On Mon, Aug 22, 2011 at 4:01 PM, Richard Guenther
<richard.guent...@gmail.com> wrote:
> On Mon, Aug 22, 2011 at 2:05 PM, Artem Shinkarov
> <artyom.shinkar...@gmail.com> wrote:
>> 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?
>
> Well, there is no problem in having the only valid mask operand for
> VEC_COND_EXPRs being either a comparison or a {-1,...} / {0,....} mask.
> Just the C parser has to transform mask ? vec1 : vec2 to mask != 0 ?
> vec1 : vec2.
This happens already in the new version of patch (not submitted yet).
> This comparison can be eliminated by optimization passes
> that
> either replace it by the real comparison computing the mask or just
> propagating the information this mask is already {-1,...} / {0,....} by simply
> dropping the comparison against zero.
This is not a problem, because the backend recognizes these patterns,
so no optimization is needed in this part.
> For the backends I'd have vcond patterns for both an embedded comparison
> and for a mask. (Now we can rewind the discussion a bit and allow
> arbitrary masks and define a vcond with a mask operand to do bitwise
> selection - what matters is the C frontend semantics which we need to
> translate to what the middle-end thinks of a VEC_COND_EXPR, they
> do not have to agree).
But it seems like another combinatorial explosion here. Considering
what Richard said in his e-mail, in order to support "generic" vcond,
we just need to enumerate all the possible cases. Or I didn't
understand right?
I mean, I don't mind of course, but it seems to me that it would be
cleaner to have one generic enough pattern.
Is there seriously no way to pass something from optab into the backend??
> If the mask is computed by a function you are of course out of luck,
> but I don't see how you'd manage to infer knowledge from nowhere either.
Well, take simpler example
a = {0};
for ( ; *p; p += 16)
a &= pattern > (vec)*p;
res = a ? v0 : v1;
In this case it is simple to analyse that a is a comparison, but you
cannot embed the operations of a into VEC_COND_EXPR.
Ok, I am testing the patch that removes hooks. Could you push a little
bit the backend-patterns business?
Thanks,
Artem.
> Richard.
>
>>
>> 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.
>>>>
>>>
>>
>
