[Bug middle-end/39514] [4.4 Regression] unreported change to packed bitfields

["ebotcazou at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #7 from ebotcazou at gcc dot gnu dot org  2009-03-23 11:47 -------
> I'd like to see this message, which is on by default, for the change
> introduced in r132614.  I haven't figured out how to determine if the offset
> for a field has changed, just the alignment.  I'll attach my current patch,
> where the message should be "Perhaps the alignment of packed bit-field <x>
> has changed in GCC 4.4", which is not terribly useful.

I think that's better (without the "perhaps", the alignment is always changed)
than "Offset" because this also can change the padding of the structure:

struct s
{
  struct
  {
    __attribute__ ((aligned (2))) int d:32;
  } __attribute__ ((packed)) c;
  unsigned short int e;
};

yields (4, 8) with 4.3.3 and (4, 6) with mainline.  This apparently will be so
uncommon in C/C++ (packed bit-field with alignment attribute) that I'm not sure
sure we need to tune it further.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39514