Chapter 5, 6, and 7 of the ocaml/kaleidoscope tutorial
and fix some tabs in chapter 3 and 4.


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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+                      "http://www.w3.org/TR/html4/strict.dtd">
+
+<html>
+<head>
+  <title>Kaleidoscope: Extending the Language: Control Flow</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <meta name="author" content="Erick Tryzelaar">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Extending the Language: Control Flow</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 5
+  <ol>
+    <li><a href="#intro">Chapter 5 Introduction</a></li>
+    <li><a href="#ifthen">If/Then/Else</a>
+    <ol>
+      <li><a href="#iflexer">Lexer Extensions</a></li>
+      <li><a href="#ifast">AST Extensions</a></li>
+      <li><a href="#ifparser">Parser Extensions</a></li>
+      <li><a href="#ifir">LLVM IR</a></li>
+      <li><a href="#ifcodegen">Code Generation</a></li>
+    </ol>
+    </li>
+    <li><a href="#for">'for' Loop Expression</a>
+    <ol>
+      <li><a href="#forlexer">Lexer Extensions</a></li>
+      <li><a href="#forast">AST Extensions</a></li>
+      <li><a href="#forparser">Parser Extensions</a></li>
+      <li><a href="#forir">LLVM IR</a></li>
+      <li><a href="#forcodegen">Code Generation</a></li>
+    </ol>
+    </li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="OCamlLangImpl6.html">Chapter 6</a>: Extending the Language:
+User-defined Operators</li>
+</ul>
+
+<div class="doc_author">
+	<p>
+		Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
+		and <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a>
+	</p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 5 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 5 of the "<a href="index.html">Implementing a language
+with LLVM</a>" 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.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="ifthen">If/Then/Else</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Extending Kaleidoscope to support if/then/else is quite straightforward.  It
+basically requires adding lexer 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.</p>
+
+<p>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:
+</p>
+
+<div class="doc_code">
+<pre>
+def fib(x)
+  if x &lt; 3 then
+    1
+  else
+    fib(x-1)+fib(x-2);
+</pre>
+</div>
+
+<p>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.</p>
+
+<p>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.
+</p>
+
+<p>Now that we know what we "want", lets break this down into its constituent
+pieces.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="iflexer">Lexer Extensions for
+If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+
+<div class="doc_text">
+
+<p>The lexer extensions are straightforward.  First we add new variants
+for the relevant tokens:</p>
+
+<div class="doc_code">
+<pre>
+  (* control *)
+  | If | Then | Else | For | In
+</pre>
+</div>
+
+<p>Once we have that, we recognize the new keywords in the lexer. This is pretty simple
+stuff:</p>
+
+<div class="doc_code">
+<pre>
+      ...
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | "if" -&gt; [&lt; 'Token.If; stream &gt;]
+      | "then" -&gt; [&lt; 'Token.Then; stream &gt;]
+      | "else" -&gt; [&lt; 'Token.Else; stream &gt;]
+      | "for" -&gt; [&lt; 'Token.For; stream &gt;]
+      | "in" -&gt; [&lt; 'Token.In; stream &gt;]
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifast">AST Extensions for
+ If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>To represent the new expression we add a new AST variant for it:</p>
+
+<div class="doc_code">
+<pre>
+type expr =
+  ...
+  (* variant for if/then/else. *)
+  | If of expr * expr * expr
+</pre>
+</div>
+
+<p>The AST variant just has pointers to the various subexpressions.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifparser">Parser Extensions for
+If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>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:</p>
+
+<div class="doc_code">
+<pre>
+let rec parse_primary = parser
+  ...
+  (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
+  | [&lt; 'Token.If; c=parse_expr;
+       'Token.Then ?? "expected 'then'"; t=parse_expr;
+       'Token.Else ?? "expected 'else'"; e=parse_expr &gt;] -&gt;
+      Ast.If (c, t, e)
+</pre>
+</div>
+
+<p>Next we hook it up as a primary expression:</p>
+
+<div class="doc_code">
+<pre>
+let rec parse_primary = parser
+  ...
+  (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
+  | [&lt; 'Token.If; c=parse_expr;
+       'Token.Then ?? "expected 'then'"; t=parse_expr;
+       'Token.Else ?? "expected 'else'"; e=parse_expr &gt;] -&gt;
+      Ast.If (c, t, e)
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifir">LLVM IR for If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>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.
