New submission from Serhiy Storchaka <[email protected]>:
The optimization is skipped if lnotab contains 255. It was very uncommon in
older versions (only when the function contains very large expressions, larger
than hundreds of lines or bytecode instructions), but in 3.8 this situation is
common.
For example:
[x
for x in a if x]
1 0 BUILD_LIST 0
2 LOAD_FAST 0 (.0)
>> 4 FOR_ITER 12 (to 18)
2 6 STORE_FAST 1 (x)
8 LOAD_FAST 1 (x)
10 POP_JUMP_IF_FALSE 16
1 12 LOAD_FAST 1 (x)
14 LIST_APPEND 2
>> 16 JUMP_ABSOLUTE 4
>> 18 RETURN_VALUE
if x:
if (y and
z):
foo()
else:
bar()
1 0 LOAD_NAME 0 (x)
2 POP_JUMP_IF_FALSE 20
2 4 LOAD_NAME 1 (y)
6 POP_JUMP_IF_FALSE 18
3 8 LOAD_NAME 2 (z)
2 10 POP_JUMP_IF_FALSE 18
4 12 LOAD_NAME 3 (foo)
14 CALL_FUNCTION 0
16 POP_TOP
>> 18 JUMP_FORWARD 6 (to 26)
6 >> 20 LOAD_NAME 4 (bar)
22 CALL_FUNCTION 0
24 POP_TOP
>> 26 LOAD_CONST 0 (None)
28 RETURN_VALUE
You can see non-optimized jumps to jumps (from 10 to 16 and from 6 and 10 to 16
correspondingly).
This is a consequence of two features: ability to encode negative line
differences in lnotab and setting lines for both outer and inner expressions.
Two ways to solve this issue:
1. Move optimizations from Python/peephole.c to Python/compile.c (see
issue32477 and issue33318). This is a new feature and it is too late for 3.8.
2. Make the peepholer to work with lnotab containing 255.
Pablo, are you interesting?
----------
components: Interpreter Core
messages: 345108
nosy: benjamin.peterson, pablogsal, serhiy.storchaka, vstinner, yselivanov
priority: normal
severity: normal
status: open
title: Peeephole optimizer does not optimize functions with multiline
expressions
type: behavior
versions: Python 3.8, Python 3.9
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Python tracker <[email protected]>
<https://bugs.python.org/issue37213>
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