Fix BLAS.gemm to update C when alpha==0 and beta!=1

Also included tests.

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

@@ -303,8 +303,8 @@ private[spark] object BLAS extends Serializable with Logging {
       C: DenseMatrix): Unit = {
     require(!C.isTransposed,
       "The matrix C cannot be the product of a transpose() call. C.isTransposed must be false.")
-    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 => gemm(alpha, sparse, B, beta, C)

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

@@ -200,8 +200,14 @@ class BLASSuite extends SparkFunSuite {
     val C10 = C1.copy
     val C11 = C1.copy
     val C12 = 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)
@@ -248,6 +254,16 @@ class BLASSuite extends SparkFunSuite {
     assert(C10 ~== expected2 absTol 1e-15)
     assert(C11 ~== expected3 absTol 1e-15)
     assert(C12 ~== 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") {