Tangential Question (purely curious): How to you validate your code produces the same (presumedly correct) results as the non-optimized (presumedly HLL) code fo

Tangential Question (purely curious): How to you validate your code produces the same (presumedly correct) results as the non-optimized (presumedly HLL) code for something as complex and variable as DNA analysis? I run into an analogous concern when people talk about "we need to rewrite this exhaustively tested with decades of track record Fortran 77 code in (whatever the new hotness in PLT is)".