Re: [PATCH 1/4] merge-recursive: silence -Wxor-used-as-pow warning

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 25, 2020, 17:27 UTC
Message-ID: <xmqq4kwjcupj.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20200125053723.GA744673@coredump.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 27 quoted lines

> The merge-recursive code uses stage number constants like this:
>
>   add = &ci->ren1->dst_entry->stages[2 ^ 1];
>   ...
>   add = &ci->ren2->dst_entry->stages[3 ^ 1];
>
> The xor has the effect of flipping the "1" bit, so that "2 ^ 1" becomes
> "3" and "3 ^ 1" becomes "2", which correspond to the "ours" and "theirs"
> stages respectively.
>
> Unfortunately, clang-10 and up issue a warning for this code:
>
>   merge-recursive.c:1759:40: error: result of '2 ^ 1' is 3; did you mean '1 << 1' (2)? [-Werror,-Wxor-used-as-pow]
>                   add = &ci->ren1->dst_entry->stages[2 ^ 1];
>                                                      ~~^~~
>                                                      1 << 1
>   merge-recursive.c:1759:40: note: replace expression with '0x2 ^ 1' to silence this warning
>
> We could silence it by using 0x2, as the compiler mentions. Or by just
> using the constants "2" and "3" directly. But after digging into it, I
> do think this bit-flip is telling us something. If we just wrote:
>
>   add = &ci->ren2->dst_entry->stages[2];
>
> for the second one, you might think that "ren2" and "2" correspond. But
> they don't. The logic is: ren2 is theirs, which is stage 3, but we
> are interested in the opposite side's stage, so flip it to 2.

So, the logical name for "^1" operator applied to 2 (ours) and 3 (theirs) is "the_other_side()"? the_other_side(theirs) == ours.

I would have written (5 - side) instead of (side ^ 1) if I were writing this, though.

> So let's keep the bit-flipping, but let's also put it behind a named
> function, which will make its purpose a bit clearer. This also has the
> side effect of suppressing the warning (and an optimizing compiler
> should be able to easily turn it into a constant as before).

OK. Now I see you named it flip_stage(), which is even better than "the-other-side" above. Makes sense.

I still think ((2 + 3) - two_or_three_to_be_flipped) easier to reason about than the bit flipping, as the implementation detail, though.

Thanks.