Re: Template notation

Salt_Peter wrote:


  Quite curiously, passing by reference may in some cases actually
produce less efficient code (depending on the compiler, of course).

  Using gcc 3.3.5 with maximum optimizations (-O3 -march=pentium4)
this code:

  template<typename T>
  inline const T& max(T const& a, T const& b) { return a < b ? b : a; }
  int foo(int a, int b) { return max(a, b); }

produces this asm:

..LFB5:
        pushl %ebp
..LCFI0:
        movl %esp, %ebp
..LCFI1:
        movl 12(%ebp), %edx
        leal 8(%ebp), %eax
        leal 12(%ebp), %ecx
        cmpl %edx, 8(%ebp)
        popl %ebp
        cmovge %eax, %ecx
        movl (%ecx), %eax
        ret

  Removing the references should *in theory* produce an identical
result, but it doesn't. The code:

  template<typename T>
  inline const T max(T const a, T const b) { return a < b ? b : a; }
  int foo(int a, int b) { return max(a, b); }

produces this asm:

..LFB5:
        pushl %ebp
..LCFI0:
        movl %esp, %ebp
..LCFI1:
        movl 8(%ebp), %ecx
        movl 12(%ebp), %eax
        popl %ebp
        cmpl %eax, %ecx
        cmovge %ecx, %eax
        ret

  The 10 opcodes have been reduced to 8. Granted, I'm not a Pentium4
expert and I can't tell for sure that the latter code is faster than
the former, but I'm pretty convinced.