------- Comment #40 from matz at suse dot de 2006-01-23 11:21 -------
Mark, re your comment #38: (my comment #39 actually came before, but I forgot
to press "Commit" :-/ ) the #pragma pack(1) does not influence DECL_PACKED.
It is only set by attribute(packed). That's why the difference of behaviour
between a struct under #pragma pack(1) vs. a struct with an attribute packed
occurs.
I agree that it conceptually makes sense to implicitely have a zero-width
bit-field never be DECL_PACKED (though this would deviate from pre-3.4).
The ugly thing is, that current code in GCC seems to handle exactly this case
differently. For instance in place_field(), we have this code:
if (PCC_BITFIELD_TYPE_MATTERS
&& ! targetm.ms_bitfield_layout_p (rli->t)
&& TREE_CODE (field) == FIELD_DECL
&& type != error_mark_node
&& DECL_BIT_FIELD (field)
&& ! DECL_PACKED (field)
&& maximum_field_alignment == 0
&& ! integer_zerop (DECL_SIZE (field))
....
the body contains a call to ADJUST_FIELD_ALIGN then. So if this is a
zero-sized bitfield, then this ADJUST won't be done no matter what DECL_PACKED
is, and it seems that this is wanted here (in difference to the other place
in layout_decl, where zero-sized bitfield simply weren't handled). The
comment above this code says that it's purpose is compatibility with
PCC, so perhaps struct with zero-sized bitfields weren't handled at all
by PCC, and this is a non-issue. I don't know.
Otherwise it might be, that both places need to be handled the same way
to not risk inconsistencies. It currently looks as if it also isn't
handled consistently right now (another call to ADJUST_... misses to
test DECL_PACKED at all for instance). Double-sigh.
Anyway, I'll attach my current patch which implements the suggested
behaviour, including zero-bitfield == !DECL_PACKED in layout_decl.
And also a small testprogram showing information about different struct
under different settings. It shows inconsistencies in 3.3, and with
the patch 4.1 is more consistent (plus the case in wine, namely of using
#pragma pack(1) still does the same as pre-3.4).
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22275
