[SPARK-9175] [MLLIB] BLAS.gemm fails to update matrix C when alpha==0 and beta!=1

Fix BLAS.gemm to update matrix C when alpha==0 and beta!=1
Also include unit tests to verify the fix.

mengxr brkyvz

Author: Meihua Wu <[email protected]>

Closes #7503 from rotationsymmetry/fix_BLAS_gemm and squashes the following commits:

fce199c [Meihua Wu] Fix BLAS.gemm to update C when alpha==0 and beta!=1

(cherry picked from commit ff3c72d)
Signed-off-by: Xiangrui Meng <[email protected]>

Author: rotationsymmetry

mllib/src/main/scala/org/apache/spark/mllib/linalg/BLAS.scala

@@ -247,8 +247,8 @@ private[spark] object BLAS extends Serializable with Logging {
       B: DenseMatrix,
       beta: Double,
       C: DenseMatrix): Unit = {
-    if (alpha == 0.0) {
-      logDebug("gemm: alpha is equal to 0. Returning C.")
+    if (alpha == 0.0 && beta == 1.0) {
+      logDebug("gemm: alpha is equal to 0 and beta is equal to 1. Returning C.")
     } else {
       A match {
         case sparse: SparseMatrix =>

mllib/src/test/scala/org/apache/spark/mllib/linalg/BLASSuite.scala

@@ -147,8 +147,14 @@ class BLASSuite extends FunSuite {
     val C6 = C1.copy
     val C7 = C1.copy
     val C8 = C1.copy
+    val C13 = C1.copy
+    val C14 = C1.copy
+    val C15 = C1.copy
+    val C16 = C1.copy
     val expected2 = new DenseMatrix(4, 2, Array(2.0, 1.0, 4.0, 2.0, 4.0, 0.0, 4.0, 3.0))
     val expected3 = new DenseMatrix(4, 2, Array(2.0, 2.0, 4.0, 2.0, 8.0, 0.0, 6.0, 6.0))
+    val expected4 = new DenseMatrix(4, 2, Array(5.0, 0.0, 10.0, 5.0, 0.0, 0.0, 5.0, 0.0))
+    val expected5 = C1.copy
 
     gemm(1.0, dA, B, 2.0, C1)
     gemm(1.0, sA, B, 2.0, C2)
@@ -181,6 +187,15 @@ class BLASSuite extends FunSuite {
     assert(C6 ~== expected2 absTol 1e-15)
     assert(C7 ~== expected3 absTol 1e-15)
     assert(C8 ~== expected3 absTol 1e-15)
+
+    gemm(0, dA, B, 5, C13)
+    gemm(0, sA, B, 5, C14)
+    gemm(0, dA, B, 1, C15)
+    gemm(0, sA, B, 1, C16)
+    assert(C13 ~== expected4 absTol 1e-15)
+    assert(C14 ~== expected4 absTol 1e-15)
+    assert(C15 ~== expected5 absTol 1e-15)
+    assert(C16 ~== expected5 absTol 1e-15)
   }
 
   test("gemv") {