New submission from Ned Batchelder <n...@nedbatchelder.com>:
CPython 3.8.0b2 behaves differently than previous releases: it no longer
optimizes away "if 0:" branches. This is something that has been done since at
least 2.4. It was done in 3.8.0b1, but no longer is.
Was this a purposeful change? Why? It seems like 3.8 is adding more
optimizations, why was this removed?
Test code (/tmp/no0.py):
import dis
import sys
print(sys.version)
def with_if_0():
print(1)
if 0:
print(2)
print(3)
dis.dis(with_if_0)
$ /usr/local/pythonz/pythons/CPython-3.8.0b2/bin/python3.8 /tmp/no0.py
3.8.0b2 (default, Jul 4 2019, 22:38:04)
[Clang 10.0.0 (clang-1000.10.44.4)]
7 0 LOAD_GLOBAL 0 (print)
2 LOAD_CONST 1 (1)
4 CALL_FUNCTION 1
6 POP_TOP
8 8 LOAD_CONST 2 (0)
10 POP_JUMP_IF_FALSE 20
9 12 LOAD_GLOBAL 0 (print)
14 LOAD_CONST 3 (2)
16 CALL_FUNCTION 1
18 POP_TOP
10 >> 20 LOAD_GLOBAL 0 (print)
22 LOAD_CONST 4 (3)
24 CALL_FUNCTION 1
26 POP_TOP
28 LOAD_CONST 0 (None)
30 RETURN_VALUE
$ /usr/local/pythonz/pythons/CPython-3.8.0b1/bin/python3.8 /tmp/no0.py
3.8.0b1 (default, Jun 4 2019, 21:21:14)
[Clang 10.0.0 (clang-1000.10.44.4)]
7 0 LOAD_GLOBAL 0 (print)
2 LOAD_CONST 1 (1)
4 CALL_FUNCTION 1
6 POP_TOP
10 8 LOAD_GLOBAL 0 (print)
10 LOAD_CONST 4 (3)
12 CALL_FUNCTION 1
14 POP_TOP
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
$ python3.7 /tmp/no0.py
3.7.3 (default, Apr 10 2019, 10:27:53)
[Clang 10.0.0 (clang-1000.10.44.4)]
7 0 LOAD_GLOBAL 0 (print)
2 LOAD_CONST 1 (1)
4 CALL_FUNCTION 1
6 POP_TOP
10 8 LOAD_GLOBAL 0 (print)
10 LOAD_CONST 2 (3)
12 CALL_FUNCTION 1
14 POP_TOP
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
$ python2.7 /tmp/no0.py
2.7.14 (default, Oct 4 2017, 09:45:53)
[GCC 4.2.1 Compatible Apple LLVM 9.0.0 (clang-900.0.37)]
7 0 LOAD_CONST 1 (1)
3 PRINT_ITEM
4 PRINT_NEWLINE
10 5 LOAD_CONST 2 (3)
8 PRINT_ITEM
9 PRINT_NEWLINE
10 LOAD_CONST 0 (None)
13 RETURN_VALUE
$ python2.4 /tmp/no0.py
2.4.6 (#1, Jun 18 2016, 17:49:14)
[GCC 4.2.1 Compatible Apple LLVM 7.0.2 (clang-700.1.81)]
7 0 LOAD_CONST 1 (1)
3 PRINT_ITEM
4 PRINT_NEWLINE
10 5 LOAD_CONST 2 (3)
8 PRINT_ITEM
9 PRINT_NEWLINE
10 LOAD_CONST 0 (None)
13 RETURN_VALUE
----------
keywords: 3.8regression
messages: 347296
nosy: nedbat
priority: normal
severity: normal
status: open
title: 3.8.0b2 no longer optimizes away "if 0:" ?
versions: Python 3.8
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue37500>
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