[SYSTEMDS-3653] Ultra Sparse Right MM Optimization

Right side Ultra sparse optimizations goring from 8.525 to 4.575
on 100 repetitions of 100k by 1000 dense %*% 1000 by 1000 with 30 non zeros.

Closes #1952

src/main/java/org/apache/sysds/runtime/matrix/data/LibMatrixMult.java

@@ -49,6 +49,7 @@
 import org.apache.sysds.runtime.data.SparseBlockCSR;
 import org.apache.sysds.runtime.data.SparseBlockFactory;
 import org.apache.sysds.runtime.data.SparseBlockMCSR;
+import org.apache.sysds.runtime.data.SparseRow;
 import org.apache.sysds.runtime.data.SparseRowScalar;
 import org.apache.sysds.runtime.data.SparseRowVector;
 import org.apache.sysds.runtime.functionobjects.SwapIndex;
@@ -194,7 +195,7 @@ public static MatrixBlock matrixMult(MatrixBlock m1, MatrixBlock m2, MatrixBlock
 			(!fixedRet && isUltraSparseMatrixMult(m1, m2, m1Perm));
 		boolean sparse = !fixedRet && !ultraSparse && !m1Perm
 			&& isSparseOutputMatrixMult(m1, m2);
-		
+
 		// allocate output
 		if(ret == null)
 			ret = new MatrixBlock(m1.rlen, m2.clen, ultraSparse | sparse);
@@ -1718,7 +1719,6 @@ else if( leftUS || m1Perm )
 			matrixMultUltraSparseLeft(m1, m2, ret, rl, ru);
 		else
 			matrixMultUltraSparseRight(m1, m2, ret, rl, ru);
-		//no need to recompute nonzeros because maintained internally
 	}
 
 	private static void matrixMultUltraSparseSelf(MatrixBlock m1, MatrixBlock ret, int rl, int ru) {
@@ -1926,10 +1926,14 @@ private static void matrixMultUltraSparseSparseSparseLeftRowGeneric(int i, int a
 
 
 	private static void matrixMultUltraSparseRight(MatrixBlock m1, MatrixBlock m2, MatrixBlock ret, int rl, int ru) {
-		if(!ret.isInSparseFormat() && ret.getDenseBlock().isContiguous())
+		if(ret.isInSparseFormat()){
+			if(m1.isInSparseFormat())
+				matrixMultUltraSparseRightSparseMCSRLeftSparseOut(m1, m2, ret, rl, ru);
+			else
+				matrixMultUltraSparseRightDenseLeftSparseOut(m1, m2, ret, rl, ru);
+		}
+		else if(ret.getDenseBlock().isContiguous())
 			matrixMultUltraSparseRightDenseOut(m1, m2, ret, rl, ru);
-		else if(m1.isInSparseFormat() && ret.isInSparseFormat())
-			matrixMultUltraSparseRightSparseMCSRLeftSparseOut(m1, m2, ret, rl, ru);
 		else
 			matrixMultUltraSparseRightGeneric(m1, m2, ret, rl, ru);
 	}
@@ -1990,6 +1994,39 @@ private static void matrixMultUltraSparseRightSparseMCSRLeftSparseOut(MatrixBloc
 		}
 	}
 
+	private static void matrixMultUltraSparseRightDenseLeftSparseOut(MatrixBlock m1, MatrixBlock m2, MatrixBlock ret, int rl, int ru) {
+		final int cd = m1.clen;
+		final DenseBlock  a = m1.denseBlock;
+		final SparseBlock b = m2.sparseBlock;
+		final SparseBlockMCSR c = (SparseBlockMCSR) ret.sparseBlock;
+
+		for(int k = 0; k < cd; k++){
+			if(b.isEmpty(k))
+				continue; // skip emptry rows right side.
+			final int bpos = b.pos(k);
+			final int blen = b.size(k);
+			final int[] bixs = b.indexes(k);
+			final double[] bvals = b.values(k);
+			for(int i = rl; i < ru; i++) 
+				mmDenseMatrixSparseRow(bpos, blen, bixs, bvals, k, i, a, c);
+		}
+	}
+
+	private static void mmDenseMatrixSparseRow(int bpos, int blen, int[] bixs, double[] bvals, int k, int i,
+		DenseBlock a, SparseBlockMCSR c) {
+		final double[] aval = a.values(i);
+		final int apos = a.pos(i);
+		if(!c.isAllocated(i))
+			c.allocate(i, Math.max(blen, 2));
+		final SparseRowVector srv = (SparseRowVector) c.get(i); // guaranteed
+		for(int j = bpos; j < bpos + blen; j++) { // right side columns
+			final int bix = bixs[j];
+			final double bval = bvals[j];
+			srv.add(bix, bval * aval[apos + k]);
+		}
+
+	}
+
 	private static void matrixMultUltraSparseRightGeneric(MatrixBlock m1, MatrixBlock m2, MatrixBlock ret, int rl, int ru) {
 		final int cd = m1.clen;