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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#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 '``NamedValues``' 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
+``NamedValues`` 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
+NamedValues 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:
+
+.. code-block:: c++
+
+    static std::map<std::string, AllocaInst*> NamedValues;
+
+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:: c++
+
+    /// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of
+    /// the function.  This is used for mutable variables etc.
+    static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
+                                              const std::string &VarName) {
+      IRBuilder<> TmpB(&TheFunction->getEntryBlock(),
+                     TheFunction->getEntryBlock().begin());
+      return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0,
+                               VarName.c_str());
+    }
+
+This funny looking code creates an IRBuilder object that is pointing at
+the first instruction (.begin()) 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:: c++
+
+    Value *VariableExprAST::Codegen() {
+      // Look this variable up in the function.
+      Value *V = NamedValues[Name];
+      if (V == 0) return ErrorV("Unknown variable name");
+
+      // Load the value.
+      return Builder.CreateLoad(V, Name.c_str());
+    }
+
+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 ``ForExprAST::Codegen`` (see the `full code listing <#code>`_ for
+the unabridged code):
+
+.. code-block:: c++
+
+      Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+      // Create an alloca for the variable in the entry block.
+      AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+
+        // Emit the start code first, without 'variable' in scope.
+      Value *StartVal = Start->Codegen();
+      if (StartVal == 0) return 0;
+
+      // Store the value into the alloca.
+      Builder.CreateStore(StartVal, Alloca);
+      ...
+
+      // Compute the end condition.
+      Value *EndCond = End->Codegen();
+      if (EndCond == 0) return EndCond;
+
+      // Reload, increment, and restore the alloca.  This handles the case where
+      // the body of the loop mutates the variable.
+      Value *CurVar = Builder.CreateLoad(Alloca);
+      Value *NextVar = Builder.CreateFAdd(CurVar, StepVal, "nextvar");
+      Builder.CreateStore(NextVar, Alloca);
+      ...
+
+This code is virtually identical to the code `before we allowed mutable
+variables <LangImpl5.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:: c++
+
+    /// CreateArgumentAllocas - Create an alloca for each argument and register the
+    /// argument in the symbol table so that references to it will succeed.
+    void PrototypeAST::CreateArgumentAllocas(Function *F) {
+      Function::arg_iterator AI = F->arg_begin();
+      for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) {
+        // Create an alloca for this variable.
+        AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]);
+
+        // Store the initial value into the alloca.
+        Builder.CreateStore(AI, Alloca);
+
+        // Add arguments to variable symbol table.
+        NamedValues[Args[Idx]] = Alloca;
+      }
+    }
+
+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 ``FunctionAST::Codegen``
+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:: c++
+
+        // Set up the optimizer pipeline.  Start with registering info about how the
+        // target lays out data structures.
+        OurFPM.add(new DataLayout(*TheExecutionEngine->getDataLayout()));
+        // Promote allocas to registers.
+        OurFPM.add(createPromoteMemoryToRegisterPass());
+        // Do simple "peephole" optimizations and bit-twiddling optzns.
+        OurFPM.add(createInstructionCombiningPass());
+        // Reassociate expressions.
+        OurFPM.add(createReassociatePass());
+
+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:: c++
+
+     int main() {
+       // Install standard binary operators.
+       // 1 is lowest precedence.
+       BinopPrecedence['='] = 2;
+       BinopPrecedence['<'] = 10;
+       BinopPrecedence['+'] = 20;
+       BinopPrecedence['-'] = 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:: c++
+
+    Value *BinaryExprAST::Codegen() {
+      // Special case '=' because we don't want to emit the LHS as an expression.
+      if (Op == '=') {
+        // Assignment requires the LHS to be an identifier.
+        VariableExprAST *LHSE = dynamic_cast<VariableExprAST*>(LHS);
+        if (!LHSE)
+          return ErrorV("destination of '=' must be a variable");
+
+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:: c++
+
+        // Codegen the RHS.
+        Value *Val = RHS->Codegen();
+        if (Val == 0) return 0;
+
+        // Look up the name.
+        Value *Variable = NamedValues[LHSE->getName()];
+        if (Variable == 0) return ErrorV("Unknown variable name");
+
+        Builder.CreateStore(Val, Variable);
+        return 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:: c++
+
+    enum Token {
+      ...
+      // var definition
+      tok_var = -13
+    ...
+    }
+    ...
+    static int gettok() {
+    ...
+        if (IdentifierStr == "in") return tok_in;
+        if (IdentifierStr == "binary") return tok_binary;
+        if (IdentifierStr == "unary") return tok_unary;
+        if (IdentifierStr == "var") return tok_var;
+        return tok_identifier;
+    ...
+
+The next step is to define the AST node that we will construct. For
+var/in, it looks like this:
+
+.. code-block:: c++
+
+    /// VarExprAST - Expression class for var/in
+    class VarExprAST : public ExprAST {
+      std::vector<std::pair<std::string, ExprAST*> > VarNames;
+      ExprAST *Body;
+    public:
+      VarExprAST(const std::vector<std::pair<std::string, ExprAST*> > &varnames,
+                 ExprAST *body)
+      : VarNames(varnames), Body(body) {}
+
+      virtual Value *Codegen();
+    };
+
+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:: c++
+
+    /// primary
+    ///   ::= identifierexpr
+    ///   ::= numberexpr
+    ///   ::= parenexpr
+    ///   ::= ifexpr
+    ///   ::= forexpr
+    ///   ::= varexpr
+    static ExprAST *ParsePrimary() {
+      switch (CurTok) {
+      default: return Error("unknown token when expecting an expression");
+      case tok_identifier: return ParseIdentifierExpr();
+      case tok_number:     return ParseNumberExpr();
+      case '(':            return ParseParenExpr();
+      case tok_if:         return ParseIfExpr();
+      case tok_for:        return ParseForExpr();
+      case tok_var:        return ParseVarExpr();
+      }
+    }
+
+Next we define ParseVarExpr:
+
+.. code-block:: c++
+
+    /// varexpr ::= 'var' identifier ('=' expression)?
