docs: Sphinxify `docs/tutorial/`

Sorry for the massive commit, but I just wanted to knock this one down
and it is really straightforward.

There are still a couple trivial (i.e. not related to the content)
things left to fix:

- Use of raw HTML links where :doc:`...` and :ref:`...` could be used
  instead. If you are a newbie and want to help fix this it would make
  for some good bite-sized patches; more experienced developers should
  be focusing on adding new content (to this tutorial or elsewhere, but
  please _do not_ waste your time on formatting when there is such dire
  need for documentation (see docs/SphinxQuickstartTemplate.rst to get
  started writing)).

- Highlighting of the kaleidoscope code blocks (currently left as bare
  `::`).  I will be working on writing a custom Pygments highlighter for
  this, mostly as training for maintaining the `llvm` code-block's lexer
  in-tree. I want to do this because I am extremely unhappy with how it
  just "gives up" on the slightest deviation from the expected syntax
  and leaves the whole code-block un-highlighted.

  More generally I am looking at writing some Sphinx extensions and
  keeping them in-tree as well, to support common use cases that
  currently have no good solution (like "monospace text inside a link").

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@169343 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 1726 insertions, 0 deletions

diff --git a/docs/tutorial/OCamlLangImpl7.rst b/docs/tutorial/OCamlLangImpl7.rst
new file mode 100644
index 0000000000..07da3a8ff9
--- /dev/null
+++ b/docs/tutorial/OCamlLangImpl7.rst
@@ -0,0 +1,1726 @@
+=======================================================
+Kaleidoscope: Extending the Language: Mutable Variables
+=======================================================
+
+.. contents::
+   :local:
+
+Written by `Chris Lattner <mailto:sabre@nondot.org>`_ and `Erick
+Tryzelaar <mailto:idadesub@users.sourceforge.net>`_
+
+Chapter 7 Introduction
+======================
+
+Welcome to Chapter 7 of the "`Implementing a language with
+LLVM <index.html>`_" tutorial. In chapters 1 through 6, we've built a
+very respectable, albeit simple, `functional programming
+language <http://en.wikipedia.org/wiki/Functional_programming>`_. In our
+journey, we learned some parsing techniques, how to build and represent
+an AST, how to build LLVM IR, and how to optimize the resultant code as
+well as JIT compile it.
+
+While Kaleidoscope is interesting as a functional language, the fact
+that it is functional makes it "too easy" to generate LLVM IR for it. In
+particular, a functional language makes it very easy to build LLVM IR
+directly in `SSA
+form <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_.
+Since LLVM requires that the input code be in SSA form, this is a very
+nice property and it is often unclear to newcomers how to generate code
+for an imperative language with mutable variables.
+
+The short (and happy) summary of this chapter is that there is no need
+for your front-end to build SSA form: LLVM provides highly tuned and
+well tested support for this, though the way it works is a bit
+unexpected for some.
+
+Why is this a hard problem?
+===========================
+
+To understand why mutable variables cause complexities in SSA
+construction, consider this extremely simple C example:
+
+.. code-block:: c
+
+    int G, H;
+    int test(_Bool Condition) {
+      int X;
+      if (Condition)
+        X = G;
+      else
+        X = H;
+      return X;
+    }
+
+In this case, we have the variable "X", whose value depends on the path
+executed in the program. Because there are two different possible values
+for X before the return instruction, a PHI node is inserted to merge the
+two values. The LLVM IR that we want for this example looks like this:
+
+.. code-block:: llvm
+
+    @G = weak global i32 0   ; type of @G is i32*
+    @H = weak global i32 0   ; type of @H is i32*
+
+    define i32 @test(i1 %Condition) {
+    entry:
+      br i1 %Condition, label %cond_true, label %cond_false
+
+    cond_true:
+      %X.0 = load i32* @G
+      br label %cond_next
+
+    cond_false:
+      %X.1 = load i32* @H
+      br label %cond_next
+
+    cond_next:
+      %X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
+      ret i32 %X.2
+    }
+
+In this example, the loads from the G and H global variables are
+explicit in the LLVM IR, and they live in the then/else branches of the
+if statement (cond\_true/cond\_false). In order to merge the incoming
+values, the X.2 phi node in the cond\_next block selects the right value
+to use based on where control flow is coming from: if control flow comes
+from the cond\_false block, X.2 gets the value of X.1. Alternatively, if
+control flow comes from cond\_true, it gets the value of X.0. The intent
+of this chapter is not to explain the details of SSA form. For more
+information, see one of the many `online
+references <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_.
+
+The question for this article is "who places the phi nodes when lowering
+assignments to mutable variables?". The issue here is that LLVM
+*requires* that its IR be in SSA form: there is no "non-ssa" mode for
+it. However, SSA construction requires non-trivial algorithms and data
+structures, so it is inconvenient and wasteful for every front-end to
+have to reproduce this logic.
+
+Memory in LLVM
+==============
+
+The 'trick' here is that while LLVM does require all register values to
+be in SSA form, it does not require (or permit) memory objects to be in
+SSA form. In the example above, note that the loads from G and H are
+direct accesses to G and H: they are not renamed or versioned. This
+differs from some other compiler systems, which do try to version memory
+objects. In LLVM, instead of encoding dataflow analysis of memory into
+the LLVM IR, it is handled with `Analysis
+Passes <../WritingAnLLVMPass.html>`_ which are computed on demand.
+
+With this in mind, the high-level idea is that we want to make a stack
+variable (which lives in memory, because it is on the stack) for each
+mutable object in a function. To take advantage of this trick, we need
+to talk about how LLVM represents stack variables.
+
+In LLVM, all memory accesses are explicit with load/store instructions,
+and it is carefully designed not to have (or need) an "address-of"
+operator. Notice how the type of the @G/@H global variables is actually
+"i32\*" even though the variable is defined as "i32". What this means is
+that @G defines *space* for an i32 in the global data area, but its
+*name* actually refers to the address for that space. Stack variables
+work the same way, except that instead of being declared with global
+variable definitions, they are declared with the `LLVM alloca
+instruction <../LangRef.html#i_alloca>`_:
+
+.. code-block:: llvm
+
+    define i32 @example() {
+    entry:
+      %X = alloca i32           ; type of %X is i32*.
+      ...
+      %tmp = load i32* %X       ; load the stack value %X from the stack.
+      %tmp2 = add i32 %tmp, 1   ; increment it
+      store i32 %tmp2, i32* %X  ; store it back
+      ...
+
+This code shows an example of how you can declare and manipulate a stack
+variable in the LLVM IR. Stack memory allocated with the alloca
+instruction is fully general: you can pass the address of the stack slot
+to functions, you can store it in other variables, etc. In our example
+above, we could rewrite the example to use the alloca technique to avoid
+using a PHI node:
+
+.. code-block:: llvm
+
+    @G = weak global i32 0   ; type of @G is i32*
+    @H = weak global i32 0   ; type of @H is i32*
+
+    define i32 @test(i1 %Condition) {
+    entry:
+      %X = alloca i32           ; type of %X is i32*.
+      br i1 %Condition, label %cond_true, label %cond_false
+
+    cond_true:
+      %X.0 = load i32* @G
+            store i32 %X.0, i32* %X   ; Update X
+      br label %cond_next
+
+    cond_false:
+      %X.1 = load i32* @H
+            store i32 %X.1, i32* %X   ; Update X
+      br label %cond_next
+
+    cond_next:
+      %X.2 = load i32* %X       ; Read X
+      ret i32 %X.2
+    }
+
+With this, we have discovered a way to handle arbitrary mutable
+variables without the need to create Phi nodes at all:
+
+#. Each mutable variable becomes a stack allocation.
