Patches item #1764087, was opened at 2007-07-30 18:06
Message generated for change (Comment added) made by rhettinger
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Category: Parser/Compiler
Group: Python 2.6
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: Paul Pogonyshev (doublep)
>Assigned to: Raymond Hettinger (rhettinger)
Summary: tiny addition to peephole optimizer
Initial Comment:
Tiny optimization that replaces Replace STORE_FAST x LOAD_FAST x with DUP_TOP
STORE_FAST x (for the same slot x). Targeted at quite common case:
foo = self.foo
if foo > 0:
...
No regression, seems to do what it is supposed to do on several test cases.
However, there is drawback:
def f(x):
y = x.y
if y is not None:
print y
is disassembled as follows:
2 0 LOAD_FAST 0 (x)
3 LOAD_ATTR 0 (y)
6 DUP_TOP
3 7 STORE_FAST 1 (y)
10 LOAD_CONST 0 (None)
13 COMPARE_OP 9 (is not)
16 JUMP_IF_FALSE 9 (to 28)
19 POP_TOP
4 20 LOAD_FAST 1 (y)
23 PRINT_ITEM
24 PRINT_NEWLINE
25 JUMP_FORWARD 1 (to 29)
>> 28 POP_TOP
>> 29 LOAD_CONST 0 (None)
32 RETURN_VALUE
Note that STORE_FAST "migrated" to another line. I'm not sure how important
that is.
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>Comment By: Raymond Hettinger (rhettinger)
Date: 2007-07-31 01:58
Message:
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I'll look at this one again. It was tried once and rejected for some
reason that I've since forgotten (lack of measurable speed-up, interference
with some other optimization, or some such).
----------------------------------------------------------------------
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