[Bug target/95762] Failure to optimize __builtin_convertvector from vector of 16 chars to vector of 16 shorts in a single instruction on AVX2

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95762

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2020-06-19
             Status|UNCONFIRMED                 |NEW
                 CC|                            |jakub at gcc dot gnu.org,
                   |                            |rguenth at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We're currently representing this as a .VEC_CONVERT IFN lowered at veclower
time to

  _4 = [vec_unpack_lo_expr] a_1(D);
  _5 = [vec_unpack_hi_expr] a_1(D);
  _2 = {_4, _5};

rather than using a NOP_EXPR as would be possible now.  I suppose we should
remove .VEC_CONVERT again for vector integer conversions and directly
use NOP_EXPRs plus make sure to lower those when not supported.  Not
sure if __builtin_convertvector also supports integer<->float conversions.