Revert r103213. It broke several sections of live website.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103219 91177308-0d34-0410-b5e6-96231b3b80d8
author: mike-m <mikem.llvm@gmail.com> 2010-05-07 00:28:04 +0000
committer: mike-m <mikem.llvm@gmail.com> 2010-05-07 00:28:04 +0000
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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: Mutable Variables / SSA
+         construction</title>
+  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
+  <meta name="author" content="Chris Lattner">
+  <link rel="stylesheet" href="../llvm.css" type="text/css">
+</head>
+
+<body>
+
+<div class="doc_title">Kaleidoscope: Extending the Language: Mutable Variables</div>
+
+<ul>
+<li><a href="index.html">Up to Tutorial Index</a></li>
+<li>Chapter 7
+  <ol>
+    <li><a href="#intro">Chapter 7 Introduction</a></li>
+    <li><a href="#why">Why is this a hard problem?</a></li>
+    <li><a href="#memory">Memory in LLVM</a></li>
+    <li><a href="#kalvars">Mutable Variables in Kaleidoscope</a></li>
+    <li><a href="#adjustments">Adjusting Existing Variables for
+     Mutation</a></li>
+    <li><a href="#assignment">New Assignment Operator</a></li>
+    <li><a href="#localvars">User-defined Local Variables</a></li>
+    <li><a href="#code">Full Code Listing</a></li>
+  </ol>
+</li>
+<li><a href="LangImpl8.html">Chapter 8</a>: Conclusion and other useful LLVM
+ tidbits</li>
+</ul>
+
+<div class="doc_author">
+  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="intro">Chapter 7 Introduction</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Welcome to Chapter 7 of the "<a href="index.html">Implementing a language
+with LLVM</a>" tutorial.  In chapters 1 through 6, we've built a very
+respectable, albeit simple, <a 
+href="http://en.wikipedia.org/wiki/Functional_programming">functional
+programming language</a>.  In our journey, we learned some parsing techniques,
+how to build and represent an AST, how to build LLVM IR, and how to optimize
+the resultant code as well as JIT compile it.</p>
+
+<p>While Kaleidoscope is interesting as a functional language, the fact that it
+is functional makes it "too easy" to generate LLVM IR for it.  In particular, a 
+functional language makes it very easy to build LLVM IR directly in <a 
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">SSA form</a>.
+Since LLVM requires that the input code be in SSA form, this is a very nice
+property and it is often unclear to newcomers how to generate code for an
+imperative language with mutable variables.</p>
+
+<p>The short (and happy) summary of this chapter is that there is no need for
+your front-end to build SSA form: LLVM provides highly tuned and well tested
+support for this, though the way it works is a bit unexpected for some.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="why">Why is this a hard problem?</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+To understand why mutable variables cause complexities in SSA construction, 
+consider this extremely simple C example:
+</p>
+
+<div class="doc_code">
+<pre>
+int G, H;
+int test(_Bool Condition) {
+  int X;
+  if (Condition)
+    X = G;
+  else
+    X = H;
+  return X;
+}
+</pre>
+</div>
+
+<p>In this case, we have the variable "X", whose value depends on the path 
+executed in the program.  Because there are two different possible values for X
+before the return instruction, a PHI node is inserted to merge the two values.
+The LLVM IR that we want for this example looks like this:</p>
+
+<div class="doc_code">
+<pre>
+@G = weak global i32 0   ; type of @G is i32*
+@H = weak global i32 0   ; type of @H is i32*
+
+define i32 @test(i1 %Condition) {
+entry:
+	br i1 %Condition, label %cond_true, label %cond_false
+
+cond_true:
+	%X.0 = load i32* @G
+	br label %cond_next
+
+cond_false:
+	%X.1 = load i32* @H
+	br label %cond_next
+
+cond_next:
+	%X.2 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
+	ret i32 %X.2
+}
+</pre>
+</div>
+
+<p>In this example, the loads from the G and H global variables are explicit in
+the LLVM IR, and they live in the then/else branches of the if statement
+(cond_true/cond_false).  In order to merge the incoming values, the X.2 phi node
+in the cond_next block selects the right value to use based on where control 
+flow is coming from: if control flow comes from the cond_false block, X.2 gets
+the value of X.1.  Alternatively, if control flow comes from cond_true, it gets
+the value of X.0.  The intent of this chapter is not to explain the details of
+SSA form.  For more information, see one of the many <a 
+href="http://en.wikipedia.org/wiki/Static_single_assignment_form">online 
+references</a>.</p>
+
+<p>The question for this article is "who places the phi nodes when lowering 
+assignments to mutable variables?".  The issue here is that LLVM 
+<em>requires</em> that its IR be in SSA form: there is no "non-ssa" mode for it.
