------- Comment #29 from mark at codesourcery dot com 2006-01-18 23:00 ------- Subject: Re: [3.4/4.0/4.1/4.2 Regression] bitfield layout change (regression?)
I think that we should do as follows. Preserve the original value of maximum_field_alignment when doing #pragma pack. Then, for zero-width bitfields, we should align to the minimum of the original maximum_field_alignment and the otherwise natural alignment. The difference between this and the last proposed patch is that I don't think we should entirely ignore maximum_field_alignment for zero-width bitfields; if "long long" as a field will only have (say) 2-byte alignment on some embedded target where structure-packing is the default, then a "long long : 0;" bitfield should only force 4-byte alignment. However, that's an abstract argument; I'm not actually sure what existing practice was with older versions of GCC. Again, in the abstract, I think the example in Comment #12 ought to have size 8 on both IA32 and AMD64 architectures. I can't see any good justification for size 12, with a PCC_BITFIELD_TYPES_MATTER ABI. And, I think that the size of the structure with #pragma pack(1) ought to be the same as with __attribute__((packed)). So, my concern with the patch in Comment #12 is that it would ignore the pre-set maximum_field_alignment on targets with default structure packing; other than that, I think it looks fine. -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22275
