On Fri, Oct 5, 2012 at 8:15 PM, DJ Delorie <d...@redhat.com> wrote:
>
>
>> Why do you need to change varasm.c at all? The hunks seem to be
>> completely separate of the attribute.
>
> Because static constructors have fields in the original order, not the
> reversed order. Otherwise code like this is miscompiled:
Err - the struct also has fields in the original order - only the bit positions
of the fields are different because of the layouting option.
> struct foo a = { 1, 2, 3 };
>
> because the 1, 2, 3 are in the C layout order, but the underlying data
> needs to be stored in the reversed order.
And you expect no other code looks at fields of a structure and its
initializer? It's bad to keep this not in-sync. Thus I don't think it's
viable to re-order fields just because bit allocation is reversed.
>> which will severely pessimize bitfield accesses to structs with the
>> bitfield-order attribute.
>
> The typical use-case for this feature is memory-mapped hardware, where
> pessimum access is preferred anyway.
I doubt that, looking at constraints for strict volatile bitfields.
>> so you are supporting this as #pragma. Which ends up tacking
>> bit_order to each type. Rather than this, why not operate similar
>> to the packed pragma, thus, adjust a global variable in
>> stor-layout.c.
>
> Because when I first proposed this feature, I was told to do it this
> way.
>
>> I don't see a value in attaching 'native' or 'msb'/'lsb' if it is
>> equal to 'native'. You un-necessarily pessimize code generation (is
>> different code even desired for a "no-op" bit_order attribute?).
>
> If the attribute corresponds to the native mode, it should be a no-op.
> The pessimizing only happens when the fields are actually in reverse
> order.
No, because you check for the attribute presence.
>> So no, I don't like this post-process layouting thing. It's a
>> layouting mode so it should have effects at bitfield layout time.
>
> The actual reversal happens in stor-layout.c. Everything else is
> there to compensate for a possible non-linear layout.
