comment about aliasing in AddMatMatDivMat.

src/cudamatrix/cu-matrix.cc

@@ -1012,6 +1012,8 @@ void CuMatrixBase<Real>::AddMatBlocks(Real alpha, const CuMatrixBase<Real> &A,
   }
 }
 
+/// dst = a * b / c (by element; when c = 0, dst = a)
+/// dst can be an alias of a, b or c safely and get expected result.
 template<typename Real>
 void CuMatrixBase<Real>::AddMatMatDivMat(const CuMatrixBase<Real> &A,
                     const CuMatrixBase<Real> &B, const CuMatrixBase<Real> &C) {

src/cudamatrix/cu-matrix.h

@@ -416,6 +416,7 @@ class CuMatrixBase {
   /// A = alpha * x * y^T + A .
   void AddVecVec(Real alpha, const CuVectorBase<Real> &x, const CuVectorBase<Real> &y);
   /// *this = a * b / c (by element; when c = 0, *this = a)
+  /// *this can be an alias of a, b or c safely and get expected result.
   void AddMatMatDivMat(const CuMatrixBase<Real> &A, const CuMatrixBase<Real> &B, const CuMatrixBase<Real> &C);
 
   /// *this = beta * *this + alpha * M M^T, for symmetric matrices.  It only