+</p>
+
+<p>To motivate the code we want to produce, lets take a look at a simple
+example.  Consider:</p>
+
+<div class="doc_code">
+<pre>
+extern foo();
+extern bar();
+def baz(x) if x then foo() else bar();
+</pre>
+</div>
+
+<p>If you disable optimizations, the code you'll (soon) get from Kaleidoscope
+looks like this:</p>
+
+<div class="doc_code">
+<pre>
+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
+}
+</pre>
+</div>
+
+<p>To visualize the control flow graph, you can use a nifty feature of the LLVM
+'<a href="http://llvm.org/cmds/opt.html">opt</a>' tool.  If you put this LLVM IR
+into "t.ll" and run "<tt>llvm-as &lt; t.ll | opt -analyze -view-cfg</tt>", <a
+href="../ProgrammersManual.html#ViewGraph">a window will pop up</a> and you'll
+see this graph:</p>
+
+<center><img src="LangImpl5-cfg.png" alt="Example CFG" width="423"
+height="315"></center>
+
+<p>Another way to get this is to call "<tt>Llvm_analysis.view_function_cfg
+f</tt>" or "<tt>Llvm_analysis.view_function_cfg_only f</tt>" (where <tt>f</tt>
+is a "<tt>Function</tt>") 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.</p>
+
+<p>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 "<tt><a href="../LangRef.html#i_fcmp">fcmp</a> one</tt>"
+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.</p>
+
+<p>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?</p>
+
+<p>The answer to this question involves an important SSA operation: the
+<a href="http://en.wikipedia.org/wiki/Static_single_assignment_form">Phi
+operation</a>.  If you're not familiar with SSA, <a
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">the wikipedia
+article</a> 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".</p>
+
+<p>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
+<em>not</em> 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:</p>
+
+<ol>
+<li>Code that involves user variables: <tt>x = 1; x = x + 1; </tt></li>
+<li>Values that are implicit in the structure of your AST, such as the Phi node
+in this case.</li>
+</ol>
+
+<p>In <a href="OCamlLangImpl7.html">Chapter 7</a> 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.</p>
+
+<p>Okay, enough of the motivation and overview, lets generate code!</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="ifcodegen">Code Generation for
+If/Then/Else</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>In order to generate code for this, we implement the <tt>Codegen</tt> method
+for <tt>IfExprAST</tt>:</p>
+
+<div class="doc_code">
+<pre>
+let rec codegen_expr = function
+  ...
+  | Ast.If (cond, then_, else_) -&gt;
+      let cond = codegen_expr cond in
+
+      (* Convert condition to a bool by comparing equal to 0.0 *)
+      let zero = const_float double_type 0.0 in
+      let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
+</pre>
+</div>
+
+<p>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.</p>
+
+<div class="doc_code">
+<pre>
+      (* Grab the first block so that we might later add the conditional branch
+       * to it at the end of the function. *)
+      let start_bb = insertion_block builder in
+      let the_function = block_parent start_bb in
+
+      let then_bb = append_block "then" the_function in
+      position_at_end then_bb builder;
+</pre>
+</div>
+
+<p>
+As opposed to the <a href="LangImpl5.html">C++ tutorial</a>, we have to build
+our basic blocks bottom up since we can't have dangling BasicBlocks.  We start
+off by saving a pointer to the first block (which might not be the entry
+block), which we'll need to build a conditional branch later.  We do this by
+asking the <tt>builder</tt> for the current BasicBlock.  The fourth line
+gets the current Function object that is being built.  It gets this by the
+<tt>start_bb</tt> for its "parent" (the function it is currently embedded
+into).</p>
+
+<p>Once it has that, it creates one block.  It is automatically appended into
+the function's list of blocks.</p>
+
+<div class="doc_code">
+<pre>
+      (* Emit 'then' value. *)
+      position_at_end then_bb builder;
+      let then_val = codegen_expr then_ in
+
+      (* Codegen of 'then' can change the current block, update then_bb for the
+       * phi. We create a new name because one is used for the phi node, and the
+       * other is used for the conditional branch. *)
+      let new_then_bb = insertion_block builder in
+</pre>
+</div>
+
+<p>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.  :)</p>
+
+<p>Once the insertion point is set, we recursively codegen the "then" expression
+from the AST.</p>
+
+<p>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.</p>
+
+<div class="doc_code">
+<pre>
+      (* Emit 'else' value. *)
+      let else_bb = append_block "else" the_function in
+      position_at_end else_bb builder;
+      let else_val = codegen_expr else_ in
+
+      (* Codegen of 'else' can change the current block, update else_bb for the
+       * phi. *)
+      let new_else_bb = insertion_block builder in
+</pre>
+</div>
+
+<p>Code generation for the 'else' block is basically identical to codegen for
+the 'then' block.</p>
+
+<div class="doc_code">
+<pre>
+      (* Emit merge block. *)
+      let merge_bb = append_block "ifcont" the_function in
+      position_at_end merge_bb builder;
+      let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
+      let phi = build_phi incoming "iftmp" builder in
+</pre>
+</div>
+
+<p>The first two lines here are now familiar: the first adds the "merge" block
+to the Function object.  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.</p>
+
+<div class="doc_code">
+<pre>
+      (* Return to the start block to add the conditional branch. *)
+      position_at_end start_bb builder;
+      ignore (build_cond_br cond_val then_bb else_bb builder);
+</pre>
+</div>
+
+<p>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
+LLVMBuilder, so it is still inserting into the block that the condition
+went into.  This is why we needed to save the "start" block.</p>
+
+<div class="doc_code">
+<pre>
+      (* Set a unconditional branch at the end of the 'then' block and the
+       * 'else' block to the 'merge' block. *)
+      position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
+      position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
+
+      (* Finally, set the builder to the end of the merge block. *)
+      position_at_end merge_bb builder;
+
+      phi
+</pre>
+</div>
+
+<p>To finish off the blocks, we create an unconditional branch
+to the merge block.  One interesting (and very important) aspect of the LLVM IR
+is that it <a href="../LangRef.html#functionstructure">requires all basic blocks
+to be "terminated"</a> with a <a href="../LangRef.html#terminators">control flow
+instruction</a> such as return or branch.  This means that all control flow,
+<em>including fall throughs</em> must be made explicit in the LLVM IR.  If you
+violate this rule, the verifier will emit an error.
