2017-08-21 14:11 GMT+02:00, Eric Brunel <eric.bru...@pragmadev.com>:
> 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 an example that works (not written by me, it's from cliki.net):
(defun fib (n)
"Tail-recursive computation of the nth element."
(check-type n (integer 0 *))
(labels ((fib-aux (n f1 f2)
(if (zerop n)
f1
(fib-aux (1- n) f2 (+ f1 f2)))))
(fib-aux n 0 1)))
The generated C code uses goto, as you can see:
static cl_object LC1fib_aux(cl_object v1n, cl_object v2f1, cl_object v3f2)
{
cl_object env0;
const cl_env_ptr cl_env_copy = ecl_process_env();
cl_object value0;
ecl_cs_check(cl_env_copy,value0);
{
TTL:
if (!(ecl_zerop(v1n))) { goto L1; }
value0 = v2f1;
cl_env_copy->nvalues = 1;
return value0;
L1:;
v1n = ecl_one_minus(v1n);
{
cl_object v4;
v4 = v3f2;
v3f2 = ecl_plus(v2f1,v3f2);
v2f1 = v4;
}
goto TTL;
}
}
Paul
>
> Here is the behavior I'm getting for this simple Lisp function:
>
> (defun enumerate (l &optional (index 0) (result nil))
> (cond
> ((null l) (reverse result))
> (t (enumerate (cdr l) (+ index 1) (cons (list index (car l))
> result)))
> )
> )
>
> 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;
> ecl_cs_check(cl_env_copy,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 {
> i++;
> v2index = va_arg(args,cl_object);
> }
> if (i >= narg) {
> v3result = ECL_NIL;
> } else {
> i++;
> v3result = va_arg(args,cl_object);
> }
> }
> va_end(args);
> if (!(v1l==ECL_NIL)) { goto L3; }
> value0 = cl_reverse(v3result);
> return value0;
> L3:;
> 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
> far.
>
> 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.
>
>
