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-=======================================================
-Kaleidoscope: Extending the Language: Mutable Variables
-=======================================================
-
-.. contents::
-   :local:
-
-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#alloca-instruction>`_:
-
-.. 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#first-class-types>`_ 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 "sroa" 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: clang uses 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 <#id1>`_
-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#code-generation-for-the-for-loop>`_. 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>`_
-