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!
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.
B* to your function and call foo() on that pointer (with D1 and D2 deriving from B), this might call D1::foo or D2::foo -- recursive or not, the compiler cannot know.call to jmp and that's it (more or less). Gcc is able to do it with function pointers, do you have any idea why vtable would be any different (right now it does tail-calls for virtual functions too)?call with jmp (though, with additional constraints on observable effects as stated above, and to a slightly different code location). However, a non-static dispatch is a little bit more involved than a simple call or a function pointer in the general case (in the easiest case it is equivalent to a function pointer). It certainly works reliably (always) with static dispatch of virtual functions, and it may (or may not) work with dynamic dispatch. Why or why not in one particular case is hard to tell.
foo ends with return foo(), then surely it can be replaced with a jmp? It's recursive, so it's going to know which foo to call, regardless of virtual. The only exception I can think of is if somebody explicitly calls d -> Base::foo() where d is a pointer of Derived type. Then the first call goes to Base::foo(), and the recursive calls go to Derived::foo(). Is this correct? And is this the only reason (but a very good reason!) why virtual is difficult?