Re: VEC_COND_EXPR optimizations

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

On Fri, Jul 31, 2020 at 1:35 PM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Thu, Jul 30, 2020 at 9:49 AM Marc Glisse <marc.glisse@inria.fr> wrote:
> >
> > When vector comparisons were forced to use vec_cond_expr, we lost a number
> > of optimizations (my fault for not adding enough testcases to prevent
> > that). This patch tries to unwrap vec_cond_expr a bit so some
> > optimizations can still happen.
> >
> > I wasn't planning to add all those transformations together, but adding
> > one caused a regression, whose fix introduced a second regression, etc.
> >
> > Using a simple fold_binary internally looks like an ok compromise to me.
> > It remains cheap enough (not recursive, and vector instructions are not
> > that frequent), while still allowing more than const_binop (X|0 or X&X for
> > instance). The transformations are quite conservative with :s and folding
> > only if everything simplifies, we may want to relax this later. And of
> > course we are going to miss things like a?b:c + a?c:b -> b+c.
> >
> > In terms of number of operations, some transformations turning 2
> > VEC_COND_EXPR into VEC_COND_EXPR + BIT_IOR_EXPR + BIT_NOT_EXPR might not
> > look like a gain... I expect the bit_not disappears in most cases, and
> > VEC_COND_EXPR looks more costly than a simpler BIT_IOR_EXPR.
> >
> > I am a bit confused that with avx512 we get types like "vector(4)
> > <signed-boolean:2>" with :2 and not :1 (is it a hack so true is 1 and not
> > -1?), but that doesn't matter for this patch.
> >
> > Regtest+bootstrap on x86_64-pc-linux-gnu
>
> +  (with
> +   {
> +     tree rhs1, rhs2 = NULL;
> +     rhs1 = fold_binary (op, type, @1, @3);
> +     if (rhs1 && is_gimple_val (rhs1))
> +       rhs2 = fold_binary (op, type, @2, @3);
>
> ICK.  I guess a more match-and-simplify way would be
>
>    (with
>     {
>       tree rhs1, rhs2;
>       gimple_match_op op (gimple_match_cond::UNCOND, op,
>                                       type, @1, @3);
>       if (op.resimplify (NULL, valueize)
>           && gimple_simplified_result_is_gimple_val (op))
>        {
>          rhs1 = op.ops[0];
>          ... other operand ...
>        }
>
> now in theory we could invent some new syntax for this, like
>
>  (simplify
>   (op (vec_cond:s @0 @1 @2) @3)
>   (vec_cond @0 (op:x @1 @3) (op:x @2 @3)))
>
> and pick something better instead of :x (:s is taken,
> would be 'simplified', :c is taken would be 'constexpr', ...).
>
> _Maybe_ just
>
>  (simplify
>   (op (vec_cond:s @0 @1 @2) @3)
>   (vec_cond:x @0 (op @1 @3) (op @2 @3)))
>
> which would have the same practical meaning as passing
> NULL for the seq argument to simplification - do not allow
> any intermediate stmt to be generated.

Note I specifically do not like those if (it-simplifies) checks
because we already would code-generate those anyway.  For

(simplify
  (plus (vec_cond:s @0 @1 @2) @3)
  (vec_cond @0 (plus @1 @3) (plus @2 @3)))

we get

                    res_op->set_op (VEC_COND_EXPR, type, 3);
                    res_op->ops[0] = captures[1];
                    res_op->ops[0] = unshare_expr (res_op->ops[0]);
                    {
                      tree _o1[2], _r1;
                      _o1[0] = captures[2];
                      _o1[1] = captures[4];
                      gimple_match_op tem_op (res_op->cond.any_else
(), PLUS_EXPR, TREE_TYPE (_o1[0]), _o1[0], _o1[1]);
                      tem_op.resimplify (lseq, valueize);
                      _r1 = maybe_push_res_to_seq (&tem_op, lseq);  (****)
                      if (!_r1) return false;
                      res_op->ops[1] = _r1;
                    }
                    {
                      tree _o1[2], _r1;
                      _o1[0] = captures[3];
                      _o1[1] = captures[4];
                      gimple_match_op tem_op (res_op->cond.any_else
(), PLUS_EXPR, TREE_TYPE (_o1[0]), _o1[0], _o1[1]);
                      tem_op.resimplify (lseq, valueize);
                      _r1 = maybe_push_res_to_seq (&tem_op, lseq);  (***)
                      if (!_r1) return false;
                      res_op->ops[2] = _r1;
                    }
                    res_op->resimplify (lseq, valueize);
                    return true;

and the only change required would be to pass NULL to maybe_push_res_to_seq
here instead of lseq at the (***) marked points.

Richard.

> The other "simple" patterns look good, you can commit
> them separately if you like.
>
> Richard.
>
> > 2020-07-30  Marc Glisse  <marc.glisse@inria.fr>
> >
> >         PR tree-optimization/95906
> >         PR target/70314
> >         * match.pd ((c ? a : b) op d, (c ? a : b) op (c ? d : e),
> >         (v ? w : 0) ? a : b, c1 ? c2 ? a : b : b): New transformations.
> >
> >         * gcc.dg/tree-ssa/andnot-2.c: New file.
> >         * gcc.dg/tree-ssa/pr95906.c: Likewise.
> >         * gcc.target/i386/pr70314.c: Likewise.
> >
> > --
> > Marc Glisse