Dear all,
I would like to learn about constant folding optimisation in Python. It seems
to be implemented in Python/ast_opt.c. In order to get impression I used
python3 and dis module:
$ python3 -V
Python 3.7.3
Arithmetics expression is folded as expected:
>>> dis.dis(compile('1 * 2', filename='<string>', mode='eval',
>>> optimize=2))
1 0 LOAD_CONST 0 (2)
2 RETURN_VALUE
On the contrary, comparison remains for runtime:
>>> dis.dis(compile('1 < 2', filename='<string>', mode='eval',
>>> optimize=2))
1 0 LOAD_CONST 0 (1)
2 LOAD_CONST 1 (2)
4 COMPARE_OP 0 (<)
6 RETURN_VALUE
In function fold_unaryop (though comparison is a binary operation) in
Python/ast_opt.c is remark: /* Fold not into comparison */
Is there a reason why comparison (== != < > <= >=) is not folded? I would
expect it handled in fold_binop.
Besides comparison there are other expressions that might be optimized out
due to constant expression. Ternary operator with constant condition True
has IF optimized out (just some dead code remains):
>>> dis.dis(compile('"a" if True else "b"', filename='<string>',
>>> mode='eval', optimize=2))
1 0 LOAD_CONST 1 ('a')
2 RETURN_VALUE
4 LOAD_CONST 2 ('b')
6 RETURN_VALUE
On the contrary, the same ternary operator with constant condition False
still loads the constat and contains POP_JUMP_IF_FALSE:
>>> dis.dis(compile('"a" if False else "b"', filename='<string>',
>>> mode='eval', optimize=2))
1 0 LOAD_CONST 0 (False)
2 POP_JUMP_IF_FALSE 8
4 LOAD_CONST 1 ('a')
6 RETURN_VALUE
>> 8 LOAD_CONST 2 ('b')
10 RETURN_VALUE
Any ideas or links, please?
Best regards,
Davis
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