Recursive Functions - 2023.1 English

Recursive functions cannot be synthesized. This applies to functions that can form multiple recursions:

unsigned foo (unsigned n) 
{  
    if (n == 0 || n == 1) return 1;  
    return (foo(n-2) + foo(n-1)); 
} 

Vitis HLS also does not support tail recursion, in which there is a finite number of function calls.

unsigned foo (unsigned m, unsigned n)  
{  
    if (m == 0) return n;  
    if (n == 0) return m; 
    return foo(n, m%n); 
} 

In C++, templates can implement tail recursion and can then be used for synthesizable tail-recursive designs.