Hi,
> when expanding code for
> struct a {short a,b,c,d;} *a;
> a->c;
>
> we first produce correct BLKmode MEM rtx representing the whole
> structure *a,
> then we use adjust_address to turn it into HImode MEM and finally
> extract_bitfield is used to offset it by 32 bits to get correct value.
I tried to create a test case as follows and stepped thru the code.
cat test.c
struct a {short a,b,c,d;};
void foo (struct a *a)
{
a->c = 1;
}
First I get a DImode MEM rtx not BLKmode:
(mem:DI (reg/f:DI 87) [2 *a_2(D)+0 S8 A16])
and adjust_address does not clear the MEM_EXPR
thus to_rtx = adjust_address (to_rtx, mode1, 0) returns:
(mem:HI (reg/f:DI 87) [2 *a_2(D)+0 S2 A16])
and then set_mem_attributes_minus_bitpos (to_rtx, to, 0, bitpos) does:
(mem:HI (reg/f:DI 87) [3 a_2(D)->c+-4 S6 A16])
which looks perfectly OK.
But with your patch the RTX looks different:
(mem:DI (reg/f:DI 87) [3 a_2(D)->c+-4 S6 A16])
which looks inconsistent, and not like an improvement.
Especially when the mode of the target_rtx may be used
somewhere in store_field:
/* If TEMP is not a PARALLEL (see below) and its mode and that of
TARGET
are both BLKmode, both must be in memory and BITPOS must be
aligned
on a byte boundary. If so, we simply do a block copy.
Likewise for
a BLKmode-like TARGET. */
if (GET_CODE (temp) != PARALLEL
&& GET_MODE (temp) == BLKmode
|| (MEM_P (target)
&& GET_MODE_CLASS (GET_MODE (target)) == MODE_INT
and I see store_fixed_bit_field also looks at the memory mode:
if (MEM_P (op0))
{
machine_mode mode = GET_MODE (op0);
if (GET_MODE_BITSIZE (mode) == 0
|| GET_MODE_BITSIZE (mode) > GET_MODE_BITSIZE (word_mode))
mode = word_mode;
mode = get_best_mode (bitsize, bitnum, bitregion_start,
bitregion_end,
MEM_ALIGN (op0), mode, MEM_VOLATILE_P (op0));
Bernd.
