https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63495
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2014-10-09
CC| |jakub at gcc dot gnu.org,
| |jsm28 at gcc dot gnu.org
Ever confirmed|0 |1
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
This started with r205685.
The problem is in the x86 32-bit ABI, which decreases alignment of fields to 32
bits if the field has integral mode, or DFmode/DCmode or complex integral mode.
int
x86_field_alignment (tree field, int computed)
{
enum machine_mode mode;
tree type = TREE_TYPE (field);
if (TARGET_64BIT || TARGET_ALIGN_DOUBLE)
return computed;
mode = TYPE_MODE (strip_array_types (type));
if (mode == DFmode || mode == DCmode
|| GET_MODE_CLASS (mode) == MODE_INT
|| GET_MODE_CLASS (mode) == MODE_COMPLEX_INT)
return MIN (32, computed);
return computed;
}
When the struct has only a single field, it is QImode, therefore despite the
user forced alignment the alignment is lowered to 32 bits.
Now, when actually laying out struct s fields, such as:
struct T { char a; struct s s; } t = { 1, { 2 } };
it actually is given 64-bit alignment, because ADJUST_FIELD_ALIGN is not used
in that case:
if (! packed_p && ! DECL_USER_ALIGN (decl))
{
/* Some targets (i.e. i386, VMS) limit struct field alignment
to a lower boundary than alignment of variables unless
it was overridden by attribute aligned. */
#ifdef BIGGEST_FIELD_ALIGNMENT
DECL_ALIGN (decl)
= MIN (DECL_ALIGN (decl), (unsigned) BIGGEST_FIELD_ALIGNMENT);
#endif
#ifdef ADJUST_FIELD_ALIGN
DECL_ALIGN (decl) = ADJUST_FIELD_ALIGN (decl, DECL_ALIGN (decl));
#endif
}
As do_type_align for FIELD_DECLs sets DECL_USER_ALIGN:
static inline void
do_type_align (tree type, tree decl)
{
if (TYPE_ALIGN (type) > DECL_ALIGN (decl))
{
DECL_ALIGN (decl) = TYPE_ALIGN (type);
if (TREE_CODE (decl) == FIELD_DECL)
DECL_USER_ALIGN (decl) = TYPE_USER_ALIGN (type);
}
}
if TYPE_USER_ALIGN is set, IMHO ADJUST_FIELD_ALIGN and corresponding
BIGGEST_FIELD_ALIGNMENT will never be used.
