autovectorization test

vtest.cc

#include <iostream>
#include <cmath>

// clang++15 vtest.cc -O2 -ffast-math -Rpass=loop-vectorize -mavx2 -o vtest

struct Stretch {
  float kb;
  float r0;
  int I1;
  int I2;
};

float stretch_energy(Stretch*  stretch_begin,
                     Stretch* stretch_end,
                     float* pos) {
  float Energy = 0.0;
  for (auto stretch = stretch_begin; stretch < stretch_end; ++stretch) {
    int I1 = stretch->I1, I2 = stretch->I2;
    float x1 = pos[I1+0], y1 = pos[I1+1], z1 = pos[I1+2];
    float x2 = pos[I2+0], y2 = pos[I2+1], z2 = pos[I2+2];
    float xdist = x1 - x2, ydist = y1 - y2, zdist = z1 - z2;
    float dist = std::sqrt(xdist*xdist+ydist*ydist+zdist*zdist);
    float StretchDeviation = dist - stretch->r0;
    Energy += stretch->kb * StretchDeviation * StretchDeviation;
  }
  return Energy;
}

int main(int argc, const char* argv[]) {
  float pos[12] = {0.0, 19.0,3.0, 10.0, 7.0, 80.0,
                   20.0, 15.0,17.0, 25.0, 44.0, 23.0 };
  Stretch stretch[] = { {10.0, 2.0, 0, 3}, {20.0, 3.0, 6, 9} };
  float energy = stretch_energy(&stretch[0], &stretch[2], pos);
  std::cout << "Energy = " << energy << "\n";
  return 0;
}