+    //                    (',' identifier ('=' expression)?)* 'in' expression
+    static ExprAST *ParseVarExpr() {
+      getNextToken();  // eat the var.
+
+      std::vector<std::pair<std::string, ExprAST*> > VarNames;
+
+      // At least one variable name is required.
+      if (CurTok != tok_identifier)
+        return Error("expected identifier after var");
+
+The first part of this code parses the list of identifier/expr pairs
+into the local ``VarNames`` vector.
+
+.. code-block:: c++
+
+      while (1) {
+        std::string Name = IdentifierStr;
+        getNextToken();  // eat identifier.
+
+        // Read the optional initializer.
+        ExprAST *Init = 0;
+        if (CurTok == '=') {
+          getNextToken(); // eat the '='.
+
+          Init = ParseExpression();
+          if (Init == 0) return 0;
+        }
+
+        VarNames.push_back(std::make_pair(Name, Init));
+
+        // End of var list, exit loop.
+        if (CurTok != ',') break;
+        getNextToken(); // eat the ','.
+
+        if (CurTok != tok_identifier)
+          return Error("expected identifier list after var");
+      }
+
+Once all the variables are parsed, we then parse the body and create the
+AST node:
+
+.. code-block:: c++
+
+      // At this point, we have to have 'in'.
+      if (CurTok != tok_in)
+        return Error("expected 'in' keyword after 'var'");
+      getNextToken();  // eat 'in'.
+
+      ExprAST *Body = ParseExpression();
+      if (Body == 0) return 0;
+
+      return new VarExprAST(VarNames, Body);
+    }
+
+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:: c++
+
+    Value *VarExprAST::Codegen() {
+      std::vector<AllocaInst *> OldBindings;
+
+      Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+      // Register all variables and emit their initializer.
+      for (unsigned i = 0, e = VarNames.size(); i != e; ++i) {
+        const std::string &VarName = VarNames[i].first;
+        ExprAST *Init = VarNames[i].second;
+
+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:: c++
+
+        // 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'.
+        Value *InitVal;
+        if (Init) {
+          InitVal = Init->Codegen();
+          if (InitVal == 0) return 0;
+        } else { // If not specified, use 0.0.
+          InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0));
+        }
+
+        AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+        Builder.CreateStore(InitVal, Alloca);
+
+        // Remember the old variable binding so that we can restore the binding when
+        // we unrecurse.
+        OldBindings.push_back(NamedValues[VarName]);
+
+        // Remember this binding.
+        NamedValues[VarName] = Alloca;
+      }
+
+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:: c++
+
+      // Codegen the body, now that all vars are in scope.
+      Value *BodyVal = Body->Codegen();
+      if (BodyVal == 0) return 0;
+
+Finally, before returning, we restore the previous variable bindings:
+
+.. code-block:: c++
+
+      // Pop all our variables from scope.
+      for (unsigned i = 0, e = VarNames.size(); i != e; ++i)
+        NamedValues[VarNames[i].first] = OldBindings[i];
+
+      // Return the body computation.
+      return BodyVal;
+    }
+
+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
+    clang++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
+    # Run
+    ./toy
+
+Here is the code:
+
+.. code-block:: c++
+
+    #include "llvm/DerivedTypes.h"
+    #include "llvm/ExecutionEngine/ExecutionEngine.h"
+    #include "llvm/ExecutionEngine/JIT.h"
+    #include "llvm/IRBuilder.h"
+    #include "llvm/LLVMContext.h"
+    #include "llvm/Module.h"
+    #include "llvm/PassManager.h"
+    #include "llvm/Analysis/Verifier.h"
+    #include "llvm/Analysis/Passes.h"
+    #include "llvm/DataLayout.h"
+    #include "llvm/Transforms/Scalar.h"
+    #include "llvm/Support/TargetSelect.h"
+    #include <cstdio>
+    #include <string>
+    #include <map>
+    #include <vector>
+    using namespace llvm;
+
+    //===----------------------------------------------------------------------===//
+    // Lexer
+    //===----------------------------------------------------------------------===//
+
+    // The lexer returns tokens [0-255] if it is an unknown character, otherwise one
+    // of these for known things.
+    enum Token {
+      tok_eof = -1,
+
+      // commands
+      tok_def = -2, tok_extern = -3,
+
+      // primary
+      tok_identifier = -4, tok_number = -5,
+
+      // control
+      tok_if = -6, tok_then = -7, tok_else = -8,
+      tok_for = -9, tok_in = -10,
+
+      // operators
+      tok_binary = -11, tok_unary = -12,
+
+      // var definition
+      tok_var = -13
+    };
+
+    static std::string IdentifierStr;  // Filled in if tok_identifier
+    static double NumVal;              // Filled in if tok_number
+
+    /// gettok - Return the next token from standard input.
+    static int gettok() {
+      static int LastChar = ' ';
+
+      // Skip any whitespace.
+      while (isspace(LastChar))
+        LastChar = getchar();
+
+      if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
+        IdentifierStr = LastChar;
+        while (isalnum((LastChar = getchar())))
+          IdentifierStr += LastChar;
+
+        if (IdentifierStr == "def") return tok_def;
+        if (IdentifierStr == "extern") return tok_extern;
+        if (IdentifierStr == "if") return tok_if;
+        if (IdentifierStr == "then") return tok_then;
+        if (IdentifierStr == "else") return tok_else;
+        if (IdentifierStr == "for") return tok_for;
+        if (IdentifierStr == "in") return tok_in;
+        if (IdentifierStr == "binary") return tok_binary;
+        if (IdentifierStr == "unary") return tok_unary;
+        if (IdentifierStr == "var") return tok_var;
+        return tok_identifier;
+      }
+
+      if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
+        std::string NumStr;
+        do {
+          NumStr += LastChar;
+          LastChar = getchar();
+        } while (isdigit(LastChar) || LastChar == '.');
+
+        NumVal = strtod(NumStr.c_str(), 0);
+        return tok_number;
+      }
+
+      if (LastChar == '#') {
+        // Comment until end of line.
