10

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

ildjarn
  • 62,044
  • 9
  • 127
  • 211
Voivoid
  • 461
  • 3
  • 11
  • when modifying your code, did you delete the `fac` function ? – SirDarius Mar 08 '11 at 12:52
  • so the function which does not get optimized is actually never called ? or are you talking about the tail call in the fist function? – Euqil Jul 29 '11 at 11:14
  • @Voivoid: Is the function actually _called_ in the assembly? (Probably, but thought I'd ask) – Mooing Duck Sep 27 '11 at 21:44
  • Just FYI, [there's no reason to use the `/Ox` optimization switch in preference to `/O2`](http://stackoverflow.com/questions/5063334/what-is-the-difference-between-the-ox-and-o2-compiler-options). The name "full optimization" for `/Ox` is somewhat deceptive (at least in contemporary versions); you actually get at least the same level of optimization, if not more, with `/O2`. – Cody Gray - on strike Jul 31 '12 at 16:24

5 Answers5

1

I tried the following code

#include "stdafx.h"

int f( size_t i, int x )
{
    return ( ( i < 2 ) ? x : f( i - 1, i * x ) );
}

int f( size_t i )
{
    return ( f( i, 1 ) );
}

int _tmain(int argc, _TCHAR* argv[])
{
    {
        f( 0 );
    }

    return 0;
}

and used the full optimization /Ox but I did not get the tail recursion. So it seems that MS VC++ 2010 does not support the tail recursion.

Vlad from Moscow
  • 11
  • 1
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
Necrolis
  • 25,836
  • 3
  • 63
  • 101
0

Try making the functions explicitly inline – furthermore, what optimization level are you using?

Konrad Rudolph
  • 530,221
  • 131
  • 937
  • 1,214
0

I don't know if it will work, but try to replace if ... else with single return statement:

return (x == 1) ? res : fac_aux( x - 1, res * x );
tomac
  • 66
  • 3
0

Looks weird, are you doing some kind of incremental compile. Other than that, it might be the fact that compiler gets confused by the multiple parameters, in the working version there's effectively only one parameter, somehow the optimization doesn't qualify anymore.

You could try making the res parameter a global, I its know messy bad practice, but it might work.

Sounds like a compiler bug/feature.

/Tony

SpagnumMoss
  • 104
  • 6