Re: [PATCH] gimple-match: Do not try UNCOND optimization with COND_LEN.

[Richard Sandiford <richard.sandiford@arm.com>]

Richard Biener <richard.guenther@gmail.com> writes:
> On Mon, Oct 16, 2023 at 11:59 PM Richard Sandiford
> <richard.sandiford@arm.com> wrote:
>>
>> Robin Dapp <rdapp.gcc@gmail.com> writes:
>> >> Why are the contents of this if statement wrong for COND_LEN?
>> >> If the "else" value doesn't matter, then the masked form can use
>> >> the "then" value for all elements.  I would have expected the same
>> >> thing to be true of COND_LEN.
>> >
>> > Right, that one was overly pessimistic.  Removed.
>> >
>> >> But isn't the test whether res_op->code itself is an internal_function?
>> >> In other words, shouldn't it just be:
>> >>
>> >>       if (internal_fn_p (res_op->code)
>> >>        && internal_fn_len_index (as_internal_fn (res_op->code)) != -1)
>> >>      return true;
>> >>
>> >> maybe_resimplify_conditional_op should already have converted to an
>> >> internal function where possible, and if combined_fn (res_op->code)
>> >> does any extra conversion on the fly, that conversion won't be reflected
>> >> in res_op.
>> >
>> > I went through some of our test cases and believe most of the problems
>> > are due to situations like the following:
>> >
>> > In vect-cond-arith-2.c we have (on riscv)
>> >   vect_neg_xi_14.4_23 = -vect_xi_13.3_22;
>> >   vect_res_2.5_24 = .COND_LEN_ADD ({ -1, ... }, vect_res_1.0_17, vect_neg_xi_14.4_23, vect_res_1.0_17, _29, 0);
>> >
>> > On aarch64 this is a situation that matches the VEC_COND_EXPR
>> > simplification that I disabled with this patch.  We valueized
>> > to _26 = vect_res_1.0_17 - vect_xi_13.3_22 and then create
>> > vect_res_2.5_24 = VEC_COND_EXPR <loop_mask_22, _26, vect_res_1.0_19>;
>> > This is later re-assembled into a COND_SUB.
>> >
>> > As we have two masks or COND_LEN we cannot use a VEC_COND_EXPR to
>> > achieve the same thing.  Would it be possible to create a COND_OP
>> > directly instead, though?  I tried the following (not very polished
>> > obviously):
>> >
>> > -      new_op.set_op (VEC_COND_EXPR, res_op->type,
>> > -                    res_op->cond.cond, res_op->ops[0],
>> > -                    res_op->cond.else_value);
>> > -      *res_op = new_op;
>> > -      return gimple_resimplify3 (seq, res_op, valueize);
>> > +      if (!res_op->cond.len)
>> > +       {
>> > +         new_op.set_op (VEC_COND_EXPR, res_op->type,
>> > +                        res_op->cond.cond, res_op->ops[0],
>> > +                        res_op->cond.else_value);
>> > +         *res_op = new_op;
>> > +         return gimple_resimplify3 (seq, res_op, valueize);
>> > +       }
>> > +      else if (seq && *seq && is_gimple_assign (*seq))
>> > +       {
>> > +         new_op.code = gimple_assign_rhs_code (*seq);
>> > +         new_op.type = res_op->type;
>> > +         new_op.num_ops = gimple_num_ops (*seq) - 1;
>> > +         new_op.ops[0] = gimple_assign_rhs1 (*seq);
>> > +         if (new_op.num_ops > 1)
>> > +           new_op.ops[1] = gimple_assign_rhs2 (*seq);
>> > +         if (new_op.num_ops > 2)
>> > +           new_op.ops[2] = gimple_assign_rhs2 (*seq);
>> > +
>> > +         new_op.cond = res_op->cond;
>> > +
>> > +         gimple_match_op bla2;
>> > +         if (convert_conditional_op (&new_op, &bla2))
>> > +           {
>> > +             *res_op = bla2;
>> > +             // SEQ should now be dead.
>> > +             return true;
>> > +           }
>> > +       }
>> >
>> > This would make the other hunk (check whether it was a LEN
>> > and try to recreate it) redundant I hope.
>> >
>> > I don't know enough about valueization, whether it's always
>> > safe to do that and other implications.  On riscv this seems
>> > to work, though and the other backends never go through the LEN
>> > path.  If, however, this is a feasible direction it could also
>> > be done for the non-LEN targets?
>>
>> I don't know much about valueisation either :)  But it does feel
>> like we're working around the lack of a LEN form of COND_EXPR.
>> In other words, it seems odd that we can do:
>>
>>   IFN_COND_LEN_ADD (mask, a, 0, b, len, bias)
>>
>> but we can't do:
>>
>>   IFN_COND_LEN (mask, a, b, len, bias)
>>
>> There seems to be no way of applying a length without also finding an
>> operation to perform.
>
> Indeed .. maybe - _maybe_ we want to scrap VEC_COND_EXPR for
> IFN_COND{,_LEN} to be more consistent here?

Yeah, sounds like it could be worthwhile.  But I suppose we still need
VEC_COND_EXPR itself because it's a generic front-end operation that
needs to be lowered.  So it might be worth starting with an ifn for the
LEN form and seeing whether the non-LEN form should switch over.

Thanks,
Richard