+        do LastChar = getchar();
+        while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
+
+        if (LastChar != EOF)
+          return gettok();
+      }
+
+      // Check for end of file.  Don't eat the EOF.
+      if (LastChar == EOF)
+        return tok_eof;
+
+      // Otherwise, just return the character as its ascii value.
+      int ThisChar = LastChar;
+      LastChar = getchar();
+      return ThisChar;
+    }
+
+    //===----------------------------------------------------------------------===//
+    // Abstract Syntax Tree (aka Parse Tree)
+    //===----------------------------------------------------------------------===//
+
+    /// ExprAST - Base class for all expression nodes.
+    class ExprAST {
+    public:
+      virtual ~ExprAST() {}
+      virtual Value *Codegen() = 0;
+    };
+
+    /// NumberExprAST - Expression class for numeric literals like "1.0".
+    class NumberExprAST : public ExprAST {
+      double Val;
+    public:
+      NumberExprAST(double val) : Val(val) {}
+      virtual Value *Codegen();
+    };
+
+    /// VariableExprAST - Expression class for referencing a variable, like "a".
+    class VariableExprAST : public ExprAST {
+      std::string Name;
+    public:
+      VariableExprAST(const std::string &name) : Name(name) {}
+      const std::string &getName() const { return Name; }
+      virtual Value *Codegen();
+    };
+
+    /// UnaryExprAST - Expression class for a unary operator.
+    class UnaryExprAST : public ExprAST {
+      char Opcode;
+      ExprAST *Operand;
+    public:
+      UnaryExprAST(char opcode, ExprAST *operand)
+        : Opcode(opcode), Operand(operand) {}
+      virtual Value *Codegen();
+    };
+
+    /// BinaryExprAST - Expression class for a binary operator.
+    class BinaryExprAST : public ExprAST {
+      char Op;
+      ExprAST *LHS, *RHS;
+    public:
+      BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs)
+        : Op(op), LHS(lhs), RHS(rhs) {}
+      virtual Value *Codegen();
+    };
+
+    /// CallExprAST - Expression class for function calls.
+    class CallExprAST : public ExprAST {
+      std::string Callee;
+      std::vector<ExprAST*> Args;
+    public:
+      CallExprAST(const std::string &callee, std::vector<ExprAST*> &args)
+        : Callee(callee), Args(args) {}
+      virtual Value *Codegen();
+    };
+
+    /// IfExprAST - Expression class for if/then/else.
+    class IfExprAST : public ExprAST {
+      ExprAST *Cond, *Then, *Else;
+    public:
+      IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else)
+      : Cond(cond), Then(then), Else(_else) {}
+      virtual Value *Codegen();
+    };
+
+    /// ForExprAST - Expression class for for/in.
+    class ForExprAST : public ExprAST {
+      std::string VarName;
+      ExprAST *Start, *End, *Step, *Body;
+    public:
+      ForExprAST(const std::string &varname, ExprAST *start, ExprAST *end,
+                 ExprAST *step, ExprAST *body)
+        : VarName(varname), Start(start), End(end), Step(step), Body(body) {}
+      virtual Value *Codegen();
+    };
+
+    /// VarExprAST - Expression class for var/in
+    class VarExprAST : public ExprAST {
+      std::vector<std::pair<std::string, ExprAST*> > VarNames;
+      ExprAST *Body;
+    public:
+      VarExprAST(const std::vector<std::pair<std::string, ExprAST*> > &varnames,
+                 ExprAST *body)
+      : VarNames(varnames), Body(body) {}
+
+      virtual Value *Codegen();
+    };
+
+    /// PrototypeAST - This class represents the "prototype" for a function,
+    /// which captures its name, and its argument names (thus implicitly the number
+    /// of arguments the function takes), as well as if it is an operator.
+    class PrototypeAST {
+      std::string Name;
+      std::vector<std::string> Args;
+      bool isOperator;
+      unsigned Precedence;  // Precedence if a binary op.
+    public:
+      PrototypeAST(const std::string &name, const std::vector<std::string> &args,
+                   bool isoperator = false, unsigned prec = 0)
+      : Name(name), Args(args), isOperator(isoperator), Precedence(prec) {}
+
+      bool isUnaryOp() const { return isOperator && Args.size() == 1; }
+      bool isBinaryOp() const { return isOperator && Args.size() == 2; }
+
+      char getOperatorName() const {
+        assert(isUnaryOp() || isBinaryOp());
+        return Name[Name.size()-1];
+      }
+
+      unsigned getBinaryPrecedence() const { return Precedence; }
+
+      Function *Codegen();
+
+      void CreateArgumentAllocas(Function *F);
+    };
+
+    /// FunctionAST - This class represents a function definition itself.
+    class FunctionAST {
+      PrototypeAST *Proto;
+      ExprAST *Body;
+    public:
+      FunctionAST(PrototypeAST *proto, ExprAST *body)
+        : Proto(proto), Body(body) {}
+
+      Function *Codegen();
+    };
+
+    //===----------------------------------------------------------------------===//
+    // Parser
+    //===----------------------------------------------------------------------===//
+
+    /// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
+    /// token the parser is looking at.  getNextToken reads another token from the
+    /// lexer and updates CurTok with its results.
