--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/docs/tutorial/LangImpl5.rst	Thu Dec 12 14:37:49 2013 +0900
@@ -0,0 +1,1607 @@
+==================================================
+Kaleidoscope: Extending the Language: Control Flow
+==================================================
+
+.. contents::
+   :local:
+
+Chapter 5 Introduction
+======================
+
+Welcome to Chapter 5 of the "`Implementing a language with
+LLVM <index.html>`_" tutorial. Parts 1-4 described the implementation of
+the simple Kaleidoscope language and included support for generating
+LLVM IR, followed by optimizations and a JIT compiler. Unfortunately, as
+presented, Kaleidoscope is mostly useless: it has no control flow other
+than call and return. This means that you can't have conditional
+branches in the code, significantly limiting its power. In this episode
+of "build that compiler", we'll extend Kaleidoscope to have an
+if/then/else expression plus a simple 'for' loop.
+
+If/Then/Else
+============
+
+Extending Kaleidoscope to support if/then/else is quite straightforward.
+It basically requires adding support for this "new" concept to the
+lexer, parser, AST, and LLVM code emitter. This example is nice, because
+it shows how easy it is to "grow" a language over time, incrementally
+extending it as new ideas are discovered.
+
+Before we get going on "how" we add this extension, lets talk about
+"what" we want. The basic idea is that we want to be able to write this
+sort of thing:
+
+::
+
+    def fib(x)
+      if x < 3 then
+        1
+      else
+        fib(x-1)+fib(x-2);
+
+In Kaleidoscope, every construct is an expression: there are no
+statements. As such, the if/then/else expression needs to return a value
+like any other. Since we're using a mostly functional form, we'll have
+it evaluate its conditional, then return the 'then' or 'else' value
+based on how the condition was resolved. This is very similar to the C
+"?:" expression.
+
+The semantics of the if/then/else expression is that it evaluates the
+condition to a boolean equality value: 0.0 is considered to be false and
+everything else is considered to be true. If the condition is true, the
+first subexpression is evaluated and returned, if the condition is
+false, the second subexpression is evaluated and returned. Since
+Kaleidoscope allows side-effects, this behavior is important to nail
+down.
+
+Now that we know what we "want", lets break this down into its
+constituent pieces.
+
+Lexer Extensions for If/Then/Else
+---------------------------------
+
+The lexer extensions are straightforward. First we add new enum values
+for the relevant tokens:
+
+.. code-block:: c++
+
+      // control
+      tok_if = -6, tok_then = -7, tok_else = -8,
+
+Once we have that, we recognize the new keywords in the lexer. This is
+pretty simple stuff:
+
+.. code-block:: c++
+
+        ...
+        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;
+        return tok_identifier;
+
+AST Extensions for If/Then/Else
+-------------------------------
+
+To represent the new expression we add a new AST node for it:
+
+.. code-block:: c++
+
+    /// 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();
+    };
+
+The AST node just has pointers to the various subexpressions.
+
+Parser Extensions for If/Then/Else
+----------------------------------
+
+Now that we have the relevant tokens coming from the lexer and we have
+the AST node to build, our parsing logic is relatively straightforward.
+First we define a new parsing function:
+
+.. code-block:: c++
+
+    /// 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);
+    }
+
+Next we hook it up as a primary expression:
+
+.. code-block:: c++
+
+    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();
+      }
+    }
+
+LLVM IR for If/Then/Else
+------------------------
+
+Now that we have it parsing and building the AST, the final piece is
+adding LLVM code generation support. This is the most interesting part
+of the if/then/else example, because this is where it starts to
+introduce new concepts. All of the code above has been thoroughly
+described in previous chapters.
