Tail recursion in gcc/g++

问题:

I've tried to search, but was unable to find: what are requisites for functions so that gcc would optimize the tail recursion? Is there any reference or list that would contain the most important cases? Since this is version specific, of my interests are 4.6.3 or higher versions (the newer the better). However, in fact, any concrete reference would be greatly appreciated.

Thanks in advance!

回答1:

With -O2 enabled, gcc will perform tail call optimization, if possible. Now, the question is: When is it possible?

It is possible to eliminate a single recursive call whenever the recursive call happens either immediately before or inside the (single) return statement, so there are no observable side effects afterwards other than possibly returning a different value. This includes returning expressions involving the ternary operator.
Note that even if there are several recursive calls in the return expression (such as in the well-known fibonacci example: return (n==1) ? 1 : fib(n-1)+fib(n-2);), tail call recursion is still applied, but only ever to the last recursive call.

As an obvious special case, if the compiler can prove that the recursive function (and its arguments) qualifies as constant expression, it can (up to a configurable maximum recursion depth, by default 500) eliminate not only the tail call, but the entire execution of the function. This is something GCC does routinely and quite reliably at -O2, which even includes calls to certain library functions such as strlen on literals.