text: ///////////////////////////////////////////////////////////////////////
  // File:        dotproductfma.cpp
  // Description: Architecture-specific dot-product function.
  // Author:      Stefan Weil
  //
  // (C) Copyright 2015, Google Inc.
  // Licensed under the Apache License, Version 2.0 (the "License");
  // you may not use this file except in compliance with the License.
  // You may obtain a copy of the License at
  // http://www.apache.org/licenses/LICENSE-2.0
  // Unless required by applicable law or agreed to in writing, software
  // distributed under the License is distributed on an "AS IS" BASIS,
  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  // See the License for the specific language governing permissions and
  // limitations under the License.
  ///////////////////////////////////////////////////////////////////////
  
  #if defined(__FMA__)
  
  #include <immintrin.h>
  #include <cstdint>
  #include "dotproduct.h"
  
  namespace tesseract {
  
  // Computes and returns the dot product of the n-vectors u and v.
  // Uses Intel FMA intrinsics to access the SIMD instruction set.
  double DotProductFMA(const double* u, const double* v, int n) {
    const unsigned quot = n / 8;
    const unsigned rem = n % 8;
    __m256d t0 = _mm256_setzero_pd();
    __m256d t1 = _mm256_setzero_pd();
    for (unsigned k = 0; k < quot; k++) {
      __m256d f0 = _mm256_loadu_pd(u);
      __m256d f1 = _mm256_loadu_pd(v);
      t0 = _mm256_fmadd_pd(f0, f1, t0);
      u += 4;
      v += 4;
      __m256d f2 = _mm256_loadu_pd(u);
      __m256d f3 = _mm256_loadu_pd(v);
      t1 = _mm256_fmadd_pd(f2, f3, t1);
      u += 4;
      v += 4;
    }
    t0 = _mm256_hadd_pd(t0, t1);
    alignas(32) double tmp[4];
    _mm256_store_pd(tmp, t0);
    double result = tmp[0] + tmp[1] + tmp[2] + tmp[3];
    for (unsigned k = 0; k < rem; k++) {
      result += *u++ * *v++;
    }
    return result;
  }
  
  }  // namespace tesseract.
  
  #endif
  