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//! Fibonacci example.
//!
//! Jit compile a function at runtime (generate native host code) to compute the fibonacci sequence
//! to demonstrate the [`juicebox_asm`] crate.
use juicebox_asm::insn::*;
use juicebox_asm::Runtime;
use juicebox_asm::{Asm, Imm64, Label, Reg64};
const fn fib_rs(n: u64) -> u64 {
match n {
0 => 0,
1 => 1,
_ => fib_rs(n - 2) + fib_rs(n - 1),
}
}
fn main() {
let mut asm = Asm::new();
let mut lp = Label::new();
let mut end = Label::new();
// Reference implementation:
//
// int fib(int n) {
// int tmp = 0;
// int prv = 1;
// int sum = 0;
// loop:
// if (n == 0) goto end;
// tmp = sum;
// sum += prv;
// prv = tmp;
// --n;
// goto loop;
// end:
// return sum;
// }
// SystemV abi:
// rdi -> first argument
// rax -> return value
let n = Reg64::rdi;
let sum = Reg64::rax;
let tmp = Reg64::rcx;
let prv = Reg64::rbx;
asm.mov(tmp, Imm64::from(0));
asm.mov(prv, Imm64::from(1));
asm.mov(sum, Imm64::from(0));
asm.bind(&mut lp);
asm.test(n, n);
asm.jz(&mut end);
asm.mov(tmp, sum);
asm.add(sum, prv);
asm.mov(prv, tmp);
asm.dec(n);
asm.jmp(&mut lp);
asm.bind(&mut end);
asm.ret();
// Write out JIT code for visualization.
// Disassemble for example with `ndisasm -b 64 jit.asm`.
let code = asm.into_code();
std::fs::write("jit.asm", &code).unwrap();
// Move code into executable page and get function pointer to it.
let mut rt = Runtime::new();
let fib = unsafe { rt.add_code::<extern "C" fn(u64) -> u64>(code) };
for n in 0..15 {
let fib_jit = fib(n);
println!("fib({}) = {}", n, fib_jit);
assert_eq!(fib_jit, fib_rs(n));
}
}
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