Re: Vector Comparison patch

[Artem Shinkarov <artyom dot shinkaroff at gmail dot com>]

On Mon, Aug 22, 2011 at 4:34 PM, Richard Guenther
<richard.guenther@gmail.com> wrote:
> On Mon, Aug 22, 2011 at 5:21 PM, Artem Shinkarov
> <artyom.shinkaroff@gmail.com> wrote:
>> On Mon, Aug 22, 2011 at 4:01 PM, Richard Guenther
>> <richard.guenther@gmail.com> wrote:
>>> On Mon, Aug 22, 2011 at 2:05 PM, Artem Shinkarov
>>> <artyom.shinkaroff@gmail.com> wrote:
>>>> On Mon, Aug 22, 2011 at 12:25 PM, Richard Guenther
>>>> <richard.guenther@gmail.com> wrote:
>>>>> On Mon, Aug 22, 2011 at 12:53 AM, Artem Shinkarov
>>>>> <artyom.shinkaroff@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.
>
> I mean for
>
> ?mask = v1 < v2 ? {-1,...} : {0,...};
> ?val = VCOND_EXPR <mask != 0, v3, v4>;
>
> optimizers can see how mask is defined and drop the != 0 test or replace
> it by v1 < v2.

Yes, sure.

>>> 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?
>
> Well, the question is still what VCOND_EXPR and thus the vcond pattern
> semantically does for a non-comparison operand. ?I'd argue that using
> the bitwise selection semantic gives us maximum flexibility and a native
> instruction with AMD XOP. ?This non-comparison VCOND_EXPR is
> also easy to implement in the middle-end expansion code if there is
> no native instruction for it - by simply emitting the bitwise operations.
>
> But I have the feeling we are talking past each other ...?

I am all for the bitwise behaviour in the backend pattern, that is
something that I rely on at the moment. What I don't want to have is
the same behaviour in the frontend. So If we can guarantee, that we
add != 0, when we don't know the "nature" of the mask, then I am
perfectly fine with the back-end having bitwise-selection behaviour.

>> 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??
>
> You can pass operands. ?And information is implicitly encoded in the name.

I didn't quite get that, could you give an example?

>>> 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.
>
> Sure, but if the above is C source the frontend would generate
> res = a != 0 ? v0 : v1; initially. ?An optimization pass could still
> track this information and replace VEC_COND_EXPR <a != 0, v0, v1>
> with VEC_COND_EXPR <a, v0, v1> (no existing one would track
> vector contents though).

Yeah, sure. My point is, that we must be able to pass this information
in the backend, that we checked everything, and we are sure that a is
a corerct mask, please don't add any != 0 to it.

>> Ok, I am testing the patch that removes hooks. Could you push a little
>> bit the backend-patterns business?
>
> Well, I suppose we're waiting for Uros here. ?I hadn't much luck with
> fiddling with the mode-iterator stuff myself.
>
> Richard.
>

Ok, fine. The patch is coming soon.


Artem.