+#. Each read of the variable becomes a load from the stack.
+#. Each update of the variable becomes a store to the stack.
+#. Taking the address of a variable just uses the stack address
+   directly.
+
+While this solution has solved our immediate problem, it introduced
+another one: we have now apparently introduced a lot of stack traffic
+for very simple and common operations, a major performance problem.
+Fortunately for us, the LLVM optimizer has a highly-tuned optimization
+pass named "mem2reg" that handles this case, promoting allocas like this
+into SSA registers, inserting Phi nodes as appropriate. If you run this
+example through the pass, for example, you'll get:
+
+.. code-block:: bash
+
+    $ llvm-as < example.ll | opt -mem2reg | llvm-dis
+    @G = weak global i32 0
+    @H = weak global i32 0
+
+    define i32 @test(i1 %Condition) {
+    entry:
+      br i1 %Condition, label %cond_true, label %cond_false
+
+    cond_true:
+      %X.0 = load i32* @G
+      br label %cond_next
+
+    cond_false:
+      %X.1 = load i32* @H
+      br label %cond_next
+
+    cond_next:
+      %X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
+      ret i32 %X.01
+    }
+
+The mem2reg pass implements the standard "iterated dominance frontier"
+algorithm for constructing SSA form and has a number of optimizations
+that speed up (very common) degenerate cases. The mem2reg optimization
+pass is the answer to dealing with mutable variables, and we highly
+recommend that you depend on it. Note that mem2reg only works on
+variables in certain circumstances:
+
+#. mem2reg is alloca-driven: it looks for allocas and if it can handle
+   them, it promotes them. It does not apply to global variables or heap
+   allocations.
+#. mem2reg only looks for alloca instructions in the entry block of the
+   function. Being in the entry block guarantees that the alloca is only
+   executed once, which makes analysis simpler.
+#. mem2reg only promotes allocas whose uses are direct loads and stores.
+   If the address of the stack object is passed to a function, or if any
+   funny pointer arithmetic is involved, the alloca will not be
+   promoted.
+#. mem2reg only works on allocas of `first
+   class <../LangRef.html#t_classifications>`_ values (such as pointers,
+   scalars and vectors), and only if the array size of the allocation is
+   1 (or missing in the .ll file). mem2reg is not capable of promoting
+   structs or arrays to registers. Note that the "scalarrepl" pass is
+   more powerful and can promote structs, "unions", and arrays in many
+   cases.
+
+All of these properties are easy to satisfy for most imperative
+languages, and we'll illustrate it below with Kaleidoscope. The final
+question you may be asking is: should I bother with this nonsense for my
+front-end? Wouldn't it be better if I just did SSA construction
+directly, avoiding use of the mem2reg optimization pass? In short, we
+strongly recommend that you use this technique for building SSA form,
+unless there is an extremely good reason not to. Using this technique
+is:
+
+-  Proven and well tested: llvm-gcc and clang both use this technique
+   for local mutable variables. As such, the most common clients of LLVM
+   are using this to handle a bulk of their variables. You can be sure
+   that bugs are found fast and fixed early.
+-  Extremely Fast: mem2reg has a number of special cases that make it
+   fast in common cases as well as fully general. For example, it has
+   fast-paths for variables that are only used in a single block,
+   variables that only have one assignment point, good heuristics to
+   avoid insertion of unneeded phi nodes, etc.
+-  Needed for debug info generation: `Debug information in
+   LLVM <../SourceLevelDebugging.html>`_ relies on having the address of
+   the variable exposed so that debug info can be attached to it. This
+   technique dovetails very naturally with this style of debug info.
+
+If nothing else, this makes it much easier to get your front-end up and
+running, and is very simple to implement. Lets extend Kaleidoscope with
+mutable variables now!
+
+Mutable Variables in Kaleidoscope
+=================================
+
+Now that we know the sort of problem we want to tackle, lets see what
+this looks like in the context of our little Kaleidoscope language.
+We're going to add two features:
+
+#. The ability to mutate variables with the '=' operator.
+#. The ability to define new variables.
+
+While the first item is really what this is about, we only have
+variables for incoming arguments as well as for induction variables, and
+redefining those only goes so far :). Also, the ability to define new
+variables is a useful thing regardless of whether you will be mutating
+them. Here's a motivating example that shows how we could use these:
+
+::
+
+    # Define ':' for sequencing: as a low-precedence operator that ignores operands
+    # and just returns the RHS.
+    def binary : 1 (x y) y;
+
+    # Recursive fib, we could do this before.
+    def fib(x)
+      if (x < 3) then
+        1
+      else
+        fib(x-1)+fib(x-2);
+
+    # Iterative fib.
+    def fibi(x)
+      var a = 1, b = 1, c in
+      (for i = 3, i < x in
+         c = a + b :
+         a = b :
+         b = c) :
+      b;
+
+    # Call it.
+    fibi(10);
+
+In order to mutate variables, we have to change our existing variables
+to use the "alloca trick". Once we have that, we'll add our new
+operator, then extend Kaleidoscope to support new variable definitions.
+
+Adjusting Existing Variables for Mutation
+=========================================
+
+The symbol table in Kaleidoscope is managed at code generation time by
+the '``named_values``' map. This map currently keeps track of the LLVM
+"Value\*" that holds the double value for the named variable. In order
+to support mutation, we need to change this slightly, so that it
+``named_values`` holds the *memory location* of the variable in
+question. Note that this change is a refactoring: it changes the
+structure of the code, but does not (by itself) change the behavior of
+the compiler. All of these changes are isolated in the Kaleidoscope code
+generator.
+
+At this point in Kaleidoscope's development, it only supports variables
+for two things: incoming arguments to functions and the induction
+variable of 'for' loops. For consistency, we'll allow mutation of these
+variables in addition to other user-defined variables. This means that
+these will both need memory locations.
+
+To start our transformation of Kaleidoscope, we'll change the
+``named_values`` map so that it maps to AllocaInst\* instead of Value\*.
+Once we do this, the C++ compiler will tell us what parts of the code we
+need to update:
+
+**Note:** the ocaml bindings currently model both ``Value*``'s and
+``AllocInst*``'s as ``Llvm.llvalue``'s, but this may change in the future
+to be more type safe.
+
+.. code-block:: ocaml
+
+    let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
+
+Also, since we will need to create these alloca's, we'll use a helper
+function that ensures that the allocas are created in the entry block of
+the function:
+
+.. code-block:: ocaml
+
+    (* Create an alloca instruction in the entry block of the function. This
+     * is used for mutable variables etc. *)
+    let create_entry_block_alloca the_function var_name =
+      let builder = builder_at (instr_begin (entry_block the_function)) in
+      build_alloca double_type var_name builder
+
+This funny looking code creates an ``Llvm.llbuilder`` object that is
+pointing at the first instruction of the entry block. It then creates an
+alloca with the expected name and returns it. Because all values in
+Kaleidoscope are doubles, there is no need to pass in a type to use.
+
+With this in place, the first functionality change we want to make is to
+variable references. In our new scheme, variables live on the stack, so
+code generating a reference to them actually needs to produce a load
+from the stack slot:
+
+.. code-block:: ocaml
+
+    let rec codegen_expr = function
+      ...
