Vector API - Request to expose horizontal adds

[August Nagro]

Isn't `myVector.reduceLanes(ADD, myMask)` the same as horizontal addition
instruction?

On Mon, Apr 26, 2021 at 4:13 PM Paul Sandoz <paul.sandoz at oracle.com> wrote:

> HI Stephane,
>
> [Apologies for the delay in replying. Emails from non-members were queued
> up for approval and we forgot to approve ‘em]
>
> Right now there is no direct support for such a hardware instruction. IIRC
> is used indirectly to perform reductions when composed with itself or other
> vector instructions.
>
> It's possible to emulate it using rearrange/shuffle operations [*], but
> that will result in many vector hardware instructions (and optimization of
> such operations are still work in progress). In principle C2 could detect a
> pattern of vector operations and replace with a horizontal add, but that is
> likely fragile and perhaps too specialized to be worth the cost.
>
> It’s an unusual instruction in some respects, not quite fitting in with
> the API design of lanewise operations, nor reduction operations. Such
> addition would require some special API treatment, perhaps a new special
> form of VectorOperators.Operator.
>
> I am inclined to sit and wait to see how important this is, then look more
> broadly at other similar kinds of hardware instructions, to see if a common
> pattern emerges that we can capture, before committing to a special case.
>
> Paul.
>
> [*]
> IntVector v1 = IntVector.fromArray(SPECIES, new int[]{10, 11, 12, 13, 14,
> 15, 16, 17}, 0);
> IntVector v2 = IntVector.fromArray(SPECIES, new int[]{20, 21, 22, 23, 24,
> 25, 26, 27}, 0);
>
> VectorShuffle<Integer> s1 = SPECIES.shuffleFromArray(new int[]{0, 2, -8,
> -6, 4, 6, -4, -2}, 0);
> VectorShuffle<Integer> s2 = SPECIES.shuffleFromArray(new int[]{1, 3, -7,
> -5, 5, 7, -3, -1}, 0);
> // emulate _mm256_hadd_epi32
> IntVector v3 = v1.rearrange(s1, v2);
> IntVector v4 = v1.rearrange(s2, v2);
> IntVector v5 = v3.add(v4);
>
> > On Mar 23, 2021, at 4:14 PM, Stephane Dubedat <
> stephane.dubedat2 at mail.dcu.ie> wrote:
> >
> > Hi, this is my first message on an openjdk mailing list, I hope this is
> > appropriate.
> > I am currently translating a C# SIMD codebase to the Java Vector API and
> > the only operation that I don't know how to translate is Horizontal Add.
> > In C# it is Sse3.HorizontalAdd(sum, sum); and correspond to the following
> > intrinsic __m128d _mm_hadd_pd (__m128d a, __m128d b)
> > So my question is: do you plan to expose this operation ? And in the
> > meantime how can I efficiently achieve this?
> >
> > Thanks,
> > respectfully,
> > Stephane Dubedat
> >
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