Best way to fill an array of Struct of Float_vs with an array of Struct of floats in VC

[Krishna Narasimhan]

But, am I correct in assuming that interleave works only with structures
that have exactly two entries. From what I read the simdize has the ability
to load from struct arrays into struct vector arrays.



[please enable javascript to see the address]> wrote:

> Hi,
>
> so first of all the new documentation that I promised:
> https://web-docs.gsi.de/~mkretz/Vc-master/group__Simdize.html
>
> Vc::simdize will help you implement the pattern Krishna asked for easily
> and
> conveniently. It's certainly quite a load on the compiler, but not on the
> generated code.
>
> On Wednesday 26 August 2015 12:28:30 Jaroslav Malec wrote:
> > As far as I know, in the current implementation you can use *scatter* and
> > *gather* for these operations.
> > Please correct me If this is a misconception.
>
> You are right about *can*. But unless you don't care for efficiency, avoid
> gather and scatter as much as possible. Gather and scatter are a crutch for
> situations that really cannot be improved to better suited memory
> patterns. In
> this case you have a regular memory pattern. Gather and scatter works for
> arbitrary (which includes regular, of course) memory patterns.
>
> For regular memory patterns you want to go for (de)interleaving instead of
> gather/scatter. Consider
>
> mem: [a0 b0 a1 b1 a2 b2 a3 b3 a4 b4 ... ]
>
> vector_load(&mem[0]) -> {a0 b0 a1 b1}
> vector_load(&mem[4]) -> {a2 b2 a3 b3}
>
> But now you really wanted to have {a0 a1 a2 a3} and {b0 b1 b2 b3}. Via a
> *deinterleave* operation (consisting of multiple efficient instructions)
> you
> can transform the two vector loads to the desired vectors.
>
> If you instead do gathers you basically do:
>
> scalar_load(&mem[0]) -> a0
> scalar_load(&mem[2]) -> a1
> scalar_load(&mem[4]) -> a2
> scalar_load(&mem[6]) -> a3
> scalar_load(&mem[1]) -> b0
> scalar_load(&mem[3]) -> b1
> scalar_load(&mem[5]) -> b2
> scalar_load(&mem[7]) -> b3
> combine(a0, a1, a2, a3) -> {a0 a1 a2 a3}
> combine(b0, b1, b2, b3) -> {b0 b1 b2 b3}
>
> Especially the large number of loads is what you want to avoid. Many SIMD
> codes are quickly limited on the load or store ports of the CPU.
>
> With AVX2 you get gather instructions. That eases the load on the CPU a
> little
> bit. But the instruction is more expensive that you'd like it to be.
>
> Regarding the *deinterleave* operations. There's lots of work on
> abstracting
> them in Vc. I'm sure the documentation isn't good enough yet... You could
> help
> by making specific request for documentation on
> https://github.com/VcDevel/Vc/issues.
>
> Hope this helps!
> Cheers,
>   Matthias
>
> >
> > If I'm on the right track on this, could you provide some code examples
> > that relate
> > to what Krishna has in mind? It could be contained in the documentation
> > also.
> > We would both appreciate it.
> >
> > Thanks.
> >
> > Jaroslav Malec
> >
> > tel. 731 160 847
> >
>[please enable javascript to see the address]>:
> > > Am working on 0.7. Any particular exampke where you load such
> structures
> > > into vectors?
> > >
>[please enable javascript to see the address]> wrote:
> > >> Hello Krishna,
> > >>
> > >> On Thursday 13 August 2015 16:48:16 Krishna Narasimhan wrote:
> > >> > Struct A{
> > >> >
> > >> >    float a,b,c
> > >> >
> > >> > }
> > >> >
> > >> > Struct A_v
> > >> > {
> > >> >
> > >> >     float_v a_v,b_v,c_v
> > >> >
> > >> > }
> > >> >
> > >> > void main()
> > >> > {
> > >> >
> > >> >    A a = new A[1000];
> > >> >    A_v a_v = new A_v[1000/float_v::Size]
> > >> >    // Assume here there is a code that populates a with values
> > >> >
> > >> >    I want to write a code here which would basically move values of
> a
> > >>
> > >> into
> > >>
> > >> > a_v because deinterleave doesnt work. Is there a form of fill() or
> > >>
> > >> store()
> > >>
> > >> > or load() I can use here?
> > >>
> > >> I have written lots of such code for Vc. I recommend you take a look
> at
> > >> Vc
> > >> master. Are you currently working on 0.7 or on master?
> > >>
> > >> In any case. Your mail prompted me to work on my documentation. I'll
> > >> hopefully
> > >> have a new section on Vc::simdize<T> up today. I'll let you know.
> > >>
> > >> Cheers,
> > >>
> > >>   Matthias
> > >>
> > >> _______________________________________________
> > >> Vc mailing list
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> > >
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>
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-- 
-----------------------------------------------------
I dare do all that may become a man; Who dares do more, is none - Macbeth,
twelfh night!
Regards
       Krishna
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