+      | Ast.Variable name ->
+          let v = try Hashtbl.find named_values name with
+            | Not_found -> raise (Error "unknown variable name")
+          in
+          (* Load the value. *)
+          build_load v name builder
+
+As you can see, this is pretty straightforward. Now we need to update
+the things that define the variables to set up the alloca. We'll start
+with ``codegen_expr Ast.For ...`` (see the `full code listing <#code>`_
+for the unabridged code):
+
+.. code-block:: ocaml
+
+      | Ast.For (var_name, start, end_, step, body) ->
+          let the_function = block_parent (insertion_block builder) in
+
+          (* Create an alloca for the variable in the entry block. *)
+          let alloca = create_entry_block_alloca the_function var_name in
+
+          (* Emit the start code first, without 'variable' in scope. *)
+          let start_val = codegen_expr start in
+
+          (* Store the value into the alloca. *)
+          ignore(build_store start_val alloca builder);
+
+          ...
+
+          (* Within the loop, the variable is defined equal to the PHI node. If it
+           * shadows an existing variable, we have to restore it, so save it
+           * now. *)
+          let old_val =
+            try Some (Hashtbl.find named_values var_name) with Not_found -> None
+          in
+          Hashtbl.add named_values var_name alloca;
+
+          ...
+
+          (* Compute the end condition. *)
+          let end_cond = codegen_expr end_ in
+
+          (* Reload, increment, and restore the alloca. This handles the case where
+           * the body of the loop mutates the variable. *)
+          let cur_var = build_load alloca var_name builder in
+          let next_var = build_add cur_var step_val "nextvar" builder in
+          ignore(build_store next_var alloca builder);
+          ...
+
+This code is virtually identical to the code `before we allowed mutable
+variables <OCamlLangImpl5.html#forcodegen>`_. The big difference is that
+we no longer have to construct a PHI node, and we use load/store to
+access the variable as needed.
+
+To support mutable argument variables, we need to also make allocas for
+them. The code for this is also pretty simple:
+
+.. code-block:: ocaml
+
+    (* Create an alloca for each argument and register the argument in the symbol
+     * table so that references to it will succeed. *)
+    let create_argument_allocas the_function proto =
+      let args = match proto with
+        | Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -> args
+      in
+      Array.iteri (fun i ai ->
+        let var_name = args.(i) in
+        (* Create an alloca for this variable. *)
+        let alloca = create_entry_block_alloca the_function var_name in
+
+        (* Store the initial value into the alloca. *)
+        ignore(build_store ai alloca builder);
+
+        (* Add arguments to variable symbol table. *)
+        Hashtbl.add named_values var_name alloca;
+      ) (params the_function)
+
+For each argument, we make an alloca, store the input value to the
+function into the alloca, and register the alloca as the memory location
+for the argument. This method gets invoked by ``Codegen.codegen_func``
+right after it sets up the entry block for the function.
+
+The final missing piece is adding the mem2reg pass, which allows us to
+get good codegen once again:
+
+.. code-block:: ocaml
+
+    let main () =
+      ...
+      let the_fpm = PassManager.create_function Codegen.the_module in
+
+      (* Set up the optimizer pipeline.  Start with registering info about how the
+       * target lays out data structures. *)
+      DataLayout.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
+
+      (* Promote allocas to registers. *)
+      add_memory_to_register_promotion the_fpm;
+
+      (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
+      add_instruction_combining the_fpm;
+
+      (* reassociate expressions. *)
+      add_reassociation the_fpm;
+
+It is interesting to see what the code looks like before and after the
+mem2reg optimization runs. For example, this is the before/after code
+for our recursive fib function. Before the optimization:
+
+.. code-block:: llvm
+
+    define double @fib(double %x) {
+    entry:
+      %x1 = alloca double
+      store double %x, double* %x1
+      %x2 = load double* %x1
+      %cmptmp = fcmp ult double %x2, 3.000000e+00
+      %booltmp = uitofp i1 %cmptmp to double
+      %ifcond = fcmp one double %booltmp, 0.000000e+00
+      br i1 %ifcond, label %then, label %else
+
+    then:    ; preds = %entry
+      br label %ifcont
+
+    else:    ; preds = %entry
+      %x3 = load double* %x1
+      %subtmp = fsub double %x3, 1.000000e+00
+      %calltmp = call double @fib(double %subtmp)
+      %x4 = load double* %x1
+      %subtmp5 = fsub double %x4, 2.000000e+00
+      %calltmp6 = call double @fib(double %subtmp5)
+      %addtmp = fadd double %calltmp, %calltmp6
+      br label %ifcont
+
+    ifcont:    ; preds = %else, %then
+      %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
+      ret double %iftmp
+    }
+
+Here there is only one variable (x, the input argument) but you can
+still see the extremely simple-minded code generation strategy we are
+using. In the entry block, an alloca is created, and the initial input
+value is stored into it. Each reference to the variable does a reload
+from the stack. Also, note that we didn't modify the if/then/else
+expression, so it still inserts a PHI node. While we could make an
+alloca for it, it is actually easier to create a PHI node for it, so we
+still just make the PHI.
+
+Here is the code after the mem2reg pass runs:
+
+.. code-block:: llvm
+
+    define double @fib(double %x) {
+    entry:
+      %cmptmp = fcmp ult double %x, 3.000000e+00
+      %booltmp = uitofp i1 %cmptmp to double
+      %ifcond = fcmp one double %booltmp, 0.000000e+00
+      br i1 %ifcond, label %then, label %else
+
+    then:
+      br label %ifcont
+
+    else:
+      %subtmp = fsub double %x, 1.000000e+00
+      %calltmp = call double @fib(double %subtmp)
+      %subtmp5 = fsub double %x, 2.000000e+00
+      %calltmp6 = call double @fib(double %subtmp5)
+      %addtmp = fadd double %calltmp, %calltmp6
+      br label %ifcont
+
+    ifcont:    ; preds = %else, %then
+      %iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
+      ret double %iftmp
+    }
+
+This is a trivial case for mem2reg, since there are no redefinitions of
+the variable. The point of showing this is to calm your tension about
+inserting such blatent inefficiencies :).
+
+After the rest of the optimizers run, we get:
+
+.. code-block:: llvm
+
+    define double @fib(double %x) {
+    entry:
+      %cmptmp = fcmp ult double %x, 3.000000e+00
+      %booltmp = uitofp i1 %cmptmp to double
+      %ifcond = fcmp ueq double %booltmp, 0.000000e+00
+      br i1 %ifcond, label %else, label %ifcont
+
+    else:
+      %subtmp = fsub double %x, 1.000000e+00
+      %calltmp = call double @fib(double %subtmp)
+      %subtmp5 = fsub double %x, 2.000000e+00
+      %calltmp6 = call double @fib(double %subtmp5)
+      %addtmp = fadd double %calltmp, %calltmp6
+      ret double %addtmp
+
+    ifcont:
+      ret double 1.000000e+00
+    }
+
+Here we see that the simplifycfg pass decided to clone the return
+instruction into the end of the 'else' block. This allowed it to
+eliminate some branches and the PHI node.
+
+Now that all symbol table references are updated to use stack variables,
+we'll add the assignment operator.
+
+New Assignment Operator
+=======================
+
+With our current framework, adding a new assignment operator is really
+simple. We will parse it just like any other binary operator, but handle
+it internally (instead of allowing the user to define it). The first
+step is to set a precedence:
+
+.. code-block:: ocaml
+
+    let main () =
+      (* Install standard binary operators.
+       * 1 is the lowest precedence. *)
+      Hashtbl.add Parser.binop_precedence '=' 2;
+      Hashtbl.add Parser.binop_precedence '<' 10;
+      Hashtbl.add Parser.binop_precedence '+' 20;
+      Hashtbl.add Parser.binop_precedence '-' 20;
+      ...
