = 10.times.map {
       find_object_in_recycled_slot(addresses)
     }
@@ -125,7 +126,7 @@ def test_many_collisions
   def test_complex_hash_keys
     list_of_objects = big_list
     hash = list_of_objects.hash
-    GC.verify_compaction_references
+    GC.verify_compaction_references(toward: :empty)
     assert_equal hash, list_of_objects.hash
   end
 end