From 23f5da9983036cf37dedc0511a930fcbc7a8bc0e Mon Sep 17 00:00:00 2001
From: Johannes Stoelp <johannes.stoelp@gmail.com>
Date: Mon, 27 Feb 2023 22:41:39 +0100
Subject: Add fibonacci example

---
 examples/fib.rs | 73 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 examples/fib.rs

diff --git a/examples/fib.rs b/examples/fib.rs
new file mode 100644
index 0000000..c4233da
--- /dev/null
+++ b/examples/fib.rs
@@ -0,0 +1,73 @@
+use juicebox_asm::prelude::*;
+use juicebox_asm::rt::Runtime;
+
+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 rt = Runtime::new(&code);
+    let fib = unsafe { rt.as_fn::<extern "C" fn(u64) -> u64>() };
+
+    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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