Eliminate constant inputs and output constants for simple operations

[Fidget-Spinner]

• BINARY_OP Constant propagation for _BINARY_OP #115758
• TO_BOOL Constant propagation for _TO_BOOL #115759

Linked PRs

• GH-115506: Improve handling of constants in tier two #124809

[gvanrossum]

A little more detail would be helpful. Even just an example.

[Fidget-Spinner]

Here's the design:

At every point where we propagate constants, we also try to eliminate the last n stack input entries.
E.g. for _BINARY_OP_ADD_INT, we eliminate left and right. To create something robust, we shall autogenerate the net stack effects of all opcodes. The algorithm is as follows:

int zap_all_constants() {
    // zap_all_constants
    // We probably don't want to store non-immortal constants, as they complicate reference counting.
    if (!_PyObject_IsImmortal(const_val) {
        return 0;
    }
    stack_effect = -stack_inputs(opcode);
    _PyUopInstruction *start = curr_instr;
    while (op_is_pure(start) && stack_effect > 0) {
        stack_effect += net_stack_effect(start);
        start--;
    }
    if (stack_effect != 0) {
        return 0;
    }
    for (; start < curr_instr; start++) {
        REPLACE_OP(start, _NOP, 0, 0);
    }
    return 1;
}

// Do this outside, because some ops have more than one stack output (e.g. a PUSH_NULL):
if (zap_all_constants(...)) {
    REPLACE_OP(curr_instr, _LOAD_CONST_INLINE_BORROW, 0, const_val);
}

This also means we will never overshoot the buffer, because anything that requires the old _SHRINK_STACK won't be const propagated.

[markshannon]

What would the code in the redundancy elimination pass for _BINARY_OP_ADD_INT actually look like?

[markshannon]

Can you make seperate issues for BINARY_OP, which is quite complex, and TO_BOOL, which is very simple in comparison?