bpo-45843: Optimize constant comparisons/contains

[thatbirdguythatuknownot]

https://bugs.python.org/issue45843

[Fidget-Spinner]

Thanks for the really impressive compiler work.

I hope I understand the code correctly. It's effectively doing constant folding for comparison expressions where both sides are consts right? So 1<2 goes from

LOAD_CONST 1
LOAD_CONST 2
COMPARE_OP 0

to

LOAD_CONST 0 (True)

My question is how common do people write things like 1<2 in production code? Constant folding for things like 10*64 make sense for readability, but I don't see how 1<2 is useful.

[markshannon]

I was surprised that this isn't handled by the AST optimizer.
In fact there is "to do" for precisely this:
https://github.com/python/cpython/blob/main/Python/ast_opt.c#L621

@thatbirdguythatuknownot could you implement this in the AST optimizer?

[markshannon]

I suspect that no one would write if 1 < 2:, but there is an expectation that Python will fold constants and that the body of

if 0:
     sensitive_internal_debugging_code

is removed from the bytecode.

So this is probably worth doing from a consistency point of view, even if it makes no difference in terms of performance.

[thatbirdguythatuknownot]

I suspect that no one would write if 1 < 2:, but there is an expectation that Python will fold constants and that the body of

if 0:
     sensitive_internal_debugging_code

is removed from the bytecode.

So this is probably worth doing from a consistency point of view, even if it makes no difference in terms of performance.

@markshannon Should I revert the changes made to Python/compiler.c? I have made changes to Python/ast_opt.c.

[markshannon]

I don't understand why you need all the extra code.

All you need to do in fold_compare is check that both sides of the comparisons are constants and perform the comparison.
See fold_subscr for an example.

[thatbirdguythatuknownot]

I don't understand why you need all the extra code.

All you need to do in fold_compare is check that both sides of the comparisons are constants and perform the comparison. See fold_subscr for an example.

@markshannon Do I return if there is a comparison that does not have a constant? Otherwise, I'm handling it with the rest of the constant comparisons.

[isidentical]

Closing since this issue got rejected on the bug tracker.