Use memcpy Function to Optimize Generated Code for Vector Assignments

Use `memcpy` Function to Optimize Generated Code for Vector Assignments

You can use the Use memcpy for vector assignment parameter to optimize generated code for vector assignments by replacing for loops with memcpy function calls. The memcpy function is more efficient than for-loop controlled element assignment for large data sets. This optimization improves execution speed.

Selecting the Use memcpy for vector assignment parameter enables the associated parameter Memcpy threshold (bytes), which allows you to specify the minimum array size in bytes for which memcpy function calls should replace for loops in the generated code. For more information, see Use memcpy for vector assignment and Memcpy threshold (bytes). In considering whether to use this optimization,

Verify that your target supports the memcpy function.
Determine whether your model uses signal vector assignments (such as Y=expression) to move large amounts of data, for example, using the Selector block.

To apply this optimization,

Consider first generating code without this optimization and measuring its execution speed, to establish a baseline for evaluating the optimized assignment.
Select Use memcpy for vector assignment and examine the setting of Memcpy threshold (bytes), which by default specifies 64 bytes as the minimum array size for which memcpy function calls replace for loops. Based on the array sizes used in your application's signal vector assignments, and target environment considerations that might bear on the threshold selection, accept the default or specify another array size.
Generate code, and measure its execution speed against your baseline or previous iterations. Iterate on steps 2 and 3 until you achieve an optimal result.

Note

The memcpy optimization may not occur under certain conditions, including when other optimizations have a higher precedence than the memcpy optimization, or when the generated code is originating from Target Language Compiler (TLC) code, such as a TLC file associated with an S-function block.

Note

If you are licensed for Embedded Coder^® software, you can use a code replacement library (CRL) to provide your own custom implementation of the memcpy function to be used in generated model code. For more information, see Memory Function Code Replacement.

Example Model

To examine the result of using the Use memcpy for vector assignment parameter on the generated vector assignment code, create a model that generates signal vector assignments. For example,

Use In, Out, and Selector blocks to create the following model.
Open Model Explorer and configure the Signal Attributes for the In1 and In2 source blocks. For each, set Port dimensions to [1,100], and set Data type to int32. Apply the changes and save the model. In this example, the model has the name vectorassign.
For each Selector block, set the Index parameter to 1:50. Set the Input port size parameter to 100.

Generate Code

The Use memcpy for vector assignment parameter is on by default. To turn off the parameter, go to the Optimization pane and clear the Use memcpy for vector assignment parameter.
Go to the Code Generation > Report pane of the Configuration Parameters dialog box and select the Create code generation report parameter and the Open report automatically parameter. Then go to the Code Generation pane, select the Generate code only option, and generate code for the model. When code generation completes, the HTML code generation report is displayed.

In the HTML code generation report, click the vectorassign.c section and inspect the model step function. Notice that the vector assignments are implemented using for loops.

/* Model step function */
void vectorassign_step(void)
{
  int32_T i;
  for (i = 0; i < 50; i++) {
    /* Outport: '<Root>/Out1' incorporates:
     *  Inport: '<Root>/In1'
     */
    vectorassign_Y.Out1[i] = vectorassign_U.In1[i];

    /* Outport: '<Root>/Out2' incorporates:
     *  Inport: '<Root>/In2'
     */
    vectorassign_Y.Out2[i] = vectorassign_U.In2[i];
  }
}

Generate Code with Optimization

Go to the Optimization pane of the Configuration Parameters dialog box and select the Use memcpy for vector assignment option. Leave the Memcpy threshold (bytes) option at its default setting of 64. Apply the changes and regenerate code for the model. When code generation completes, the HTML code generation report again is displayed.

In the HTML code generation report, click the vectorassign.c section and inspect the model output function. Notice that the vector assignments now are implemented using memcpy function calls.

/* Model step function */
void vectorassign_step(void)
{
  /* Outport: '<Root>/Out1' incorporates:
   *  Inport: '<Root>/In1'
   */
  memcpy(&vectorassign_Y.Out1[0], &vectorassign_U.In1[0], 50U * sizeof(real_T));

  /* Outport: '<Root>/Out2' incorporates:
   *  Inport: '<Root>/In2'
   */
  memcpy(&vectorassign_Y.Out2[0], &vectorassign_U.In2[0], 50U * sizeof(real_T));
}