vs2010 c++ tail call optimization

Question

Consider the following code:

int fac_aux( int x, int res ) {
    if( x == 1 ) return res;
    else return fac_aux( x - 1, res * x );
}

int fac( int x ) {
    return fac_aux( x, 1 );
}

int main() {
    int x = fac( 50 );

    std::cout << x;
    return 0;
}

According to generated asm file everything is ok, tail call is optimized.

Try to replace

int x = fac( 50 );

with

int x = fac_aux( 50, 1 );

Strange enough, but tail call optimization is disappeared. As far as I remember there was no such a strange compiler behaviour in VS2008. Any ideas why these things happen and how to be sure of tail call optimization is done?

; Function compile flags: /Ogtp

Tried both /O2 and /Ox optimization flags. Are there any other compiler options that matter?

Edit: VS2012 manages to do the optimization

Answer 1

when the original is compiled, the assembly at the callsite has partial inlining of fac_aux, specifically the x - 1 part, which is required for the tail recursion, but using fac_aux prevents the partial inlining and thus the tail recursion optimization:

TestThin.fac_aux 013B1000   CMP ECX,1
013B1003                    JE SHORT TestThin.013B100E
013B1005                    IMUL EAX,ECX
013B1008                    DEC ECX
013B1009                    CMP ECX,1
013B100C                    JNZ SHORT TestThin.013B1005
013B100E                    RETN
013B100F                    INT3
TestThin.main 013B1010      MOV EAX,32
013B1015                    LEA ECX,DWORD PTR DS:[EAX-1] ;notice the partial inlining of x - 1
013B1018                    CALL TestThin.fac_aux