+    static int CurTok;
+    static int getNextToken() {
+      return CurTok = gettok();
+    }
+
+    /// BinopPrecedence - This holds the precedence for each binary operator that is
+    /// defined.
+    static std::map<char, int> BinopPrecedence;
+
+    /// GetTokPrecedence - Get the precedence of the pending binary operator token.
+    static int GetTokPrecedence() {
+      if (!isascii(CurTok))
+        return -1;
+
+      // Make sure it's a declared binop.
+      int TokPrec = BinopPrecedence[CurTok];
+      if (TokPrec <= 0) return -1;
+      return TokPrec;
+    }
+
+    /// Error* - These are little helper functions for error handling.
+    ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
+    PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
+    FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
+
+    static ExprAST *ParseExpression();
+
+    /// identifierexpr
+    ///   ::= identifier
+    ///   ::= identifier '(' expression* ')'
+    static ExprAST *ParseIdentifierExpr() {
+      std::string IdName = IdentifierStr;
+
+      getNextToken();  // eat identifier.
+
+      if (CurTok != '(') // Simple variable ref.
+        return new VariableExprAST(IdName);
+
+      // Call.
+      getNextToken();  // eat (
+      std::vector<ExprAST*> Args;
+      if (CurTok != ')') {
+        while (1) {
+          ExprAST *Arg = ParseExpression();
+          if (!Arg) return 0;
+          Args.push_back(Arg);
+
+          if (CurTok == ')') break;
+
+          if (CurTok != ',')
+            return Error("Expected ')' or ',' in argument list");
+          getNextToken();
+        }
+      }
+
+      // Eat the ')'.
+      getNextToken();
+
+      return new CallExprAST(IdName, Args);
+    }
+
+    /// numberexpr ::= number
+    static ExprAST *ParseNumberExpr() {
+      ExprAST *Result = new NumberExprAST(NumVal);
+      getNextToken(); // consume the number
+      return Result;
+    }
+
+    /// parenexpr ::= '(' expression ')'
+    static ExprAST *ParseParenExpr() {
+      getNextToken();  // eat (.
+      ExprAST *V = ParseExpression();
+      if (!V) return 0;
+
+      if (CurTok != ')')
+        return Error("expected ')'");
+      getNextToken();  // eat ).
+      return V;
+    }
+
+    /// ifexpr ::= 'if' expression 'then' expression 'else' expression
+    static ExprAST *ParseIfExpr() {
+      getNextToken();  // eat the if.
+
+      // condition.
+      ExprAST *Cond = ParseExpression();
+      if (!Cond) return 0;
+
+      if (CurTok != tok_then)
+        return Error("expected then");
+      getNextToken();  // eat the then
+
+      ExprAST *Then = ParseExpression();
+      if (Then == 0) return 0;
+
+      if (CurTok != tok_else)
+        return Error("expected else");
+
+      getNextToken();
+
+      ExprAST *Else = ParseExpression();
+      if (!Else) return 0;
+
+      return new IfExprAST(Cond, Then, Else);
+    }
+
+    /// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
+    static ExprAST *ParseForExpr() {
+      getNextToken();  // eat the for.
+
+      if (CurTok != tok_identifier)
+        return Error("expected identifier after for");
+
+      std::string IdName = IdentifierStr;
+      getNextToken();  // eat identifier.
+
+      if (CurTok != '=')
+        return Error("expected '=' after for");
+      getNextToken();  // eat '='.
+
+
+      ExprAST *Start = ParseExpression();
+      if (Start == 0) return 0;
+      if (CurTok != ',')
+        return Error("expected ',' after for start value");
+      getNextToken();
+
+      ExprAST *End = ParseExpression();
+      if (End == 0) return 0;
+
+      // The step value is optional.
+      ExprAST *Step = 0;
+      if (CurTok == ',') {
+        getNextToken();
+        Step = ParseExpression();
+        if (Step == 0) return 0;
+      }
+
+      if (CurTok != tok_in)
+        return Error("expected 'in' after for");
+      getNextToken();  // eat 'in'.
+
+      ExprAST *Body = ParseExpression();
+      if (Body == 0) return 0;
+
+      return new ForExprAST(IdName, Start, End, Step, Body);
+    }
+
+    /// varexpr ::= 'var' identifier ('=' expression)?
+    //                    (',' identifier ('=' expression)?)* 'in' expression
+    static ExprAST *ParseVarExpr() {
+      getNextToken();  // eat the var.
+
+      std::vector<std::pair<std::string, ExprAST*> > VarNames;
+
+      // At least one variable name is required.
+      if (CurTok != tok_identifier)
+        return Error("expected identifier after var");
+
+      while (1) {
+        std::string Name = IdentifierStr;
+        getNextToken();  // eat identifier.
+
+        // Read the optional initializer.
+        ExprAST *Init = 0;
+        if (CurTok == '=') {
+          getNextToken(); // eat the '='.
+
+          Init = ParseExpression();
+          if (Init == 0) return 0;
+        }
+
+        VarNames.push_back(std::make_pair(Name, Init));
+
+        // End of var list, exit loop.
+        if (CurTok != ',') break;
+        getNextToken(); // eat the ','.
+
+        if (CurTok != tok_identifier)
+          return Error("expected identifier list after var");
+      }
+
+      // At this point, we have to have 'in'.
+      if (CurTok != tok_in)
+        return Error("expected 'in' keyword after 'var'");
+      getNextToken();  // eat 'in'.