+
+To motivate the code we want to produce, lets take a look at a simple
+example. Consider:
+
+::
+
+    extern foo();
+    extern bar();
+    def baz(x) if x then foo() else bar();
+
+If you disable optimizations, the code you'll (soon) get from
+Kaleidoscope looks like this:
+
+.. code-block:: llvm
+
+    declare double @foo()
+
+    declare double @bar()
+
+    define double @baz(double %x) {
+    entry:
+      %ifcond = fcmp one double %x, 0.000000e+00
+      br i1 %ifcond, label %then, label %else
+
+    then:       ; preds = %entry
+      %calltmp = call double @foo()
+      br label %ifcont
+
+    else:       ; preds = %entry
+      %calltmp1 = call double @bar()
+      br label %ifcont
+
+    ifcont:     ; preds = %else, %then
+      %iftmp = phi double [ %calltmp, %then ], [ %calltmp1, %else ]
+      ret double %iftmp
+    }
+
+To visualize the control flow graph, you can use a nifty feature of the
+LLVM '`opt <http://llvm.org/cmds/opt.html>`_' tool. If you put this LLVM
+IR into "t.ll" and run "``llvm-as < t.ll | opt -analyze -view-cfg``", `a
+window will pop up <../ProgrammersManual.html#ViewGraph>`_ and you'll
+see this graph:
+
+.. figure:: LangImpl5-cfg.png
+   :align: center
+   :alt: Example CFG
+
+   Example CFG
+
+Another way to get this is to call "``F->viewCFG()``" or
+"``F->viewCFGOnly()``" (where F is a "``Function*``") either by
+inserting actual calls into the code and recompiling or by calling these
+in the debugger. LLVM has many nice features for visualizing various
+graphs.
+
+Getting back to the generated code, it is fairly simple: the entry block
+evaluates the conditional expression ("x" in our case here) and compares
+the result to 0.0 with the "``fcmp one``" instruction ('one' is "Ordered
+and Not Equal"). Based on the result of this expression, the code jumps
+to either the "then" or "else" blocks, which contain the expressions for
+the true/false cases.
+
+Once the then/else blocks are finished executing, they both branch back
+to the 'ifcont' block to execute the code that happens after the
+if/then/else. In this case the only thing left to do is to return to the
+caller of the function. The question then becomes: how does the code
+know which expression to return?
+
+The answer to this question involves an important SSA operation: the
+`Phi
+operation <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_.
+If you're not familiar with SSA, `the wikipedia
+article <http://en.wikipedia.org/wiki/Static_single_assignment_form>`_
+is a good introduction and there are various other introductions to it
+available on your favorite search engine. The short version is that
+"execution" of the Phi operation requires "remembering" which block
+control came from. The Phi operation takes on the value corresponding to
+the input control block. In this case, if control comes in from the
+"then" block, it gets the value of "calltmp". If control comes from the
+"else" block, it gets the value of "calltmp1".
+
+At this point, you are probably starting to think "Oh no! This means my
+simple and elegant front-end will have to start generating SSA form in
+order to use LLVM!". Fortunately, this is not the case, and we strongly
+advise *not* implementing an SSA construction algorithm in your
+front-end unless there is an amazingly good reason to do so. In
+practice, there are two sorts of values that float around in code
+written for your average imperative programming language that might need
+Phi nodes:
+
+#. Code that involves user variables: ``x = 1; x = x + 1;``
+#. Values that are implicit in the structure of your AST, such as the
+   Phi node in this case.
+
+In `Chapter 7 <LangImpl7.html>`_ of this tutorial ("mutable variables"),
+we'll talk about #1 in depth. For now, just believe me that you don't
+need SSA construction to handle this case. For #2, you have the choice
+of using the techniques that we will describe for #1, or you can insert
+Phi nodes directly, if convenient. In this case, it is really really
+easy to generate the Phi node, so we choose to do it directly.
+
+Okay, enough of the motivation and overview, lets generate code!
+
+Code Generation for If/Then/Else
+--------------------------------
+
+In order to generate code for this, we implement the ``Codegen`` method
+for ``IfExprAST``:
+
+.. code-block:: c++
+
+    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");
+
+This code is straightforward and similar to what we saw before. We emit
+the expression for the condition, then compare that value to zero to get
+a truth value as a 1-bit (bool) value.
+
+.. code-block:: c++
+
+      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);
+
+This code creates the basic blocks that are related to the if/then/else
+statement, and correspond directly to the blocks in the example above.
+The first line gets the current Function object that is being built. It
+gets this by asking the builder for the current BasicBlock, and asking
+that block for its "parent" (the function it is currently embedded
+into).
