https://git.ruby-lang.org/ruby.git/commit/?id=f9b9d1c580

From f9b9d1c58052fa566bddc7fcc902ef123902bdc2 Mon Sep 17 00:00:00 2001
From: Aaron Patterson <tenderlove@r...>
Date: Tue, 1 Jun 2021 14:34:34 -0700
Subject: Use the current object as the compaction index

Instead of keeping track of the current bit plane, keep track of the
actual slot when compacting.  This means we don't need to re-scan
objects inside the same bit plane when we continue with movement
---
 gc.c | 101 ++++++++++++++++++++++++++++++++++++++++++-------------------------
 1 file changed, 64 insertions(+), 37 deletions(-)

diff --git a/gc.c b/gc.c
index 3844333..7f1ff7c 100644
--- a/gc.c
+++ b/gc.c
@@ -683,7 +683,7 @@ typedef struct rb_heap_struct { https://github.com/ruby/ruby/blob/trunk/gc.c#L683
     struct list_head pages;
     struct heap_page *sweeping_page; /* iterator for .pages */
     struct heap_page *compact_cursor;
-    size_t compact_cursor_index;
+    RVALUE * compact_cursor_index;
 #if GC_ENABLE_INCREMENTAL_MARK
     struct heap_page *pooled_pages;
 #endif
@@ -4895,11 +4895,48 @@ unlock_page_body(rb_objspace_t *objspace, struct heap_page_body *body) https://github.com/ruby/ruby/blob/trunk/gc.c#L4895
     }
 }
 
+static inline bool
+try_move_in_plane(rb_objspace_t *objspace, rb_heap_t *heap, intptr_t p, bits_t bits, VALUE dest)
+{
+    if (bits) {
+        do {
+            if (bits & 1) {
+                /* We're trying to move "p" */
+                objspace->rcompactor.considered_count_table[BUILTIN_TYPE((VALUE)p)]++;
+
+                if (gc_is_moveable_obj(objspace, (VALUE)p)) {
+                    /* We were able to move "p" */
+                    objspace->rcompactor.moved_count_table[BUILTIN_TYPE((VALUE)p)]++;
+                    objspace->rcompactor.total_moved++;
+
+                    bool from_freelist = false;
+
+                    if (BUILTIN_TYPE(dest) == T_NONE) {
+                        from_freelist = true;
+                    }
+
+                    gc_move(objspace, (VALUE)p, dest);
+                    gc_pin(objspace, (VALUE)p);
+                    heap->compact_cursor_index = (RVALUE *)p;
+                    if (from_freelist) {
+                        FL_SET((VALUE)p, FL_FROM_FREELIST);
+                    }
+
+                    return true;
+                }
+            }
+            p += sizeof(RVALUE);
+            bits >>= 1;
+        } while (bits);
+    }
+
+    return false;
+}
+
 static short
 try_move(rb_objspace_t *objspace, rb_heap_t *heap, struct heap_page *sweep_page, VALUE dest)
 {
     struct heap_page * cursor = heap->compact_cursor;
-    char from_freelist = 0;
 
     GC_ASSERT(!MARKED_IN_BITMAP(GET_HEAP_MARK_BITS(dest), dest));
 
@@ -4907,49 +4944,39 @@ try_move(rb_objspace_t *objspace, rb_heap_t *heap, struct heap_page *sweep_page, https://github.com/ruby/ruby/blob/trunk/gc.c#L4944
      * T_NONE, it is an object that just got freed but hasn't been
      * added to the freelist yet */
 
-    if (BUILTIN_TYPE(dest) == T_NONE) {
-        from_freelist = 1;
-    }
-
     while(1) {
-        size_t index = heap->compact_cursor_index;
+        size_t index;
 
         bits_t *mark_bits = cursor->mark_bits;
         bits_t *pin_bits = cursor->pinned_bits;
-        RVALUE * p = cursor->start;
-        RVALUE * offset = p - NUM_IN_PAGE(p);
+        RVALUE * p;
+
+        if (heap->compact_cursor_index) {
+            index = BITMAP_INDEX(heap->compact_cursor_index);
+            p = heap->compact_cursor_index;
+            GC_ASSERT(cursor == GET_HEAP_PAGE(p));
+        } else {
+            index = 0;
+            p = cursor->start;
+        }
+
+        bits_t bits = mark_bits[index] & ~pin_bits[index];
+
+        bits >>= NUM_IN_PAGE(p);
+        if (try_move_in_plane(objspace, heap, (intptr_t)p, bits, dest)) return 1;
+
+        if (index == 0) {
+            p = cursor->start + (BITS_BITLENGTH - NUM_IN_PAGE(cursor->start));
+        } else {
+            p = cursor->start + (BITS_BITLENGTH - NUM_IN_PAGE(cursor->start)) + (BITS_BITLENGTH * index);
+        }
 
         /* Find an object to move and move it. Movable objects must be
          * marked, so we iterate using the marking bitmap */
-        for (size_t i = index; i < HEAP_PAGE_BITMAP_LIMIT; i++) {
+        for (size_t i = index + 1; i < HEAP_PAGE_BITMAP_LIMIT; i++) {
             bits_t bits = mark_bits[i] & ~pin_bits[i];
-
-            if (bits) {
-                p = offset + i * BITS_BITLENGTH;
-
-                do {
-                    if (bits & 1) {
-                        /* We're trying to move "p" */
-                        objspace->rcompactor.considered_count_table[BUILTIN_TYPE((VALUE)p)]++;
-
-                        if (gc_is_moveable_obj(objspace, (VALUE)p)) {
-                            /* We were able to move "p" */
-                            objspace->rcompactor.moved_count_table[BUILTIN_TYPE((VALUE)p)]++;
-                            objspace->rcompactor.total_moved++;
-                            gc_move(objspace, (VALUE)p, dest);
-                            gc_pin(objspace, (VALUE)p);
-                            heap->compact_cursor_index = i;
-                            if (from_freelist) {
-                                FL_SET((VALUE)p, FL_FROM_FREELIST);
-                            }
-
-                            return 1;
-                        }
-                    }
-                    p++;
-                    bits >>= 1;
-                } while (bits);
-            }
+            if (try_move_in_plane(objspace, heap, (intptr_t)p, bits, dest)) return 1;
+            p += BITS_BITLENGTH;
         }
 
         /* We couldn't find a movable object on the compact cursor, so lets
-- 
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