+
+      ExprAST *Body = ParseExpression();
+      if (Body == 0) return 0;
+
+      return new VarExprAST(VarNames, Body);
+    }
+
+    /// primary
+    ///   ::= identifierexpr
+    ///   ::= numberexpr
+    ///   ::= parenexpr
+    ///   ::= ifexpr
+    ///   ::= forexpr
+    ///   ::= varexpr
+    static ExprAST *ParsePrimary() {
+      switch (CurTok) {
+      default: return Error("unknown token when expecting an expression");
+      case tok_identifier: return ParseIdentifierExpr();
+      case tok_number:     return ParseNumberExpr();
+      case '(':            return ParseParenExpr();
+      case tok_if:         return ParseIfExpr();
+      case tok_for:        return ParseForExpr();
+      case tok_var:        return ParseVarExpr();
+      }
+    }
+
+    /// unary
+    ///   ::= primary
+    ///   ::= '!' unary
+    static ExprAST *ParseUnary() {
+      // If the current token is not an operator, it must be a primary expr.
+      if (!isascii(CurTok) || CurTok == '(' || CurTok == ',')
+        return ParsePrimary();
+
+      // If this is a unary operator, read it.
+      int Opc = CurTok;
+      getNextToken();
+      if (ExprAST *Operand = ParseUnary())
+        return new UnaryExprAST(Opc, Operand);
+      return 0;
+    }
+
+    /// binoprhs
+    ///   ::= ('+' unary)*
+    static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+      // If this is a binop, find its precedence.
+      while (1) {
+        int TokPrec = GetTokPrecedence();
+
+        // If this is a binop that binds at least as tightly as the current binop,
+        // consume it, otherwise we are done.
+        if (TokPrec < ExprPrec)
+          return LHS;
+
+        // Okay, we know this is a binop.
+        int BinOp = CurTok;
+        getNextToken();  // eat binop
+
+        // Parse the unary expression after the binary operator.
+        ExprAST *RHS = ParseUnary();
+        if (!RHS) return 0;
+
+        // If BinOp binds less tightly with RHS than the operator after RHS, let
+        // the pending operator take RHS as its LHS.
+        int NextPrec = GetTokPrecedence();
+        if (TokPrec < NextPrec) {
+          RHS = ParseBinOpRHS(TokPrec+1, RHS);
+          if (RHS == 0) return 0;
+        }
+
+        // Merge LHS/RHS.
+        LHS = new BinaryExprAST(BinOp, LHS, RHS);
+      }
+    }
+
+    /// expression
+    ///   ::= unary binoprhs
+    ///
+    static ExprAST *ParseExpression() {
+      ExprAST *LHS = ParseUnary();
+      if (!LHS) return 0;
+
+      return ParseBinOpRHS(0, LHS);
+    }
+
+    /// prototype
+    ///   ::= id '(' id* ')'
+    ///   ::= binary LETTER number? (id, id)
+    ///   ::= unary LETTER (id)
+    static PrototypeAST *ParsePrototype() {
+      std::string FnName;
+
+      unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary.
+      unsigned BinaryPrecedence = 30;
+
+      switch (CurTok) {
+      default:
+        return ErrorP("Expected function name in prototype");
+      case tok_identifier:
+        FnName = IdentifierStr;
+        Kind = 0;
+        getNextToken();
+        break;
+      case tok_unary:
+        getNextToken();
+        if (!isascii(CurTok))
+          return ErrorP("Expected unary operator");
+        FnName = "unary";
+        FnName += (char)CurTok;
+        Kind = 1;
+        getNextToken();
+        break;
+      case tok_binary:
+        getNextToken();
+        if (!isascii(CurTok))
+          return ErrorP("Expected binary operator");
+        FnName = "binary";
+        FnName += (char)CurTok;
+        Kind = 2;
+        getNextToken();
+
+        // Read the precedence if present.
+        if (CurTok == tok_number) {
+          if (NumVal < 1 || NumVal > 100)
+            return ErrorP("Invalid precedecnce: must be 1..100");
+          BinaryPrecedence = (unsigned)NumVal;
+          getNextToken();
+        }
+        break;
+      }
+
+      if (CurTok != '(')
+        return ErrorP("Expected '(' in prototype");
+
+      std::vector<std::string> ArgNames;
+      while (getNextToken() == tok_identifier)
+        ArgNames.push_back(IdentifierStr);
+      if (CurTok != ')')
+        return ErrorP("Expected ')' in prototype");
+
+      // success.
+      getNextToken();  // eat ')'.
+
+      // Verify right number of names for operator.
+      if (Kind && ArgNames.size() != Kind)
+        return ErrorP("Invalid number of operands for operator");
+
+      return new PrototypeAST(FnName, ArgNames, Kind != 0, BinaryPrecedence);
+    }
+
+    /// definition ::= 'def' prototype expression
+    static FunctionAST *ParseDefinition() {
+      getNextToken();  // eat def.
+      PrototypeAST *Proto = ParsePrototype();
+      if (Proto == 0) return 0;
+
+      if (ExprAST *E = ParseExpression())
+        return new FunctionAST(Proto, E);
+      return 0;
+    }
+
+    /// toplevelexpr ::= expression
+    static FunctionAST *ParseTopLevelExpr() {
+      if (ExprAST *E = ParseExpression()) {
+        // Make an anonymous proto.
+        PrototypeAST *Proto = new PrototypeAST("", std::vector<std::string>());
+        return new FunctionAST(Proto, E);
+      }
+      return 0;
+    }
+
+    /// external ::= 'extern' prototype
+    static PrototypeAST *ParseExtern() {
+      getNextToken();  // eat extern.
+      return ParsePrototype();
+    }
+
+    //===----------------------------------------------------------------------===//
+    // Code Generation
+    //===----------------------------------------------------------------------===//
+
+    static Module *TheModule;
+    static IRBuilder<> Builder(getGlobalContext());
+    static std::map<std::string, AllocaInst*> NamedValues;
+    static FunctionPassManager *TheFPM;
+
+    Value *ErrorV(const char *Str) { Error(Str); return 0; }
+
+    /// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of
+    /// the function.  This is used for mutable variables etc.
