[Bug target/71310] Bitfields cause load hit store with smaller store and larger load

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71310

Segher Boessenkool <segher at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2016-05-27
     Ever confirmed|0                           |1

--- Comment #2 from Segher Boessenkool <segher at gcc dot gnu.org> ---
At expand time this is

  skb_5(D)->csum_valid = 1;
  _1 = BIT_FIELD_REF <*skb_5(D), 64, 64>;
  _2 = _1 & 27021597764222976;
  _7 = _2 != 0;
  _8 = (int) _7;
  return _8;

The RMW of writing csum_valid is done as a SImode memory access.
The BIT_FIELD_REF is DImode however.  Why?