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.