[MINOR] Avoid unnecessary overhead in createvar instructions

This patch makes a minor performance improvement to the createvar
instruction execution (which happens for every non-scalar operator). In
detail, the need for creating unique file names (from one instruction),
led to unnecessary string concatenation and thus object allocation. We
now reuse the existing thread-local string builders as used for
instruction generation.

On a special-case scenario with ~1M loop iterations over tiny data (100
values), this patch improved the createvar overhead from 22.1s to 5.6s
(and overall from 49s to 33s).

src/main/java/org/apache/sysds/runtime/instructions/InstructionUtils.java

@@ -1008,4 +1008,12 @@ public static String concatOperands(String... inputs) {
 		sb.append(inputs[inputs.length-1]);
 		return sb.toString();
 	}
+
+	public static String concatStrings(String... inputs) {
+		StringBuilder sb = _strBuilders.get();
+		sb.setLength(0); //reuse allocated space
+		for( int i=0; i<inputs.length; i++ )
+			sb.append(inputs[i]);
+		return sb.toString();
+	}
 }

src/main/java/org/apache/sysds/runtime/instructions/cp/VariableCPInstruction.java

@@ -521,9 +521,8 @@ public void processInstruction(ExecutionContext ec) {
 				//(existing objects gets cleared through rmvar instructions)
 				String fname = getInput2().getName();
 				// check if unique filename needs to be generated
-				if( Boolean.parseBoolean(getInput3().getName()) ) {
-					fname = fname + '_' + _uniqueVarID.getNextID();
-				}
+				if( Boolean.parseBoolean(getInput3().getName()) )
+					fname = getUniqueFileName(fname);
 				MatrixObject obj = new MatrixObject(getInput1().getValueType(), fname);
 				//clone meta data because it is updated on copy-on-write, otherwise there
 				//is potential for hidden side effects between variables.
@@ -544,9 +543,8 @@ else if( getInput1().getDataType() == DataType.TENSOR ) {
 				//(existing objects gets cleared through rmvar instructions)
 				String fname = getInput2().getName();
 				// check if unique filename needs to be generated
-				if( Boolean.parseBoolean(getInput3().getName()) ) {
-					fname = fname + '_' + _uniqueVarID.getNextID();
-				}
+				if( Boolean.parseBoolean(getInput3().getName()) )
+					fname = getUniqueFileName(fname);
 				CacheableData<?> obj = new TensorObject(getInput1().getValueType(), fname);
 				//clone meta data because it is updated on copy-on-write, otherwise there
 				//is potential for hidden side effects between variables.
@@ -560,6 +558,8 @@ else if( getInput1().getDataType() == DataType.TENSOR ) {
 			}
 			else if( getInput1().getDataType() == DataType.FRAME ) {
 				String fname = getInput2().getName();
+				if( Boolean.parseBoolean(getInput3().getName()) )
+					fname = getUniqueFileName(fname);
 				FrameObject fobj = new FrameObject(fname);
 				fobj.setMetaData((MetaData)metadata.clone());
 				fobj.setFileFormatProperties(_formatProperties);
@@ -1257,4 +1257,8 @@ public boolean isVariableCastInstruction() {
 			|| opcode == VariableOperationCode.CastAsDoubleVariable
 			|| opcode == VariableOperationCode.CastAsBooleanVariable;
 	}
+
+	public static String getUniqueFileName(String fname) {
+		return InstructionUtils.concatStrings(fname, "_", String.valueOf(_uniqueVarID.getNextID()));
+	}
 }