Why isn't g++ tail call optimizing while gcc is?

Question

I wanted to check whether g++ supports tail calling so I wrote this simple program to check it: http://ideone.com/hnXHv

using namespace std;

size_t st;

v         


        

Answer 1

Because you're passing a temporary std::string object to the PrintStackTop(std::string) function. This object is allocated on the stack and thus prevent the tail call optimization.

I modified your code:

void PrintStackTopStr(char const*const type)
{
    int stack_top;
    if(st == 0) st = (size_t) &stack_top;
    cout << "In " << type << " call version, the stack top is: " << (st - (size_t) &stack_top) << endl;
}

int RealTailCallFactorial(int n, int a = 1)
{
    PrintStackTopStr("tail");
    if(n < 2)
        return a;
    return RealTailCallFactorial(n - 1, n * a);
}

Compile with: g++ -O2 -fno-exceptions -o tailcall tailcall.cpp

And it now uses the tail call optimisation. You can see it in action if you use the -S flag to produce the assembly:

L39:
        imull   %ebx, %esi
        subl    $1, %ebx
L38:
        movl    $LC2, (%esp)
        call    __Z16PrintStackTopStrPKc
        cmpl    $1, %ebx
        jg      L39

You see the recursive call inlined as a loop (jg L39).