Re: [PATCH] Simplify vec_unpack of uniform_vector_p constructors in match.pd.

[Richard Biener <richard.guenther@gmail.com>]

On Sat, May 21, 2022 at 5:31 PM Roger Sayle <roger@nextmovesoftware.com> wrote:
>
>
> 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?

I think we need a way to query whether the target can do a VEC_DUPLICATE_EXPR.
Currently we only ever have them for VL vectors and expand via
expand_vector_broadcast which eventually simply gives up when there's no
vec_duplicate or vec_init optabs suitable.

IIRC with the VEC_PERM extension we should be able to handle
VEC_DUPLICATE via VEC_PERM?  (but we don't yet accept a scalar
input, just V1<mode>?)

I see most targets have picked up vec_duplicate but sparc, but still
we'd need to check the specific mode.  I think we can disregart
vec_init checking and only apply the transforms when vec_duplicate
is available.

Richard.

>
> 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
> --
>