diff --git a/zjit/src/lib.rs b/zjit/src/lib.rs
index 655a85aca6..c93145cb7d 100644
--- a/zjit/src/lib.rs
+++ b/zjit/src/lib.rs
@@ -1,5 +1,3 @@
-mod zjit;
-
 extern "C" fn zjit_init() {
     println!("zjit_init");
 }
@@ -8,3 +6,109 @@ extern "C" fn zjit_init() {
 pub extern "C" fn rb_zjit_parse_option() -> bool {
     false
 }
+
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+pub struct InsnId(usize);
+#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
+pub struct BlockId(usize);
+
+// TODO: replace with VALUE
+#[derive(Debug)]
+enum RubyValue {
+    Nil,
+}
+
+#[derive(Debug)]
+enum Insn {
+    Param { idx: usize },
+    Const { val: RubyValue },
+    Return { val: InsnId },
+}
+
+#[derive(Debug)]
+struct Block {
+    params: Vec<InsnId>,
+    insns: Vec<InsnId>,
+}
+
+impl Block {
+    fn new() -> Block {
+        Block { params: vec![], insns: vec![] }
+    }
+}
+
+#[derive(Debug)]
+struct Function {
+    entry_block: BlockId,
+    insns: Vec<Insn>,
+    blocks: Vec<Block>,
+}
+
+impl Function {
+    fn new() -> Function {
+        Function { blocks: vec![Block::new()], insns: vec![], entry_block: BlockId(0) }
+    }
+
+    fn push_insn(&mut self, block: BlockId, insn: Insn) -> InsnId {
+        let id = InsnId(self.insns.len());
+        self.insns.push(insn);
+        // Add the insn to the block
+        self.blocks[block.0].insns.push(id);
+        id
+    }
+}
+
+enum RubyOpcode {
+    Putnil,
+    Leave,
+}
+
+struct FrameState {
+    stack: Vec<InsnId>,
+}
+
+impl FrameState {
+    fn new() -> FrameState {
+        FrameState { stack: vec![] }
+    }
+
+    fn push(&mut self, val: InsnId) {
+        self.stack.push(val);
+    }
+
+    fn pop(&mut self) -> InsnId {
+        self.stack.pop().expect("Bytecode stack mismatch")
+    }
+}
+
+fn to_ssa(opcodes: &Vec<RubyOpcode>) -> Function {
+    let mut result = Function::new();
+    let mut state = FrameState::new();
+    let block = result.entry_block;
+    for opcode in opcodes {
+        match opcode {
+            RubyOpcode::Putnil => {
+                state.push(result.push_insn(block, Insn::Const { val: RubyValue::Nil }));
+            },
+            RubyOpcode::Leave => {
+                result.push_insn(block, Insn::Return { val: state.pop() });
+            },
+        }
+    }
+    result
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::*;
+
+    #[test]
+    fn test() {
+        let opcodes = vec![
+            RubyOpcode::Putnil,
+            RubyOpcode::Leave,
+        ];
+        let function = to_ssa(&opcodes);
+        println!("zjit {function:?}");
+    }
+}
diff --git a/zjit/src/main.rs b/zjit/src/main.rs
deleted file mode 100644
index 89774abe85..0000000000
--- a/zjit/src/main.rs
+++ /dev/null
@@ -1,26 +0,0 @@
-mod cruby;
-
-#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
-pub struct InsnId(usize);
-#[derive(Copy, Clone, Eq, PartialEq, Hash, Debug)]
-pub struct BlockId(usize);
-
-enum Insn {
-    Param { idx: usize },
-}
-
-#[derive(Debug)]
-struct Block {
-    params: Vec<InsnId>,
-    insns: Vec<InsnId>,
-}
-
-#[derive(Debug)]
-struct Function {
-    name: String,
-    entry_block: BlockId,
-}
-
-fn main() {
-    println!("zjit");
-}