Part 2: Portable Intrinsics

Using intrinsics while retaining code portability can be challenging. The way I prefer to do it is to create a C++ class that represents a vector, and that provides a thin wrapper around the intrinsic data and functions.

For example, include/floatvec.h contains a simple FloatVec class that defines a single vector of floats. The interface to the FloatVec class is the same regardless of whether SSE2, AVX or another vectorisation scheme is used. It is only the implementation that changes. For example, the start of the class is as follows;

/** A simple class that provides a portable, optimised
    vector of floats
*/
class FloatVec
{
private:
    #ifdef __AVX__
        // if we have AVX, then we have a 8xfloat vector
        __m256 v;
        #define FLOATVEC_SIZE 8
    #else
    #ifdef __SSE2__
        // if we have SSE2, then we have a 4xfloat vector
        __m128 v;
        #define FLOATVEC_SIZE 4
    #else
        // in the general case, we will create a float array of 
        // size 8 (we can choose whatever size we want)
        float v[8];
        #define FLOATVEC_SIZE 8
    #endif
    #endif

If AVX is available, then the #ifdef __AVX__ is true, and so the FloatVec class has a single member variable called v, which is of type __m256.

If AVX isn’t available, but SSE2 is, then #ifdef __SSE2__ is true. This means that now FloatVec contains a single member variable valled v, which is of type __m128.

If neither AVX nor SSE2 are available, then FloatVec contains a single member varable called v, which is an array of eight floats.

The interface to FloatVec provides some useful functions;

The last function is worth looking at deeper, as it shows how #ifdef guards are used to provide a different implementation depending on whether AVX, SSE2 or something else is available.

/** Add two vectors together */
inline FloatVec FloatVec::operator+(const FloatVec &other) const
{
    #ifdef __AVX__
        return FloatVec( _mm256_add_ps(v, other.v) );
    #else
    #ifdef __SSE2__
        return FloatVec( _mm_add_ps(v, other.v) );
    #else
        FloatVec result;

        #pragma omp simd
        for (int i=0; i<size(); ++i)
        {
            result.v[i] = v[i] + other.v[i];
        }

        return result;
    #endif
    #endif
}

The first thing to note is that this function is declared inline, meaning that it is automatically expanded by the compiler, with the function call replaced by the code within the function at compile time. This is to ensure that we don’t suffer any penalties from calling a function.

Next, we have three implementations;

Using FloatVec to abstract vector instrinsics

Now we will use FloatVec to vectorise the loop.cpp example that we have been using throughout the workshop. Create a new file called portable.cpp and copy into it;

#include "workshop.h"
#include "floatvec.h"

int main(int argc, char **argv)
{
    const int size = 512;

    auto a = workshop::Array<float>(size);
    auto b = workshop::Array<float>(size);
    auto c = workshop::Array<float>(size);

    for (int i=0; i<size; ++i)
    {
        a[i] = 1.0*(i+1);
        b[i] = 2.5*(i+1);
        c[i] = 0.0;
    }

    auto vec_a = FloatVec::fromArray(a);
    auto vec_b = FloatVec::fromArray(b);
    auto vec_c = FloatVec::fromArray(c);

    const int vec_size = vec_a.size();

    auto timer = workshop::start_timer();

    for (int j=0; j<100000; ++j)
    {
        for (int i=0; i<size; ++i)
        {
            c[i] = a[i] + b[i];
        }
    }

    auto duration = workshop::get_duration(timer);

    timer = workshop::start_timer();

    for (int j=0; j<100000; ++j)
    {   
        for (int i=0; i<vec_size; ++i)
        {
            vec_c[i] = vec_a[i] + vec_b[i];
        }
    }

    std::cout << "c = ";

    for (int i=0; i<c.size(); ++i)
    {
        std::cout << c[i] << " ";
    }

    std::cout << std::endl;

    c = FloatVec::toArray(vec_c);

    std::cout << "vec_c = ";

    for (int i=0; i<c.size(); ++i)
    {
        std::cout << c[i] << " ";
    }

    std::cout << std::endl;

    auto vector_duration = workshop::get_duration(timer);

    std::cout << "The standard loop took " << duration
              << " microseconds to complete." << std::endl;

    std::cout << "The vectorised loop took " << vector_duration
              << " microseconds to complete." << std::endl;

    return 0;
}

We will compile this for AVX, SSE and omp simd using;

g++ -O2 -msse2 --std=c++14 -Iinclude portable.cpp -o sse_portable
g++ -O2 -mavx --std=c++14 -Iinclude portable.cpp -o avx_portable
g++ -O2 -fopenmp-simd --std=c++14 -Iinclude portable.cpp -o omp_portable

Run the three executables and compare their speed, using;

./sse_portable
./avx_portable
./omp_portable

Note that, in this example, we have printed out the value of c and vec_c so that you can see that the results are all the same.

On my computer I find that the standard loop takes about 32 ms to run.

Exercises

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