## Algorithm-Based Code Replacement

You can develop a code replacement library to optimize the performance of certain math functions based on the computation or approximation algorithm configured for a block.

For example, you can configure:

• The Reciprocal Sqrt block to use the `Newton-Raphson` or `Exact` computation method.

• The Trigonometric Function block, with Function set to `sin`, `cos`, `sincos`, or `atan2`, to use the approximation method `CORDIC`, `Lookup`, or `None`.

To develop a code replacement library, use either the interactive or programmatic approach. For more information, see Develop a Code Replacement Library.

### Interactively Develop a Code Replacement Library

1. Open the Code Replacement Tool (crtool), from the MATLAB command line with the following command:

`>>crtool`
2. Create a table.

1. From the crtool context menu, select File > New Table.

2. In the right pane, name the table `crl_rsqrt`. Click Apply.

3. Create an entry. From the crtool context menu, select File > New entry > Function.

4. Create entry parameters. In the Function drop-down list, select `rSqrt`. Algorithm information appears in the crtool. For this example, set the parameter to Newton Raphson.

5. Create the conceptual representation. The conceptual representation describes the signature of the function that you want to replace. In the Conceptual function subsection of the crtool, specify the return argument, `y1`, and the input argument, `u1`, by using the Data Type of double and the Argument Type of Scalar.

Specify a Name for the replacement function under Function prototype.

6. Create the implementation representation. The implementation representation describes the signature of the optimization function. For this example, to specify that the implementation arguments have the same order and properties as the conceptual arguments, select the Make conceptual and implementation argument types the same check box.

7. Specify build information. Click the Build Information tab to open the build requirements pane. Specify the files (source, header, object) that the code generator requires for code replacement. For this example, you do not need to specify build information.

8. Validate and save the table. In the Mapping Information tab, click Validate entry. In the crtool context menu, select File > Save table > Save.

9. Register a code replacement library. Registration creates a library composed of the tables that you specify. Select File > Generate registration file. In the Generate registration file dialog box, fill out these fields:

To use your code replacement library, refresh your current MATLAB session with the command:

`>>sl_refresh_customizations`

10. Verify the code replacement library. From the MATLAB command line, open the library by using the Code Replacement Viewer and verify that the table and entry are correctly specified. For more information, see Verify Code Replacement Library. Configure your model to use the code replacement library, generate code, and verify that replacement occurs as expected. If unexpected behavior occurs, examine the hit and miss logs to troubleshoot the issues.

### Programmatically Develop a Code Replacement Library

1. Open the programmatic interface from the MATLAB menu by selecting New > Function.

2. Create a table.

1. Create a function to call your code replacement library table. The function cannot have arguments or return a table object.

2. Create a table object by calling `RTW.TflTable`.

```function hTable = crl_rsqrt() % Create a function to call the code replacement library table %% Create a table object hTable = RTW.TflTable; ```
3. Create an entry. Because this example replaces a function, create a code replacement entry in your table by calling the entry function `RTW.TflCFunctionEntry`.

```function hTable = crl_rsqrt() % Create a code replacement library table %% Create a table object hTable = RTW.TflTable; %% Create an entry hEntry = RTW.TflCFunctionEntry; ```
4. Create entry parameters. Because this examples replaces a function, create entry parameters by calling the function `setTflCFunctionEntryParameters`.

To use the algorithm for a function in an entry definition, set the `EntryInfoAlgorithm` property in a call to the function `setTflCFunctionEntryParameters` as `RTW_Newton_Raphson`.

```function hTable = crl_rsqrt() % Create a code replacement library table %% Create a table object hTable = RTW.TflTable; %% Create an entry hEntry = RTW.TflCFunctionEntry; %% Create entry parameters hEntry.setTflCFunctionEntryParameters(... 'Key', 'rSqrt', ... 'Priority', 80, ... 'ImplementationName', 'rsqrt_newton', ... 'ImplementationHeaderFile', 'rsqrt.h', ... 'EntryInfoAlgorithm', 'RTW_NEWTON_RAPHSON');```
5. Create the conceptual representation. The conceptual representation describes the signature of the function that you want to replace. To explicitly specify argument properties, call the function `createAndAddConceptualArg`.

