This patch simplifies vec_unpack_hi_expr/vec_unpack_lo_expr of a uniform
constructor or vec_duplicate operand.  The motivation is from PR 105621
where after optimization, we're left with:

  vect_cst__21 = {c_8(D), c_8(D), c_8(D), c_8(D)};
  vect_iftmp.7_4 = [vec_unpack_hi_expr] vect_cst__21;

It turns out that there are no constant folding/simplification patterns
in match.pd, but the above can be simplified further to the equivalent:

  _20 = (long int) c_8(D);
  vect_iftmp.7_4 = [vec_duplicate_expr] _20;

which on x86-64 results in one less instruction, replacing pshufd $0
then punpackhq, with punpcklqdq.  This transformation is also useful
for helping CSE to spot that unpack_hi and unpack_lo are equivalent.

This patch has been tested on x86_64-pc-linux-gnu with make bootstrap
and make -k check with no new failures.  Ok for mainline?


2022-05-21  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
        * match.pd (simplify vec_unpack_hi): Simplify VEC_UNPACK_*_EXPR
        of uniform vector constructors and vec_duplicate.

gcc/testsuite/ChangeLog
        * g++.dg/vect/pr105621.cc: New test case.


Thanks in advance,
Roger
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