Question

According to answers to that question:
Which, if any, C++ compilers do tail-recursion optimization?
it seems, that compiler should do tail-recursion optimization.

But I’ve tried proposed options and it seems that compiler can’t do this optimization in case of template functions. Could it be fixed somehow?

Answer 1

I don’t use the MS compilers, but GCC can certainly do tail-recursion optimisation for templates. Given this function:

template <typename T>
T f( T t ) {
   cout << t << endl;
   if ( t == 0 ) {
      return t;
   }
   return f( t - 1 );
}

The code produced is:

    5   T f( T t ) {
    6       cout << t << endl;
-   0x401362    <main+22>:      mov    %esi,0x4(%esp)
-   0x401366    <main+26>:      movl   $0x4740c0,(%esp)
-   0x40136d    <main+33>:      call   0x448620 <_ZNSolsEi>
-   0x401372    <main+38>:      mov    %eax,%ebx
    7      if ( t == 0 ) {
-   0x4013a5    <main+89>:      test   %esi,%esi
-   0x4013a7    <main+91>:      je     0x4013c8 <main+124>
    8         return t;
    9      }
    10     return f( t - 1 );
-   0x4013a9    <main+93>:      dec    %esi
-   0x4013aa    <main+94>:      jmp    0x401362 <main+22>
    11  }

You can see that the recursive call has been turned into a jump back to the start of the function. This optimisation is only performed by GCC if the code is compiled with optimisations enabled (-O2 in this case) – perhaps the same is true for MS C++?