eagerly initialize ivar table when index is small enough

When the inline cache is written, the iv table will contain an entry for
the instance variable.  If we get an inline cache hit, then we know the
iv table must contain a value for the index written to the inline cache.

If the index in the inline cache is larger than the list on the object,
but *smaller* than the iv index table on the class, then we can just
eagerly allocate the iv list to be the same size as the iv index table.

This avoids duplicate work of checking frozen as well as looking up the
index for the particular instance variable name.

4 files changed, 50 insertions, 20 deletions

diff --git a/benchmark/vm_ivar_init.yml b/benchmark/vm_ivar_init.yml
new file mode 100644
index 0000000000..c6f1633907
--- /dev/null
+++ b/benchmark/vm_ivar_init.yml
@@ -0,0 +1,14 @@
+prelude: |
+  class C
+    def initialize
+      @a = nil
+      @b = nil
+      @c = nil
+      @d = nil
+      @e = nil
+    end
+  end
+benchmark:
+  vm_ivar_init: |
+    C.new
+loop_count: 30000000
diff --git a/internal/variable.h b/internal/variable.h
index d5d0cccbfb..34ad4e1199 100644
--- a/internal/variable.h
+++ b/internal/variable.h
@@ -52,6 +52,7 @@ VALUE rb_gvar_get(ID);
 VALUE rb_gvar_set(ID, VALUE);
 VALUE rb_gvar_defined(ID);
 void rb_const_warn_if_deprecated(const rb_const_entry_t *, VALUE, ID);
+void rb_init_iv_list(VALUE obj, uint32_t len, uint32_t newsize, st_table * index_tbl);
 MJIT_SYMBOL_EXPORT_END
 
 static inline bool
diff --git a/variable.c b/variable.c
index 7ebbcf85b8..669d47e8d3 100644
--- a/variable.c
+++ b/variable.c
@@ -1402,6 +1402,28 @@ rb_obj_transient_heap_evacuate(VALUE obj, int promote)
 }
 #endif
 
+void
+rb_init_iv_list(VALUE obj, uint32_t len, uint32_t newsize, st_table * index_tbl)
+{
+    VALUE *ptr = ROBJECT_IVPTR(obj);
+    VALUE *newptr;
+
+    if (RBASIC(obj)->flags & ROBJECT_EMBED) {
+        newptr = obj_ivar_heap_alloc(obj, newsize);
+        MEMCPY(newptr, ptr, VALUE, len);
+        RBASIC(obj)->flags &= ~ROBJECT_EMBED;
+        ROBJECT(obj)->as.heap.ivptr = newptr;
+    } else {
+        newptr = obj_ivar_heap_realloc(obj, len, newsize);
+    }
+
+    for (; len < newsize; len++) {
+        newptr[len] = Qundef;
+    }
+    ROBJECT(obj)->as.heap.numiv = newsize;
+    ROBJECT(obj)->as.heap.iv_index_tbl = index_tbl;
+}
+
 static VALUE
 obj_ivar_set(VALUE obj, ID id, VALUE val)
 {
@@ -1419,25 +1441,8 @@ obj_ivar_set(VALUE obj, ID id, VALUE val)
 
     len = ROBJECT_NUMIV(obj);
     if (len <= ivup.index) {
-        VALUE *ptr = ROBJECT_IVPTR(obj);
-        VALUE *newptr;
         uint32_t newsize = iv_index_tbl_newsize(&ivup);
-
-        if (RBASIC(obj)->flags & ROBJECT_EMBED) {
-            newptr = obj_ivar_heap_alloc(obj, newsize);
-            MEMCPY(newptr, ptr, VALUE, len);
-            RBASIC(obj)->flags &= ~ROBJECT_EMBED;
-            ROBJECT(obj)->as.heap.ivptr = newptr;
-        }
-        else {
-            newptr = obj_ivar_heap_realloc(obj, len, newsize);
-        }
-
-        for (; len < newsize; len++) {
-            newptr[len] = Qundef;
-        }
-        ROBJECT(obj)->as.heap.numiv = newsize;
-        ROBJECT(obj)->as.heap.iv_index_tbl = ivup.u.iv_index_tbl;
+        rb_init_iv_list(obj, len, newsize, ivup.u.iv_index_tbl);
     }
     RB_OBJ_WRITE(obj, &ROBJECT_IVPTR(obj)[ivup.index], val);
 
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 51b3c40010..f8b2f707ca 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -1213,11 +1213,21 @@ vm_setivar(VALUE obj, ID id, VALUE val, const rb_iseq_t *iseq, IVC ic, const str
 	    VALUE *ptr = ROBJECT_IVPTR(obj);
 	    index = !is_attr ? ic->entry->index : vm_cc_attr_index(cc)-1;
 
-	    if (RB_DEBUG_COUNTER_INC_UNLESS(ivar_set_ic_miss_oorange, index < ROBJECT_NUMIV(obj))) {
+	    if (index < ROBJECT_NUMIV(obj)) {
 		RB_OBJ_WRITE(obj, &ptr[index], val);
 		RB_DEBUG_COUNTER_INC(ivar_set_ic_hit);
 		return val; /* inline cache hit */
-	    }
+	    } else {
+                st_table * iv_idx_tbl = RCLASS_IV_INDEX_TBL(rb_class_real(klass));
+                if (index < iv_idx_tbl->num_entries) {
+                    rb_init_iv_list(obj, ROBJECT_NUMIV(obj), iv_idx_tbl->num_entries, iv_idx_tbl);
+                    ptr = ROBJECT_IVPTR(obj);
+                    RB_OBJ_WRITE(obj, &ptr[index], val);
+                    RB_DEBUG_COUNTER_INC(ivar_set_ic_hit);
+                    return val; /* inline cache hit */
+                }
+            }
+            RB_DEBUG_COUNTER_INC(ivar_set_ic_miss_oorange);
 	}
 	else {
 	    struct st_table *iv_index_tbl = ROBJECT_IV_INDEX_TBL(obj);