Learn OCaml the Hard Way is a series about learning OCaml from the ground up:

Modern compilers are composed by multiple stages: parsers, optimizers, linkers, and assemblers. Let’s go through it one by one to have a better understanding of the OCaml compiler. Here’s the OCaml’s compilation pipeline:

source: https://dev.realworldocaml.org/compiler-frontend.html

From Real World OCaml:

Each source file represents a compilation unit that is built separately. The compiler generates intermediate files with different filename extensions to use as it advances through the compilation stages. The linker takes a collection of compiled units and produces a standalone executable or library archive that can be reused by other applications.

We can easily go through intermediate representations via poking into these files.

Abstract Syntax Tree (AST)

OCaml’s metaprogramming ability relies on manipulating ASTs (Parsetrees). ppx_tools is a set of tools for people who want to write programs with such ability. We can use it to obtain the AST of a source code.

Assume we have the following OCaml code:

type t = | Alice | Bob | Charlie | David

let test v =
  match v with
  | Alice   -> 100
  | Bob     -> 101
  | Charlie -> 102
  | David   -> 103

We can get the AST of it via the following command:

$ $ ocamlfind ppx_tools/dumpast t.ml
t.ml
==>
[{pstr_desc =
   Pstr_type (Recursive,
    [{ptype_name = {txt = "t"}; ptype_params = []; ptype_cstrs = [];
      ptype_kind =
       Ptype_variant
        [{pcd_name = {txt = "Alice"}; pcd_args = Pcstr_tuple [];
          pcd_res = None};
         {pcd_name = {txt = "Bob"}; pcd_args = Pcstr_tuple [];
          pcd_res = None};
         {pcd_name = {txt = "Charlie"}; pcd_args = Pcstr_tuple [];
          pcd_res = None};
         {pcd_name = {txt = "David"}; pcd_args = Pcstr_tuple [];
          pcd_res = None}];
      ptype_private = Public; ptype_manifest = None}])};
 {pstr_desc =
   Pstr_value (Nonrecursive,
    [{pvb_pat = {ppat_desc = Ppat_var {txt = "test"}; ppat_loc_stack = []};
      pvb_expr =
       {pexp_desc =
         Pexp_fun (Nolabel, None,
          {ppat_desc = Ppat_var {txt = "v"}; ppat_loc_stack = []},
          {pexp_desc =
            Pexp_match
             ({pexp_desc = Pexp_ident {txt = Lident "v"};
               pexp_loc_stack = []},
             [{pc_lhs =
                {ppat_desc = Ppat_construct ({txt = Lident "Alice"}, None);
                 ppat_loc_stack = []};
               pc_guard = None;
               pc_rhs =
                {pexp_desc = Pexp_constant (Pconst_integer ("100", None));
                 pexp_loc_stack = []}};
              {pc_lhs =
                {ppat_desc = Ppat_construct ({txt = Lident "Bob"}, None);
                 ppat_loc_stack = []};
               pc_guard = None;
               pc_rhs =
                {pexp_desc = Pexp_constant (Pconst_integer ("101", None));
                 pexp_loc_stack = []}};
              {pc_lhs =
                {ppat_desc = Ppat_construct ({txt = Lident "Charlie"}, None);
                 ppat_loc_stack = []};
               pc_guard = None;
               pc_rhs =
                {pexp_desc = Pexp_constant (Pconst_integer ("102", None));
                 pexp_loc_stack = []}};
              {pc_lhs =
                {ppat_desc = Ppat_construct ({txt = Lident "David"}, None);
                 ppat_loc_stack = []};
               pc_guard = None;
               pc_rhs =
                {pexp_desc = Pexp_constant (Pconst_integer ("103", None));
                 pexp_loc_stack = []}}]);
           pexp_loc_stack = []});
        pexp_loc_stack = []}}])}]
=========

The output is pretty straight forward. The Parsetree documentation is a good reference to understand it.

Typedtree

For the simplicity of the article, we will skip the detail of type inferencing and checking here. They will (hopefully) be explained in the future.

Lambda

The first code generation phase in the OCaml pipeline is to create a Lambda Form. Lambda form discards higher-level constructs such as modules, pattern matching, and objects.

  • Modules and objects are replaced with records and function pointers.
  • Pattern Matchings are compiled into optimized automatas.
  • Block and values are mapped to the runtime memory model.

We can obtain the lambda form via the following command:

$ ocamlc -dlambda -c t.ml
(setglobal T!
  (let
    (test/85 =
       (function v/87 : int
         (switch* v/87
          case int 0: 100
          case int 1: 101
          case int 2: 102
          case int 3: 103)))
    (makeblock 0 test/85)))

Lambda Form is explicitly undocumented and can change across compiler revisions.

For more detail, see The Compiler Backend: Bytecode and Native code - Real World OCaml.

We can generate both bytecodes and native binaries from the Lambda Form.

Bytecode and js_of_ocaml

We can obtain the bytecode with the following command:

$ ocamlc -dinstr t.ml
	branch L2
L1:	acc 0
	switch 6 5 4 3/
L6:	const 100
	return 1
L5:	const 101
	return 1
L4:	const 102
	return 1
L3:	const 103
	return 1
L2:	closure L1, 0
	push
	acc 0
	makeblock 1, 0
	pop 1
	setglobal T!

The bytecode can be executed with ocamlrun, a portable interpreter for OCaml’s bytecode.

The OCaml bytecode is based on a stack-based VM. The instruction set of the Caml Virtual Machine is documented here.

Since the OCaml bytecode is quite stable. We can generate a target-specified code (such as Javascript for the web) from it without recompiling any library.

  • js_of_ocaml is a compiler from OCaml bytecode programs to Javascript.
  • Caramel is an Erlang backend to the OCaml compiler.

Here’s a simple example of using js_of_ocaml:

$ ocamlfind ocamlc -package js_of_ocaml -package js_of_ocaml-ppx \
          -linkpkg -o t.byte t.ml
$ js_of_ocaml t.byte

Native Compilation

Finally, we can generate native binaries with ocamlopt t.ml. You can get ocamlopt to output the assembly with the -S flag.

If we want the best performance (and we usually does). We should use the compiler with flambda optimizers. you can install a flambda-optimized OCaml with opam switch, such as:

$ opam switch create 4.11.1+flambda

Extra: Top-level

The OCaml top-level supports loading both source code or bytecodes. To load a source code, use the #mod_use command. To load a bytecode, use #load.

References

Like this post? Subscribe to Learn OCaml the Hard Way to get more!