private static final long getResolvedDataTypeIndex(DataType dataType, DataTypeManagerDB dtm) {

		long dataTypeId = dtm.getID(dataType);
		if (dataTypeId == DataTypeManager.NULL_DATATYPE_ID) {
			return DataTypeManager.NULL_DATATYPE_ID;
		}
		return dataTypeId & ID_TO_INDEX_MASK;
	}
Get a bit-field datatype instance for a given ID.  The upper byte of the ID will be ignored.
@param id bit-field datatype ID
@param dtm data type manager
@return bit-field data type
Ghidra/Framework/SoftwareModeling/src/main/java/ghidra/program/database/data/BitFieldDBDataType.java