+However, SSA construction requires non-trivial algorithms and data structures,
+so it is inconvenient and wasteful for every front-end to have to reproduce this
+logic.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="memory">Memory in LLVM</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>The 'trick' here is that while LLVM does require all register values to be
+in SSA form, it does not require (or permit) memory objects to be in SSA form.
+In the example above, note that the loads from G and H are direct accesses to
+G and H: they are not renamed or versioned.  This differs from some other
+compiler systems, which do try to version memory objects.  In LLVM, instead of
+encoding dataflow analysis of memory into the LLVM IR, it is handled with <a 
+href="../WritingAnLLVMPass.html">Analysis Passes</a> which are computed on
+demand.</p>
+
+<p>
+With this in mind, the high-level idea is that we want to make a stack variable
+(which lives in memory, because it is on the stack) for each mutable object in
+a function.  To take advantage of this trick, we need to talk about how LLVM
+represents stack variables.
+</p>
+
+<p>In LLVM, all memory accesses are explicit with load/store instructions, and
+it is carefully designed not to have (or need) an "address-of" operator.  Notice
+how the type of the @G/@H global variables is actually "i32*" even though the 
+variable is defined as "i32".  What this means is that @G defines <em>space</em>
+for an i32 in the global data area, but its <em>name</em> actually refers to the
+address for that space.  Stack variables work the same way, except that instead of 
+being declared with global variable definitions, they are declared with the 
+<a href="../LangRef.html#i_alloca">LLVM alloca instruction</a>:</p>
+
+<div class="doc_code">
+<pre>
+define i32 @example() {
+entry:
+	%X = alloca i32           ; type of %X is i32*.
+	...
+	%tmp = load i32* %X       ; load the stack value %X from the stack.
+	%tmp2 = add i32 %tmp, 1   ; increment it
+	store i32 %tmp2, i32* %X  ; store it back
+	...
+</pre>
+</div>
+
+<p>This code shows an example of how you can declare and manipulate a stack
+variable in the LLVM IR.  Stack memory allocated with the alloca instruction is
+fully general: you can pass the address of the stack slot to functions, you can
+store it in other variables, etc.  In our example above, we could rewrite the
+example to use the alloca technique to avoid using a PHI node:</p>
+
+<div class="doc_code">
+<pre>
+@G = weak global i32 0   ; type of @G is i32*
+@H = weak global i32 0   ; type of @H is i32*
+
+define i32 @test(i1 %Condition) {
+entry:
+	%X = alloca i32           ; type of %X is i32*.
