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Give meaning to the oparg for BUILD_MAP: estimated size of the dictionary.

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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.
This commit is contained in:
Raymond Hettinger 2007-12-18 21:24:09 +00:00
parent 3c887b2802
commit fd7ed407d7
6 changed files with 26 additions and 6 deletions
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6
Python/compile.c
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@ -2922,11 +2922,9 @@ compiler_visit_expr(struct compiler *c, expr_ty e)
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));