Problems encountred during vectorization of code using SSE intrinsics

[priyanka06]

I have been struggling with vectorizing a particular application for sometime now and I have tried everything. From autovectorization, to handcoded SSE intrinsics. But somehow I am unable to obtain speedup on my stencil based application.

Following is a snippet of my current code, which I have vectorized using SSE intrinsics.

//#pragma ivdep
 for ( i = STENCIL; i < z - STENCIL; i+=4 )
 {
  it = it2 + i;
  __m128 center = _mm_mul_ps(_mm_load_ps(&p2[it]),C00_i);

  u_j4 = _mm_load_ps(&p2[i+j*it_j-it_j4+k*it_k]); //Line 180
  u_j3 = _mm_load_ps(&p2[i+j*it_j-it_j3+k*it_k]);
  u_j2 = _mm_load_ps(&p2[i+j*it_j-it_j2+k*it_k]);
  u_j1 = _mm_load_ps(&p2[i+j*it_j-it_j +k*it_k]);
  u_j8 = _mm_load_ps(&p2[i+j*it_j+it_j4+k*it_k]);
  u_j7 = _mm_load_ps(&p2[i+j*it_j+it_j3+k*it_k]);
  u_j6 = _mm_load_ps(&p2[i+j*it_j+it_j2+k*it_k]);
  u_j5 = _mm_load_ps(&p2[i+j*it_j+it_j +k*it_k]);

  __m128 tmp2i = _mm_mul_ps(_mm_add_ps(u_j4,u_j8),X4_i);
  __m128 tmp3 = _mm_mul_ps(_mm_add_ps(u_j3,u_j7),X3_i);
  __m128 tmp4 = _mm_mul_ps(_mm_add_ps(u_j2,u_j6),X2_i);
  __m128 tmp5 = _mm_mul_ps(_mm_add_ps(u_j1,u_j5),X1_i);

  __m128 tmp6 = _mm_add_ps(_mm_add_ps(tmp2i,tmp3),_mm_add_ps(tmp4,tmp5));
  __m128 tmp7 = _mm_add_ps(tmp6,center);

  _mm_store_ps(&tmp2,tmp7); //Line 196

 }

When I compile (icc) the above code without #pragma ivdep I get the following message:
remark: loop was not vectorized: existence of vector dependence.
vector dependence: assumed FLOW dependence between tmp2 line 196 and tmp2 line 196.
vector dependence: assumed ANTI dependence between tmp2 line 196 and tmp2 line 196.
When I compile (icc) it with the #pragma ivdep, I get the following message:
remark: loop was not vectorized: unsupported data type. //Line 180
Why is there a dependence suggested for Line 196? How can I eliminate the suggested vector dependence?

[Thomas_W_Intel]

When you already have vectorized the code with intrinsics, the compiler cannot auto-vectorize it: You already have done the vectorization. If you want to investigate in auto-vectorization, you have to go back to the scalar version.

[jimdempseyatthecove]

Consider writing your sse intrinsics to use fewer temps and to interleave the loads with the multiplys Jim Dempsey