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Customize Match and Replacement Process

During the build process, the code generator uses:

  • Preset match criteria to identify functions and operators for which application-specific implementations replace default implementations.

  • Preset replacement function signatures.

It is possible that preset match criteria and preset replacement function signatures do not completely meet your function and operator replacement needs. For example:

  • You want to replace an operator with a particular fixed-point implementation function only when fraction lengths are within a particular range.

  • When a match occurs, you want to modify your replacement function signature based on compile-time information, such as passing fraction-length values into the function.

To add extra logic into the code replacement match and replacement process, create custom code replacement table entries. With custom entries, you can specify additional match criteria and modify the replacement function signature to meet application needs.

To create a custom code replacement entry:

  1. Create a custom code replacement entry class, derived from RTW.TflCFunctionEntryML (for function replacement) or RTW.TflCOperationEntryML (for operator replacement).

  2. In your derived class, implement a do_match method with a fixed preset signature as a MATLAB® function. In your do_match method, create a copy of an existing type of code replacement entry. On the copy, provide either or both of the following customizations that instantiate the class:

    • Add match criteria that the base class does not provide. The base class provides a match based on:

      • Argument number

      • Argument name

      • Signedness

      • Word size

      • Slope (if not specified with wildcards)

      • Bias (if not specified with wildcards)

      • Math modes, such as saturation and rounding

      • Operator or function key

    • Modify the implementation signature by adding additional arguments or setting constant input argument values. You can inject a constant value, such as an input scaling value, as an additional argument to the replacement function.

  3. Create code replacement entries that instantiate the custom entry class.

  4. Register a library containing the code replacement table that includes your entries.

During code generation, the code replacement match process tries to match function or operator call sites with the base class of your derived entry class. If the process finds a match, the software calls your do_match method to execute your additional match logic (if any) and your replacement function customizations (if any).

Customize Match and Replacement Process for Operators

This example shows how to create custom code replacement entries that add logic to the code match and replacement process for a scalar operation. Custom entries specify additional match criteria or modify the replacement function signature to meet application needs.

For example:

  • When fraction lengths are within a specific range, replace an operator with a fixed-point implementation function.

  • When a match occurs, modify the replacement function signature based on compile-time information, such as passing fraction-length values into the function.

This example modifies a fixed-point addition replacement such that the implementation function passes in the fraction lengths of the input and output data types as arguments.

To create custom code replacement entries that add logic to the code replacement match and replacement process:

  1. Create a class, for example TflCustomOperationEntry, that is derived from the base class RTW.TflCOperationEntryML. The derived class defines a do_match method with the following signature:

    function ent = do_match(hThis, ...
        hCSO, ...
        targetBitPerChar, ...
        targetBitPerShort, ...
        targetBitPerInt, ...
        targetBitPerLong, ...
        targetBitPerLongLong)

    In the do_match signature:

    • ent is the return handle. If the match succeeds, ent is returned as a TflCOperationEntry handle. If the match fails, enty is returned as empty or as an error message that you specify by calling error in the entry.

    • hThis is the handle to the class instance.

    • hCSO is a handle to an object that the code generator creates for querying the library for a replacement.

    • Remaining arguments are the number of bits for various data types of the current target.

    The do_match method creates a copy of an existing code replacement entry type and adds match criteria that the base class does not provide. The method makes modifications to the implementation signature. In this case, the do_match method relies on the base class for checking word size and signedness. do_match must match only the number of conceptual arguments to the value 3 (two inputs and one output) and the bias for each argument to value 0. If the code generator finds a match, do_match:

    • Sets the return handle.

    • Removes slope and bias wild cards from the conceptual arguments (the match is for specific slope and bias values).

    • Writes fraction-length values for the inputs and output into replacement function arguments 3, 4, and 5.

    You can create and add three additional implementation function arguments for passing fraction lengths in the class definition or in each code replacement entry definition that instantiates this class. This example creates the arguments, adds them to a code replacement table definition file, and sets them to specific values in the class definition code.

    classdef TflCustomOperationEntry < RTW.TflCOperationEntryML
  methods
    function ent = do_match(hThis, ...
        hCSO, ... %#ok
        targetBitPerChar, ... %#ok
        targetBitPerShort, ... %#ok
        targetBitPerInt, ... %#ok
        targetBitPerLong, ... %#ok
        targetBitPerLongLong) %#ok

      % DO_MATCH - Create a custom match function. The base class
      % checks the types of the arguments prior to calling this
      % method. This class will check additional data and can
      % modify the implementation function.

      % The base class checks word size and signedness. Slopes and biases
      % have been wildcarded, so the only additional checking to do is
      % to check that the biases are zero and that there are only three
      % conceptual arguments (one output, two inputs)

      ent = []; % default the return to empty, indicating the match failed

      if length(hCSO.ConceptualArgs) == 3 && ...
          hCSO.ConceptualArgs(1).Type.Bias == 0 && ...
          hCSO.ConceptualArgs(2).Type.Bias == 0 && ...
          hCSO.ConceptualArgs(3).Type.Bias == 0

        % Modify the default implementation. Since this is a
        % generator entry, a concrete entry is created using this entry
        % as a template. The type of entry being created is a standard
        % TflCOperationEntry. Using the standard operation entry
        % provides required information, and you do not need
        % a custom match function.
        ent = RTW.TflCOperationEntry(hThis);

        % Set the fraction-length values in the implementation function.
        ent.Implementation.Arguments(3).Value = ...
           -1.0*hCSO.ConceptualArgs(2).Type.FixedExponent;
        ent.Implementation.Arguments(4).Value = ...
           -1.0*hCSO.ConceptualArgs(3).Type.FixedExponent;
        ent.Implementation.Arguments(5).Value = ...
           -1.0*hCSO.ConceptualArgs(1).Type.FixedExponent;
      end
    end
  end
end

    Exit the class folder and return to the previous working folder.

