c++ - 需要 std::vector 优化

Question

我编写了一个使用 std::vector<double> 的例程相当沉重。它运行得相当慢，AQTime 似乎暗示我正在构建大量的 vector ，但我不确定为什么会这样。对于某些上下文，我的示例运行迭代了 10 次。每次迭代将 3 个约 400 点的 c 数组复制到 vector 中，并创建 3 个新的相同大小的 vector 用于输出。每个输出点可能是来自 2 个输入 vector 的最多 20 个点相加的结果，最坏的情况是 10*400*3*2*20 = 480,000 次取消引用。令人难以置信的是，探查器表明某些 std::方法被调用了 4600 万次。我怀疑我做错了什么!

部分代码:

vector<double>gdbChannel::GetVector() {
   if (fHaveDoubleData & (fLength > 0)) {
      double * pD = getDoublePointer();
      vector<double>v(pD, pD + fLength);

      return v;
   } else {
      throw(Exception("attempt to retrieve vector on empty line")); ;
   }
}

void gdbChannel::SaveVector(GX_HANDLE _hLine, const vector<double> & V) {
   if (hLine != _hLine) {
      GetLine(_hLine, V.size(), true);
   }
   GX_DOUBLE * pData = getDoublePointer();
   memcpy(pData, &V[0], V.size()*sizeof(V[0]));
   ReplaceData();
}

///This routine gets called 10 times
 bool SpecRatio::DoWork(GX_HANDLE_PTR pLine) {
   if (!(hKin.GetLine(*pLine, true) && hUin.GetLine(*pLine, true) && hTHin.GetLine(*pLine, true))) {
      return true;
   }
   vector<double>vK = hKin.GetVector();
   vector<double>vU = hUin.GetVector();
   vector<double>vTh = hTHin.GetVector();

   if ((vK.size() == 0) || (vU.size() == 0) || (vTh.size() == 0)) {
      return true;
   }
   ///TODO: confirm all vectors the same lenghth
   len = vK.size();
   vUK.clear();  // these 3 vectors are declared as private class members
   vUTh.clear();
   vThK.clear();
   vUK.reserve(len);
   vUTh.reserve(len);
   vThK.reserve(len);

   // TODO: ensure everything is same fidincr, fidstart and length

   for (int i = 0; i < len; i++) {
      if (vK.at(i) < MinK) {
         vUK.push_back(rDUMMY);
         vUTh.push_back(rDUMMY);
         vThK.push_back(rDUMMY);
      } else {
         vUK.push_back(RatioPoint(vU, vK, i, UMin, KMin));
         vUTh.push_back(RatioPoint(vU, vTh, i, UMin, ThMin));
         vThK.push_back(RatioPoint(vTh, vK, i, ThMin, KMin));
      }

   }
   hUKout.setFidParams(hKin);
   hUKout.SaveVector(*pLine, vUK);
   hUTHout.setFidParams(hKin);
   hUTHout.SaveVector(*pLine, vUTh);
   hTHKout.setFidParams(hKin);
   hTHKout.SaveVector(*pLine, vThK);
   return TestError();
}

double SpecRatio::VValue(vector<double>V, int Index) {
   double result;
   if ((Index < 0) || (Index >= len)) {
      result = 0;

   } else {
      try {
         result = V.at(Index);
         if (OasisUtils::isDummy(result)) {
            result = 0;
         }
      }
      catch (out_of_range) {
         result = 0;
      }
   }
   return result;
}

double SpecRatio::RatioPoint(vector<double>Num, vector<double>Denom, int Index, double NumMin, double DenomMin) {
   double num = VValue(Num, Index);
   double denom = VValue(Denom, Index);
   int s = 0;
   // Search equalled 10 in this case
   while (((num < NumMin) || (denom < DenomMin)) && (s < Search)) {
      num += VValue(Num, Index - s) + VValue(Num, Index + s);
      denom += VValue(Denom, Index - s) + VValue(Denom, Index + s);
      s++;
   }
   if ((num < NumMin) || (denom < DenomMin)) {
      return rDUMMY;
   } else {
      return num / denom;
   }

}

最严重的 AQTime 违规行为是:

std::_Uninit_copy >, double *, std::allocator > 3.65 secs and 115731 Hits

std::_Construct 1.69 secs and 46450637 Hits

std::_Vector_const_iterator >::operator !=1.66 secs and 46566395 Hits and so on...

std::allocator<double>::construct ,
operator new ,
std::_Vector_const_iterator<double, std::allocator<double> >::operator ++ , std::_Vector_const_iterator<double, std::allocator<double> >::operator * std::_Vector_const_iterator<double, std::allocator<double> >::operator ==

每个都被调用超过 4600 万次。

我显然做错了什么导致创建所有这些对象。谁能看到我的错误？

Answer 1

这是因为您正在按值传递函数参数。每次按值传递 std::vector 时，都必须制作 vector 的完整拷贝。

改变这些:

double SpecRatio::VValue(vector<double>V, int Index) {

double SpecRatio::RatioPoint(vector<double>Num, vector<double>Denom...

收件人:

double SpecRatio::VValue(const vector<double> &V, int Index)

double SpecRatio::RatioPoint(const vector<double> &Num, const vector<double> &Denom...

因为对于您的使用，您实际上从来不需要制作这些载体的单独拷贝。