+
+Once it has that, it creates three blocks. Note that it passes
+"TheFunction" into the constructor for the "then" block. This causes the
+constructor to automatically insert the new block into the end of the
+specified function. The other two blocks are created, but aren't yet
+inserted into the function.
+
+Once the blocks are created, we can emit the conditional branch that
+chooses between them. Note that creating new blocks does not implicitly
+affect the IRBuilder, so it is still inserting into the block that the
+condition went into. Also note that it is creating a branch to the
+"then" block and the "else" block, even though the "else" block isn't
+inserted into the function yet. This is all ok: it is the standard way
+that LLVM supports forward references.
+
+.. code-block:: c++
+
+      // 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();
+
+After the conditional branch is inserted, we move the builder to start
+inserting into the "then" block. Strictly speaking, this call moves the
+insertion point to be at the end of the specified block. However, since
+the "then" block is empty, it also starts out by inserting at the
+beginning of the block. :)
+
+Once the insertion point is set, we recursively codegen the "then"
+expression from the AST. To finish off the "then" block, we create an
+unconditional branch to the merge block. One interesting (and very
+important) aspect of the LLVM IR is that it `requires all basic blocks
+to be "terminated" <../LangRef.html#functionstructure>`_ with a `control
+flow instruction <../LangRef.html#terminators>`_ such as return or
+branch. This means that all control flow, *including fall throughs* must
+be made explicit in the LLVM IR. If you violate this rule, the verifier
+will emit an error.
+
+The final line here is quite subtle, but is very important. The basic
+issue is that when we create the Phi node in the merge block, we need to
+set up the block/value pairs that indicate how the Phi will work.
+Importantly, the Phi node expects to have an entry for each predecessor
+of the block in the CFG. Why then, are we getting the current block when
+we just set it to ThenBB 5 lines above? The problem is that the "Then"
+expression may actually itself change the block that the Builder is
+emitting into if, for example, it contains a nested "if/then/else"
+expression. Because calling Codegen recursively could arbitrarily change
+the notion of the current block, we are required to get an up-to-date
+value for code that will set up the Phi node.
+
+.. code-block:: c++
+
+      // 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();
+
+Code generation for the 'else' block is basically identical to codegen
+for the 'then' block. The only significant difference is the first line,
+which adds the 'else' block to the function. Recall previously that the
+'else' block was created, but not added to the function. Now that the
+'then' and 'else' blocks are emitted, we can finish up with the merge
+code:
+
+.. code-block:: c++
+
+      // 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;
+    }
+
+The first two lines here are now familiar: the first adds the "merge"
+block to the Function object (it was previously floating, like the else
+block above). The second block changes the insertion point so that newly
+created code will go into the "merge" block. Once that is done, we need
+to create the PHI node and set up the block/value pairs for the PHI.
+
+Finally, the CodeGen function returns the phi node as the value computed
+by the if/then/else expression. In our example above, this returned
+value will feed into the code for the top-level function, which will
+create the return instruction.
+
+Overall, we now have the ability to execute conditional code in
+Kaleidoscope. With this extension, Kaleidoscope is a fairly complete
+language that can calculate a wide variety of numeric functions. Next up
+we'll add another useful expression that is familiar from non-functional
+languages...
+
+'for' Loop Expression
+=====================
+
+Now that we know how to add basic control flow constructs to the
+language, we have the tools to add more powerful things. Lets add
+something more aggressive, a 'for' expression:
+
+::
+
+     extern putchard(char)
+     def printstar(n)
+       for i = 1, i < n, 1.0 in
+         putchard(42);  # ascii 42 = '*'
+
+     # print 100 '*' characters
+     printstar(100);
+
+This expression defines a new variable ("i" in this case) which iterates
+from a starting value, while the condition ("i < n" in this case) is
+true, incrementing by an optional step value ("1.0" in this case). If
+the step value is omitted, it defaults to 1.0. While the loop is true,
+it executes its body expression. Because we don't have anything better
+to return, we'll just define the loop as always returning 0.0. In the
+future when we have mutable variables, it will get more useful.
