Viper: Unary minus changes rhs variable in place, even if assigned to new lhs variable.

[rkompass]

Checks

• I agree to follow the MicroPython Code of Conduct to ensure a safe and respectful space for everyone.

• I've searched for existing issues matching this bug, and didn't find any.

Port, board and/or hardware

w600, RP2, others

MicroPython version

latest, e.g.: MicroPython v1.23.0-preview.344.gb1ac266bb on 2024-04-29; Raspberry Pi Pico W with RP2040

Reproduction

@micropython.viper
def vip_unaryminus() -> int:
    a = 5
    b = -a          
    return a+b

print(vip_unaryminus())   # -10  <---- error, should be 0

@micropython.viper
def vip_tuple_um():
    a:int = int(-5)
    b = -a          
    return (a, b)

print(vip_tuple_um())    # (5, 5)  <---- error, should be (-5, 5)

Expected behaviour

0            # as 5 and -5 cancel out, so 5 + (-5) = 0
(-5, 5)

Observed behaviour

-10
(5, 5)

Additional Information

No, I've provided everything above.
My interpretation of this errorneaous behaviour is in the issue heading.

Thank you, btw., for improving viper permanently.

[rkompass]

After having a look at 3c445f6 I suspected that unary invert would exhibit the same error and so it does:

@micropython.viper
def vip_invert() -> int:
    a = 5
    b = ~a          
    return a+b

print(vip_invert())    # -12  <---- error, should be -1

@micropython.viper
def vip_tuple_inv():
    a:int = int(5)
    b = ~a          
    return (a, b)

print(vip_tuple_inv())    # (-6, -6)  <---- error, should be (5, -6)

Looking at the PR I suspect that the logic

int reg = REG_RET;
emit_pre_pop_reg_flexible(emit, &vtype, &reg, reg, reg);
ASM_OPERATION_REG(emit->as, reg);
emit_post_push_reg(emit, vtype, reg);

emits code to change the register in place, which only works if the register is not used anymore.

The right approach would be to defer the register modification to happen with the lhs register after introducing it when doing the the assignment.