+
+<p>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.</p>
+
+<p>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...</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="for">'for' Loop Expression</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>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:</p>
+
+<div class="doc_code">
+<pre>
+ extern putchard(char);
+ def printstar(n)
+   for i = 1, i &lt; n, 1.0 in
+     putchard(42);  # ascii 42 = '*'
+
+ # print 100 '*' characters
+ printstar(100);
+</pre>
+</div>
+
+<p>This expression defines a new variable ("i" in this case) which iterates from
+a starting value, while the condition ("i &lt; 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.</p>
+
+<p>As before, lets talk about the changes that we need to Kaleidoscope to
+support this.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forlexer">Lexer Extensions for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The lexer extensions are the same sort of thing as for if/then/else:</p>
+
+<div class="doc_code">
+<pre>
+  ... in Token.token ...
+  (* control *)
+  | If | Then | Else
+  <b>| For | In</b>
+
+  ... in Lexer.lex_ident...
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | "if" -&gt; [&lt; 'Token.If; stream &gt;]
+      | "then" -&gt; [&lt; 'Token.Then; stream &gt;]
+      | "else" -&gt; [&lt; 'Token.Else; stream &gt;]
+      <b>| "for" -&gt; [&lt; 'Token.For; stream &gt;]
+      | "in" -&gt; [&lt; 'Token.In; stream &gt;]</b>
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forast">AST Extensions for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The AST variant is just as simple.  It basically boils down to capturing
+the variable name and the constituent expressions in the node.</p>
+
+<div class="doc_code">
+<pre>
+type expr =
+  ...
+  (* variant for for/in. *)
+  | For of string * expr * expr * expr option * expr
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forparser">Parser Extensions for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>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:</p>
+
+<div class="doc_code">
+<pre>
+let rec parse_primary = parser
+  ...
+  (* forexpr
+        ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
+  | [&lt; 'Token.For;
+       'Token.Ident id ?? "expected identifier after for";
+       'Token.Kwd '=' ?? "expected '=' after for";
+       stream &gt;] -&gt;
+      begin parser
+        | [&lt;
+             start=parse_expr;
+             'Token.Kwd ',' ?? "expected ',' after for";
+             end_=parse_expr;
+             stream &gt;] -&gt;
+            let step =
+              begin parser
+              | [&lt; 'Token.Kwd ','; step=parse_expr &gt;] -&gt; Some step
+              | [&lt; &gt;] -&gt; None
+              end stream
+            in
+            begin parser
+            | [&lt; 'Token.In; body=parse_expr &gt;] -&gt;
+                Ast.For (id, start, end_, step, body)
+            | [&lt; &gt;] -&gt;
+                raise (Stream.Error "expected 'in' after for")
+            end stream
+        | [&lt; &gt;] -&gt;
+            raise (Stream.Error "expected '=' after for")
+      end stream
+</pre>
+</div>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forir">LLVM IR for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>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):
+</p>
+
+<div class="doc_code">
+<pre>
+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 = add 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
+}
+</pre>
+</div>
+
+<p>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.</p>
+
+</div>
+
+<!-- ======================================================================= -->
+<div class="doc_subsubsection"><a name="forcodegen">Code Generation for
+the 'for' Loop</a></div>
+<!-- ======================================================================= -->
+
+<div class="doc_text">
+
+<p>The first part of Codegen is very simple: we just output the start expression
+for the loop value:</p>
+
+<div class="doc_code">
+<pre>
+let rec codegen_expr = function
+  ...
+  | Ast.For (var_name, start, end_, step, body) -&gt;
+      (* Emit the start code first, without 'variable' in scope. *)
+      let start_val = codegen_expr start in
+</pre>
+</div>
+
+<p>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).</p>
+
+<div class="doc_code">
+<pre>
+      (* Make the new basic block for the loop header, inserting after current
+       * block. *)
+      let preheader_bb = insertion_block builder in
+      let the_function = block_parent preheader_bb in
+      let loop_bb = append_block "loop" the_function in
+
+      (* Insert an explicit fall through from the current block to the
+       * loop_bb. *)
+      ignore (build_br loop_bb builder);
+</pre>
+</div>
+
+<p>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.</p>
+
+<div class="doc_code">
+<pre>
+      (* Start insertion in loop_bb. *)
+      position_at_end loop_bb builder;
+
+      (* Start the PHI node with an entry for start. *)
+      let variable = build_phi [(start_val, preheader_bb)] var_name builder in
+</pre>
+</div>
+
+<p>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!).</p>
+
+<div class="doc_code">
+<pre>
+      (* Within the loop, the variable is defined equal to the PHI node. If it
+       * shadows an existing variable, we have to restore it, so save it
+       * now. *)
+      let old_val =
+        try Some (Hashtbl.find named_values var_name) with Not_found -&gt; None
+      in
+      Hashtbl.add named_values var_name 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 *)
+      ignore (codegen_expr body);
+</pre>
+</div>
+
+<p>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 <tt>old_val</tt> (which will be None if there is
+no shadowed variable).</p>
+
+<p>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.</p>
+
+<div class="doc_code">
+<pre>
+      (* Emit the step value. *)
+      let step_val =
+        match step with
+        | Some step -&gt; codegen_expr step
+        (* If not specified, use 1.0. *)
+        | None -&gt; const_float double_type 1.0
+      in
+
+      let next_var = build_add variable step_val "nextvar" builder in
+</pre>
+</div>
+
+<p>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.
