On Mon, Aug 22, 2011 at 5:43 PM, Artem Shinkarov
<artyom.shinkar...@gmail.com> wrote:
> On Mon, Aug 22, 2011 at 4:34 PM, Richard Guenther
> <richard.guent...@gmail.com> wrote:
>> On Mon, Aug 22, 2011 at 5:21 PM, Artem Shinkarov
>> <artyom.shinkar...@gmail.com> wrote:
>>> 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.
>>
>> 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.
Well, the C frontend would simply always add that != 0 (because it
doesn't know).
>>> 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?
It was a larger variant of "no, apart from what is obvious".
>>>> 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.
But all masks are correct as soon as they appear in a VEC_COND_EXPR.
That's the whole point of the bitwise semantics. It's only the C frontend
that needs to be careful to impose its stricter semantics.
Richard.
