[Python-Dev] CACHE opcode in Python 3.11 bytecode

[Matthieu Dartiailh]

Hi all,

I am in the slow process of adding support for Python 3.11 in the bytecode project (https://github.com/MatthieuDartiailh/bytecode).

While attempting to update some tests I stumbled upon the need to include CACHE opcode to get things to work. For example, one can use bytecode to manually assemble the bytecode for the function:

def f():
    return 24 < 42

Under Python 3.10 it would look like:

f.__code__ = Bytecode(

[

    Instr("LOAD_CONST", 24),

        Instr("LOAD_CONST", 42),

        Instr("COMPARE_OP", Compare.LT),

    Instr("RETURN_VALUE"),                    ]

).to_code()

Under Python 3.11 I had to go to:

f.__code__ = Bytecode(

[

    Instr("RESUME", 0), Instr("LOAD_CONST", 24),

        Instr("LOAD_CONST", 42),

        Instr("COMPARE_OP", Compare.LT), Instr("CACHE", 0), Instr("CACHE", 0),

    Instr("RETURN_VALUE"),                    ]

).to_code()

Reading the doc for the dis module I understand the need for the RESUME instruction. However the documentation is rather vague in regard of CACHE.

In particular when using the first version, the code in the function ends up looking like '\x97\x00d\x00d\x01k\x00\x00\x00\x00\x00' even though bytecode generated '\x97\x00d\x00d\x01k\x00S\x00'. One can "see" that the two caches (\x00\x00\x00\x00) have been added automatically but the return disappeared. Is this a bug in 3.11 and if not where can I find more details regarding where one expect CACHE instructions to be present ?

Best

Matthieu C. Dartiailh

PS: I know the mailing list is going to be retired but I did not yet got everything configured for Discourse.

[Jeremiah Gabriel Pascual]

The CACHE opcode is a new 3.11+ opcode which is kind of a NOP but is used by some other opcodes to store cache information for specialization. A map of <opcode number> -> <number of CACHEs needed> is in `dis._inline_cache_entries` if that helps with this.
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