feat(wpcarro/scratch): Rewrite Python compiler in OCaml r/5093

Just to see how productive I could be in OCaml with little familiarity. Overall
I really like it.

Change-Id: I8affc65a5ee86a29d4f8c01426529ae9948660f9
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6934
Reviewed-by: wpcarro <wpcarro@gmail.com>
Autosubmit: wpcarro <wpcarro@gmail.com>
Tested-by: BuildkiteCI

4 files changed, 154 insertions, 0 deletions

diff --git a/users/wpcarro/scratch/compiler/.envrc b/users/wpcarro/scratch/compiler/.envrc
new file mode 100644
index 0000000000..ff7eea1f7a
--- /dev/null
+++ b/users/wpcarro/scratch/compiler/.envrc
@@ -0,0 +1,3 @@
+source_up
+
+use_nix
diff --git a/users/wpcarro/scratch/compiler/.gitignore b/users/wpcarro/scratch/compiler/.gitignore
new file mode 100644
index 0000000000..25414be2a9
--- /dev/null
+++ b/users/wpcarro/scratch/compiler/.gitignore
@@ -0,0 +1,2 @@
+a.out
+*.cmi
\ No newline at end of file
diff --git a/users/wpcarro/scratch/compiler/register_vm.ml b/users/wpcarro/scratch/compiler/register_vm.ml
new file mode 100644
index 0000000000..aa5062cb24
--- /dev/null
+++ b/users/wpcarro/scratch/compiler/register_vm.ml
@@ -0,0 +1,140 @@
+(*
+  Rewriting the Python implementation of the register VM in OCaml to see how
+  how much imperative/mutative programming OCaml allows.
+
+  Note: Some of this code is intentionally not written in a functional style
+  because one of the goals was to see how similar this OCaml implementation
+  could be to the Python implementation.
+
+  Conclusion: It's pretty easily to switch between the two languages.
+
+  Usage: Recommended compilation settings I hastily found online:
+  $ ocamlopt -w +A-42-48 -warn-error +A-3-44 ./register_vm.ml && ./a.out
+
+  Formatting:
+  $ ocamlformat --inplace --enable-outside-detected-project ./register_vm.ml
+ *)
+
+type reg = X | Y | Res
+type binop = int -> int -> int
+
+type ast =
+  | Const of int
+  | Add of ast * ast
+  | Sub of ast * ast
+  | Mul of ast * ast
+  | Div of ast * ast
+
+type opcode0 =
+  | Op0AssignRegLit of reg * int
+  | Op0AssignRegReg of reg * reg
+  | Op0BinOp of binop * reg * reg * reg
+  | Op0PushReg of reg
+  | Op0PopAndSet of reg
+  | Op0Null
+
+type opcode1 =
+  | Op1AssignRegLit of int * int
+  | Op1AssignRegReg of int * int
+  | Op1BinOp of (int -> int -> int) * int * int * int
+  | Op1PushReg of int
+  | Op1PopAndSet of int
+  | Op1Null
+
+type opcodes0 = opcode0 array
+type opcodes1 = opcode1 array
+
+let registers : int array = Array.make 8 0
+let stack : int Stack.t = Stack.create ()
+let reg_idx (r : reg) : int = match r with X -> 0 | Y -> 1 | Res -> 2
+
+let reg_name (r : reg) : string =
+  match r with X -> "x" | Y -> "y" | Res -> "res"
+
+let print_opcodes0 (xs : opcodes0) : opcodes0 =
+  let print_opcode x =
+    match x with
+    | Op0AssignRegLit (r, x) -> Printf.printf "%s <- %d\n" (reg_name r) x
+    | Op0AssignRegReg (dst, src) ->
+        Printf.printf "%s <- $%s\n" (reg_name dst) (reg_name src)
+    | Op0PushReg src -> Printf.printf "push $%s\n" (reg_name src)
