Robin Dapp <rdapp....@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.
Does IFN_COND_LEN make conceptual sense on RVV? If so, would defining
it solve some of these problems?
I suppose in the worst case, IFN_COND_LEN is equivalent to IFN_COND_LEN_IOR
with a zero input (and extended to floats). So if the target can do
IFN_COND_LEN_IOR, it could implement IFN_COND_LEN using the same instruction.
Thanks,
Richard
