[Bug target/102583] [x86] Failure to optimize 32-byte integer vector conversion to 16-byte float vector properly when converting upper part with -mavx2

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102583

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-10-04
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We're lacking stripping of a load that's only used partially I guess. 
forwprop1
generates

  _1 = *srcp_11(D);
  _2 = BIT_FIELD_REF <_1, 32, 128>;
  _3 = (float) _2;
  _4 = BIT_FIELD_REF <_1, 32, 160>;
  _5 = (float) _4;
  _6 = BIT_FIELD_REF <_1, 32, 192>;
  _7 = (float) _6;
  _8 = BIT_FIELD_REF <_1, 32, 224>;
  _9 = (float) _8;
  _12 = VEC_PERM_EXPR <_1, _1, { 4, 5, 6, 7, 4, 5, 6, 7 }>;
  _15 = BIT_FIELD_REF <_12, 128, 0>;
  _16 = (vector(4) float) _15;

and the second forwprop then sees

  _1 = *srcp_3(D);
  _4 = VEC_PERM_EXPR <_1, _1, { 4, 5, 6, 7, 4, 5, 6, 7 }>;
  _5 = BIT_FIELD_REF <_4, 128, 0>;
  _6 = (vector(4) float) _5;

in a single-use chain we probably want to try swapping the
BIT_FIELD_REF and the VEC_PERM_EXPR so that we expose the
BIT_FIELD_REF directly to the load which we'd already handle.