constant folding - why not more

[Barry Scott]

> On 10 Nov 2020, at 14:45, David Kolovratník <david at kolovratnik.net> wrote:
> 
> 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

I do not have answers to your questions, but I would suggest that you look at 3.9
or even 3.10a2 to see if this is still the case.

Barry

> 
> 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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