On Wed, Dec 20, 2017 at 2:48 PM, Richard Sandiford
<richard.sandif...@linaro.org> wrote:
> Richard Biener <richard.guent...@gmail.com> writes:
>> On Tue, Dec 12, 2017 at 4:46 PM, Richard Sandiford
>> <richard.sandif...@linaro.org> wrote:
>>> Richard Biener <richard.guent...@gmail.com> writes:
>>>> On Sun, Dec 10, 2017 at 12:20 AM, Richard Sandiford
>>>> <richard.sandif...@linaro.org> wrote:
>>>>> This patch changes vec_perm_indices from a plain vec<> to a class
>>>>> that stores a canonicalised permutation, using the same encoding
>>>>> as for VECTOR_CSTs. This means that vec_perm_indices now carries
>>>>> information about the number of vectors being permuted (currently
>>>>> always 1 or 2) and the number of elements in each input vector.
>>>>
>>>> Before I dive into the C++ details can you explain why it needs this
>>>> info and how it encodes it for variable-length vectors? To interleave
>>>> two vectors you need sth like { 0, N, 1, N+1, ... }, I'm not sure we
>>>> can directly encode N here, can we? extract even/odd should just
>>>> work as { 0, 2, 4, 6, ...} without knowledge of whether we permute
>>>> one or two vectors (the one vector case just has two times the same
>>>> vector) or how many elements each of the vectors (or the result) has.
>>>
>>> One of the later patches switches the element types to HOST_WIDE_INT,
>>> so that we can represent all ssizetypes. Then there's a poly_int
>>> patch (not yet posted) to make that poly_int64, so that we can
>>> represent the N even for variable-length vectors.
>>>
>>> The class needs to know the number of elements because that affects
>>> the canonical representation. E.g. extract even on fixed-length
>>> vectors with both inputs the same should be { 0, 2, 4, ..., 0, 2, 4 ... },
>>> which we can't encode as a simple series. Interleave low with both
>>> inputs the same should be { 0, 0, 1, 1, ... } for both fixed-length and
>>> variable-length vectors.
>>
>> Huh? extract even is { 0, 2, 4, 6, 8 ... } indexes in the selection vector
>> are referencing concat'ed input vectors. So yes, for two same vectors
>> that's effectively { 0, 2, 4, ..., 0, 2, 4, ... } but I don't see why
>> that should
>> be the canonical form?
>
> Current practice is to use the single-input form where possible,
> if both inputs are the same (see e.g. the VEC_PERM_EXPR handling
> in fold-const.c). It means that things like:
>
> _1 = VEC_PERM_EXPR <a, a, { 0, 2, 4, 6, 0, 2, 4, 6 }>;
> _2 = VEC_PERM_EXPR <a, a, { 0, 2, 4, 6, 8, 10, 12, 14 }>;
> _3 = VEC_PERM_EXPR <a, b, { 0, 2, 4, 6, 0, 2, 4, 6 }>;
>
> get folded to the same sequence, and so can be CSEd.
>
> We could instead convert the single-input form to use the two-input
> selector, but that would be harder. The advantage of treating the
> single-input form as canonical is that it works even for irregular
> permutes.
Ok, I see. Maybe adding a comment along this helps.
Thanks,
Richard.
> Thanks,
> Richard
>
>>> Also, operator[] is supposed to return an in-range selector even if
>>> the selector element is only implicitly encoded. So we need to know
>>> the number of input elements there.
>>>
>>> Separating the number of input elements into the number of inputs
>>> and the number of elements per input isn't really necessary, but made
>>> it easier to provide routines for testing whether all selected
>>> elements come from a particular input, and for rotating the selector
>>> by a whole number of inputs.
>>>
>>> Thanks,
>>> Richard
