How to define lower limit at while code generation?

Hi,

I am trying to generate C++ code for my function with Codegen. It has variable data size but has limits,min 100, max 1000. I know I can define upto limit but is it possible to define down limit?

Thanks for your help,

Answer 1

[Ryan Livingston]

There is unfortunately no way to define a lower bound for a size. In this case, if you define the upper bound, Coder will assume the size could be anything up to that bound.

[Emre Doruk]

Hi Ryan,

Thanks for your answer. Is there a way for work around? Because lack of down limit causes unneccessary complexity in generated code. I try to handle it with defining fixed size (expected max size) and manage this size issue in C++ code. But I am not sure is this way has better for performance.

[Ryan Livingston]

That fixed size approach could work. In some cases you can also give value range hints to MATLAB Coder by using MATLAB assert to prune dead code. This may help but isn't a guarantee. For example

function y = foo(x)
t = zeros(x);
s = size(t,1);
assert(s > 100);
if s < 100
    y = 1;
else
    y = -1;
end
end

>> codegen foo -args 1 -config:lib -report

generates

double foo(double x)
{
  (void)x;
  return -1.0;
}

So you can use assert to give hints near the usage of the size lower bound. Since you can't specify lower bounds on the matrix size, the assert needs to be based on a variable whose value is the size rather than doing assert(size(t,1) > 100).

I'll give the warning that these asserts are hints which may or may not result in code optimization based on Coder heuristics. They can be useful but I'd warn against going down a rabbit hole without direct evidence of benefit in your use case.