+'<tt>next_var</tt>' will be the value of the loop variable on the next iteration
+of the loop.</p>
+
+<div class="doc_code">
+<pre>
+      (* Compute the end condition. *)
+      let end_cond = codegen_expr end_ in
+
+      (* Convert condition to a bool by comparing equal to 0.0. *)
+      let zero = const_float double_type 0.0 in
+      let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
+</pre>
+</div>
+
+<p>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.</p>
+
+<div class="doc_code">
+<pre>
+      (* Create the "after loop" block and insert it. *)
+      let loop_end_bb = insertion_block builder in
+      let after_bb = append_block "afterloop" the_function in
+
+      (* Insert the conditional branch into the end of loop_end_bb. *)
+      ignore (build_cond_br end_cond loop_bb after_bb builder);
+
+      (* Any new code will be inserted in after_bb. *)
+      position_at_end after_bb builder;
+</pre>
+</div>
+
+<p>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.</p>
+
+<div class="doc_code">
+<pre>
+      (* Add a new entry to the PHI node for the backedge. *)
+      add_incoming (next_var, loop_end_bb) variable;
+
+      (* Restore the unshadowed variable. *)
+      begin match old_val with
+      | Some old_val -&gt; Hashtbl.add named_values var_name old_val
+      | None -&gt; ()
+      end;
+
+      (* for expr always returns 0.0. *)
+      const_null double_type
+</pre>
+</div>
+
+<p>The final code handles various cleanups: now that we have the
+"<tt>next_var</tt>" 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 <tt>Codegen.codegen_expr</tt>.</p>
+
+<p>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 <a href="OCamlLangImpl6.html">user-defined operators</a>
+to our poor innocent language.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="code">Full Code Listing</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+Here is the complete code listing for our running example, enhanced with the
+if/then/else and for expressions..  To build this example, use:
+</p>
+
+<div class="doc_code">
+<pre>
+# Compile
+ocamlbuild toy.byte
+# Run
+./toy.byte
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<dl>
+<dt>_tags:</dt>
+<dd class="doc_code">
+<pre>
+&lt;{lexer,parser}.ml&gt;: use_camlp4, pp(camlp4of)
+&lt;*.{byte,native}&gt;: g++, use_llvm, use_llvm_analysis
+&lt;*.{byte,native}&gt;: use_llvm_executionengine, use_llvm_target
+&lt;*.{byte,native}&gt;: use_llvm_scalar_opts, use_bindings
+</pre>
+</dd>
+
+<dt>myocamlbuild.ml:</dt>
+<dd class="doc_code">
+<pre>
+open Ocamlbuild_plugin;;
+
+ocaml_lib ~extern:true "llvm";;
+ocaml_lib ~extern:true "llvm_analysis";;
+ocaml_lib ~extern:true "llvm_executionengine";;
+ocaml_lib ~extern:true "llvm_target";;
+ocaml_lib ~extern:true "llvm_scalar_opts";;
+
+flag ["link"; "ocaml"; "g++"] (S[A"-cc"; A"g++"]);;
+dep ["link"; "ocaml"; "use_bindings"] ["bindings.o"];;
+</pre>
+</dd>
+
+<dt>token.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer Tokens
+ *===----------------------------------------------------------------------===*)
+
+(* The lexer returns these 'Kwd' if it is an unknown character, otherwise one of
+ * these others for known things. *)
+type token =
+  (* commands *)
+  | Def | Extern
+
+  (* primary *)
+  | Ident of string | Number of float
+
+  (* unknown *)
+  | Kwd of char
+
+  (* control *)
+  | If | Then | Else
+  | For | In
+</pre>
+</dd>
+
+<dt>lexer.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Lexer
+ *===----------------------------------------------------------------------===*)
+
+let rec lex = parser
+  (* Skip any whitespace. *)
+  | [&lt; ' (' ' | '\n' | '\r' | '\t'); stream &gt;] -&gt; lex stream
+
+  (* identifier: [a-zA-Z][a-zA-Z0-9] *)
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+
+  (* number: [0-9.]+ *)
+  | [&lt; ' ('0' .. '9' as c); stream &gt;] -&gt;
+      let buffer = Buffer.create 1 in
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+
+  (* Comment until end of line. *)
+  | [&lt; ' ('#'); stream &gt;] -&gt;
+      lex_comment stream
+
+  (* Otherwise, just return the character as its ascii value. *)
+  | [&lt; 'c; stream &gt;] -&gt;
+      [&lt; 'Token.Kwd c; lex stream &gt;]
+
+  (* end of stream. *)
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+
+and lex_number buffer = parser
+  | [&lt; ' ('0' .. '9' | '.' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_number buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      [&lt; 'Token.Number (float_of_string (Buffer.contents buffer)); stream &gt;]
+
+and lex_ident buffer = parser
+  | [&lt; ' ('A' .. 'Z' | 'a' .. 'z' | '0' .. '9' as c); stream &gt;] -&gt;
+      Buffer.add_char buffer c;
+      lex_ident buffer stream
+  | [&lt; stream=lex &gt;] -&gt;
+      match Buffer.contents buffer with
