1 files changed, 22 insertions, 2 deletions
diff --git a/shape.h b/shape.h
index 194cd296a2..c634055cb6 100644
--- a/shape.h
+++ b/shape.h
@@ -16,8 +16,18 @@ STATIC_ASSERT(shape_id_num_bits, SHAPE_ID_NUM_BITS == sizeof(shape_id_t) * CHAR_
 #define SHAPE_ID_FL_FROZEN (SHAPE_FL_FROZEN << SHAPE_ID_OFFSET_NUM_BITS)
 #define SHAPE_ID_FL_HAS_OBJECT_ID (SHAPE_FL_HAS_OBJECT_ID << SHAPE_ID_OFFSET_NUM_BITS)
 #define SHAPE_ID_FL_TOO_COMPLEX (SHAPE_FL_TOO_COMPLEX << SHAPE_ID_OFFSET_NUM_BITS)
+#define SHAPE_ID_FL_EMBEDDED (SHAPE_FL_EMBEDDED << SHAPE_ID_OFFSET_NUM_BITS)
 #define SHAPE_ID_FL_NON_CANONICAL_MASK (SHAPE_FL_NON_CANONICAL_MASK << SHAPE_ID_OFFSET_NUM_BITS)
-#define SHAPE_ID_READ_ONLY_MASK (~SHAPE_ID_FL_FROZEN)
+
+#define SHAPE_ID_HEAP_INDEX_BITS 3
+#define SHAPE_ID_HEAP_INDEX_OFFSET (SHAPE_ID_NUM_BITS - SHAPE_ID_HEAP_INDEX_BITS)
+#define SHAPE_ID_HEAP_INDEX_MAX ((1 << SHAPE_ID_HEAP_INDEX_BITS) - 1)
+#define SHAPE_ID_HEAP_INDEX_MASK (SHAPE_ID_HEAP_INDEX_MAX << SHAPE_ID_HEAP_INDEX_OFFSET)
+
+// The interpreter doesn't care about embeded or frozen status when reading ivars.
+// So we normalize shape_id by clearing these bits to improve cache hits.
+// JITs however might care about it.
+#define SHAPE_ID_READ_ONLY_MASK (~(SHAPE_ID_FL_FROZEN | SHAPE_ID_FL_EMBEDDED | SHAPE_ID_HEAP_INDEX_MASK))
 
 typedef uint32_t redblack_id_t;
 
@@ -72,6 +82,7 @@ enum shape_flags {
     SHAPE_FL_FROZEN             = 1 << 0,
     SHAPE_FL_HAS_OBJECT_ID      = 1 << 1,
     SHAPE_FL_TOO_COMPLEX        = 1 << 2,
+    SHAPE_FL_EMBEDDED           = 1 << 3,
 
     SHAPE_FL_NON_CANONICAL_MASK = SHAPE_FL_FROZEN | SHAPE_FL_HAS_OBJECT_ID,
 };
@@ -189,10 +200,19 @@ rb_shape_canonical_p(shape_id_t shape_id)
     return !(shape_id & SHAPE_ID_FL_NON_CANONICAL_MASK);
 }
 
+static inline uint8_t
+rb_shape_heap_index(shape_id_t shape_id)
+{
+    return (uint8_t)((shape_id & SHAPE_ID_HEAP_INDEX_MASK) >> SHAPE_ID_HEAP_INDEX_OFFSET);
+}
+
 static inline shape_id_t
 rb_shape_root(size_t heap_id)
 {
-    return (shape_id_t)(heap_id + FIRST_T_OBJECT_SHAPE_ID);
+    shape_id_t heap_index = (shape_id_t)heap_id;
+
+    shape_id_t shape_id = (heap_index + FIRST_T_OBJECT_SHAPE_ID);
+    return shape_id | ((heap_index + 1) << SHAPE_ID_HEAP_INDEX_OFFSET) | SHAPE_ID_FL_EMBEDDED;
 }
 
 static inline bool