On the IA64, the following record,
typedef struct sptr_struct
{
long unsigned int phase: 48;
short unsigned int thread: 16;
void *addr;
} sptr_t;
is assigned a BLKmode rather a TImode, and I was wondering whether
this is a requirement of the IA64 ABI, or a coincidental result of
various target configuration defintions?
The final determination of the mode assigned to this struct is
made in compute_record_mode(). The logic first tentatively assigns
a TImode (128 bits) as expected, in the second branch of this if
statement (GCC version 3.3.2):
/* If we only have one real field; use its mode. This only applies to
RECORD_TYPE. This does not apply to unions. */
if (TREE_CODE (type) == RECORD_TYPE && mode != VOIDmode)
TYPE_MODE (type) = mode;
else
TYPE_MODE (type) = mode_for_size_tree (TYPE_SIZE (type), MODE_INT, 1);
and then reverses that decision in the subsequent if statement:
/* If structure's known alignment is less than what the scalar
mode would need, and it matters, then stick with BLKmode. */
if (TYPE_MODE (type) != BLKmode
&& STRICT_ALIGNMENT
&& ! (TYPE_ALIGN (type) >= BIGGEST_ALIGNMENT
|| TYPE_ALIGN (type) >= GET_MODE_ALIGNMENT (TYPE_MODE (type))))
{
/* If this is the only reason this type is BLKmode, then
don't force containing types to be BLKmode. */
TYPE_NO_FORCE_BLK (type) = 1;
TYPE_MODE (type) = BLKmode;
}
primarily because STRICT_ALIGNMENT is asserted, and BIGGEST_ALIGNMENT is 128 in
config/ia64/ia64.h:
#define STRICT_ALIGNMENT 1
/* Optional x86 80-bit float, quad-precision 128-bit float, and quad-word
128 bit integers all require 128 bit alignment. */
#define BIGGEST_ALIGNMENT 128
And this configuration parameter in config/ia64/ia64.h may also have led
to the decision to force 64 bit alignment for this structure (this is asserted
on most targets):
/* Define this if you wish to imitate the way many other C compilers handle
alignment of bitfields and the structures that contain them.
The behavior is that the type written for a bit-field (`int', `short', or
other integer type) imposes an alignment for the entire structure, as if the
structure really did contain an ordinary field of that type. In addition,
the bit-field is placed within the structure so that it would fit within such
a field, not crossing a boundary for it. */
#define PCC_BITFIELD_TYPE_MATTERS 1
--------------------
Question: If we assume that a TImode would've been a more efficient mode
to represent the record type above, would it not have been acceptable for
the compiler to promote the alignment of this type to 128, given there
are no apparent restrictions otherwise, or are there other C conventions
at work that dictate otherwise? Is there a configuration tweak that
would've led to using TImode rather than BLKmode?