+
+Now that the parser knows the precedence of the binary operator, it
+takes care of all the parsing and AST generation. We just need to
+implement codegen for the assignment operator. This looks like:
+
+.. code-block:: ocaml
+
+    let rec codegen_expr = function
+          begin match op with
+          | '=' ->
+              (* Special case '=' because we don't want to emit the LHS as an
+               * expression. *)
+              let name =
+                match lhs with
+                | Ast.Variable name -> name
+                | _ -> raise (Error "destination of '=' must be a variable")
+              in
+
+Unlike the rest of the binary operators, our assignment operator doesn't
+follow the "emit LHS, emit RHS, do computation" model. As such, it is
+handled as a special case before the other binary operators are handled.
+The other strange thing is that it requires the LHS to be a variable. It
+is invalid to have "(x+1) = expr" - only things like "x = expr" are
+allowed.
+
+.. code-block:: ocaml
+
+              (* Codegen the rhs. *)
+              let val_ = codegen_expr rhs in
+
+              (* Lookup the name. *)
+              let variable = try Hashtbl.find named_values name with
+              | Not_found -> raise (Error "unknown variable name")
+              in
+              ignore(build_store val_ variable builder);
+              val_
+          | _ ->
+                ...
+
+Once we have the variable, codegen'ing the assignment is
+straightforward: we emit the RHS of the assignment, create a store, and
+return the computed value. Returning a value allows for chained
+assignments like "X = (Y = Z)".
+
+Now that we have an assignment operator, we can mutate loop variables
+and arguments. For example, we can now run code like this:
+
+::
+
+    # Function to print a double.
+    extern printd(x);
+
+    # Define ':' for sequencing: as a low-precedence operator that ignores operands
+    # and just returns the RHS.
+    def binary : 1 (x y) y;
+
+    def test(x)
+      printd(x) :
+      x = 4 :
+      printd(x);
+
+    test(123);
+
+When run, this example prints "123" and then "4", showing that we did
+actually mutate the value! Okay, we have now officially implemented our
+goal: getting this to work requires SSA construction in the general
+case. However, to be really useful, we want the ability to define our
+own local variables, lets add this next!
+
+User-defined Local Variables
+============================
+
+Adding var/in is just like any other other extensions we made to
+Kaleidoscope: we extend the lexer, the parser, the AST and the code
+generator. The first step for adding our new 'var/in' construct is to
+extend the lexer. As before, this is pretty trivial, the code looks like
+this:
+
+.. code-block:: ocaml
+
+    type token =
+      ...
+      (* var definition *)
+      | Var
+
+    ...
+
+    and lex_ident buffer = parser
+          ...
+          | "in" -> [< 'Token.In; stream >]
+          | "binary" -> [< 'Token.Binary; stream >]
+          | "unary" -> [< 'Token.Unary; stream >]
+          | "var" -> [< 'Token.Var; stream >]
+          ...
+
+The next step is to define the AST node that we will construct. For
+var/in, it looks like this:
+
+.. code-block:: ocaml
+
+    type expr =
+      ...
+      (* variant for var/in. *)
+      | Var of (string * expr option) array * expr
+      ...
+
+var/in allows a list of names to be defined all at once, and each name
+can optionally have an initializer value. As such, we capture this
+information in the VarNames vector. Also, var/in has a body, this body
+is allowed to access the variables defined by the var/in.
+
+With this in place, we can define the parser pieces. The first thing we
+do is add it as a primary expression:
+
+.. code-block:: ocaml
+
+    (* primary
+     *   ::= identifier
+     *   ::= numberexpr
+     *   ::= parenexpr
+     *   ::= ifexpr
+     *   ::= forexpr
+     *   ::= varexpr *)
+    let rec parse_primary = parser
+      ...
+      (* varexpr
+       *   ::= 'var' identifier ('=' expression?
+       *             (',' identifier ('=' expression)?)* 'in' expression *)
+      | [< 'Token.Var;
+           (* At least one variable name is required. *)
+           'Token.Ident id ?? "expected identifier after var";
+           init=parse_var_init;
+           var_names=parse_var_names [(id, init)];
+           (* At this point, we have to have 'in'. *)
+           'Token.In ?? "expected 'in' keyword after 'var'";
+           body=parse_expr >] ->
+          Ast.Var (Array.of_list (List.rev var_names), body)
+
+    ...
+
+    and parse_var_init = parser
+      (* read in the optional initializer. *)
+      | [< 'Token.Kwd '='; e=parse_expr >] -> Some e
+      | [< >] -> None
+
+    and parse_var_names accumulator = parser
+      | [< 'Token.Kwd ',';
+           'Token.Ident id ?? "expected identifier list after var";
+           init=parse_var_init;
+           e=parse_var_names ((id, init) :: accumulator) >] -> e
+      | [< >] -> accumulator
+
+Now that we can parse and represent the code, we need to support
+emission of LLVM IR for it. This code starts out with:
+
+.. code-block:: ocaml
+
+    let rec codegen_expr = function
+      ...
+      | Ast.Var (var_names, body)
+          let old_bindings = ref [] in
+
+          let the_function = block_parent (insertion_block builder) in
+
+          (* Register all variables and emit their initializer. *)
+          Array.iter (fun (var_name, init) ->
+
+Basically it loops over all the variables, installing them one at a
+time. For each variable we put into the symbol table, we remember the
+previous value that we replace in OldBindings.
+
+.. code-block:: ocaml
+
+            (* Emit the initializer before adding the variable to scope, this
+             * prevents the initializer from referencing the variable itself, and
+             * permits stuff like this:
+             *   var a = 1 in
+             *     var a = a in ...   # refers to outer 'a'. *)
+            let init_val =
+              match init with
+              | Some init -> codegen_expr init
+              (* If not specified, use 0.0. *)
+              | None -> const_float double_type 0.0
+            in
+
+            let alloca = create_entry_block_alloca the_function var_name in
+            ignore(build_store init_val alloca builder);
+
+            (* Remember the old variable binding so that we can restore the binding
+             * when we unrecurse. *)
+
+            begin
+              try
+                let old_value = Hashtbl.find named_values var_name in
+                old_bindings := (var_name, old_value) :: !old_bindings;
+              with Not_found > ()
+            end;
+
+            (* Remember this binding. *)
+            Hashtbl.add named_values var_name alloca;
+          ) var_names;
+
+There are more comments here than code. The basic idea is that we emit
+the initializer, create the alloca, then update the symbol table to
+point to it. Once all the variables are installed in the symbol table,
+we evaluate the body of the var/in expression:
+
+.. code-block:: ocaml
+
+          (* Codegen the body, now that all vars are in scope. *)
+          let body_val = codegen_expr body in
+
+Finally, before returning, we restore the previous variable bindings:
+
+.. code-block:: ocaml
+
+          (* Pop all our variables from scope. *)
+          List.iter (fun (var_name, old_value) ->
+            Hashtbl.add named_values var_name old_value
+          ) !old_bindings;
+
+          (* Return the body computation. *)
+          body_val
+
+The end result of all of this is that we get properly scoped variable
+definitions, and we even (trivially) allow mutation of them :).
+
+With this, we completed what we set out to do. Our nice iterative fib
+example from the intro compiles and runs just fine. The mem2reg pass
+optimizes all of our stack variables into SSA registers, inserting PHI
+nodes where needed, and our front-end remains simple: no "iterated
+dominance frontier" computation anywhere in sight.
