------- Comment #33 from mark at codesourcery dot com 2006-01-19 16:59 ------- Subject: Re: [3.4/4.0/4.1/4.2 Regression] bitfield layout change (regression?)
matz at suse dot de wrote: > ------- Comment #32 from matz at suse dot de 2006-01-19 14:44 ------- > Mark, I agree that it's saner when both structures (with #pragma pack and > attribute packed) have the same length of 8 on i686 and x86_64 (because > the bitfield was declared 'int' in difference to 'long' for instance). > > Then I have a question to clarify if I understood correctly: by remembering > the original maximum_field_alignment and using that for zero-sized bitfields > you want to use the absolute first, default, m_f_a, or the one last set > before the innermost #pragma pack? Consider an example like so, and lets > assume the initial max field alignment was 4: > > mfa == 4 > #pragma pack(2) // 1 mfa == 2 > #pragma pack(1) // 2 mfa == 1 > #pragma pack() // 3 mfa == 4 > #pragma pack (push,2) // 4 mfa == 2 > #pragma pack (push,1) // 5 mfa == 1 > #pragma pop // 6 mfa == 2 > #pragma pop // 7 mfa == 4 > > With what would you constrain the alignment of a zero sized bitfield > at each of the seven points? I'm suggesting that a zero-width bitfield have the same alignment influence at all 7 points. In your example, mfa would be effectively four at all 7 points. > What if the initial mfa is 0 (i.e. not set)? Then, it should be zero at all seven points, meaning that a zero-width bitfield of type T would force alignment to whatever alignment a field of type T would have. > Should -fpack-struct=... (which influences the initial mfa) influence > that constraint too, or not? That's harder, but I think it should. The point is basically that, independent of #pragma pack, a zero-width bitfield should give you the alignment it would give you oustide of the #pragma packs. > My opinion is, that at each of the seven points above we should constrain > with the initial mfa (i.e. 4 in the example above), as adjusted by > the -fpack-struct command line option. Good, we agree! > Another point: If we make the structure with attribute packed on both x86 and > x86-64 be eight long (to agree with the behaviour of using pragma), then we > do add another variant unfortunately. In pre 3.4 that structure was 12 on > x86-64 (which I think was an actual error). That's unfortunate, but given that things are moving around, I think we should go for maximum sanity. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22275
