Recursive Functions - 2022.1 English

Vitis High-Level Synthesis User Guide (UG1399)

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2022.1 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.

Note: Virtual Functions are not supported.