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

[Richard Sandiford <richard.sandiford@linaro.org>]

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.

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