+      | "def" -&gt; [&lt; 'Token.Def; stream &gt;]
+      | "extern" -&gt; [&lt; 'Token.Extern; stream &gt;]
+      | "if" -&gt; [&lt; 'Token.If; stream &gt;]
+      | "then" -&gt; [&lt; 'Token.Then; stream &gt;]
+      | "else" -&gt; [&lt; 'Token.Else; stream &gt;]
+      | "for" -&gt; [&lt; 'Token.For; stream &gt;]
+      | "in" -&gt; [&lt; 'Token.In; stream &gt;]
+      | id -&gt; [&lt; 'Token.Ident id; stream &gt;]
+
+and lex_comment = parser
+  | [&lt; ' ('\n'); stream=lex &gt;] -&gt; stream
+  | [&lt; 'c; e=lex_comment &gt;] -&gt; e
+  | [&lt; &gt;] -&gt; [&lt; &gt;]
+</pre>
+</dd>
+
+<dt>ast.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Abstract Syntax Tree (aka Parse Tree)
+ *===----------------------------------------------------------------------===*)
+
+(* expr - Base type for all expression nodes. *)
+type expr =
+  (* variant for numeric literals like "1.0". *)
+  | Number of float
+
+  (* variant for referencing a variable, like "a". *)
+  | Variable of string
+
+  (* variant for a binary operator. *)
+  | Binary of char * expr * expr
+
+  (* variant for function calls. *)
+  | Call of string * expr array
+
+  (* variant for if/then/else. *)
+  | If of expr * expr * expr
+
+  (* variant for for/in. *)
+  | For of string * expr * expr * expr option * expr
+
+(* proto - This type represents the "prototype" for a function, which captures
+ * its name, and its argument names (thus implicitly the number of arguments the
+ * function takes). *)
+type proto = Prototype of string * string array
+
+(* func - This type represents a function definition itself. *)
+type func = Function of proto * expr
+</pre>
+</dd>
+
+<dt>parser.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===---------------------------------------------------------------------===
+ * Parser
+ *===---------------------------------------------------------------------===*)
+
+(* binop_precedence - This holds the precedence for each binary operator that is
+ * defined *)
+let binop_precedence:(char, int) Hashtbl.t = Hashtbl.create 10
+
+(* precedence - Get the precedence of the pending binary operator token. *)
+let precedence c = try Hashtbl.find binop_precedence c with Not_found -&gt; -1
+
+(* primary
+ *   ::= identifier
+ *   ::= numberexpr
+ *   ::= parenexpr
+ *   ::= ifexpr
+ *   ::= forexpr *)
+let rec parse_primary = parser
+  (* numberexpr ::= number *)
+  | [&lt; 'Token.Number n &gt;] -&gt; Ast.Number n
+
+  (* parenexpr ::= '(' expression ')' *)
+  | [&lt; 'Token.Kwd '('; e=parse_expr; 'Token.Kwd ')' ?? "expected ')'" &gt;] -&gt; e
+
+  (* identifierexpr
+   *   ::= identifier
+   *   ::= identifier '(' argumentexpr ')' *)
+  | [&lt; 'Token.Ident id; stream &gt;] -&gt;
+      let rec parse_args accumulator = parser
+        | [&lt; e=parse_expr; stream &gt;] -&gt;
+            begin parser
+              | [&lt; 'Token.Kwd ','; e=parse_args (e :: accumulator) &gt;] -&gt; e
+              | [&lt; &gt;] -&gt; e :: accumulator
+            end stream
+        | [&lt; &gt;] -&gt; accumulator
+      in
+      let rec parse_ident id = parser
+        (* Call. *)
+        | [&lt; 'Token.Kwd '(';
+             args=parse_args [];
+             'Token.Kwd ')' ?? "expected ')'"&gt;] -&gt;
+            Ast.Call (id, Array.of_list (List.rev args))
+
+        (* Simple variable ref. *)
+        | [&lt; &gt;] -&gt; Ast.Variable id
+      in
+      parse_ident id stream
+
+  (* ifexpr ::= 'if' expr 'then' expr 'else' expr *)
+  | [&lt; 'Token.If; c=parse_expr;
+       'Token.Then ?? "expected 'then'"; t=parse_expr;
+       'Token.Else ?? "expected 'else'"; e=parse_expr &gt;] -&gt;
+      Ast.If (c, t, e)
+
+  (* forexpr
+        ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression *)
+  | [&lt; 'Token.For;
+       'Token.Ident id ?? "expected identifier after for";
+       'Token.Kwd '=' ?? "expected '=' after for";
+       stream &gt;] -&gt;
+      begin parser
+        | [&lt;
+             start=parse_expr;
+             'Token.Kwd ',' ?? "expected ',' after for";
+             end_=parse_expr;
+             stream &gt;] -&gt;
+            let step =
+              begin parser
+              | [&lt; 'Token.Kwd ','; step=parse_expr &gt;] -&gt; Some step
+              | [&lt; &gt;] -&gt; None
+              end stream
+            in
+            begin parser
+            | [&lt; 'Token.In; body=parse_expr &gt;] -&gt;
+                Ast.For (id, start, end_, step, body)
+            | [&lt; &gt;] -&gt;
+                raise (Stream.Error "expected 'in' after for")
+            end stream
+        | [&lt; &gt;] -&gt;
+            raise (Stream.Error "expected '=' after for")
+      end stream
+
+  | [&lt; &gt;] -&gt; raise (Stream.Error "unknown token when expecting an expression.")
