Re: [07/13] Make vec_perm_indices use new vector encoding

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Dec 12, 2017 at 4:46 PM, Richard Sandiford
<richard.sandiford@linaro.org> wrote:
> Richard Biener <richard.guenther@gmail.com> writes:
>> On Sun, Dec 10, 2017 at 12:20 AM, Richard Sandiford
>> <richard.sandiford@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?

> 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