```function hTable = crl_rsqrt() % Create a code replacement library table %% Create a table object hTable = RTW.TflTable; %% Create an entry hEntry = RTW.TflCFunctionEntry; %% Create entry parameters hEntry.setTflCFunctionEntryParameters(... 'Key', 'rSqrt', ... 'Priority', 80, ... 'ImplementationName', 'rsqrt_newton', ... 'ImplementationHeaderFile', 'rsqrt.h', ... 'EntryInfoAlgorithm', 'RTW_NEWTON_RAPHSON'); %% Create the conceptual representation createAndAddConceptualArg(hEntry, 'RTW.TflArgNumeric', ... 'Name', 'y1', ... 'IOType', 'RTW_IO_OUTPUT', ... 'DataTypeMode', 'double'); createAndAddConceptualArg(hEntry, 'RTW.TflArgNumeric', ... 'Name', 'u1', ... 'DataTypeMode', 'double');```
6. Create the implementation representation. The implementation representation describes the signature of the optimization function. To specify that the implementation arguments have the same order and properties as the conceptual arguments, call the function `copyConceptualArgsToImplementation`. Add the complete entry to the table by calling the function `addEntry`.

```function hTable = crl_rsqrt() % Create a code replacement library table %% Create a table object hTable = RTW.TflTable; %% Create an entry hEntry = RTW.TflCFunctionEntry; %% Create entry parameters hEntry.setTflCFunctionEntryParameters(... 'Key', 'rSqrt', ... 'Priority', 80, ... 'ImplementationName', 'rsqrt_newton', ... 'ImplementationHeaderFile', 'rsqrt.h', ... 'EntryInfoAlgorithm', 'RTW_NEWTON_RAPHSON'); %% Create the conceptual representation createAndAddConceptualArg(hEntry, 'RTW.TflArgNumeric', ... 'Name', 'y1', ... 'IOType', 'RTW_IO_OUTPUT', ... 'DataTypeMode', 'double'); createAndAddConceptualArg(hEntry, 'RTW.TflArgNumeric', ... 'Name', 'u1', ... 'DataTypeMode', 'double'); %% Create the Implementation Representation copyConceptualArgsToImplementation(hEntry); %% Add the entry to the table hTable.addEntry(hEntry); ```
7. Specify build information. In the entry parameters, specify files (header, source, object) that the code generator needs for code replacement. For this example, build information is not required.

8. Validate and save the customization file. From the MATLAB menu, save this customization file by selecting File > Save. From the command line, validate the code replacement library table by calling it:

`>> hTable = crl_rsqrt`
9. Register the code replacement library. Registration creates a code replacement library by defining the library name, code replacement tables, and other information. Create a registration file by using these specifications:

```function rtwTargetInfo(cm) cm.registerTargetInfo(@loc_register_crl); end function this = loc_register_crl this(1) = RTW.TflRegistry; this(1).Name = 'CRL for algorithm function replacement'; this(1).TableList = {'crl_rsqrt.m'}; % table created in this example this(1).TargetHWDeviceType = {'*'}; this(1).Description = 'Example code replacement library'; end ```

To use your code replacement library, refresh your current MATLAB session with the command:

`>>sl_refresh_customizations`

10. Verify the code replacement library. From the MATLAB command line, open the library by using the Code Replacement Viewer and verify that the table and entry are correctly specified. For more information, see Verify Code Replacement Library. Configure your model to use the code replacement library, generate code, and verify that replacement occurs as expected. If unexpected behavior occurs, examine the hit and miss logs to troubleshoot the issues.