Issue with O2/O3 optimisation using Intel compiler 2017 update 2

Hello,

While trying to compare parallelism between OMP, MPI, Cuda and OpenACC, I've encountered some dangerous behavior using the "Intel ICPC 17.0.2 20170213". The code below is solving the classical temperature distribution in 2D (just for test):

// /opt/intel/bin/icpc -O2 main.cpp && time ./a.out
#include <iostream>
#include <cmath>

int main()
{

  const int nx = 2800;
  const int ny = 2800;
  const float lx = 1.f;
  const float ly = 1.f;
  float dx = lx / (float)nx;
  float dy = ly / (float)ny;
  const float lambda = 0.001f;
  float dt = 0.01f/lambda/(1.0f/(dx*dx)/12.f+1.0f/(dy*dy)/12.f);
  float dtdx = dt*lambda/dx/dx/12.f;
  float dtdy = dt*lambda/dy/dy/12.f;

  int nxt = nx + 4;
  int nyt = ny + 4;

  float* T = new float[nxt*nyt];
  float* Told = new float[nxt*nyt];

  for (int i = 0; i < nxt; i++) {
    for (int j = 0; j < nyt; j++) {
      int ind = i*nyt + j;
      T[ind] = 1.0f + exp(-32.f*(pow((float)(j-nyt/2)/(float)nyt,2)+pow((float)(i-nxt/2)/(float)nxt,2)));
      Told[ind] = T[ind];
    }
  }

  for (int step=0; step<1000; step++) {

    for (int i = 2; i < nxt-2; i++) {
      for (int j = 2; j < nyt-2; j++) {
        int ind = i*nyt + j;
        T[ind] = Told[ind] + dtdx * (-Told[(i-2)*nyt + (j)]+16.f*Told[(i-1)*nyt + (j)]-30.f*Told[(i)*nyt + (j)]+16.f*Told[(i+1)*nyt + (j)]-Told[(i+2)*nyt + (j)])
        + dtdy * (-Told[(i)*nyt + (j-2)]+16.f*Told[(i)*nyt + (j-1)]-30.f*Told[(i)*nyt + (j)]+16.f*Told[(i)*nyt + (j+1)]-Told[(i)*nyt + (j+2)]);
      }
    }

    for (int i = 0; i < nxt*nyt; i++) {
        Told[i] = T[i];
    }

  }

  float sum = 0.0f;
  for (int i = 0; i < nxt*nyt; i++) {
      sum += T[i];
  }

  std::cout << sum/(float)(nxt*nyt) << std::endl;

  return 0;
}

After 1000 "time" iterations, the code is supposed to give the results: 1.08712 (using O0 or O1 compile flag)

Without any parallelism (no openmp), the code is compiled with O2 optimisation and gives the following (wrong) results: 1.09783

/opt/intel/bin/icpc -O2 main.cpp && time ./a.out

Using LLVM C++ (Apple) or GNU G++, with O2 or even O3 optimization flag, gives good results (1.08712).

It seems that Intel compiler with O2 or O3 compile flag does aggressive optimizations on floating-point data. In order to get the good result, -fp-model precise flag needs to be added to the command line.

Why does the O2 flag create such aggressive optimizations? (while GNU or LLVM does not) I mean, I know that O3 can be a dangerous flag to use but I thought O2 was, a least, usable. If you are not aware of the -fp-model flag, your results may be completely wrong...

Thank you.