+    static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
+                                              const std::string &VarName) {
+      IRBuilder<> TmpB(&TheFunction->getEntryBlock(),
+                     TheFunction->getEntryBlock().begin());
+      return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0,
+                               VarName.c_str());
+    }
+
+    Value *NumberExprAST::Codegen() {
+      return ConstantFP::get(getGlobalContext(), APFloat(Val));
+    }
+
+    Value *VariableExprAST::Codegen() {
+      // Look this variable up in the function.
+      Value *V = NamedValues[Name];
+      if (V == 0) return ErrorV("Unknown variable name");
+
+      // Load the value.
+      return Builder.CreateLoad(V, Name.c_str());
+    }
+
+    Value *UnaryExprAST::Codegen() {
+      Value *OperandV = Operand->Codegen();
+      if (OperandV == 0) return 0;
+
+      Function *F = TheModule->getFunction(std::string("unary")+Opcode);
+      if (F == 0)
+        return ErrorV("Unknown unary operator");
+
+      return Builder.CreateCall(F, OperandV, "unop");
+    }
+
+    Value *BinaryExprAST::Codegen() {
+      // Special case '=' because we don't want to emit the LHS as an expression.
+      if (Op == '=') {
+        // Assignment requires the LHS to be an identifier.
+        VariableExprAST *LHSE = dynamic_cast<VariableExprAST*>(LHS);
+        if (!LHSE)
+          return ErrorV("destination of '=' must be a variable");
+        // Codegen the RHS.
+        Value *Val = RHS->Codegen();
+        if (Val == 0) return 0;
+
+        // Look up the name.
+        Value *Variable = NamedValues[LHSE->getName()];
+        if (Variable == 0) return ErrorV("Unknown variable name");
+
+        Builder.CreateStore(Val, Variable);
+        return Val;
+      }
+
+      Value *L = LHS->Codegen();
+      Value *R = RHS->Codegen();
+      if (L == 0 || R == 0) return 0;
+
+      switch (Op) {
+      case '+': return Builder.CreateFAdd(L, R, "addtmp");
+      case '-': return Builder.CreateFSub(L, R, "subtmp");
+      case '*': return Builder.CreateFMul(L, R, "multmp");
+      case '<':
+        L = Builder.CreateFCmpULT(L, R, "cmptmp");
+        // Convert bool 0/1 to double 0.0 or 1.0
+        return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
+                                    "booltmp");
+      default: break;
+      }
+
+      // If it wasn't a builtin binary operator, it must be a user defined one. Emit
+      // a call to it.
+      Function *F = TheModule->getFunction(std::string("binary")+Op);
+      assert(F && "binary operator not found!");
+
+      Value *Ops[2] = { L, R };
+      return Builder.CreateCall(F, Ops, "binop");
+    }
+
+    Value *CallExprAST::Codegen() {
+      // Look up the name in the global module table.
+      Function *CalleeF = TheModule->getFunction(Callee);
+      if (CalleeF == 0)
+        return ErrorV("Unknown function referenced");
+
+      // If argument mismatch error.
+      if (CalleeF->arg_size() != Args.size())
+        return ErrorV("Incorrect # arguments passed");
+
+      std::vector<Value*> ArgsV;
+      for (unsigned i = 0, e = Args.size(); i != e; ++i) {
+        ArgsV.push_back(Args[i]->Codegen());
+        if (ArgsV.back() == 0) return 0;
+      }
+
+      return Builder.CreateCall(CalleeF, ArgsV, "calltmp");
+    }
+
+    Value *IfExprAST::Codegen() {
+      Value *CondV = Cond->Codegen();
+      if (CondV == 0) return 0;
+
+      // Convert condition to a bool by comparing equal to 0.0.
+      CondV = Builder.CreateFCmpONE(CondV,
+                                  ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                    "ifcond");
+
+      Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+      // Create blocks for the then and else cases.  Insert the 'then' block at the
+      // end of the function.
+      BasicBlock *ThenBB = BasicBlock::Create(getGlobalContext(), "then", TheFunction);
+      BasicBlock *ElseBB = BasicBlock::Create(getGlobalContext(), "else");
+      BasicBlock *MergeBB = BasicBlock::Create(getGlobalContext(), "ifcont");
+
+      Builder.CreateCondBr(CondV, ThenBB, ElseBB);
+
+      // Emit then value.
+      Builder.SetInsertPoint(ThenBB);
+
+      Value *ThenV = Then->Codegen();
+      if (ThenV == 0) return 0;
+
+      Builder.CreateBr(MergeBB);
+      // Codegen of 'Then' can change the current block, update ThenBB for the PHI.
+      ThenBB = Builder.GetInsertBlock();
+
+      // Emit else block.
+      TheFunction->getBasicBlockList().push_back(ElseBB);
+      Builder.SetInsertPoint(ElseBB);
+
+      Value *ElseV = Else->Codegen();
+      if (ElseV == 0) return 0;
+
+      Builder.CreateBr(MergeBB);
+      // Codegen of 'Else' can change the current block, update ElseBB for the PHI.
+      ElseBB = Builder.GetInsertBlock();
+
+      // Emit merge block.
+      TheFunction->getBasicBlockList().push_back(MergeBB);
+      Builder.SetInsertPoint(MergeBB);
+      PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), 2,
+                                      "iftmp");
+
+      PN->addIncoming(ThenV, ThenBB);
+      PN->addIncoming(ElseV, ElseBB);
+      return PN;
+    }
+
+    Value *ForExprAST::Codegen() {
+      // 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:
+
+      Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+      // Create an alloca for the variable in the entry block.
+      AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+
+      // Emit the start code first, without 'variable' in scope.