+
+Full Code Listing
+=================
+
+Here is the complete code listing for our running example, enhanced with
+mutable variables and var/in support. To build this example, use:
+
+.. code-block:: bash
+
+    # Compile
+    ocamlbuild toy.byte
+    # Run
+    ./toy.byte
+
+Here is the code:
+
+\_tags:
+    ::
+
+        <{lexer,parser}.ml>: use_camlp4, pp(camlp4of)
+        <*.{byte,native}>: g++, use_llvm, use_llvm_analysis
+        <*.{byte,native}>: use_llvm_executionengine, use_llvm_target
+        <*.{byte,native}>: use_llvm_scalar_opts, use_bindings
+
+myocamlbuild.ml:
+    .. code-block:: ocaml
+
+        open Ocamlbuild_plugin;;
+
+        ocaml_lib ~extern:true "llvm";;
+        ocaml_lib ~extern:true "llvm_analysis";;
+        ocaml_lib ~extern:true "llvm_executionengine";;
+        ocaml_lib ~extern:true "llvm_target";;
+        ocaml_lib ~extern:true "llvm_scalar_opts";;
+
+        flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"; A"-cclib"; A"-rdynamic"]);;
+        dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
+
+token.ml:
+    .. code-block:: ocaml
+
+        (*===----------------------------------------------------------------------===
+         * Lexer Tokens
+         *===----------------------------------------------------------------------===*)
+
+        (* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
+         * these others for known things. *)
+        type token =
+          (* commands *)
+          | Def | Extern
+
+          (* primary *)
+          | Ident of string | Number of float
+
+          (* unknown *)
+          | Kwd of char
+
+          (* control *)
+          | If | Then | Else
+          | For | In
+
+          (* operators *)
+          | Binary | Unary
+
+          (* var definition *)
+          | Var
+
+lexer.ml:
+    .. code-block:: ocaml
+
+        (*===----------------------------------------------------------------------===
+         * Lexer
+         *===----------------------------------------------------------------------===*)
+
+        let rec lex = parser
+          (* Skip any whitespace. *)
+          | [< ' (' ' | '\n' | '\r' | '\t'); stream >] -> lex stream
+
+          (* identifier: [a-zA-Z][a-zA-Z0-9] *)
+          | [< ' ('A' .. 'Z' | 'a' .. 'z' as c); stream >] ->
+              let buffer = Buffer.create 1 in
+              Buffer.add_char buffer c;
+              lex_ident buffer stream
+
+          (* number: [0-9.]+ *)
+          | [< ' ('0' .. '9' as c); stream >] ->
+              let buffer = Buffer.create 1 in
+              Buffer.add_char buffer c;
+              lex_number buffer stream
+
+          (* Comment until end of line. *)
+          | [< ' ('#'); stream >] ->
+              lex_comment stream
+
+          (* Otherwise, just return the character as its ascii value. *)
+          | [< 'c; stream >] ->
+              [< 'Token.Kwd c; lex stream >]
+
+          (* end of stream. *)
+          | [< >] -> [< >]
+
+        and lex_number buffer = parser
+          | [< ' ('0' .. '9' | '.' as c); stream >] ->
+              Buffer.add_char buffer c;
+              lex_number buffer stream
+          | [< stream=lex >] ->
+              [< 'Token.Number (float_of_string (Buffer.contents buffer)); stream >]
+
+        and lex_ident buffer = parser
+          | [< ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream >] ->
+              Buffer.add_char buffer c;
+              lex_ident buffer stream
+          | [< stream=lex >] ->
+              match Buffer.contents buffer with
+              | "def" -> [< 'Token.Def; stream >]
+              | "extern" -> [< 'Token.Extern; stream >]
+              | "if" -> [< 'Token.If; stream >]
+              | "then" -> [< 'Token.Then; stream >]
+              | "else" -> [< 'Token.Else; stream >]
+              | "for" -> [< 'Token.For; stream >]
+              | "in" -> [< 'Token.In; stream >]
+              | "binary" -> [< 'Token.Binary; stream >]
+              | "unary" -> [< 'Token.Unary; stream >]
+              | "var" -> [< 'Token.Var; stream >]
+              | id -> [< 'Token.Ident id; stream >]
+
+        and lex_comment = parser
+          | [< ' ('\n'); stream=lex >] -> stream
+          | [< 'c; e=lex_comment >] -> e
+          | [< >] -> [< >]
+
+ast.ml:
+    .. code-block:: ocaml
+
+        (*===----------------------------------------------------------------------===
+         * Abstract Syntax Tree (aka Parse Tree)
+         *===----------------------------------------------------------------------===*)
+
+        (* expr - Base type for all expression nodes. *)
+        type expr =
+          (* variant for numeric literals like "1.0". *)
+          | Number of float
+
+          (* variant for referencing a variable, like "a". *)
+          | Variable of string
+
+          (* variant for a unary operator. *)
+          | Unary of char * expr
+
+          (* variant for a binary operator. *)
+          | Binary of char * expr * expr
+
+          (* variant for function calls. *)
+          | Call of string * expr array
+
+          (* variant for if/then/else. *)
+          | If of expr * expr * expr
+
+          (* variant for for/in. *)
+          | For of string * expr * expr * expr option * expr
+
+          (* variant for var/in. *)
+          | Var of (string * expr option) array * expr
+
+        (* proto - This type represents the "prototype" for a function, which captures
+         * its name, and its argument names (thus implicitly the number of arguments the
+         * function takes). *)
+        type proto =
+          | Prototype of string * string array
+          | BinOpPrototype of string * string array * int
+
+        (* func - This type represents a function definition itself. *)
+        type func = Function of proto * expr
+
+parser.ml:
+    .. code-block:: ocaml
+
+        (*===---------------------------------------------------------------------===
+         * Parser
+         *===---------------------------------------------------------------------===*)
+
+        (* binop_precedence - This holds the precedence for each binary operator that is
+         * defined *)
+        let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
+
+        (* precedence - Get the precedence of the pending binary operator token. *)
+        let precedence c = try Hashtbl.find binop_precedence c with Not_found -> -1
+
+        (* primary
+         *   ::= identifier
+         *   ::= numberexpr
+         *   ::= parenexpr
+         *   ::= ifexpr
+         *   ::= forexpr
+         *   ::= varexpr *)
+        let rec parse_primary = parser
+          (* numberexpr ::= number *)
+          | [< 'Token.Number n >] -> Ast.Number n
+
+          (* parenexpr ::= '(' expression ')' *)
+          | [< 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" >] -> e
+
+          (* identifierexpr
+           *   ::= identifier
+           *   ::= identifier '(' argumentexpr ')' *)
+          | [< 'Token.Ident id; stream >] ->
+              let rec parse_args accumulator = parser
+                | [< e=parse_expr; stream >] ->
+                    begin parser
+                      | [< 'Token.Kwd ','; e=parse_args (e :: accumulator) >] -> e
+                      | [< >] -> e :: accumulator
+                    end stream
+                | [< >] -> accumulator
+              in
+              let rec parse_ident id = parser
+                (* Call. *)
+                | [< 'Token.Kwd '(';
+                     args=parse_args [];
+                     'Token.Kwd ')' ?? "expected ')'">] ->
+                    Ast.Call (id, Array.of_list (List.rev args))
+
+                (* Simple variable ref. *)
+                | [< >] -> Ast.Variable id
+              in
+              parse_ident id stream
+
+          (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
+          | [< 'Token.If; c=parse_expr;
+               'Token.Then ?? "expected 'then'"; t=parse_expr;
+               'Token.Else ?? "expected 'else'"; e=parse_expr >] ->
+              Ast.If (c, t, e)
+
+          (* forexpr
+                ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
+          | [< 'Token.For;
+               'Token.Ident id ?? "expected identifier after for";
+               'Token.Kwd '=' ?? "expected '=' after for";
+               stream >] ->
+              begin parser
+                | [<
+                     start=parse_expr;
+                     'Token.Kwd ',' ?? "expected ',' after for";
+                     end_=parse_expr;
+                     stream >] ->
+                    let step =
+                      begin parser
+                      | [< 'Token.Kwd ','; step=parse_expr >] -> Some step
+                      | [< >] -> None
+                      end stream
+                    in
+                    begin parser
+                    | [< 'Token.In; body=parse_expr >] ->
+                        Ast.For (id, start, end_, step, body)
+                    | [< >] ->
+                        raise (Stream.Error "expected 'in' after for")
+                    end stream
+                | [< >] ->
+                    raise (Stream.Error "expected '=' after for")
+              end stream
+
+          (* varexpr
+           *   ::= 'var' identifier ('=' expression?