+
+As before, lets talk about the changes that we need to Kaleidoscope to
+support this.
+
+Lexer Extensions for the 'for' Loop
+-----------------------------------
+
+The lexer extensions are the same sort of thing as for if/then/else:
+
+.. code-block:: c++
+
+      ... in enum Token ...
+      // control
+      tok_if = -6, tok_then = -7, tok_else = -8,
+      tok_for = -9, tok_in = -10
+
+      ... in gettok ...
+      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;
+      return tok_identifier;
+
+AST Extensions for the 'for' Loop
+---------------------------------
+
+The AST node is just as simple. It basically boils down to capturing the
+variable name and the constituent expressions in the node.
+
+.. code-block:: c++
+
+    /// 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();
+    };
+
+Parser Extensions for the 'for' Loop
+------------------------------------
+
+The parser code is also fairly standard. The only interesting thing here
+is handling of the optional step value. The parser code handles it by
+checking to see if the second comma is present. If not, it sets the step
+value to null in the AST node:
+
+.. code-block:: c++
+
+    /// 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);
+    }
+
+LLVM IR for the 'for' Loop
+--------------------------
+
+Now we get to the good part: the LLVM IR we want to generate for this
+thing. With the simple example above, we get this LLVM IR (note that
+this dump is generated with optimizations disabled for clarity):
+
+.. code-block:: llvm
+
+    declare double @putchard(double)
+
+    define double @printstar(double %n) {
+    entry:
+      ; initial value = 1.0 (inlined into phi)
+      br label %loop
+
+    loop:       ; preds = %loop, %entry
+      %i = phi double [ 1.000000e+00, %entry ], [ %nextvar, %loop ]
+      ; body
+      %calltmp = call double @putchard(double 4.200000e+01)
+      ; increment
+      %nextvar = fadd double %i, 1.000000e+00
+
+      ; termination test
+      %cmptmp = fcmp ult double %i, %n
+      %booltmp = uitofp i1 %cmptmp to double
+      %loopcond = fcmp one double %booltmp, 0.000000e+00
+      br i1 %loopcond, label %loop, label %afterloop
+
+    afterloop:      ; preds = %loop
+      ; loop always returns 0.0
+      ret double 0.000000e+00
+    }
+
+This loop contains all the same constructs we saw before: a phi node,
+several expressions, and some basic blocks. Lets see how this fits
+together.
+
+Code Generation for the 'for' Loop
+----------------------------------
+
+The first part of Codegen is very simple: we just output the start
+expression for the loop value:
+
+.. code-block:: c++
+
+    Value *ForExprAST::Codegen() {
+      // Emit the start code first, without 'variable' in scope.
+      Value *StartVal = Start->Codegen();
+      if (StartVal == 0) return 0;
+
+With this out of the way, the next step is to set up the LLVM basic
+block for the start of the loop body. In the case above, the whole loop
+body is one block, but remember that the body code itself could consist
+of multiple blocks (e.g. if it contains an if/then/else or a for/in
+expression).
+
+.. code-block:: c++
+
+      // Make the new basic block for the loop header, inserting after current
+      // block.
+      Function *TheFunction = Builder.GetInsertBlock()->getParent();
+      BasicBlock *PreheaderBB = Builder.GetInsertBlock();
+      BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction);
+
+      // Insert an explicit fall through from the current block to the LoopBB.
+      Builder.CreateBr(LoopBB);
+
+This code is similar to what we saw for if/then/else. Because we will
+need it to create the Phi node, we remember the block that falls through
+into the loop. Once we have that, we create the actual block that starts
+the loop and create an unconditional branch for the fall-through between
+the two blocks.
+
+.. code-block:: c++
+
+      // Start insertion in LoopBB.
+      Builder.SetInsertPoint(LoopBB);
+
+      // Start the PHI node with an entry for Start.
+      PHINode *Variable = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), 2, VarName.c_str());
+      Variable->addIncoming(StartVal, PreheaderBB);
+
+Now that the "preheader" for the loop is set up, we switch to emitting
+code for the loop body. To begin with, we move the insertion point and
+create the PHI node for the loop induction variable. Since we already
+know the incoming value for the starting value, we add it to the Phi
+node. Note that the Phi will eventually get a second value for the
+backedge, but we can't set it up yet (because it doesn't exist!).
