A warning message caused by unused variable in non-debug mode

[gaogaotiantian]

Bug report

Bug description:

A compiler warning was introduced by 15309329b65a285cb7b3071f0f08ac964b61411b in non-debug build.

static void
completed_cycle(GCState *gcstate)
{
    PyGC_Head *not_visited = &gcstate->old[gcstate->visited_space^1].head;
    assert(gc_list_is_empty(not_visited));
    gcstate->visited_space = flip_old_space(gcstate->visited_space);
    if (gcstate->work_to_do > 0) {
        gcstate->work_to_do = 0;
    }
}

not_visited is only used in assert which will be optimized out in non debug build, thus caused the warning of unused variable. We can

• leave the warning to be (seems bad and unprecedented)
• throw out the variable and put the value into assert directly (less readable but maybe acceptable?)
• use NDEBUG to protect the variable declaration (easier to understand, but messier code)
• use some compiler specific annotation for the variable (may not be portable?)

I don't see the perfect solution here so maybe @markshannon can call the decision?

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

[AlexWaygood]

Does #117112 fix this?

[gaogaotiantian]

Ah, yes. That's option 2.

[AlexWaygood]

Fixed in https://github.com/python/cpython/commit/e28477f214276db941e715eebc8cdfb96c1207d9, I believe (please reopen if I'm incorrect!)