  2. Create and save the following code replacement table definition file, crl_table_custom_add_ufix32.m. This file defines a code replacement table that contains a single operator entry, an entry generator for unsigned 32-bit fixed-point addition operations, with arbitrary fraction-length values on the inputs and the output. The table entry:

    • Instantiates the derived class TflCustomOperationEntry from the previous step. If you want to replace word sizes and signedness attributes, you can use the same derived class, but not the same entry, because you cannot use a wild card with the WordLength and IsSigned arguments. For example, to support uint8, int8, uint16, int16, and int32, add five other distinct entries. To use different implementation functions for saturation and rounding modes other than overflow and round to floor, add entries for those match permutations.

    • Sets operator entry parameters with the call to the setTflCOperationEntryParameters function.

    • Calls the createAndAddConceptualArg function to create conceptual arguments y1, u1, and u2.

    • Calls createAndSetCImplementationReturn and createAndAddImplementationArg to define the signature for the replacement function. Three of the calls to createAndAddImplementationArg create implementation arguments to hold the fraction-length values for the inputs and output. Alternatively, the entry can omit those argument definitions. Instead, the do_match method of the derived class TflCustomOperationEntry can create and add the three implementation arguments. When the number of additional implementation arguments required can vary based on compile-time information, use the alternative approach.

    • Calls addEntry to add the entry to a code replacement table.

    function hTable = crl_table_custom_add_ufix32

hTable = RTW.TflTable;

% Add TflCustomOperationEntry
op_entry = TflCustomOperationEntry;

setTflCOperationEntryParameters(op_entry, ...
    'Key',                      'RTW_OP_ADD', ...
    'Priority',                 30, ...
    'SaturationMode',           'RTW_SATURATE_ON_OVERFLOW', ...
    'RoundingModes',            {'RTW_ROUND_FLOOR'}, ...
    'ImplementationName',       'myFixptAdd', ...
    'ImplementationHeaderFile', 'myFixptAdd.h', ...
    'ImplementationSourceFile', 'myFixptAdd.c');

createAndAddConceptualArg(op_entry, 'RTW.TflArgNumeric', ...
     'Name',       'y1', ...
     'IOType',     'RTW_IO_OUTPUT', ...
     'CheckSlope', false, ...
     'CheckBias',  false, ...
     'DataType',   'Fixed', ...
     'Scaling',    'BinaryPoint', ...
     'IsSigned',   false, ...
     'WordLength', 32);

createAndAddConceptualArg(op_entry, 'RTW.TflArgNumeric', ...
      'Name',       'u1', ...
      'IOType',     'RTW_IO_INPUT', ...
      'CheckSlope', false, ...
      'CheckBias',  false, ...
      'DataType',   'Fixed', ...
      'Scaling',    'BinaryPoint', ...
      'IsSigned',   false, ...
      'WordLength', 32);

createAndAddConceptualArg(op_entry, 'RTW.TflArgNumeric', ...
       'Name',       'u2', ...
       'IOType',     'RTW_IO_INPUT', ...
       'CheckSlope', false, ...
       'CheckBias',  false, ...
       'DataType',   'Fixed', ...
       'Scaling',    'BinaryPoint', ...
       'IsSigned',   false, ...
       'WordLength', 32);

% Specify replacement function signature
createAndSetCImplementationReturn(op_entry, 'RTW.TflArgNumeric', ...
    'Name',       'y1', ...
    'IOType',     'RTW_IO_OUTPUT', ...
    'IsSigned',   false, ...
    'WordLength', 32, ...
    'FractionLength', 0);

createAndAddImplementationArg(op_entry, 'RTW.TflArgNumeric', ...
    'Name',       'u1', ...
    'IOType',     'RTW_IO_INPUT', ...
    'IsSigned',   false, ...
    'WordLength', 32, ...
    'FractionLength', 0);

createAndAddImplementationArg(op_entry, 'RTW.TflArgNumeric', ...
    'Name',       'u2', ...
    'IOType',     'RTW_IO_INPUT', ...
    'IsSigned',   false, ...
    'WordLength', 32, ...
    'FractionLength', 0);

% Add 3 fraction-length args. Actual values are set during code generation.
createAndAddImplementationArg(op_entry, 'RTW.TflArgNumericConstant', ...
    'Name',       'fl_in1', ...
    'IOType',     'RTW_IO_INPUT', ...
    'IsSigned',   false, ...
    'WordLength', 32, ...
    'FractionLength', 0, ...
    'Value',       0);

createAndAddImplementationArg(op_entry, 'RTW.TflArgNumericConstant', ...
    'Name',       'fl_in2', ...
    'IOType',     'RTW_IO_INPUT', ...
    'IsSigned',   false, ...
    'WordLength', 32, ...
    'FractionLength', 0, ...
    'Value',       0);

createAndAddImplementationArg(op_entry, 'RTW.TflArgNumericConstant', ...
    'Name',       'fl_out', ...
    'IOType',     'RTW_IO_INPUT', ...
    'IsSigned',   false, ...
    'WordLength', 32, ...
    'FractionLength', 0, ...
    'Value',       0);

addEntry(hTable, op_entry);

  3. Check the validity of the operator entry.

    • At the command prompt, invoke the table definition file.

      tbl = crl_table_custom_add_ufix32

    • In the Code Replacement Viewer, view the table definition file.

      crviewer(crl_table_custom_add_ufix32)

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