+
+.. code-block:: c++
+
+      // 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.
+      Value *OldVal = NamedValues[VarName];
+      NamedValues[VarName] = Variable;
+
+      // 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;
+
+Now the code starts to get more interesting. Our 'for' loop introduces a
+new variable to the symbol table. This means that our symbol table can
+now contain either function arguments or loop variables. To handle this,
+before we codegen the body of the loop, we add the loop variable as the
+current value for its name. Note that it is possible that there is a
+variable of the same name in the outer scope. It would be easy to make
+this an error (emit an error and return null if there is already an
+entry for VarName) but we choose to allow shadowing of variables. In
+order to handle this correctly, we remember the Value that we are
+potentially shadowing in ``OldVal`` (which will be null if there is no
+shadowed variable).
+
+Once the loop variable is set into the symbol table, the code
+recursively codegen's the body. This allows the body to use the loop
+variable: any references to it will naturally find it in the symbol
+table.
+
+.. code-block:: c++
+
+      // 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));
+      }
+
+      Value *NextVar = Builder.CreateFAdd(Variable, StepVal, "nextvar");
+
+Now that the body is emitted, we compute the next value of the iteration
+variable by adding the step value, or 1.0 if it isn't present.
+'``NextVar``' will be the value of the loop variable on the next
+iteration of the loop.
+
+.. code-block:: c++
+
+      // Compute the end condition.
+      Value *EndCond = End->Codegen();
+      if (EndCond == 0) return EndCond;
+
+      // Convert condition to a bool by comparing equal to 0.0.
+      EndCond = Builder.CreateFCmpONE(EndCond,
+                                  ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                      "loopcond");
+
+Finally, we evaluate the exit value of the loop, to determine whether
+the loop should exit. This mirrors the condition evaluation for the
+if/then/else statement.
+
+.. code-block:: c++
+
+      // Create the "after loop" block and insert it.
+      BasicBlock *LoopEndBB = Builder.GetInsertBlock();
+      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);
+
+With the code for the body of the loop complete, we just need to finish
+up the control flow for it. This code remembers the end block (for the
+phi node), then creates the block for the loop exit ("afterloop"). Based
+on the value of the exit condition, it creates a conditional branch that
+chooses between executing the loop again and exiting the loop. Any
+future code is emitted in the "afterloop" block, so it sets the
+insertion position to it.
+
+.. code-block:: c++
+
+      // Add a new entry to the PHI node for the backedge.
+      Variable->addIncoming(NextVar, LoopEndBB);
+
+      // 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()));
+    }
+
+The final code handles various cleanups: now that we have the "NextVar"
+value, we can add the incoming value to the loop PHI node. After that,
+we remove the loop variable from the symbol table, so that it isn't in
+scope after the for loop. Finally, code generation of the for loop
+always returns 0.0, so that is what we return from
+``ForExprAST::Codegen``.
+
+With this, we conclude the "adding control flow to Kaleidoscope" chapter
+of the tutorial. In this chapter we added two control flow constructs,
+and used them to motivate a couple of aspects of the LLVM IR that are
+important for front-end implementors to know. In the next chapter of our
+saga, we will get a bit crazier and add `user-defined
+operators <LangImpl6.html>`_ to our poor innocent language.
+
+Full Code Listing
+=================
+
+Here is the complete code listing for our running example, enhanced with
+the if/then/else and for expressions.. 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
+    };
+
+    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;
+        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) {}
+      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();
+    };
+
+    /// 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).
+    class PrototypeAST {
+      std::string Name;
+      std::vector<std::string> Args;
+    public:
+      PrototypeAST(const std::string &name, const std::vector<std::string> &args)
+        : Name(name), Args(args) {}
+
+      Function *Codegen();
+    };
+
+    /// 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);
+    }
+
+    /// primary
+    ///   ::= identifierexpr
+    ///   ::= numberexpr
+    ///   ::= parenexpr
+    ///   ::= ifexpr
+    ///   ::= forexpr
+    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();
+      }
+    }
+
+    /// binoprhs
+    ///   ::= ('+' primary)*
+    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 primary expression after the binary operator.
