Previous: , Up: GNU lightning examples  


4.4 Fibonacci numbers

The code in this section calculates the Fibonacci sequence. That is modeled by the recurrence relation:

     f(0) = 0
     f(1) = f(2) = 1
     f(n) = f(n-1) + f(n-2)

The purpose of this example is to introduce branches. There are two kind of branches: backward branches and forward branches. We’ll present the calculation in a recursive and iterative form; the former only uses forward branches, while the latter uses both.

#include <stdio.h>
#include <lightning.h>

static jit_state_t *_jit;

typedef int (*pifi)(int);       /* Pointer to Int Function of Int */

int main(int argc, char *argv[])
{
  pifi       fib;
  jit_node_t *label;
  jit_node_t *call;
  jit_node_t *in;                 /* offset of the argument */
  jit_node_t *ref;                /* to patch the forward reference */
  jit_node_t *zero;               /* to patch the forward reference */

  init_jit(argv[0]);
  _jit = jit_new_state();

  label = jit_label();
        jit_prolog   ();
  in =  jit_arg      ();
        jit_getarg   (JIT_V0, in);              /* R0 = n */
 zero = jit_beqi     (JIT_R0, 0);
        jit_movr     (JIT_V0, JIT_R0);          /* V0 = R0 */
        jit_movi     (JIT_R0, 1);
  ref = jit_blei     (JIT_V0, 2);
        jit_subi     (JIT_V1, JIT_V0, 1);       /* V1 = n-1 */
        jit_subi     (JIT_V2, JIT_V0, 2);       /* V2 = n-2 */
        jit_prepare();
          jit_pushargr(JIT_V1);
        call = jit_finishi(NULL);
        jit_patch_at(call, label);
        jit_retval(JIT_V1);                     /* V1 = fib(n-1) */
        jit_prepare();
          jit_pushargr(JIT_V2);
        call = jit_finishi(NULL);
        jit_patch_at(call, label);
        jit_retval(JIT_R0);                     /* R0 = fib(n-2) */
        jit_addr(JIT_R0, JIT_R0, JIT_V1);       /* R0 = R0 + V1 */

  jit_patch(ref);                               /* patch jump */
  jit_patch(zero);                              /* patch jump */
        jit_retr(JIT_R0);

  /* call the generated code, passing 32 as an argument */
  fib = jit_emit();
  jit_clear_state();
  printf("fib(%d) = %d\n", 32, fib(32));
  jit_destroy_state();
  finish_jit();
  return 0;
}

As said above, this is the first example of dynamically compiling branches. Branch instructions have two operands containing the values to be compared, and return a jit_note_t * object to be patched.

Because labels final address are only known after calling emit, it is required to call patch or patch_at, what does tell GNU lightning that the target to patch is actually a pointer to a jit_node_t * object, otherwise, it would assume that is a pointer to a C function. Note that conditional branches do not receive a label argument, so they must be patched.

You need to call patch_at on the return of value calli, finishi, and calli if it is actually referencing a label in the jit code. All branch instructions do not receive a label argument. Note that movi is an special case, and patching it is usually done to get the final address of a label, usually to later call jmpr.

Now, here is the iterative version:

#include <stdio.h>
#include <lightning.h>

static jit_state_t *_jit;

typedef int (*pifi)(int);       /* Pointer to Int Function of Int */

int main(int argc, char *argv[])
{
  pifi       fib;
  jit_node_t *in;               /* offset of the argument */
  jit_node_t *ref;              /* to patch the forward reference */
  jit_node_t *zero;             /* to patch the forward reference */
  jit_node_t *jump;             /* jump to start of loop */
  jit_node_t *loop;             /* start of the loop */

  init_jit(argv[0]);
  _jit = jit_new_state();

        jit_prolog   ();
  in =  jit_arg      ();
        jit_getarg   (JIT_R0, in);              /* R0 = n */
 zero = jit_beqi     (JIT_R0, 0);
        jit_movr     (JIT_R1, JIT_R0);
        jit_movi     (JIT_R0, 1);
  ref = jit_blti     (JIT_R1, 2);
        jit_subi     (JIT_R2, JIT_R2, 2);
        jit_movr     (JIT_R1, JIT_R0);

  loop= jit_label();
        jit_subi     (JIT_R2, JIT_R2, 1);       /* decr. counter */
        jit_movr     (JIT_V0, JIT_R0);          /* V0 = R0 */
        jit_addr     (JIT_R0, JIT_R0, JIT_R1);  /* R0 = R0 + R1 */
        jit_movr     (JIT_R1, JIT_V0);          /* R1 = V0 */
  jump= jit_bnei     (JIT_R2, 0);               /* if (R2) goto loop; */
  jit_patch_at(jump, loop);

  jit_patch(ref);                               /* patch forward jump */
  jit_patch(zero);                              /* patch forward jump */
        jit_retr     (JIT_R0);

  /* call the generated code, passing 36 as an argument */
  fib = jit_emit();
  jit_clear_state();
  printf("fib(%d) = %d\n", 36, fib(36));
  jit_destroy_state();
  finish_jit();
  return 0;
}

This code calculates the recurrence relation using iteration (a for loop in high-level languages). There are no function calls anymore: instead, there is a backward jump (the bnei at the end of the loop).

Note that the program must remember the address for backward jumps; for forward jumps it is only required to remember the jump code, and call patch for the implicit label.


Previous: , Up: GNU lightning examples