+           *             (',' identifier ('=' expression)?)* 'in' expression *)
+          | [< 'Token.Var;
+               (* At least one variable name is required. *)
+               'Token.Ident id ?? "expected identifier after var";
+               init=parse_var_init;
+               var_names=parse_var_names [(id, init)];
+               (* At this point, we have to have 'in'. *)
+               'Token.In ?? "expected 'in' keyword after 'var'";
+               body=parse_expr >] ->
+              Ast.Var (Array.of_list (List.rev var_names), body)
+
+          | [< >] -> raise (Stream.Error "unknown token when expecting an expression.")
+
+        (* unary
+         *   ::= primary
+         *   ::= '!' unary *)
+        and parse_unary = parser
+          (* If this is a unary operator, read it. *)
+          | [< 'Token.Kwd op when op != '(' && op != ')'; operand=parse_expr >] ->
+              Ast.Unary (op, operand)
+
+          (* If the current token is not an operator, it must be a primary expr. *)
+          | [< stream >] -> parse_primary stream
+
+        (* binoprhs
+         *   ::= ('+' primary)* *)
+        and parse_bin_rhs expr_prec lhs stream =
+          match Stream.peek stream with
+          (* If this is a binop, find its precedence. *)
+          | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c ->
+              let token_prec = precedence c in
+
+              (* If this is a binop that binds at least as tightly as the current binop,
+               * consume it, otherwise we are done. *)
+              if token_prec < expr_prec then lhs else begin
+                (* Eat the binop. *)
+                Stream.junk stream;
+
+                (* Parse the primary expression after the binary operator. *)
+                let rhs = parse_unary stream in
+
+                (* Okay, we know this is a binop. *)
+                let rhs =
+                  match Stream.peek stream with
+                  | Some (Token.Kwd c2) ->
+                      (* If BinOp binds less tightly with rhs than the operator after
+                       * rhs, let the pending operator take rhs as its lhs. *)
+                      let next_prec = precedence c2 in
+                      if token_prec < next_prec
+                      then parse_bin_rhs (token_prec + 1) rhs stream
+                      else rhs
+                  | _ -> rhs
+                in
+
+                (* Merge lhs/rhs. *)
+                let lhs = Ast.Binary (c, lhs, rhs) in
+                parse_bin_rhs expr_prec lhs stream
+              end
+          | _ -> lhs
+
+        and parse_var_init = parser
+          (* read in the optional initializer. *)
+          | [< 'Token.Kwd '='; e=parse_expr >] -> Some e
+          | [< >] -> None
+
+        and parse_var_names accumulator = parser
+          | [< 'Token.Kwd ',';
+               'Token.Ident id ?? "expected identifier list after var";
+               init=parse_var_init;
+               e=parse_var_names ((id, init) :: accumulator) >] -> e
+          | [< >] -> accumulator
+
+        (* expression
+         *   ::= primary binoprhs *)
+        and parse_expr = parser
+          | [< lhs=parse_unary; stream >] -> parse_bin_rhs 0 lhs stream
+
+        (* prototype
+         *   ::= id '(' id* ')'
+         *   ::= binary LETTER number? (id, id)
+         *   ::= unary LETTER number? (id) *)
+        let parse_prototype =
+          let rec parse_args accumulator = parser
+            | [< 'Token.Ident id; e=parse_args (id::accumulator) >] -> e
+            | [< >] -> accumulator
+          in
+          let parse_operator = parser
+            | [< 'Token.Unary >] -> "unary", 1
+            | [< 'Token.Binary >] -> "binary", 2
+          in
+          let parse_binary_precedence = parser
+            | [< 'Token.Number n >] -> int_of_float n
+            | [< >] -> 30
+          in
+          parser
+          | [< 'Token.Ident id;
+               'Token.Kwd '(' ?? "expected '(' in prototype";
+               args=parse_args [];
+               'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
+              (* success. *)
+              Ast.Prototype (id, Array.of_list (List.rev args))
+          | [< (prefix, kind)=parse_operator;
+               'Token.Kwd op ?? "expected an operator";
+               (* Read the precedence if present. *)
+               binary_precedence=parse_binary_precedence;
+               'Token.Kwd '(' ?? "expected '(' in prototype";
+                args=parse_args [];
+               'Token.Kwd ')' ?? "expected ')' in prototype" >] ->
+              let name = prefix ^ (String.make 1 op) in
+              let args = Array.of_list (List.rev args) in
+
+              (* Verify right number of arguments for operator. *)
+              if Array.length args != kind
+              then raise (Stream.Error "invalid number of operands for operator")
+              else
+                if kind == 1 then
+                  Ast.Prototype (name, args)
+                else
+                  Ast.BinOpPrototype (name, args, binary_precedence)
+          | [< >] ->
+              raise (Stream.Error "expected function name in prototype")
+
+        (* definition ::= 'def' prototype expression *)
+        let parse_definition = parser
+          | [< 'Token.Def; p=parse_prototype; e=parse_expr >] ->
+              Ast.Function (p, e)
+
+        (* toplevelexpr ::= expression *)
+        let parse_toplevel = parser
+          | [< e=parse_expr >] ->
+              (* Make an anonymous proto. *)
+              Ast.Function (Ast.Prototype ("", [||]), e)
+
+        (*  external ::= 'extern' prototype *)
+        let parse_extern = parser
+          | [< 'Token.Extern; e=parse_prototype >] -> e
+
+codegen.ml:
+    .. code-block:: ocaml
+
+        (*===----------------------------------------------------------------------===
+         * Code Generation
+         *===----------------------------------------------------------------------===*)
+
+        open Llvm
+
+        exception Error of string
+
+        let context = global_context ()
+        let the_module = create_module context "my cool jit"
+        let builder = builder context
+        let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
+        let double_type = double_type context
+
+        (* Create an alloca instruction in the entry block of the function. This
+         * is used for mutable variables etc. *)
+        let create_entry_block_alloca the_function var_name =
+          let builder = builder_at context (instr_begin (entry_block the_function)) in
+          build_alloca double_type var_name builder
+
+        let rec codegen_expr = function
+          | Ast.Number n -> const_float double_type n
+          | Ast.Variable name ->
+              let v = try Hashtbl.find named_values name with
+                | Not_found -> raise (Error "unknown variable name")
+              in
+              (* Load the value. *)
+              build_load v name builder
+          | Ast.Unary (op, operand) ->
+              let operand = codegen_expr operand in
+              let callee = "unary" ^ (String.make 1 op) in
+              let callee =
+                match lookup_function callee the_module with
+                | Some callee -> callee
+                | None -> raise (Error "unknown unary operator")
+              in
+              build_call callee [|operand|] "unop" builder
+          | Ast.Binary (op, lhs, rhs) ->
+              begin match op with
+              | '=' ->
+                  (* Special case '=' because we don't want to emit the LHS as an
+                   * expression. *)
+                  let name =
+                    match lhs with
+                    | Ast.Variable name -> name
+                    | _ -> raise (Error "destination of '=' must be a variable")
+                  in
+
+                  (* Codegen the rhs. *)
+                  let val_ = codegen_expr rhs in
+
+                  (* Lookup the name. *)
+                  let variable = try Hashtbl.find named_values name with
+                  | Not_found -> raise (Error "unknown variable name")
+                  in
+                  ignore(build_store val_ variable builder);
+                  val_
+              | _ ->
+                  let lhs_val = codegen_expr lhs in
+                  let rhs_val = codegen_expr rhs in
+                  begin
+                    match op with
+                    | '+' -> build_add lhs_val rhs_val "addtmp" builder
+                    | '-' -> build_sub lhs_val rhs_val "subtmp" builder
+                    | '*' -> build_mul lhs_val rhs_val "multmp" builder
+                    | '<' ->
+                        (* Convert bool 0/1 to double 0.0 or 1.0 *)
+                        let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
+                        build_uitofp i double_type "booltmp" builder
+                    | _ ->
+                        (* If it wasn't a builtin binary operator, it must be a user defined
+                         * one. Emit a call to it. *)
+                        let callee = "binary" ^ (String.make 1 op) in
+                        let callee =
+                          match lookup_function callee the_module with
+                          | Some callee -> callee
+                          | None -> raise (Error "binary operator not found!")