+      Value *StartVal = Start->Codegen();
+      if (StartVal == 0) return 0;
+
+      // Store the value into the alloca.
+      Builder.CreateStore(StartVal, Alloca);
+
+      // Make the new basic block for the loop header, inserting after current
+      // block.
+      BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction);
+
+      // Insert an explicit fall through from the current block to the LoopBB.
+      Builder.CreateBr(LoopBB);
+
+      // Start insertion in LoopBB.
+      Builder.SetInsertPoint(LoopBB);
+
+      // 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.
+      AllocaInst *OldVal = NamedValues[VarName];
+      NamedValues[VarName] = 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.
+      if (Body->Codegen() == 0)
+        return 0;
+
+      // Emit the step value.
+      Value *StepVal;
+      if (Step) {
+        StepVal = Step->Codegen();
+        if (StepVal == 0) return 0;
+      } else {
+        // If not specified, use 1.0.
+        StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0));
+      }
+
+      // Compute the end condition.
+      Value *EndCond = End->Codegen();
+      if (EndCond == 0) return EndCond;
+
+      // Reload, increment, and restore the alloca.  This handles the case where
+      // the body of the loop mutates the variable.
+      Value *CurVar = Builder.CreateLoad(Alloca, VarName.c_str());
+      Value *NextVar = Builder.CreateFAdd(CurVar, StepVal, "nextvar");
+      Builder.CreateStore(NextVar, Alloca);
+
+      // Convert condition to a bool by comparing equal to 0.0.
+      EndCond = Builder.CreateFCmpONE(EndCond,
+                                  ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                      "loopcond");
+
+      // Create the "after loop" block and insert it.
+      BasicBlock *AfterBB = BasicBlock::Create(getGlobalContext(), "afterloop", TheFunction);
+
+      // Insert the conditional branch into the end of LoopEndBB.
+      Builder.CreateCondBr(EndCond, LoopBB, AfterBB);
+
+      // Any new code will be inserted in AfterBB.
+      Builder.SetInsertPoint(AfterBB);
+
+      // Restore the unshadowed variable.
+      if (OldVal)
+        NamedValues[VarName] = OldVal;
+      else
+        NamedValues.erase(VarName);
+
+
+      // for expr always returns 0.0.
+      return Constant::getNullValue(Type::getDoubleTy(getGlobalContext()));
+    }
+
+    Value *VarExprAST::Codegen() {
+      std::vector<AllocaInst *> OldBindings;
+
+      Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+      // Register all variables and emit their initializer.
+      for (unsigned i = 0, e = VarNames.size(); i != e; ++i) {
+        const std::string &VarName = VarNames[i].first;
+        ExprAST *Init = VarNames[i].second;
+
+        // 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'.
+        Value *InitVal;
+        if (Init) {
+          InitVal = Init->Codegen();
+          if (InitVal == 0) return 0;
+        } else { // If not specified, use 0.0.
+          InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0));
+        }
+
+        AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+        Builder.CreateStore(InitVal, Alloca);
+
+        // Remember the old variable binding so that we can restore the binding when
+        // we unrecurse.
+        OldBindings.push_back(NamedValues[VarName]);
+
+        // Remember this binding.
+        NamedValues[VarName] = Alloca;
+      }
+
+      // Codegen the body, now that all vars are in scope.
+      Value *BodyVal = Body->Codegen();
+      if (BodyVal == 0) return 0;
+
+      // Pop all our variables from scope.
+      for (unsigned i = 0, e = VarNames.size(); i != e; ++i)
+        NamedValues[VarNames[i].first] = OldBindings[i];
+
+      // Return the body computation.
+      return BodyVal;
+    }
+
+    Function *PrototypeAST::Codegen() {
+      // Make the function type:  double(double,double) etc.
+      std::vector<Type*> Doubles(Args.size(),
+                                 Type::getDoubleTy(getGlobalContext()));
+      FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
+                                           Doubles, false);
+
+      Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
+
+      // If F conflicted, there was already something named 'Name'.  If it has a
+      // body, don't allow redefinition or reextern.
+      if (F->getName() != Name) {
+        // Delete the one we just made and get the existing one.
+        F->eraseFromParent();
+        F = TheModule->getFunction(Name);
+
+        // If F already has a body, reject this.
+        if (!F->empty()) {
+          ErrorF("redefinition of function");
+          return 0;
+        }
+
+        // If F took a different number of args, reject.
+        if (F->arg_size() != Args.size()) {
+          ErrorF("redefinition of function with different # args");
+          return 0;
+        }
+      }
+
+      // Set names for all arguments.
+      unsigned Idx = 0;
+      for (Function::arg_iterator AI = F->arg_begin(); Idx != Args.size();
+           ++AI, ++Idx)
+        AI->setName(Args[Idx]);
+
+      return F;
+    }
+
+    /// CreateArgumentAllocas - Create an alloca for each argument and register the
+    /// argument in the symbol table so that references to it will succeed.
+    void PrototypeAST::CreateArgumentAllocas(Function *F) {
+      Function::arg_iterator AI = F->arg_begin();
+      for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) {
+        // Create an alloca for this variable.
+        AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]);
+
+        // Store the initial value into the alloca.
+        Builder.CreateStore(AI, Alloca);
+
+        // Add arguments to variable symbol table.
+        NamedValues[Args[Idx]] = Alloca;
+      }
+    }
+
+    Function *FunctionAST::Codegen() {
+      NamedValues.clear();
+
+      Function *TheFunction = Proto->Codegen();
+      if (TheFunction == 0)
+        return 0;
+
+      // If this is an operator, install it.
+      if (Proto->isBinaryOp())
+        BinopPrecedence[Proto->getOperatorName()] = Proto->getBinaryPrecedence();
+
+      // Create a new basic block to start insertion into.
