>>
>> hmm...
>> the above change is just not aggressive enough.
>>
>>
>> with a slightly changed test case I have now got a
>> recursion on the extract_fixed_bit_fields on ARM but
>> interestingly not on powerpc:
>>
>> #include <stdlib.h>
>>
>> typedef struct {
>> char pad;
>> int arr[0];
>> } __attribute__((packed)) str;
>>
>> str *
>> foo (int* src)
>> {
>> str *s = malloc (sizeof (str) + sizeof (int));
>> *src = s->arr[0];
>> s->arr[0] = 0x12345678;
>> return s;
>> }
>>
>> Now I think about reverting that hunk back to what I had in mind initially:
>>
>> else if (MEM_P (str_rtx)
>> && GET_MODE_BITSIZE (GET_MODE (str_rtx)) != 0
>> && GET_MODE_BITSIZE (GET_MODE (str_rtx)) < GET_MODE_BITSIZE (word_mode)
>> && bitnum % GET_MODE_BITSIZE (GET_MODE (str_rtx)) + bitsize
>>> GET_MODE_BITSIZE (GET_MODE (str_rtx)))
>> /* If the mode of str_rtx is too small or unaligned
>> fall back to word mode for subsequent logic. */
>> str_rtx = adjust_address (str_rtx, word_mode, 0);
>>
>> this fixes the recursion on the read side too. And it is limited to cases
>> where the mode does not match the bitnum and bitsize parameters.
>
> But that's not conditional on -fstrict-volatile-bitfields and frankly it
> doesn't
> make sense to me. Why is the recursion not there for
> -fno-strict-volatile-bitfields?
>
the problem here is different, than we thought. Both recursions have been
observed initially only with volatile accesses and -fstrict-volatile-bitfields.
That's why it looked like the right thing to do. But PR59134 shows clearly
that both recursions have nothing to do with strict-volatile-bitfields. They
happen just because the hardware is not always able to access unaligned data
on one instruction. And the mode in str_rtx is sometimes too restrictive.
Now in this test case, we have s, a packed structure, but malloc retruns a
BIGGEST_ALIGNMENT. And at -O2 the back-end sees the larger alignment.
But the mode of str_rtx is QImode, because that is the mode of the struct str.
This mode is only good for accessing pad, but not for arr[0].
It does never make sense to access 32 bits in QImode, especially if the memory
is un-aligned. That is how this hunk resolves the issue.
> Richard.
>
>>
>> Bernd.
>>
>>
>>>
>>>> Bernd.
>>>>
>>>>> Richard.
>>>>>
>>>>>> -Sandra