+
+(* binoprhs
+ *   ::= ('+' primary)* *)
+and parse_bin_rhs expr_prec lhs stream =
+  match Stream.peek stream with
+  (* If this is a binop, find its precedence. *)
+  | Some (Token.Kwd c) when Hashtbl.mem binop_precedence c -&gt;
+      let token_prec = precedence c in
+
+      (* If this is a binop that binds at least as tightly as the current binop,
+       * consume it, otherwise we are done. *)
+      if token_prec &lt; expr_prec then lhs else begin
+        (* Eat the binop. *)
+        Stream.junk stream;
+
+        (* Parse the primary expression after the binary operator. *)
+        let rhs = parse_primary stream in
+
+        (* Okay, we know this is a binop. *)
+        let rhs =
+          match Stream.peek stream with
+          | Some (Token.Kwd c2) -&gt;
+              (* If BinOp binds less tightly with rhs than the operator after
+               * rhs, let the pending operator take rhs as its lhs. *)
+              let next_prec = precedence c2 in
+              if token_prec &lt; next_prec
+              then parse_bin_rhs (token_prec + 1) rhs stream
+              else rhs
+          | _ -&gt; rhs
+        in
+
+        (* Merge lhs/rhs. *)
+        let lhs = Ast.Binary (c, lhs, rhs) in
+        parse_bin_rhs expr_prec lhs stream
+      end
+  | _ -&gt; lhs
+
+(* expression
+ *   ::= primary binoprhs *)
+and parse_expr = parser
+  | [&lt; lhs=parse_primary; stream &gt;] -&gt; parse_bin_rhs 0 lhs stream
+
+(* prototype
+ *   ::= id '(' id* ')' *)
+let parse_prototype =
+  let rec parse_args accumulator = parser
+    | [&lt; 'Token.Ident id; e=parse_args (id::accumulator) &gt;] -&gt; e
+    | [&lt; &gt;] -&gt; accumulator
+  in
+
+  parser
+  | [&lt; 'Token.Ident id;
+       'Token.Kwd '(' ?? "expected '(' in prototype";
+       args=parse_args [];
+       'Token.Kwd ')' ?? "expected ')' in prototype" &gt;] -&gt;
+      (* success. *)
+      Ast.Prototype (id, Array.of_list (List.rev args))
+
+  | [&lt; &gt;] -&gt;
+      raise (Stream.Error "expected function name in prototype")
+
+(* definition ::= 'def' prototype expression *)
+let parse_definition = parser
+  | [&lt; 'Token.Def; p=parse_prototype; e=parse_expr &gt;] -&gt;
+      Ast.Function (p, e)
+
+(* toplevelexpr ::= expression *)
+let parse_toplevel = parser
+  | [&lt; e=parse_expr &gt;] -&gt;
+      (* Make an anonymous proto. *)
+      Ast.Function (Ast.Prototype ("", [||]), e)
+
+(*  external ::= 'extern' prototype *)
+let parse_extern = parser
+  | [&lt; 'Token.Extern; e=parse_prototype &gt;] -&gt; e
+</pre>
+</dd>
+
+<dt>codegen.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Code Generation
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+
+exception Error of string
+
+let the_module = create_module "my cool jit"
+let builder = builder ()
+let named_values:(string, llvalue) Hashtbl.t = Hashtbl.create 10
+
+let rec codegen_expr = function
+  | Ast.Number n -&gt; const_float double_type n
+  | Ast.Variable name -&gt;
+      (try Hashtbl.find named_values name with
+        | Not_found -&gt; raise (Error "unknown variable name"))
+  | Ast.Binary (op, lhs, rhs) -&gt;
+      let lhs_val = codegen_expr lhs in
+      let rhs_val = codegen_expr rhs in
+      begin
+        match op with
+        | '+' -&gt; build_add lhs_val rhs_val "addtmp" builder
+        | '-' -&gt; build_sub lhs_val rhs_val "subtmp" builder
+        | '*' -&gt; build_mul lhs_val rhs_val "multmp" builder
+        | '&lt;' -&gt;
+            (* Convert bool 0/1 to double 0.0 or 1.0 *)
+            let i = build_fcmp Fcmp.Ult lhs_val rhs_val "cmptmp" builder in
+            build_uitofp i double_type "booltmp" builder
+        | _ -&gt; raise (Error "invalid binary operator")
+      end
+  | Ast.Call (callee, args) -&gt;
+      (* Look up the name in the module table. *)
+      let callee =
+        match lookup_function callee the_module with
+        | Some callee -&gt; callee
+        | None -&gt; raise (Error "unknown function referenced")
+      in
+      let params = params callee in
+
+      (* If argument mismatch error. *)
+      if Array.length params == Array.length args then () else
+        raise (Error "incorrect # arguments passed");
+      let args = Array.map codegen_expr args in
+      build_call callee args "calltmp" builder
+  | Ast.If (cond, then_, else_) -&gt;
+      let cond = codegen_expr cond in
+
+      (* Convert condition to a bool by comparing equal to 0.0 *)
+      let zero = const_float double_type 0.0 in
+      let cond_val = build_fcmp Fcmp.One cond zero "ifcond" builder in
+
+      (* Grab the first block so that we might later add the conditional branch
+       * to it at the end of the function. *)
+      let start_bb = insertion_block builder in
+      let the_function = block_parent start_bb in
+
+      let then_bb = append_block "then" the_function in
+
+      (* Emit 'then' value. *)
+      position_at_end then_bb builder;
+      let then_val = codegen_expr then_ in
+
+      (* Codegen of 'then' can change the current block, update then_bb for the
+       * phi. We create a new name because one is used for the phi node, and the
