vm_getivar: normalize shape_id to ignore frozen state

Freezing an object changes its `shape_id` This is necessary
so that `setivar` routines can use the `shape_id` as a cache key
and save on checking the frozen status every time.

However for `getivar` routines, this causes needless cache misses.
By clearing that bit we increase hit rate in codepaths that see
both frozen and mutable objects.

Author: byroot

shape.h

@@ -14,6 +14,7 @@ STATIC_ASSERT(shape_id_num_bits, SHAPE_ID_NUM_BITS == sizeof(shape_id_t) * CHAR_
 #define SHAPE_ID_OFFSET_MASK (SHAPE_BUFFER_SIZE - 1)
 #define SHAPE_ID_FLAGS_MASK (shape_id_t)(((1 << (SHAPE_ID_NUM_BITS - SHAPE_ID_OFFSET_NUM_BITS)) - 1) << SHAPE_ID_OFFSET_NUM_BITS)
 #define SHAPE_ID_FL_FROZEN (SHAPE_FL_FROZEN << SHAPE_ID_OFFSET_NUM_BITS)
+#define SHAPE_ID_READ_ONLY_MASK (~SHAPE_ID_FL_FROZEN)
 
 typedef uint32_t redblack_id_t;
 
@@ -110,6 +111,14 @@ RBASIC_SHAPE_ID(VALUE obj)
 #endif
 }
 
+// Same as RBASIC_SHAPE_ID but with flags that have no impact
+// on reads removed. e.g. Remove FL_FROZEN.
+static inline shape_id_t
+RBASIC_SHAPE_ID_FOR_READ(VALUE obj)
+{
+    return RBASIC_SHAPE_ID(obj) & SHAPE_ID_READ_ONLY_MASK;
+}
+
 static inline void
 RBASIC_SET_SHAPE_ID(VALUE obj, shape_id_t shape_id)
 {

vm_insnhelper.c

@@ -1215,14 +1215,13 @@ vm_getivar(VALUE obj, ID id, const rb_iseq_t *iseq, IVC ic, const struct rb_call
 {
 #if OPT_IC_FOR_IVAR
     VALUE val = Qundef;
-    shape_id_t shape_id;
     VALUE * ivar_list;
 
     if (SPECIAL_CONST_P(obj)) {
         return default_value;
     }
 
-    shape_id = RBASIC_SHAPE_ID(obj);
+    shape_id_t shape_id = RBASIC_SHAPE_ID_FOR_READ(obj);
 
     switch (BUILTIN_TYPE(obj)) {
       case T_OBJECT: