[Bug middle-end/81502] In some cases the data is moved to memory unnecessarily [partial regression]

["glisse at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81502

Marc Glisse <glisse at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2017-07-21
     Ever confirmed|0                           |1

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
.optimized dump:

int bar(void*) (void * ptr)
{
  int res;
  __m128i word;
  long unsigned int _2;
  vector(2) long long int word.3_3;
  unsigned int _4;

  <bb 2> [100.00%] [count: INV]:
  _2 = (long unsigned int) ptr_9(D);
  word = { 0, 0 };
  MEM[(char * {ref-all})&word] = _2;
  word.3_3 = word;
  word ={v} {CLOBBER};
  _4 = BIT_FIELD_REF <word.3_3, 32, 0>;
  res_5 = (int) _4;
  return res_5;

}

We missed turning the memory write into a BIT_INSERT_EXPR, and passes like PRE
missed following the bit_field_expr all the way to _2.

.combine dump:
[...]
(insn 8 3 10 2 (set (reg/v:V2DI 90 [ word ])
        (vec_concat:V2DI (reg/v/f:DI 92 [ ptr ])
            (const_int 0 [0]))) "b.c":16 3712 {vec_concatv2di}
     (expr_list:REG_DEAD (reg/v/f:DI 92 [ ptr ])
        (nil)))
(insn 10 8 15 2 (set (reg:SI 94 [ res ])
        (vec_select:SI (subreg:V4SI (reg/v:V2DI 90 [ word ]) 0)
            (parallel [
                    (const_int 0 [0])
                ]))) "b.c":20 3697 {*vec_extractv4si_0}
     (expr_list:REG_DEAD (reg/v:V2DI 90 [ word ])
        (nil)))
[...]

combine tries
(set (reg:SI 94 [ res ])
    (vec_select:SI (subreg:V4SI (vec_concat:V2DI (reg/v/f:DI 92 [ ptr ])
                (const_int 0 [0])) 0)
        (parallel [
                (const_int 0 [0])
            ])))
which we fail to simplify. The xmm1-xmm0 mov is not considered a mov by the
compiler but concatenation with 0, so not a RA problem.

The change of mode (64-bit pointer to 32-bit int) seems to play a big role in
confusing things here.