+       * other is used for the conditional branch. *)
+      let new_then_bb = insertion_block builder in
+
+      (* Emit 'else' value. *)
+      let else_bb = append_block "else" the_function in
+      position_at_end else_bb builder;
+      let else_val = codegen_expr else_ in
+
+      (* Codegen of 'else' can change the current block, update else_bb for the
+       * phi. *)
+      let new_else_bb = insertion_block builder in
+
+      (* Emit merge block. *)
+      let merge_bb = append_block "ifcont" the_function in
+      position_at_end merge_bb builder;
+      let incoming = [(then_val, new_then_bb); (else_val, new_else_bb)] in
+      let phi = build_phi incoming "iftmp" builder in
+
+      (* Return to the start block to add the conditional branch. *)
+      position_at_end start_bb builder;
+      ignore (build_cond_br cond_val then_bb else_bb builder);
+
+      (* Set a unconditional branch at the end of the 'then' block and the
+       * 'else' block to the 'merge' block. *)
+      position_at_end new_then_bb builder; ignore (build_br merge_bb builder);
+      position_at_end new_else_bb builder; ignore (build_br merge_bb builder);
+
+      (* Finally, set the builder to the end of the merge block. *)
+      position_at_end merge_bb builder;
+
+      phi
+  | Ast.For (var_name, start, end_, step, body) -&gt;
+      (* Emit the start code first, without 'variable' in scope. *)
+      let start_val = codegen_expr start in
+
+      (* Make the new basic block for the loop header, inserting after current
+       * block. *)
+      let preheader_bb = insertion_block builder in
+      let the_function = block_parent preheader_bb in
+      let loop_bb = append_block "loop" the_function in
+
+      (* Insert an explicit fall through from the current block to the
+       * loop_bb. *)
+      ignore (build_br loop_bb builder);
+
+      (* Start insertion in loop_bb. *)
+      position_at_end loop_bb builder;
+
+      (* Start the PHI node with an entry for start. *)
+      let variable = build_phi [(start_val, preheader_bb)] var_name builder in
+
+      (* Within the loop, the variable is defined equal to the PHI node. If it
+       * shadows an existing variable, we have to restore it, so save it
+       * now. *)
+      let old_val =
+        try Some (Hashtbl.find named_values var_name) with Not_found -&gt; None
+      in
+      Hashtbl.add named_values var_name 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 *)
+      ignore (codegen_expr body);
+
+      (* Emit the step value. *)
+      let step_val =
+        match step with
+        | Some step -&gt; codegen_expr step
+        (* If not specified, use 1.0. *)
+        | None -&gt; const_float double_type 1.0
+      in
+
+      let next_var = build_add variable step_val "nextvar" builder in
+
+      (* Compute the end condition. *)
+      let end_cond = codegen_expr end_ in
+
+      (* Convert condition to a bool by comparing equal to 0.0. *)
+      let zero = const_float double_type 0.0 in
+      let end_cond = build_fcmp Fcmp.One end_cond zero "loopcond" builder in
+
+      (* Create the "after loop" block and insert it. *)
+      let loop_end_bb = insertion_block builder in
+      let after_bb = append_block "afterloop" the_function in
+
+      (* Insert the conditional branch into the end of loop_end_bb. *)
+      ignore (build_cond_br end_cond loop_bb after_bb builder);
+
+      (* Any new code will be inserted in after_bb. *)
+      position_at_end after_bb builder;
+
+      (* Add a new entry to the PHI node for the backedge. *)
+      add_incoming (next_var, loop_end_bb) variable;
+
+      (* Restore the unshadowed variable. *)
+      begin match old_val with
+      | Some old_val -&gt; Hashtbl.add named_values var_name old_val
+      | None -&gt; ()
+      end;
+
+      (* for expr always returns 0.0. *)
+      const_null double_type
+
+let codegen_proto = function
+  | Ast.Prototype (name, args) -&gt;
+      (* Make the function type: double(double,double) etc. *)
+      let doubles = Array.make (Array.length args) double_type in
+      let ft = function_type double_type doubles in
+      let f =
+        match lookup_function name the_module with
+        | None -&gt; declare_function name ft the_module
+
+        (* If 'f' conflicted, there was already something named 'name'. If it
+         * has a body, don't allow redefinition or reextern. *)
+        | Some f -&gt;
+            (* If 'f' already has a body, reject this. *)
+            if block_begin f &lt;&gt; At_end f then
+              raise (Error "redefinition of function");
+
+            (* If 'f' took a different number of arguments, reject. *)
+            if element_type (type_of f) &lt;&gt; ft then
+              raise (Error "redefinition of function with different # args");
+            f
+      in
+
+      (* Set names for all arguments. *)
+      Array.iteri (fun i a -&gt;
+        let n = args.(i) in
+        set_value_name n a;
+        Hashtbl.add named_values n a;
+      ) (params f);
+      f
+
+let codegen_func the_fpm = function
+  | Ast.Function (proto, body) -&gt;
+      Hashtbl.clear named_values;