+      BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
+      Builder.SetInsertPoint(BB);
+
+      // Add all arguments to the symbol table and create their allocas.
+      Proto->CreateArgumentAllocas(TheFunction);
+
+      if (Value *RetVal = Body->Codegen()) {
+        // Finish off the function.
+        Builder.CreateRet(RetVal);
+
+        // Validate the generated code, checking for consistency.
+        verifyFunction(*TheFunction);
+
+        // Optimize the function.
+        TheFPM->run(*TheFunction);
+
+        return TheFunction;
+      }
+
+      // Error reading body, remove function.
+      TheFunction->eraseFromParent();
+
+      if (Proto->isBinaryOp())
+        BinopPrecedence.erase(Proto->getOperatorName());
+      return 0;
+    }
+
+    //===----------------------------------------------------------------------===//
+    // Top-Level parsing and JIT Driver
+    //===----------------------------------------------------------------------===//
+
+    static ExecutionEngine *TheExecutionEngine;
+
+    static void HandleDefinition() {
+      if (FunctionAST *F = ParseDefinition()) {
+        if (Function *LF = F->Codegen()) {
+          fprintf(stderr, "Read function definition:");
+          LF->dump();
+        }
+      } else {
+        // Skip token for error recovery.
+        getNextToken();
+      }
+    }
+
+    static void HandleExtern() {
+      if (PrototypeAST *P = ParseExtern()) {
+        if (Function *F = P->Codegen()) {
+          fprintf(stderr, "Read extern: ");
+          F->dump();
+        }
+      } else {
+        // Skip token for error recovery.
+        getNextToken();
+      }
+    }
+
+    static void HandleTopLevelExpression() {
+      // Evaluate a top-level expression into an anonymous function.
+      if (FunctionAST *F = ParseTopLevelExpr()) {
+        if (Function *LF = F->Codegen()) {
+          // JIT the function, returning a function pointer.
+          void *FPtr = TheExecutionEngine->getPointerToFunction(LF);
+
+          // Cast it to the right type (takes no arguments, returns a double) so we
+          // can call it as a native function.
+          double (*FP)() = (double (*)())(intptr_t)FPtr;
+          fprintf(stderr, "Evaluated to %f\n", FP());
+        }
+      } else {
+        // Skip token for error recovery.
+        getNextToken();
+      }
+    }
+
+    /// top ::= definition | external | expression | ';'
+    static void MainLoop() {
+      while (1) {
+        fprintf(stderr, "ready> ");
+        switch (CurTok) {
+        case tok_eof:    return;
+        case ';':        getNextToken(); break;  // ignore top-level semicolons.
+        case tok_def:    HandleDefinition(); break;
+        case tok_extern: HandleExtern(); break;
+        default:         HandleTopLevelExpression(); break;
+        }
+      }
+    }
+
+    //===----------------------------------------------------------------------===//
+    // "Library" functions that can be "extern'd" from user code.
+    //===----------------------------------------------------------------------===//
+
+    /// putchard - putchar that takes a double and returns 0.
+    extern "C"
+    double putchard(double X) {
+      putchar((char)X);
+      return 0;
+    }
+
+    /// printd - printf that takes a double prints it as "%f\n", returning 0.
+    extern "C"
+    double printd(double X) {
+      printf("%f\n", X);
+      return 0;
+    }
+
+    //===----------------------------------------------------------------------===//
+    // Main driver code.
+    //===----------------------------------------------------------------------===//
+
+    int main() {
+      InitializeNativeTarget();
+      LLVMContext &Context = getGlobalContext();
+
+      // Install standard binary operators.
+      // 1 is lowest precedence.
+      BinopPrecedence['='] = 2;
+      BinopPrecedence['<'] = 10;
+      BinopPrecedence['+'] = 20;
+      BinopPrecedence['-'] = 20;
+      BinopPrecedence['*'] = 40;  // highest.
+
+      // Prime the first token.
+      fprintf(stderr, "ready> ");
+      getNextToken();
+
+      // Make the module, which holds all the code.
+      TheModule = new Module("my cool jit", Context);
+
+      // Create the JIT.  This takes ownership of the module.
+      std::string ErrStr;
+      TheExecutionEngine = EngineBuilder(TheModule).setErrorStr(&ErrStr).create();
+      if (!TheExecutionEngine) {
+        fprintf(stderr, "Could not create ExecutionEngine: %s\n", ErrStr.c_str());
+        exit(1);
+      }
+
+      FunctionPassManager OurFPM(TheModule);
+
+      // Set up the optimizer pipeline.  Start with registering info about how the
+      // target lays out data structures.
+      OurFPM.add(new DataLayout(*TheExecutionEngine->getDataLayout()));
+      // Provide basic AliasAnalysis support for GVN.
+      OurFPM.add(createBasicAliasAnalysisPass());
+      // Promote allocas to registers.
+      OurFPM.add(createPromoteMemoryToRegisterPass());
+      // Do simple "peephole" optimizations and bit-twiddling optzns.
+      OurFPM.add(createInstructionCombiningPass());
+      // Reassociate expressions.
+      OurFPM.add(createReassociatePass());
+      // Eliminate Common SubExpressions.
+      OurFPM.add(createGVNPass());
+      // Simplify the control flow graph (deleting unreachable blocks, etc).
+      OurFPM.add(createCFGSimplificationPass());
+
+      OurFPM.doInitialization();
+
+      // Set the global so the code gen can use this.
+      TheFPM = &OurFPM;
+
+      // Run the main "interpreter loop" now.
+      MainLoop();
+
+      TheFPM = 0;
+
+      // Print out all of the generated code.
+      TheModule->dump();
+
+      return 0;
+    }
+
+`Next: Conclusion and other useful LLVM tidbits <LangImpl8.html>`_
+