+    | Op0PopAndSet dst -> Printf.printf "%s <- pop\n" (reg_name dst)
+    | Op0BinOp (_, lhs, rhs, dst) ->
+        Printf.printf "%s <- $%s ? $%s\n" (reg_name dst) (reg_name lhs)
+          (reg_name rhs)
+    | Op0Null -> ()
+  in
+  Array.iter print_opcode xs;
+  xs
+
+(* Mutatively add xs to ys *)
+let add_ops (xs : opcodes0) (ys : opcodes0) (i : int ref) : unit =
+  let j = ref 0 in
+  while xs.(!j) != Op0Null do
+    ys.(!i) <- xs.(!j);
+    i := !i + 1;
+    j := !j + 1
+  done
+
+let rec compile (ast : ast) : opcodes0 =
+  let result : opcodes0 = Array.make 20 Op0Null and i : int ref = ref 0 in
+  (match ast with
+  | Const x ->
+      result.(!i) <- Op0AssignRegLit (Res, x);
+      i := !i + 1
+  | Add (lhs, rhs) -> compile_bin_op ( + ) lhs rhs result i
+  | Sub (lhs, rhs) -> compile_bin_op ( - ) lhs rhs result i
+  | Mul (lhs, rhs) -> compile_bin_op ( * ) lhs rhs result i
+  | Div (lhs, rhs) -> compile_bin_op ( / ) lhs rhs result i);
+  result
+
+and compile_bin_op (f : binop) (lhs : ast) (rhs : ast) (result : opcodes0)
+    (i : int ref) =
+  add_ops (compile lhs) result i;
+  result.(!i) <- Op0PushReg Res;
+  i := !i + 1;
+  add_ops (compile rhs) result i;
+  result.(!i + 1) <- Op0PopAndSet X;
+  result.(!i) <- Op0AssignRegReg (Y, Res);
+  result.(!i + 2) <- Op0BinOp (f, X, Y, Res);
+  i := !i + 3
+
+let compile_registers (xs : opcodes0) : opcodes1 =
+  let do_compile x =
+    match x with
+    | Op0AssignRegLit (dst, x) -> Op1AssignRegLit (reg_idx dst, x)
+    | Op0AssignRegReg (dst, src) -> Op1AssignRegReg (reg_idx dst, reg_idx src)
+    | Op0PushReg src -> Op1PushReg (reg_idx src)
+    | Op0PopAndSet dst -> Op1PopAndSet (reg_idx dst)
+    | Op0BinOp (f, lhs, rhs, dst) ->
+        Op1BinOp (f, reg_idx lhs, reg_idx rhs, reg_idx dst)
+    | Op0Null -> Op1Null
+  in
+  Array.map do_compile xs
+
+let eval (xs : opcodes1) : int =
+  let ip = ref 0 in
+  while !ip < Array.length xs do
+    match xs.(!ip) with
+    | Op1AssignRegLit (dst, x) ->
+        registers.(dst) <- x;
+        ip := !ip + 1
+    | Op1AssignRegReg (dst, src) ->
+        registers.(dst) <- registers.(src);
+        ip := !ip + 1
+    | Op1PushReg src ->
+        Stack.push registers.(src) stack;
+        ip := !ip + 1
+    | Op1PopAndSet dst ->
+        registers.(dst) <- Stack.pop stack;
+        ip := !ip + 1
+    | Op1BinOp (f, lhs, rhs, dst) ->
+        registers.(dst) <- f registers.(lhs) registers.(rhs);
+        ip := !ip + 1
+    | Op1Null -> ip := !ip + 1
+  done;
+  registers.(reg_idx Res)
+;;
+
+Add (Mul (Const 2, Div (Const 100, Const 2)), Const 5)
+|> compile |> print_opcodes0 |> compile_registers |> eval |> print_int
diff --git a/users/wpcarro/scratch/compiler/shell.nix b/users/wpcarro/scratch/compiler/shell.nix
new file mode 100644
index 0000000000..ec339eb91d
--- /dev/null
+++ b/users/wpcarro/scratch/compiler/shell.nix
@@ -0,0 +1,9 @@
+{ pkgs, ... }:
+
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    ocaml
+    ocamlPackages.utop
+    ocamlformat
+  ];
+}