+      let the_function = codegen_proto proto in
+
+      (* Create a new basic block to start insertion into. *)
+      let bb = append_block "entry" the_function in
+      position_at_end bb builder;
+
+      try
+        let ret_val = codegen_expr body in
+
+        (* Finish off the function. *)
+        let _ = build_ret ret_val builder in
+
+        (* Validate the generated code, checking for consistency. *)
+        Llvm_analysis.assert_valid_function the_function;
+
+        (* Optimize the function. *)
+        let _ = PassManager.run_function the_function the_fpm in
+
+        the_function
+      with e -&gt;
+        delete_function the_function;
+        raise e
+</pre>
+</dd>
+
+<dt>toplevel.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Top-Level parsing and JIT Driver
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+open Llvm_executionengine
+
+(* top ::= definition | external | expression | ';' *)
+let rec main_loop the_fpm the_execution_engine stream =
+  match Stream.peek stream with
+  | None -&gt; ()
+
+  (* ignore top-level semicolons. *)
+  | Some (Token.Kwd ';') -&gt;
+      Stream.junk stream;
+      main_loop the_fpm the_execution_engine stream
+
+  | Some token -&gt;
+      begin
+        try match token with
+        | Token.Def -&gt;
+            let e = Parser.parse_definition stream in
+            print_endline "parsed a function definition.";
+            dump_value (Codegen.codegen_func the_fpm e);
+        | Token.Extern -&gt;
+            let e = Parser.parse_extern stream in
+            print_endline "parsed an extern.";
+            dump_value (Codegen.codegen_proto e);
+        | _ -&gt;
+            (* Evaluate a top-level expression into an anonymous function. *)
+            let e = Parser.parse_toplevel stream in
+            print_endline "parsed a top-level expr";
+            let the_function = Codegen.codegen_func the_fpm e in
+            dump_value the_function;
+
+            (* JIT the function, returning a function pointer. *)
+            let result = ExecutionEngine.run_function the_function [||]
+              the_execution_engine in
+
+            print_string "Evaluated to ";
+            print_float (GenericValue.as_float double_type result);
+            print_newline ();
+        with Stream.Error s | Codegen.Error s -&gt;
+          (* Skip token for error recovery. *)
+          Stream.junk stream;
+          print_endline s;
+      end;
+      print_string "ready&gt; "; flush stdout;
+      main_loop the_fpm the_execution_engine stream
+</pre>
+</dd>
+
+<dt>toy.ml:</dt>
+<dd class="doc_code">
+<pre>
+(*===----------------------------------------------------------------------===
+ * Main driver code.
+ *===----------------------------------------------------------------------===*)
+
+open Llvm
+open Llvm_executionengine
+open Llvm_target
+open Llvm_scalar_opts
+
+let main () =
+  (* Install standard binary operators.
+   * 1 is the lowest precedence. *)
+  Hashtbl.add Parser.binop_precedence '&lt;' 10;
+  Hashtbl.add Parser.binop_precedence '+' 20;
+  Hashtbl.add Parser.binop_precedence '-' 20;
+  Hashtbl.add Parser.binop_precedence '*' 40;    (* highest. *)
+
+  (* Prime the first token. *)
+  print_string "ready&gt; "; flush stdout;
+  let stream = Lexer.lex (Stream.of_channel stdin) in
+
+  (* Create the JIT. *)
+  let the_module_provider = ModuleProvider.create Codegen.the_module in
+  let the_execution_engine = ExecutionEngine.create the_module_provider in
+  let the_fpm = PassManager.create_function the_module_provider in
+
+  (* Set up the optimizer pipeline.  Start with registering info about how the
+   * target lays out data structures. *)
+  TargetData.add (ExecutionEngine.target_data the_execution_engine) the_fpm;
+
+  (* Do simple "peephole" optimizations and bit-twiddling optzn. *)
+  add_instruction_combining the_fpm;
+
+  (* reassociate expressions. *)
+  add_reassociation the_fpm;
+
+  (* Eliminate Common SubExpressions. *)
+  add_gvn the_fpm;
+
+  (* Simplify the control flow graph (deleting unreachable blocks, etc). *)
+  add_cfg_simplification the_fpm;
+
+  (* Run the main "interpreter loop" now. *)
+  Toplevel.main_loop the_fpm the_execution_engine stream;
+
+  (* Print out all the generated code. *)
+  dump_module Codegen.the_module
+;;
+
+main ()
+</pre>
+</dd>
+
+<dt>bindings.c</dt>
+<dd class="doc_code">
+<pre>
+#include &lt;stdio.h&gt;
+
+/* putchard - putchar that takes a double and returns 0. */
+extern double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+</pre>
+</dd>
+</dl>
+
+<a href="OCamlLangImpl6.html">Next: Extending the language: user-defined
+operators</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
+  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+  src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
+  <a href="http://validator.w3.org/check/referer"><img
+  src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="mailto:idadesub@users.sourceforge.net">Erick Tryzelaar</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date: 2007-10-17 11:05:13 -0700 (Wed, 17 Oct 2007) $
+</address>
+</body>
+</html>