+        ExprAST *RHS = ParsePrimary();
+        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
+    ///   ::= primary binoprhs
+    ///
+    static ExprAST *ParseExpression() {
+      ExprAST *LHS = ParsePrimary();
+      if (!LHS) return 0;
+
+      return ParseBinOpRHS(0, LHS);
+    }
+
+    /// prototype
+    ///   ::= id '(' id* ')'
+    static PrototypeAST *ParsePrototype() {
+      if (CurTok != tok_identifier)
+        return ErrorP("Expected function name in prototype");
+
+      std::string FnName = IdentifierStr;
+      getNextToken();
+
+      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 ')'.
+
+      return new PrototypeAST(FnName, ArgNames);
+    }
+
+    /// 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, Value*> NamedValues;
+    static FunctionPassManager *TheFPM;
+
+    Value *ErrorV(const char *Str) { Error(Str); return 0; }
+
+    Value *NumberExprAST::Codegen() {
+      return ConstantFP::get(getGlobalContext(), APFloat(Val));
+    }
+
+    Value *VariableExprAST::Codegen() {
+      // Look this variable up in the function.
+      Value *V = NamedValues[Name];
+      return V ? V : ErrorV("Unknown variable name");
+    }
+
+    Value *BinaryExprAST::Codegen() {
+      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: return ErrorV("invalid binary operator");
+      }
+    }
+
+    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:
+      //   ...
+      //   start = startexpr
+      //   goto loop
+      // loop:
+      //   variable = phi [start, loopheader], [nextvariable, loopend]
+      //   ...
+      //   bodyexpr
+      //   ...
+      // loopend:
+      //   step = stepexpr
+      //   nextvariable = variable + step
+      //   endcond = endexpr
+      //   br endcond, loop, endloop
+      // outloop:
+
+      // Emit the start code first, without 'variable' in scope.
+      Value *StartVal = Start->Codegen();
+      if (StartVal == 0) return 0;
+
+      // Make the new basic block for the loop header, inserting after current
+      // block.
+      Function *TheFunction = Builder.GetInsertBlock()->getParent();
+      BasicBlock *PreheaderBB = Builder.GetInsertBlock();
+      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);
+
+      // Start the PHI node with an entry for Start.
+      PHINode *Variable = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()), 2, VarName.c_str());
+      Variable->addIncoming(StartVal, PreheaderBB);
+
+      // 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.
+      Value *OldVal = NamedValues[VarName];
+      NamedValues[VarName] = Variable;
+
+      // 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));
+      }
+
+      Value *NextVar = Builder.CreateFAdd(Variable, StepVal, "nextvar");
+
+      // Compute the end condition.
+      Value *EndCond = End->Codegen();
+      if (EndCond == 0) return EndCond;
+
+      // 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 *LoopEndBB = Builder.GetInsertBlock();
+      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);
+
+      // Add a new entry to the PHI node for the backedge.
+      Variable->addIncoming(NextVar, LoopEndBB);
+
+      // 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()));
+    }
+
+    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]);
+
+        // Add arguments to variable symbol table.
+        NamedValues[Args[Idx]] = AI;
+      }
+
+      return F;
+    }
+
+    Function *FunctionAST::Codegen() {
+      NamedValues.clear();
+
+      Function *TheFunction = Proto->Codegen();
+      if (TheFunction == 0)
+        return 0;
+
+      // Create a new basic block to start insertion into.
+      BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
+      Builder.SetInsertPoint(BB);
+
+      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();
+      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;
+    }
+
+    //===----------------------------------------------------------------------===//
+    // Main driver code.
+    //===----------------------------------------------------------------------===//
+
+    int main() {
+      InitializeNativeTarget();
+      LLVMContext &Context = getGlobalContext();
+
+      // Install standard binary operators.
+      // 1 is lowest precedence.
+      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());
+      // 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: Extending the language: user-defined operators <LangImpl6.html>`_
+