On Mon, Aug 22, 2011 at 12:25 PM, Richard Guenther
<richard.guent...@gmail.com> wrote:
> On Mon, Aug 22, 2011 at 12:53 AM, Artem Shinkarov
> <artyom.shinkar...@gmail.com> wrote:
>> Richard
>>
>> I formalized an approach a little-bit, now it works without target
>> hooks, but some polishing is still required. I want you to comment on
>> the several important approaches that I use in the patch.
>>
>> So how does it work.
>> 1) All the vector comparisons at the level of type-checker are
>> introduced using VEC_COND_EXPR with constant selection operands being
>> {-1} and {0}. For example v0 > v1 is transformed into VEC_COND_EXPR<v0
>>> v1, {-1}, {0}>.
>>
>> 2) When optabs expand VEC_COND_EXPR, two cases are considered:
>> 2.a) first operand of VEC_COND_EXPR is comparison, in that case nothing
>> changes.
>> 2.b) first operand is something else, in that case, we specially mark
>> this case, recognize it in the backend, and do not create a
>> comparison, but use the mask as it was a result of some comparison.
>>
>> 3) In order to make sure that mask in VEC_COND_EXPR<mask, v0, v1> is a
>> vector comparison we use is_vector_comparison function, if it returns
>> false, then we replace mask with mask != {0}.
>>
>> So we end-up with the following functionality:
>> VEC_COND_EXPR<mask, v0,v1> -- if we know that mask is a result of
>> comparison of two vectors, we leave it as it is, otherwise change with
>> mask != {0}.
>>
>> Plain vector comparison a <op> b is represented with VEC_COND_EXPR,
>> which correctly expands, without creating useless masking.
>>
>>
>> Basically for me there are two questions:
>> 1) Can we perform information passing in optabs in a nicer way?
>> 2) How is_vector_comparison could be improved? I have several ideas,
>> like checking if constant vector all consists of 0 and -1, and so on.
>> But first is it conceptually fine.
>>
>> P.S. I tired to put the functionality of is_vector_comparison in
>> tree-ssa-forwprop, but the thing is that it is called only with -On,
>> which I find inappropriate, and the functionality gets more
>> complicated.
>
> Why is it inappropriate to not optimize it at -O0? If the user
> separates comparison and ?: expression it's his own fault.
Well, because all the information is there, and I perfectly envision
the case when user expressed comparison separately, just to avoid code
duplication.
Like:
mask = a > b;
res1 = mask ? v0 : v1;
res2 = mask ? v2 : v3;
Which in this case would be different from
res1 = a > b ? v0 : v1;
res2 = a > b ? v2 : v3;
> Btw, the new hook is still in the patch.
>
> I would simply always create != 0 if it isn't and let optimizers
> (tree-ssa-forwprop.c) optimize this. You still have to deal with
> non-comparison operands during expansion though, but if
> you always forced a != 0 from the start you can then simply
> interpret it as a proper comparison result (in which case I'd
> modify the backends to have an alternate pattern or directly
> expand to masking operations - using the fake comparison
> RTX is too much of a hack).
Richard, I think you didn't get the problem.
I really need the way, to pass the information, that the expression
that is in the first operand of vcond is an appropriate mask. I though
for quite a while and this hack is the only answer I found, is there a
better way to do that. I could for example introduce another
tree-node, but it would be overkill as well.
Now why do I need it so much:
I want to implement the comparison in a way that {1, 5, 0, -1} is
actually {-1,-1,-1,-1}. So whenever I am not sure that mask of
VEC_COND_EXPR is a real comparison I transform it to mask != {0} (not
always). To check the stuff, I use is_vector_comparison in
tree-vect-generic.
So I really have the difference between mask ? x : y and mask != {0} ?
x : y, otherwise I could treat mask != {0} in the backend as just
mask.
If this link between optabs and backend breaks, then the patch falls
apart. Because every time the comparison is taken out VEC_COND_EXPR, I
will have to put != {0}. Keep in mind, that I cannot always put the
comparison inside the VEC_COND_EXPR, what if it is defined in a
function-comparison, or somehow else?
So what would be an appropriate way to connect optabs and the backend?
Thanks,
Artem.
All the rest would be adjusted later.
> tree
> constant_boolean_node (int value, tree type)
> {
> - if (type == integer_type_node)
> + if (TREE_CODE (type) == VECTOR_TYPE)
> + {
> + tree tval;
> +
> + gcc_assert (TREE_CODE (TREE_TYPE (type)) == INTEGER_TYPE);
> + tval = build_int_cst (TREE_TYPE (type), value);
> + return build_vector_from_val (type, tval);
>
> as value is either 0 or 1 that won't work. Oh, I see you pass -1
> for true in the callers. But I think we should simply decide that true (1)
> means -1 for a vector boolean node (and the value parameter should
> be a bool instead). Thus,
>
> + if (TREE_CODE (type) == VECTOR_TYPE)
> + {
> + tree tval;
> +
> + gcc_assert (TREE_CODE (TREE_TYPE (type)) == INTEGER_TYPE);
> + tval = build_int_cst (TREE_TYPE (type), value ? -1 : 0);
> + return build_vector_from_val (type, tval);
>
> instead.
>
> @@ -9073,26 +9082,29 @@ fold_comparison (location_t loc, enum tr
> floating-point, we can only do some of these simplifications.) */
> if (operand_equal_p (arg0, arg1, 0))
> {
> + int true_val = TREE_CODE (type) == VECTOR_TYPE ? -1 : 0;
> + tree arg0_type = TREE_TYPE (arg0);
> +
>
> as I said this is not necessary - the FLOAT_TYPE_P and HONOR_NANS
> macros work perfectly fine on vector types.
>
> Richard.
>
>>
>> Thanks,
>> Artem.
>>
>