+	br i1 %Condition, label %cond_true, label %cond_false
+
+cond_true:
+	%X.0 = load i32* @G
+        store i32 %X.0, i32* %X   ; Update X
+	br label %cond_next
+
+cond_false:
+	%X.1 = load i32* @H
+        store i32 %X.1, i32* %X   ; Update X
+	br label %cond_next
+
+cond_next:
+	%X.2 = load i32* %X       ; Read X
+	ret i32 %X.2
+}
+</pre>
+</div>
+
+<p>With this, we have discovered a way to handle arbitrary mutable variables
+without the need to create Phi nodes at all:</p>
+
+<ol>
+<li>Each mutable variable becomes a stack allocation.</li>
+<li>Each read of the variable becomes a load from the stack.</li>
+<li>Each update of the variable becomes a store to the stack.</li>
+<li>Taking the address of a variable just uses the stack address directly.</li>
+</ol>
+
+<p>While this solution has solved our immediate problem, it introduced another
+one: we have now apparently introduced a lot of stack traffic for very simple
+and common operations, a major performance problem.  Fortunately for us, the
+LLVM optimizer has a highly-tuned optimization pass named "mem2reg" that handles
+this case, promoting allocas like this into SSA registers, inserting Phi nodes
+as appropriate.  If you run this example through the pass, for example, you'll
+get:</p>
+
+<div class="doc_code">
+<pre>
+$ <b>llvm-as &lt; example.ll | opt -mem2reg | llvm-dis</b>
+@G = weak global i32 0
+@H = weak global i32 0
+
+define i32 @test(i1 %Condition) {
+entry:
+	br i1 %Condition, label %cond_true, label %cond_false
+
+cond_true:
+	%X.0 = load i32* @G
+	br label %cond_next
+
+cond_false:
+	%X.1 = load i32* @H
+	br label %cond_next
+
+cond_next:
+	%X.01 = phi i32 [ %X.1, %cond_false ], [ %X.0, %cond_true ]
+	ret i32 %X.01
+}
+</pre>
+</div>
+
+<p>The mem2reg pass implements the standard "iterated dominance frontier"
+algorithm for constructing SSA form and has a number of optimizations that speed
+up (very common) degenerate cases. The mem2reg optimization pass is the answer to dealing 
+with mutable variables, and we highly recommend that you depend on it.  Note that
+mem2reg only works on variables in certain circumstances:</p>
+
+<ol>
+<li>mem2reg is alloca-driven: it looks for allocas and if it can handle them, it
+promotes them.  It does not apply to global variables or heap allocations.</li>
+
+<li>mem2reg only looks for alloca instructions in the entry block of the
+function.  Being in the entry block guarantees that the alloca is only executed
+once, which makes analysis simpler.</li>
+
+<li>mem2reg only promotes allocas whose uses are direct loads and stores.  If
+the address of the stack object is passed to a function, or if any funny pointer
+arithmetic is involved, the alloca will not be promoted.</li>
+
+<li>mem2reg only works on allocas of <a 
+href="../LangRef.html#t_classifications">first class</a> 
+values (such as pointers, scalars and vectors), and only if the array size
+of the allocation is 1 (or missing in the .ll file).  mem2reg is not capable of
+promoting structs or arrays to registers.  Note that the "scalarrepl" pass is
+more powerful and can promote structs, "unions", and arrays in many cases.</li>
+
+</ol>
+
+<p>
+All of these properties are easy to satisfy for most imperative languages, and
+we'll illustrate it below with Kaleidoscope.  The final question you may be
+asking is: should I bother with this nonsense for my front-end?  Wouldn't it be
+better if I just did SSA construction directly, avoiding use of the mem2reg
+optimization pass?  In short, we strongly recommend that you use this technique
+for building SSA form, unless there is an extremely good reason not to.  Using
+this technique is:</p>
+
+<ul>
+<li>Proven and well tested: llvm-gcc and clang both use this technique for local
+mutable variables.  As such, the most common clients of LLVM are using this to
+handle a bulk of their variables.  You can be sure that bugs are found fast and
+fixed early.</li>
+
+<li>Extremely Fast: mem2reg has a number of special cases that make it fast in
+common cases as well as fully general.  For example, it has fast-paths for
+variables that are only used in a single block, variables that only have one
+assignment point, good heuristics to avoid insertion of unneeded phi nodes, etc.
+</li>
+
+<li>Needed for debug info generation: <a href="../SourceLevelDebugging.html">
+Debug information in LLVM</a> relies on having the address of the variable
+exposed so that debug info can be attached to it.  This technique dovetails 
+very naturally with this style of debug info.</li>
+</ul>
+
+<p>If nothing else, this makes it much easier to get your front-end up and 
+running, and is very simple to implement.  Lets extend Kaleidoscope with mutable
+variables now!
