gh-132732: Automatically constant evaluate pure operations

[Fidget-Spinner]

• Issue: Constant evaluate/propagate pure ops automatically #132732

[python-cla-bot]

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[brandtbucher]

This is really neat!

Other than two opcodes I found that shouldn't be marked pure, I just have one thought:

Rather than rewriting the bodies like this to use the symbols-manipulating functions (which seems error-prone), would we be able to just use stackrefs to do this?

For example, _BINARY_OP_ADD_INT is defined like this:

PyObject *left_o = PyStackRef_AsPyObjectBorrow(left);
PyObject *right_o = PyStackRef_AsPyObjectBorrow(right);
// ...
res = PyStackRef_FromPyObjectSteal(res_o);

Rather than rewriting uses of these functions, could it be easier to just do something like this, since we're guranteed not to escape?

if (sym_is_const(ctx, stack_pointer[-2]) && sym_is_const(ctx, stack_pointer[-1])) {
    // Generated code to turn constant symbols into stackrefs:
    _PyStackRef left = PyStackRef_FromPyObjectBorrow(sym_get_const(ctx, stack_pointer[-2]));
    _PyStackRef right = PyStackRef_FromPyObjectBorrow(sym_get_const(ctx, stack_pointer[-1]));
    _PyStackRef res;
    // Now the actual body, same as it appears in executor_cases.c.h:
    PyObject *left_o = PyStackRef_AsPyObjectBorrow(left);
    PyObject *right_o = PyStackRef_AsPyObjectBorrow(right);
    // ...
    res = PyStackRef_FromPyObjectSteal(res_o);
    // Generated code to turn stackrefs into constant symbols:
    stack_pointer[-1] = sym_new_const(ctx, PyStackRef_AsPyObjectSteal(res));
}

I'm not too familiar with the design of the cases generator though, so maybe this is way harder or something. Either way, I'm excited to see this get in!

[brandtbucher]

Why did you un-annotate these three float ops?

[Fidget-Spinner]

They use the strange _PyFloat_FromDouble_ConsumeInputs which take stackrefs.

Should be fixed with your suggestion.

[Fidget-Spinner]

Rather than rewriting uses of these functions, could it be easier to just do something like this, since we're guranteed not to escape?

Seems feasible. I could try to rewrite all occurences of the variable with a stackref-producing const one. Let me try that.

[Fidget-Spinner]

I've verified no refleak on test_capi.test_opt locally apart from #132731 which is pre-existing.