From: Junio C Hamano <gitster@pobox.com>
Date: Wed, 16 Jan 2013 23:43:41 GMT
Subject: Re: [PATCH 1/2] fix clang -Wconstant-conversion with bit fields
Message-ID: <7vpq14vglu.fsf@alter.siamese.dyndns.org>
In-Reply-To: <1358376443-7404-1-git-send-email-apelisse@gmail.com>

Antoine Pelisse <apelisse@gmail.com> writes:

> clang incorrectly reports a constant conversion warning (implicit
> truncation to bit field) when using the "flag &= ~FLAG" form, because
> ~FLAG needs to be truncated.
>
> Convert this form to "flag = flag & ~FLAG" fixes the issue as
> the right operand now fits into the bit field.

If the "clang incorrectly reports" is already recognised by clang
folks as a bug to be fixed in clang, I'd rather not to take this
patch.

I do not think it is reasonable to expect people to remember that
they have to write "flags &= ~TO_DROP" in a longhand whenever they
are adding new code that needs to do bit-fields, so even if this
patch makes clang silent for the _current_ code, it will not stay
that way.  Something like

#define FLIP_BIT_CLR(fld,bit) do { \
	typeof(fld) *x = &(fld); \
        *x = *x & (~(bit)); \
} while (0)

may be more palapable but not by a large margin.

Yuck.