Give meaning to the oparg for BUILD_MAP: estimated size of the dictionary.
Allows dictionaries to be pre-sized (upto 255 elements) saving time lost
to re-sizes with their attendant mallocs and re-insertions.
Has zero effect on small dictionaries (5 elements or fewer), a slight
benefit for dicts upto 22 elements (because they had to resize once
anyway), and more benefit for dicts upto 255 elements (saving multiple
resizes during the build-up and reducing the number of collisions on
the first insertions). Beyond 255 elements, there is no addional benefit.
diff --git a/Python/compile.c b/Python/compile.c
index 3b0c53f..36ad8a4 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -2922,11 +2922,9 @@
case IfExp_kind:
return compiler_ifexp(c, e);
case Dict_kind:
- /* XXX get rid of arg? */
- ADDOP_I(c, BUILD_MAP, 0);
n = asdl_seq_LEN(e->v.Dict.values);
- /* We must arrange things just right for STORE_SUBSCR.
- It wants the stack to look like (value) (dict) (key) */
+ ADDOP_I(c, BUILD_MAP, (n>255 ? 255 : n));
+ n = asdl_seq_LEN(e->v.Dict.values);
for (i = 0; i < n; i++) {
VISIT(c, expr,
(expr_ty)asdl_seq_GET(e->v.Dict.values, i));