frep-loop: PHI problem with reductions

[huettern]

For parallel reductions where the accumulators are all initialized to the same value, the PHI nodes are merged into the instructions and the reduction is made on the register of initial value, instead on "new" registers

  for(unsigned i=0; i < MATSIZE; ++i) {
    #pragma unroll 1
    for(unsigned j=0; j < MATSIZE; j+=UNROLL) {
      register double acc[UNROLL];
      
      #pragma unroll
      for (int u = 0; u < UNROLL; ++u) acc[u] = 0.0; // <- All is fine if this is e.g. c[i*MATSIZE+j+u];
      
      #pragma frep infer
      for(unsigned k=0; k < MATSIZE; ++k) {
        #pragma unroll
        for (int u = 0; u < UNROLL; ++u)
        {
          acc[u] += __builtin_ssr_pop(0)*__builtin_ssr_pop(1);
        }
      }
      
      #pragma unroll
      for (int u = 0; u < UNROLL; ++u) c[i*MATSIZE+j+u] = acc[u];
    }
  }

With 0.0 this results in the wrong assembly

fmadd.d	ft5, ft1, ft0, ft3
fmadd.d	ft6, ft1, ft0, ft3
fmadd.d	ft7, ft1, ft0, ft3
# ...

For inididual initial values (c[i*MATSIZE+j+u], the problem disappears

fmadd.d	ft3, ft1, ft0, ft3
fmadd.d	ft4, ft1, ft0, ft4
fmadd.d	ft5, ft1, ft0, ft5
fmadd.d	ft6, ft1, ft0, ft6