New submission from Neal Norwitz:
Optimize list comprehensions by using the list on the stack, rather than
storing in a (local/global) variable. This reduces the size of the
stack by one pointer, reduces the bytecode size by 8, and reduces the
iterations in the eval loop by 2 + # of iterations to create the new
list. It also eliminates internal variables in varnames.
List comps in module/class scope execute faster by avoiding more costly
dict lookups in globals.
For this source code:
def f(): return [x for x in s]
Byte code before change:
1 0 BUILD_LIST 0
3 DUP_TOP
4 STORE_FAST 0 (_[1])
7 LOAD_GLOBAL 1 (s)
10 GET_ITER
>> 11 FOR_ITER 13 (to 27)
14 STORE_FAST 1 (x)
17 LOAD_FAST 0 (_[1])
20 LOAD_FAST 1 (x)
23 LIST_APPEND
24 JUMP_ABSOLUTE 11
>> 27 DELETE_FAST 0 (_[1])
30 RETURN_VALUE
New byte code:
1 0 BUILD_LIST 0
3 LOAD_GLOBAL 0 (s)
6 GET_ITER
>> 7 FOR_ITER 12 (to 22)
10 STORE_FAST 0 (x)
13 LOAD_FAST 0 (x)
16 LIST_APPEND 2
19 JUMP_ABSOLUTE 7
>> 22 RETURN_VALUE
DUP_TOP/STORE_FAST are eliminated before the loop. One LOAD_FAST is
eliminated inside the loop. LIST_APPEND is changed to reference the
value on the stack. Is it a problem to change the opcode of LIST_APPEND?
This might make debugging harder. I'm not sure how debuggers work with
list comps.
A similar optimization could probably be done to eliminate all uses of
the temporary variables (WITH_CLEANUP at least).
This patch still needs to update docs and the compiler package
implemented in Python.
----------
components: Interpreter Core
files: list-comp-opt2.patch
keywords: patch, patch
messages: 62961
nosy: nnorwitz
severity: normal
status: open
title: optimize list comprehensions
type: resource usage
versions: Python 2.6
Added file: http://bugs.python.org/file9545/list-comp-opt2.patch
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Tracker <[EMAIL PROTECTED]>
<http://bugs.python.org/issue2183>
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