Coder: Prevent code optimization replacing a variable with its constant value

Hello

I'm using Matlab coder to code generate. Example:

N = 31;
for 1:N
%calculating things
end

So my variable N has a constant value, but is used everywhere in my code to define lengths, loops etc. Now when I code generate the variable N is substituted for the value 31 everywhere in the generated C code. I want to prevent this as N, even though defined as a constant in the code, we do change often and I would like to be able to just open up the c file and change the value there and not having to code generate all over again. It can neither be an input. Is there a way to tell Matlab Coder to keep the variable N and not optimize it away?

Best Regards MC

[Owen Claxton]

Also looking for a solution to this. Cannot find anywhere

Answer 1

[Owen Claxton]

Hi Michel C,

After an indepth dive into the documentation, I stumbled upon this link: https://au.mathworks.com/help/coder/ref/codegen.html

Hopefully this works for your C code, however note that I am working on the problem in the context of ROS and C++.

By defining a global variable T in my code, like this:

function test
    global T;
    A = uint8(2);
    out1 = test_helper(A);
    fprintf('\tA: %u\n', out1);
    function output = test_helper(input)
        if input > 3
            output = uint8(2) + T;
        else
            output = uint8(1) + T;
        end
    end
end

and editting my codegen line to:

%% Some extra junk relating to what I'm working on. Hopefully it works
%% without, but just in case:
cfg = coder.config('exe');
cfg.Hardware = coder.hardware('Robot Operating System (ROS)');
cfg.HardwareImplementation.ProdHWDeviceType = 'ARM Compatible->ARM Cortex';
cfg.GenCodeOnly = true;
cfg.Hardware.BuildAction = 'None';
cfg.PreserveVariableNames = 'UserNames';
cfg.Verbosity = 'Verbose';

% Codegen:
globals_def = {'T', 1};
codegen -globals globals_def test -args {} -config cfg

MATLAB generated some definitions for the variable within the src, in a file called 'test_initialise.cpp':

//
// Academic License - for use in teaching, academic research, and meeting
// course requirements at degree granting institutions only.  Not for
// government, commercial, or other organizational use.
// File: test_initialize.cpp
//
// MATLAB Coder version            : 5.2
// C/C++ source code generated on  : 21-Sep-2021 18:06:42
//

// Include Files
#include "test_initialize.h"
#include "test_data.h"

// Function Definitions
//
// Arguments    : void
// Return Type  : void
//
void test_initialize()
{
  T = 10.0;
  isInitialized_test = true;
}

//
// File trailer for test_initialize.cpp
//
// [EOF]
//

As you can see, T is there and defined. Furthermore, in the actual 'test.cpp':

//
// Academic License - for use in teaching, academic research, and meeting
// course requirements at degree granting institutions only.  Not for
// government, commercial, or other organizational use.
// File: test.cpp
//
// MATLAB Coder version            : 5.2
// C/C++ source code generated on  : 21-Sep-2021 18:06:42
//

// Include Files
#include "test.h"
#include "test_data.h"
#include "test_initialize.h"
#include <cmath>
#include <stdio.h>

// Function Definitions
//
// Arguments    : void
// Return Type  : void
//
void test()
{
  double d;
  unsigned char u;
  if (!isInitialized_test) {
    test_initialize();
  }
  d = std::round(T + 1.0);
  if (d < 256.0) {
    if (d >= 0.0) {
      u = static_cast<unsigned char>(d);
    } else {
      u = 0U;
    }
  } else if (d >= 256.0) {
    u = MAX_uint8_T;
  } else {
    u = 0U;
  }
  printf("\tA: %u\n", u);
  fflush(stdout);
}

//
// File trailer for test.cpp
//
// [EOF]
//

You can see T is indeed contained within the script.

Best of luck!