Tail self-call optimization in generated C code?

Eric Brunel eric.brunel at pragmadev.com
Mon Aug 21 12:11:46 UTC 2017

Hello all,

I'm trying to get ECL to optimize tail self-calls in the C code 
generated from the Lisp files and it looks like I'm missing something, 
because I can't find a way to do that.

Here is the behavior I'm getting for this simple Lisp function:

(defun enumerate (l &optional (index 0) (result nil))
     ((null l) (reverse result))
     (t (enumerate (cdr l) (+ index 1) (cons (list index (car l)) 

As far as I can see, this function is properly tail-recursive, so I 
expected the generated C code to take the into account and generate a 
goto instead of a recursive call. But what I'm getting is this:

static cl_object L1enumerate(cl_narg narg, cl_object v1l, ...)
  cl_object T0, T1, T2, T3, T4;
  cl_object env0;
  const cl_env_ptr cl_env_copy = ecl_process_env();
  cl_object value0;
  if (ecl_unlikely(narg<1)) FEwrong_num_arguments_anonym();
  if (ecl_unlikely(narg>3)) FEwrong_num_arguments_anonym();
   cl_object v2index;
   cl_object v3result;
   va_list args; va_start(args,v1l);
    int i = 1;
    if (i >= narg) {
     v2index = ecl_make_fixnum(0);
    } else {
     v2index = va_arg(args,cl_object);
    if (i >= narg) {
     v3result = ECL_NIL;
    } else {
     v3result = va_arg(args,cl_object);
   if (!(v1l==ECL_NIL)) { goto L3; }
   value0 = cl_reverse(v3result);
   return value0;
   T0 = ecl_cdr(v1l);
   T1 = ecl_plus(v2index,ecl_make_fixnum(1));
   T2 = ecl_car(v1l);
   T3 = cl_list(2, v2index, T2);
   T4 = CONS(T3,v3result);
   value0 = L1enumerate(3, T0, T1, T4);
   return value0;

The recursive call is generated as a recursive call in C too...

I'm almost sure I've seen C code generated from ECL that generated self 
tail calls as goto's, and I know it works when using the bytecode, where 
you just have to compile the function. But my search for a way to 
trigger this behavior for the generated C code has been unsuccessful so 

I'm using ECL 16.1.3. I used "ecl -c ... --compile ..." to get the C 
code, but my main build uses asdf:make-build with an ASD file.

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