coder.loop.interchange

Interchange loop indices in generated code

Since R2023a

Syntax

coder.loop.interchange("loopA_ID","loopB_ID")

loopObj = coder.loop.interchange(___)

Description

coder.loop.interchange("loopA_ID","loopB_ID") prompts the code generator to interchange the for loops with index names loopA_ID and loopB_ID in the generated code.

Use this transform when accessing array elements stored in contiguous memory blocks. In such situations, the data is loaded into cache and can be reused without a cache refresh.

For more information about loop optimizations, see Optimize Loops in Generated Code.

loopObj = coder.loop.interchange(___) creates a loop control object with transformSchedule property set to coder.loop.interchange. Use the apply method to apply the transform to the specified loop.

Examples

Interchange for-Loops in Generated Code

You can use the coder.loop.interchange function to swap for-loops in the generated code. This optimization can improve cache performance when accessing array elements in your code.

Define a MATLAB® function interchangeForLoops that operates on a matrix by using two for-loops. Call the coder.loop.interchange function immediately before the for-loop to optimize.

type interchangeForLoops.m

function out = interchangeForLoops()
out = zeros(100,70);

coder.loop.interchange('i','j');
for i = 1:100
    for j = 1:70
        out(i,j) = out(i,j) + i*j;
    end
end

Generate code for this function at the command line.

codegen interchangeForLoops -config:lib

Warning: Code generation is using a coder.EmbeddedCodeConfig object. Because
Embedded Coder is not installed, this might cause some Embedded Coder features
to fail.


Code generation successful (with warnings): View report

Inspect the generated code. The indices for-loops are swapped in the generated code. The data elements are loaded into cache in a contiguous block and a larger portion of the cache is reused within the inner loop.

type(fullfile("codegen","lib","interchangeForLoops","interchangeForLoops.c"))

/*
 * File: interchangeForLoops.c
 *
 * MATLAB Coder version            : 25.2
 * C/C++ source code generated on  : 09-Aug-2025 12:56:44
 */

/* Include Files */
#include "interchangeForLoops.h"
#include "interchangeForLoops_data.h"
#include "interchangeForLoops_initialize.h"
#include "omp.h"
#include <string.h>

/* Function Definitions */
/*
 * Arguments    : double out[7000]
 * Return Type  : void
 */
void interchangeForLoops(double out[7000])
{
  int i;
  int j;
  int out_tmp;
  if (!isInitialized_interchangeForLoops) {
    interchangeForLoops_initialize();
  }
  memset(&out[0], 0, 7000U * sizeof(double));
#pragma omp parallel for num_threads(omp_get_max_threads()) private(i, out_tmp)

  for (j = 0; j < 70; j++) {
    for (i = 0; i < 100; i++) {
      out_tmp = i + 100 * j;
      out[out_tmp] += (double)((i + 1) * (j + 1));
    }
  }
}

/*
 * File trailer for interchangeForLoops.c
 *
 * [EOF]
 */

Input Arguments

loopA_ID — for-loop index name

string scalar | character vector

for loop identifier or index name to interchange, specified as a character vector a string scalar.

Data Types: char | string

loopB_ID — for-loop index name

string scalar | character vector

for loop identifier or index name to interchange, specified as a character vector a string scalar.

Data Types: char | string

Output Arguments

loopObj — Loop control object

coder.loop.Control object

Loop control object with transformSchedule property set to coder.loop.interchange.

Version History

Introduced in R2023a