Re: [PATCH] Add COMPLEX_VECTOR_INT modes

[Richard Sandiford <richard.sandiford@arm.com>]

Richard Biener <richard.guenther@gmail.com> writes:
> On Fri, Jun 9, 2023 at 11:45 AM Andrew Stubbs <ams@codesourcery.com> wrote:
>>
>> On 09/06/2023 10:02, Richard Sandiford wrote:
>> > Andrew Stubbs <ams@codesourcery.com> writes:
>> >> On 07/06/2023 20:42, Richard Sandiford wrote:
>> >>> I don't know if this helps (probably not), but we have a similar
>> >>> situation on AArch64: a 64-bit mode like V8QI can be doubled to a
>> >>> 128-bit vector or to a pair of 64-bit vectors.  We used V16QI for
>> >>> the former and "V2x8QI" for the latter.  V2x8QI is forced to come
>> >>> after V16QI in the mode list, and so it is only ever used through
>> >>> explicit choice.  But both modes are functionally vectors of 16 QIs.
>> >>
>> >> OK, that's interesting, but how do you map "complex int" vectors to that
>> >> mode? I tried to figure it out, but there's no DIVMOD support so I
>> >> couldn't just do a straight comparison.
>> >
>> > Yeah, we don't do that currently.  Instead we make TARGET_ARRAY_MODE
>> > return V2x8QI for an array of 2 V8QIs (which is OK, since V2x8QI has
>> > 64-bit rather than 128-bit alignment).  So we should use it for a
>> > complex-y type like:
>> >
>> >    struct { res_type res[2]; };
>> >
>> > In principle we should be able to do the same for:
>> >
>> >    struct { res_type a, b; };
>> >
>> > but that isn't supported yet.  I think it would need a new target hook
>> > along the lines of TARGET_ARRAY_MODE, but for structs rather than arrays.
>
> And the same should work for complex types, no?  In fact we could document
> that TARGET_ARRAY_MODE also is used for _Complex?  Note the hook
> is used for type layout and thus innocent array types (in aggregates) can end up
> with a vector mode now.

Yeah, that was deliberate.  Given that we have modes for pairs of vectors,
it seemed better to use them even without an explicit opt-in.

> Hopefully that's without bad effects (on the ABI).

Well, I won't make any guarantees :)  But we did check, and it seemed
to be handled correctly.  Most of the AArch64 ABI code is agnostic to
aggregate modes.

> That said, the hook _could_ be used just for divmod expansion without
> actually creating a complex (or array) type of vectors.
>
>> > The advantage of this from AArch64's PoV is that it extends to 3x and 4x
>> > tuples as well, whereas complex is obviously for pairs only.
>> >
>> > I don't know if it would be acceptable to use that kind of struct wrapper
>> > for the divmod code though (for the vector case only).
>>
>> Looking again, I don't think this will help because GCN does not have an
>> instruction that loads vectors that are back-to-back, hence there's
>> little benefit in adding the tuple mode.
>>
>> However, GCN does have instructions that effectively load 2, 3, or 4
>> vectors that are *interleaved*, which would be the likely case for
>> complex numbers (or pixel colour data!)
>
> that's load_lanes and I think not related here but it probably also
> needs the xN modes.

Yeah, we need the modes for that too.

I don't the modes imply that the registers can be loaded and stored
in-order using a single instruction.  That isn't possible for big-endian
AArch64, for example.  It also isn't possible for the equivalent SVE types.
But the modes are still useful in those cases, because of their use in
interleaved loads and stores (and for the ABI).

Thanks,
Richard