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="kalvars">Mutable Variables in 
+Kaleidoscope</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Now that we know the sort of problem we want to tackle, lets see what this
+looks like in the context of our little Kaleidoscope language.  We're going to
+add two features:</p>
+
+<ol>
+<li>The ability to mutate variables with the '=' operator.</li>
+<li>The ability to define new variables.</li>
+</ol>
+
+<p>While the first item is really what this is about, we only have variables
+for incoming arguments as well as for induction variables, and redefining those only
+goes so far :).  Also, the ability to define new variables is a
+useful thing regardless of whether you will be mutating them.  Here's a
+motivating example that shows how we could use these:</p>
+
+<div class="doc_code">
+<pre>
+# Define ':' for sequencing: as a low-precedence operator that ignores operands
+# and just returns the RHS.
+def binary : 1 (x y) y;
+
+# Recursive fib, we could do this before.
+def fib(x)
+  if (x &lt; 3) then
+    1
+  else
+    fib(x-1)+fib(x-2);
+
+# Iterative fib.
+def fibi(x)
+  <b>var a = 1, b = 1, c in</b>
+  (for i = 3, i &lt; x in 
+     <b>c = a + b</b> :
+     <b>a = b</b> :
+     <b>b = c</b>) :
+  b;
+
+# Call it. 
+fibi(10);
+</pre>
+</div>
+
+<p>
+In order to mutate variables, we have to change our existing variables to use
+the "alloca trick".  Once we have that, we'll add our new operator, then extend
+Kaleidoscope to support new variable definitions.
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="adjustments">Adjusting Existing Variables for
+Mutation</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>
+The symbol table in Kaleidoscope is managed at code generation time by the 
+'<tt>NamedValues</tt>' map.  This map currently keeps track of the LLVM "Value*"
+that holds the double value for the named variable.  In order to support
+mutation, we need to change this slightly, so that it <tt>NamedValues</tt> holds
+the <em>memory location</em> of the variable in question.  Note that this 
+change is a refactoring: it changes the structure of the code, but does not
+(by itself) change the behavior of the compiler.  All of these changes are 
+isolated in the Kaleidoscope code generator.</p>
+
+<p>
+At this point in Kaleidoscope's development, it only supports variables for two
+things: incoming arguments to functions and the induction variable of 'for'
+loops.  For consistency, we'll allow mutation of these variables in addition to
+other user-defined variables.  This means that these will both need memory
+locations.
+</p>
+
+<p>To start our transformation of Kaleidoscope, we'll change the NamedValues
+map so that it maps to AllocaInst* instead of Value*.  Once we do this, the C++ 
+compiler will tell us what parts of the code we need to update:</p>
+
+<div class="doc_code">
+<pre>
+static std::map&lt;std::string, AllocaInst*&gt; NamedValues;
+</pre>
+</div>
+
+<p>Also, since we will need to create these alloca's, we'll use a helper
+function that ensures that the allocas are created in the entry block of the
+function:</p>
+
+<div class="doc_code">
+<pre>
+/// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of
+/// the function.  This is used for mutable variables etc.
+static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
+                                          const std::string &amp;VarName) {
+  IRBuilder&lt;&gt; TmpB(&amp;TheFunction-&gt;getEntryBlock(),
+                 TheFunction-&gt;getEntryBlock().begin());
+  return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0,
+                           VarName.c_str());
+}
+</pre>
+</div>
+
+<p>This funny looking code creates an IRBuilder object that is pointing at
+the first instruction (.begin()) of the entry block.  It then creates an alloca
+with the expected name and returns it.  Because all values in Kaleidoscope are
+doubles, there is no need to pass in a type to use.</p>
+
+<p>With this in place, the first functionality change we want to make is to
+variable references.  In our new scheme, variables live on the stack, so code
+generating a reference to them actually needs to produce a load from the stack
+slot:</p>
+
+<div class="doc_code">
+<pre>
+Value *VariableExprAST::Codegen() {
+  // Look this variable up in the function.
+  Value *V = NamedValues[Name];
+  if (V == 0) return ErrorV("Unknown variable name");
+
+  <b>// Load the value.
+  return Builder.CreateLoad(V, Name.c_str());</b>
+}
+</pre>
+</div>
+
+<p>As you can see, this is pretty straightforward.  Now we need to update the
+things that define the variables to set up the alloca.  We'll start with 
+<tt>ForExprAST::Codegen</tt> (see the <a href="#code">full code listing</a> for
+the unabridged code):</p>
+
+<div class="doc_code">
+<pre>
+  Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+  <b>// Create an alloca for the variable in the entry block.
