std::vector vs Vc::Memory

[Tijskens Engelbert]

Dear sandro
the attachments contains the main file test.cpp and the included timer.h
i included some unrolling tests as mathias suggested for the scalar case. that helps indeed. didn’t check the simd case so far.
kindest regards,
bert

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

Dear Tijskens,

I was intrigued by your observations  and tried to reproduce them but I failed. Actually, I feel like Matthias that measuring such short minimalistic code section is really tough.

Would you be able  to share your benchmark code and the way you compile it such that I can have a more thorough look?

Best

Sandro



[please enable javascript to see the address][please enable javascript to see the address]>>:
Dear all,

I am trying to figure out how to use std::vector<float> efficiently in combination with Vc. (to have dynamic arrays and performance)

    std::vector<float> x(1024);
    for( int i=0; i<ne; ++i ) {//initialize
        x[i]=1.0;
    }
// scalar loop using std::vector
    for( int i=0; i<ne; ++i ) {
        x[i] -= 1.0;
    }
// vector loop using std::vector
    for( int i=0; i<ne; i+=Vc::float_v::Size )
    {
        Vc::float_v vx( &x[i] );
        vx -= 1.0;
        vx.store( &x[i] );
    }
// vector loop using Vc::Memory instead of std::vector
    Vc::Memory<Vc::float_v,ne> Vx;
    for( int i=0; i<ne; ++i ) {//initialize
        Vx[i] = 1.0;
    }
    Vc::float_v one(1.);
    ET_TIME_THIS
    ( "Vc::Memory<Vc::float_v,ne>  vector",
        for( int i=0; i<nv; ++i ) {
            Vx.vector(i) -= one;
        }
When i time these loops i get the following results
scalar loop using std::vector : 2162 cycles/repetition, 9.4e-07 seconds/repetition, 1   x speedup, 2.3  GHz, 100 repetitions.
vector loop using std::vector :  357 cycles/repetition, 1.6e-07 seconds/repetition, 6.04x speedup, 2.24 GHz, 100 repetitions.
vector loop using Vc::Memory  :  288 cycles/repetition, 1.2e-07 seconds/repetition, 7.49x speedup, 2.4  GHz, 100 repetitions.


is there a way to improve the vector loop using std::vector? By the way if i write the second loop as
// vector loop using std::vector
    for( int i=0; i<ne; i+=Vc::float_v::Size )
    {
        Vc::float_v vx( &x[i] );
        Vx.vector(i) -= one;
        vx.store( &x[i] );
    }
things get even worse, the speedup being only 4.2x roughly.


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--
Dr. Sandro Wenzel
PH / SFT
CERN


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