+                        in
+                        build_call callee [|lhs_val; rhs_val|] "binop" builder
+                  end
+              end
+          | Ast.Call (callee, args) ->
+              (* Look up the name in the module table. *)
+              let callee =
+                match lookup_function callee the_module with
+                | Some callee -> callee
+                | None -> raise (Error "unknown function referenced")
+              in
+              let params = params callee in
+
+              (* If argument mismatch error. *)
+              if Array.length params == Array.length args then () else
+                raise (Error "incorrect # arguments passed");
+              let args = Array.map codegen_expr args in
+              build_call callee args "calltmp" builder
+          | Ast.If (cond, then_, else_) ->
+              let cond = codegen_expr cond in
+
+              (* Convert condition to a bool by comparing equal to 0.0 *)
+              let zero = const_float double_type 0.0 in
+              let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
+
+              (* Grab the first block so that we might later add the conditional branch
+               * to it at the end of the function. *)
+              let start_bb = insertion_block builder in
+              let the_function = block_parent start_bb in
+
+              let then_bb = append_block context "then" the_function in
+
+              (* Emit 'then' value. *)
+              position_at_end then_bb builder;
+              let then_val = codegen_expr then_ in
+
+              (* Codegen of 'then' can change the current block, update then_bb for the
+               * phi. We create a new name because one is used for the phi node, and the
+               * other is used for the conditional branch. *)
+              let new_then_bb = insertion_block builder in
+
+              (* Emit 'else' value. *)
+              let else_bb = append_block context "else" the_function in
+              position_at_end else_bb builder;
+              let else_val = codegen_expr else_ in
+
+              (* Codegen of 'else' can change the current block, update else_bb for the
+               * phi. *)
+              let new_else_bb = insertion_block builder in
+
+              (* Emit merge block. *)
+              let merge_bb = append_block context "ifcont" the_function in
+              position_at_end merge_bb builder;
+              let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
+              let phi = build_phi incoming "iftmp" builder in
+
+              (* Return to the start block to add the conditional branch. *)
+              position_at_end start_bb builder;
+              ignore (build_cond_br cond_val then_bb else_bb builder);
+
+              (* Set a unconditional branch at the end of the 'then' block and the
+               * 'else' block to the 'merge' block. *)
+              position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
+              position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
+
+              (* Finally, set the builder to the end of the merge block. *)
+              position_at_end merge_bb builder;
+
+              phi
+          | Ast.For (var_name, start, end_, step, body) ->
+              (* Output this as:
+               *   var = alloca double
+               *   ...
+               *   start = startexpr
+               *   store start -> var
+               *   goto loop
+               * loop:
+               *   ...
+               *   bodyexpr
+               *   ...
+               * loopend:
+               *   step = stepexpr
+               *   endcond = endexpr
+               *
+               *   curvar = load var
+               *   nextvar = curvar + step
+               *   store nextvar -> var
+               *   br endcond, loop, endloop
+               * outloop: *)
+
+              let the_function = block_parent (insertion_block builder) in
+
+              (* Create an alloca for the variable in the entry block. *)
+              let alloca = create_entry_block_alloca the_function var_name in
+
+              (* Emit the start code first, without 'variable' in scope. *)
+              let start_val = codegen_expr start in
+
+              (* Store the value into the alloca. *)
+              ignore(build_store start_val alloca builder);
+
+              (* Make the new basic block for the loop header, inserting after current
+               * block. *)
+              let loop_bb = append_block context "loop" the_function in
+
+              (* Insert an explicit fall through from the current block to the
+               * loop_bb. *)
+              ignore (build_br loop_bb builder);
+
+              (* Start insertion in loop_bb. *)
+              position_at_end loop_bb builder;
+
+              (* Within the loop, the variable is defined equal to the PHI node. If it
+               * shadows an existing variable, we have to restore it, so save it
+               * now. *)
+              let old_val =
+                try Some (Hashtbl.find named_values var_name) with Not_found -> None
+              in
+              Hashtbl.add named_values var_name alloca;
+
+              (* Emit the body of the loop.  This, like any other expr, can change the
+               * current BB.  Note that we ignore the value computed by the body, but
+               * don't allow an error *)
+              ignore (codegen_expr body);
+
+              (* Emit the step value. *)
+              let step_val =
+                match step with
+                | Some step -> codegen_expr step
+                (* If not specified, use 1.0. *)
+                | None -> const_float double_type 1.0
+              in
+
+              (* Compute the end condition. *)
+              let end_cond = codegen_expr end_ in
+
+              (* Reload, increment, and restore the alloca. This handles the case where
+               * the body of the loop mutates the variable. *)
+              let cur_var = build_load alloca var_name builder in
+              let next_var = build_add cur_var step_val "nextvar" builder in
+              ignore(build_store next_var alloca builder);
+
+              (* Convert condition to a bool by comparing equal to 0.0. *)
+              let zero = const_float double_type 0.0 in
+              let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
+
+              (* Create the "after loop" block and insert it. *)
+              let after_bb = append_block context "afterloop" the_function in
+
+              (* Insert the conditional branch into the end of loop_end_bb. *)
+              ignore (build_cond_br end_cond loop_bb after_bb builder);
+
+              (* Any new code will be inserted in after_bb. *)
+              position_at_end after_bb builder;
+
+              (* Restore the unshadowed variable. *)
+              begin match old_val with
+              | Some old_val -> Hashtbl.add named_values var_name old_val
+              | None -> ()
+              end;
+
+              (* for expr always returns 0.0. *)
+              const_null double_type
+          | Ast.Var (var_names, body) ->
+              let old_bindings = ref [] in
+
+              let the_function = block_parent (insertion_block builder) in
+
+              (* Register all variables and emit their initializer. *)
+              Array.iter (fun (var_name, init) ->
+                (* Emit the initializer before adding the variable to scope, this
+                 * prevents the initializer from referencing the variable itself, and
+                 * permits stuff like this:
+                 *   var a = 1 in
+                 *     var a = a in ...   # refers to outer 'a'. *)
+                let init_val =
+                  match init with
+                  | Some init -> codegen_expr init
+                  (* If not specified, use 0.0. *)
+                  | None -> const_float double_type 0.0
+                in
+
+                let alloca = create_entry_block_alloca the_function var_name in
+                ignore(build_store init_val alloca builder);
+
+                (* Remember the old variable binding so that we can restore the binding
+                 * when we unrecurse. *)
+                begin
+                  try
+                    let old_value = Hashtbl.find named_values var_name in
+                    old_bindings := (var_name, old_value) :: !old_bindings;
+                  with Not_found -> ()
+                end;
+
+                (* Remember this binding. *)
+                Hashtbl.add named_values var_name alloca;
+              ) var_names;
+
+              (* Codegen the body, now that all vars are in scope. *)
+              let body_val = codegen_expr body in
+
