Tail call optimization on function with many arguments

["Lionel Villard" <villard at us dot ibm dot com>]

Hi,
 
consider this small C program:
 
  #include <stdlib.h>
  int tailoptimized(int c1, int c2, int c3, int c4, int c5, int c6)
 {
    if (c1 > c5)
        {
            if (c4 < 5)
            {
                return c3 + c2;
            }
        }
        else if (c5 > 45)
                return c6;
        return c3 + c4 + c5 + c6;
  }
 
  int nottailoptimized(int c1, int c2, int c3, int c4, int c5, int c6, int 
c7)
  {
      if (c2 > c5)
      {
          if (c2 < 5)
         {
              return c3 + c2;
         }
      }
      else if (c1 > 45)
            return c6;
      return c1 + c4 + c5 + c6;
 }
 
  int call(int count)
  {
      if (count > 100)
    {
        return tailoptimized(count *2, count, count * 3, count, count, 
count);
    }
    return nottailoptimized(count, count* 2, count, count, count, count, 
count * 4);
  }
 
  int main (int argc, char* argv[])
  {
      call(atoi(argv[0]));
  }
 
I used this command to compile it:
 
    gcc -O1 -foptimize-sibling-calls -S -c test.c
 
and generated assembly code for the 'call' function is:
 
call:
.LFB17:
    .cfi_startproc
    cmpl    $100, %edi
    jle    .L12
    leal    (%rdi,%rdi), %eax
    leal    (%rax,%rdi), %edx
    movl    %edi, %r9d
    movl    %edi, %r8d
    movl    %edi, %ecx
    movl    %edi, %esi
    movl    %eax, %edi
    jmp    tailoptimized
.L12:
    subq    $8, %rsp
    .cfi_def_cfa_offset 16
    leal    (%rdi,%rdi), %esi
    leal    0(,%rdi,4), %eax
    movl    %eax, (%rsp)
    movl    %edi, %r9d
    movl    %edi, %r8d
    movl    %edi, %ecx
    movl    %edi, %edx
    call    nottailoptimized
    addq    $8, %rsp
    .cfi_def_cfa_offset 8
    ret
    .cfi_endproc
 
As you can see, when a function takes more than 6 arguments 
(nottailoptimized in this case), TCO is turned off.
 
What's the reason for this limitation (number of registers?)? Is there a 
flag/option that would allow more than 6 arguments?
 
Thanks!
Lionel