[Bug target/101579] Suboptimal codegen for __builtin_shufflevector

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101579

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |jakub at gcc dot gnu.org

--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
As
typedef unsigned char V __attribute__((vector_size (32)));

V
foo (V x)
{
  return __builtin_shufflevector (x, x, 0, 1, 2, 0, 5, 1, 0, 1, 3, 2, 3, 0, 4,
3, 1, 2,
                                        2, 0, 4, 2, 3, 1, 1, 2, 3, 4, 1, 1, 0,
0, 5, 2);
}

V
bar (V x)
{
  return __builtin_shufflevector (x, x, 0, 3, 3, 3, 3, 4, 5, 0, 1, 5, 2, 1, 0,
1, 1, 2,
                                        3, 2, 0, 5, 4, 5, 1, 0, 1, 4, 4, 3, 4,
5, 2, 0);
}
with -O2 -mavx2 is handled, I'd say this is veclower task to determine that the
particular permutation could be cheaply implemented with two permutations of
half-sized vectors and ask the backend if it supports those.
Of course there can be other permutations that can't be implemented that way
easily and might e.g. need more half-sized permutations...