Github user dbtsai commented on a diff in the pull request:

    https://github.com/apache/spark/pull/6209#discussion_r30473300
  
    --- Diff: mllib/src/main/scala/org/apache/spark/mllib/linalg/BLAS.scala ---
    @@ -473,44 +473,161 @@ private[spark] object BLAS extends Serializable with 
Logging {
         if (alpha == 0.0) {
           logDebug("gemv: alpha is equal to 0. Returning y.")
         } else {
    -      A match {
    -        case sparse: SparseMatrix =>
    -          gemv(alpha, sparse, x, beta, y)
    -        case dense: DenseMatrix =>
    -          gemv(alpha, dense, x, beta, y)
    +      (A, x) match {
    +        case (smA: SparseMatrix, dvx: DenseVector) =>
    +          gemv(alpha, smA, dvx, beta, y)
    +        case (smA: SparseMatrix, svx: SparseVector) =>
    +          gemv(alpha, smA, svx, beta, y)
    +        case (dmA: DenseMatrix, dvx: DenseVector) =>
    +          gemv(alpha, dmA, dvx, beta, y)
    +        case (dmA: DenseMatrix, svx: SparseVector) =>
    +          gemv(alpha, dmA, svx, beta, y)
             case _ =>
    -          throw new IllegalArgumentException(s"gemv doesn't support matrix 
type ${A.getClass}.")
    +          throw new IllegalArgumentException(s"gemv doesn't support 
running on matrix type " +
    +            s"${A.getClass} and vector type ${x.getClass}.")
           }
         }
       }
     
       /**
        * y := alpha * A * x + beta * y
    -   * For `DenseMatrix` A.
    +   * For `DenseMatrix` A and `DenseVector` x.
        */
       private def gemv(
           alpha: Double,
           A: DenseMatrix,
           x: DenseVector,
           beta: Double,
    -      y: DenseVector): Unit =  {
    +      y: DenseVector): Unit = {
         val tStrA = if (A.isTransposed) "T" else "N"
         val mA = if (!A.isTransposed) A.numRows else A.numCols
         val nA = if (!A.isTransposed) A.numCols else A.numRows
         nativeBLAS.dgemv(tStrA, mA, nA, alpha, A.values, mA, x.values, 1, beta,
           y.values, 1)
       }
    + 
    +  /**
    +   * y := alpha * A * x + beta * y
    +   * For `DenseMatrix` A and `SparseVector` x.
    +   */
    +  private def gemv(
    +      alpha: Double,
    +      A: DenseMatrix,
    +      x: SparseVector,
    +      beta: Double,
    +      y: DenseVector): Unit = {
    +    val mA: Int = A.numRows
    +    val nA: Int = A.numCols
    +
    +    val Avals = A.values
    +
    +    val xIndices = x.indices
    +    val xNnz = xIndices.length
    +    val xValues = x.values
    +    val yValues = y.values
    +
    +    scal(beta, y)
     
    +    if (A.isTransposed) {
    +      var rowCounterForA = 0
    +      while (rowCounterForA < mA) {
    +        var sum = 0.0
    +        var k = 0
    +        while (k < xNnz) {
    +          sum += xValues(k) * Avals(xIndices(k) + rowCounterForA * nA)
    +          k += 1
    +        }
    +        yValues(rowCounterForA) += sum * alpha
    +        rowCounterForA += 1
    +      }
    +    } else {
    +      var rowCounterForA = 0
    +      while (rowCounterForA < mA) {
    +        var sum = 0.0
    +        var k = 0
    +        while (k < xNnz) {
    +          sum += xValues(k) * Avals(xIndices(k) * mA + rowCounterForA)
    +          k += 1
    +        }
    +        yValues(rowCounterForA) += sum * alpha
    +        rowCounterForA += 1
    +      }
    +    }
    +  }
    + 
       /**
        * y := alpha * A * x + beta * y
    -   * For `SparseMatrix` A.
    +   * For `SparseMatrix` A and `SparseVector` x.
    +   */
    +  private def gemv(
    +      alpha: Double,
    +      A: SparseMatrix,
    +      x: SparseVector,
    +      beta: Double,
    +      y: DenseVector): Unit = {
    +    val xValues = x.values
    +    val xIndices = x.indices
    +    val xNnz = xIndices.length
    +
    +    val yValues = y.values
    +
    +    val mA: Int = A.numRows
    +    val nA: Int = A.numCols
    +
    +    val Avals = A.values
    +    val Arows = if (!A.isTransposed) A.rowIndices else A.colPtrs
    +    val Acols = if (!A.isTransposed) A.colPtrs else A.rowIndices
    +
    +    scal(beta, y)
    --- End diff --
    
    ```
    if (alpha == 0.0 && beta != 0.0) {
      scal(beta, y)
      return
    }
    ```


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