Data Reference Overview

Some records in a model.rtw file, such as those corresponding to parameters and constant block I/O, can have extremely large data value vectors embedded in them. Such a vector can cause significant memory overhead during code generation because the values must be maintained as text in memory during this process.

To avoid such overhead, by default the Simulink^® software does not write out the entire data value vector into model.rtw. Instead, it writes a key called a data reference that can be used during code generation to access the data directly from Simulink. If the data is not mutated during code generation, it is efficiently streamed to disk when the actual code containing the data values is written out.

A data reference has the format SLData(index), where index is a numeric value that tells Simulink which data is being referenced. TLC directives such as GENERATE_FORMATTED_VALUE store data references in unexpanded format in memory. When the generated code is written out to disk, the data values expand to the actual values. To access the data vector of the model parameter written to the ModelParameters record in the model.rtw, use the LibGetParameterValue function.

Control Data Reference Threshold

By default, if you do not specify a value for the model configuration parameter RTWDataReferencesMinSize, the code generator assigns the value -1 to the parameter. In this case, the code generator writes a data reference of a model parameter to the model.rtw file in place of a data vector. This optimization reduces the file read/write operations, which improves the code generation time. When you specify a positive value for the RTWDataReferencesMinSize parameter, the code generator uses the value as the threshold size based on which it writes data references to the model.rtw file in place of a data vector.

To change the maximum length of a vector that can appear literally in the file, use:

set_param(0, 'RTWDataReferencesMinSize', maxlen)

Simulink replaces a vector as long or longer than maxlen with a data reference when it creates model.rtw. Specify maxlen as an integer or as inf. Specifying inf disables data references. The complete value set of every vector, however long, then appears literally in model.rtw and occupies text memory during code generation.

Setting an explicit maxlen affects only the current MATLAB^® session. To set the value across sessions, include a set_param command in your startup.m file, or automate execution of the command when MATLAB launches.

Expand Data References

You can explicitly expand a data reference by using the GENERATE_FORMATTED_VALUE built-in function with the optional third expand argument. Commands such as FEVAL may cause a data reference to be expanded to the full form.

Avoid Data Reference Expansion

Either turning off data references completely or expanding select parameters in TLC can cause significant text memory overhead during the code generation process. During most common code generation tasks, it is unnecessary to have the expanded data vector in memory and pay the price of the additional overhead. Avoid expanded data vectors unless no alternative exists.

Restart Code Generation

A model.rtw file that contains data references cannot be used in isolation to restart a custom code generation process. The data references within it become stale once the code generation process is completed. Attempting to start a code generation process using only this file may result in unpredictable behavior and memory segmentation faults.