+              (* Pop all our variables from scope. *)
+              List.iter (fun (var_name, old_value) ->
+                Hashtbl.add named_values var_name old_value
+              ) !old_bindings;
+
+              (* Return the body computation. *)
+              body_val
+
+        let codegen_proto = function
+          | Ast.Prototype (name, args) | Ast.BinOpPrototype (name, args, _) ->
+              (* Make the function type: double(double,double) etc. *)
+              let doubles = Array.make (Array.length args) double_type in
+              let ft = function_type double_type doubles in
+              let f =
+                match lookup_function name the_module with
+                | None -> declare_function name ft the_module
+
+                (* If 'f' conflicted, there was already something named 'name'. If it
+                 * has a body, don't allow redefinition or reextern. *)
+                | Some f ->
+                    (* If 'f' already has a body, reject this. *)
+                    if block_begin f <> At_end f then
+                      raise (Error "redefinition of function");
+
+                    (* If 'f' took a different number of arguments, reject. *)
+                    if element_type (type_of f) <> ft then
+                      raise (Error "redefinition of function with different # args");
+                    f
+              in
+
+              (* Set names for all arguments. *)
+              Array.iteri (fun i a ->
+                let n = args.(i) in
+                set_value_name n a;
+                Hashtbl.add named_values n a;
+              ) (params f);
+              f
+
+        (* Create an alloca for each argument and register the argument in the symbol
+         * table so that references to it will succeed. *)
+        let create_argument_allocas the_function proto =
+          let args = match proto with
+            | Ast.Prototype (_, args) | Ast.BinOpPrototype (_, args, _) -> args
+          in
+          Array.iteri (fun i ai ->
+            let var_name = args.(i) in
+            (* Create an alloca for this variable. *)
+            let alloca = create_entry_block_alloca the_function var_name in
+
+            (* Store the initial value into the alloca. *)
+            ignore(build_store ai alloca builder);
+
+            (* Add arguments to variable symbol table. *)
+            Hashtbl.add named_values var_name alloca;
+          ) (params the_function)
+
+        let codegen_func the_fpm = function
+          | Ast.Function (proto, body) ->
+              Hashtbl.clear named_values;
+              let the_function = codegen_proto proto in
+
+              (* If this is an operator, install it. *)
+              begin match proto with
+              | Ast.BinOpPrototype (name, args, prec) ->
+                  let op = name.[String.length name - 1] in
+                  Hashtbl.add Parser.binop_precedence op prec;
+              | _ -> ()
+              end;
+
+              (* Create a new basic block to start insertion into. *)
+              let bb = append_block context "entry" the_function in
+              position_at_end bb builder;
+
+              try
+                (* Add all arguments to the symbol table and create their allocas. *)
+                create_argument_allocas the_function proto;
+
+                let ret_val = codegen_expr body in
+
+                (* Finish off the function. *)
+                let _ = build_ret ret_val builder in
+
+                (* Validate the generated code, checking for consistency. *)
+                Llvm_analysis.assert_valid_function the_function;
+
+                (* Optimize the function. *)
+                let _ = PassManager.run_function the_function the_fpm in
+
+                the_function
+              with e ->
+                delete_function the_function;
+                raise e
+
+toplevel.ml:
+    .. code-block:: ocaml
+
+        (*===----------------------------------------------------------------------===
+         * Top-Level parsing and JIT Driver
+         *===----------------------------------------------------------------------===*)
+
+        open Llvm
+        open Llvm_executionengine
+
+        (* top ::= definition | external | expression | ';' *)
+        let rec main_loop the_fpm the_execution_engine stream =
+          match Stream.peek stream with
+          | None -> ()
+
+          (* ignore top-level semicolons. *)
+          | Some (Token.Kwd ';') ->
+              Stream.junk stream;
+              main_loop the_fpm the_execution_engine stream
+
+          | Some token ->
+              begin
+                try match token with
+                | Token.Def ->
+                    let e = Parser.parse_definition stream in
+                    print_endline "parsed a function definition.";
+                    dump_value (Codegen.codegen_func the_fpm e);
+                | Token.Extern ->
+                    let e = Parser.parse_extern stream in
+                    print_endline "parsed an extern.";
+                    dump_value (Codegen.codegen_proto e);
+                | _ ->
+                    (* Evaluate a top-level expression into an anonymous function. *)
+                    let e = Parser.parse_toplevel stream in
+                    print_endline "parsed a top-level expr";
+                    let the_function = Codegen.codegen_func the_fpm e in
+                    dump_value the_function;
+
+                    (* JIT the function, returning a function pointer. *)
+                    let result = ExecutionEngine.run_function the_function [||]
+                      the_execution_engine in
+
+                    print_string "Evaluated to ";
+                    print_float (GenericValue.as_float Codegen.double_type result);
+                    print_newline ();
+                with Stream.Error s | Codegen.Error s ->
+                  (* Skip token for error recovery. *)
+                  Stream.junk stream;
+                  print_endline s;
+              end;
+              print_string "ready> "; flush stdout;
+              main_loop the_fpm the_execution_engine stream
+
+toy.ml:
+    .. code-block:: ocaml
+
+        (*===----------------------------------------------------------------------===
+         * Main driver code.
+         *===----------------------------------------------------------------------===*)
+
+        open Llvm
+        open Llvm_executionengine
+        open Llvm_target
+        open Llvm_scalar_opts
+
+        let main () =
+          ignore (initialize_native_target ());
+
+          (* Install standard binary operators.
+           * 1 is the lowest precedence. *)
+          Hashtbl.add Parser.binop_precedence '=' 2;
+          Hashtbl.add Parser.binop_precedence '<' 10;
+          Hashtbl.add Parser.binop_precedence '+' 20;
+          Hashtbl.add Parser.binop_precedence '-' 20;
+          Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
+
+          (* Prime the first token. *)
+          print_string "ready> "; flush stdout;
+          let stream = Lexer.lex (Stream.of_channel stdin) in
+
+          (* Create the JIT. *)
+          let the_execution_engine = ExecutionEngine.create Codegen.the_module in
+          let the_fpm = PassManager.create_function Codegen.the_module in
+
+          (* Set up the optimizer pipeline.  Start with registering info about how the
+           * target lays out data structures. *)
+          DataLayout.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
+
+          (* Promote allocas to registers. *)
+          add_memory_to_register_promotion the_fpm;
+
+          (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
+          add_instruction_combination the_fpm;
+
+          (* reassociate expressions. *)
+          add_reassociation the_fpm;
+
+          (* Eliminate Common SubExpressions. *)
+          add_gvn the_fpm;
+
+          (* Simplify the control flow graph (deleting unreachable blocks, etc). *)
+          add_cfg_simplification the_fpm;
+
+          ignore (PassManager.initialize the_fpm);
+
+          (* Run the main "interpreter loop" now. *)
+          Toplevel.main_loop the_fpm the_execution_engine stream;
+
+          (* Print out all the generated code. *)
+          dump_module Codegen.the_module
+        ;;
+
+        main ()
+
+bindings.c
+    .. code-block:: c
+
+        #include <stdio.h>
+
+        /* putchard - putchar that takes a double and returns 0. */
+        extern double putchard(double X) {
+          putchar((char)X);
+          return 0;
+        }
+
+        /* printd - printf that takes a double prints it as "%f\n", returning 0. */
+        extern double printd(double X) {
+          printf("%f\n", X);
+          return 0;
+        }
+
+`Next: Conclusion and other useful LLVM tidbits <OCamlLangImpl8.html>`_
+