From: Junio C Hamano Date: Wed, 16 Jan 2013 23:46:18 GMT Subject: Re: [PATCH 1/2] fix clang -Wconstant-conversion with bit fields Message-ID: <7vlibsvghh.fsf@alter.siamese.dyndns.org> In-Reply-To: <7vpq14vglu.fsf@alter.siamese.dyndns.org> Junio C Hamano writes: > Antoine Pelisse 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. Double yuck. I meant palatable. In any case, I see somebody reports that more recent clang does not have this bug in the near-by message, so let's forget about this issue. Thanks.