+  AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);</b>
+  
+    // Emit the start code first, without 'variable' in scope.
+  Value *StartVal = Start-&gt;Codegen();
+  if (StartVal == 0) return 0;
+  
+  <b>// Store the value into the alloca.
+  Builder.CreateStore(StartVal, Alloca);</b>
+  ...
+
+  // Compute the end condition.
+  Value *EndCond = End-&gt;Codegen();
+  if (EndCond == 0) return EndCond;
+  
+  <b>// Reload, increment, and restore the alloca.  This handles the case where
+  // the body of the loop mutates the variable.
+  Value *CurVar = Builder.CreateLoad(Alloca);
+  Value *NextVar = Builder.CreateAdd(CurVar, StepVal, "nextvar");
+  Builder.CreateStore(NextVar, Alloca);</b>
+  ...
+</pre>
+</div>
+
+<p>This code is virtually identical to the code <a 
+href="LangImpl5.html#forcodegen">before we allowed mutable variables</a>.  The
+big difference is that we no longer have to construct a PHI node, and we use
+load/store to access the variable as needed.</p>
+
+<p>To support mutable argument variables, we need to also make allocas for them.
+The code for this is also pretty simple:</p>
+
+<div class="doc_code">
+<pre>
+/// CreateArgumentAllocas - Create an alloca for each argument and register the
+/// argument in the symbol table so that references to it will succeed.
+void PrototypeAST::CreateArgumentAllocas(Function *F) {
+  Function::arg_iterator AI = F-&gt;arg_begin();
+  for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) {
+    // Create an alloca for this variable.
+    AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]);
+
+    // Store the initial value into the alloca.
+    Builder.CreateStore(AI, Alloca);
+
+    // Add arguments to variable symbol table.
+    NamedValues[Args[Idx]] = Alloca;
+  }
+}
+</pre>
+</div>
+
+<p>For each argument, we make an alloca, store the input value to the function
+into the alloca, and register the alloca as the memory location for the
+argument.  This method gets invoked by <tt>FunctionAST::Codegen</tt> right after
+it sets up the entry block for the function.</p>
+
+<p>The final missing piece is adding the mem2reg pass, which allows us to get
+good codegen once again:</p>
+
+<div class="doc_code">
+<pre>
+    // Set up the optimizer pipeline.  Start with registering info about how the
+    // target lays out data structures.
+    OurFPM.add(new TargetData(*TheExecutionEngine-&gt;getTargetData()));
+    <b>// Promote allocas to registers.
+    OurFPM.add(createPromoteMemoryToRegisterPass());</b>
+    // Do simple "peephole" optimizations and bit-twiddling optzns.
+    OurFPM.add(createInstructionCombiningPass());
+    // Reassociate expressions.
+    OurFPM.add(createReassociatePass());
+</pre>
+</div>
+
+<p>It is interesting to see what the code looks like before and after the
+mem2reg optimization runs.  For example, this is the before/after code for our
+recursive fib function.  Before the optimization:</p>
+
+<div class="doc_code">
+<pre>
+define double @fib(double %x) {
+entry:
+	<b>%x1 = alloca double
+	store double %x, double* %x1
+	%x2 = load double* %x1</b>
+	%cmptmp = fcmp ult double %x2, 3.000000e+00
+	%booltmp = uitofp i1 %cmptmp to double
+	%ifcond = fcmp one double %booltmp, 0.000000e+00
+	br i1 %ifcond, label %then, label %else
+
+then:		; preds = %entry
+	br label %ifcont
+
+else:		; preds = %entry
+	<b>%x3 = load double* %x1</b>
+	%subtmp = fsub double %x3, 1.000000e+00
+	%calltmp = call double @fib( double %subtmp )
+	<b>%x4 = load double* %x1</b>
+	%subtmp5 = fsub double %x4, 2.000000e+00
+	%calltmp6 = call double @fib( double %subtmp5 )
+	%addtmp = fadd double %calltmp, %calltmp6
+	br label %ifcont
+
+ifcont:		; preds = %else, %then
+	%iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
+	ret double %iftmp
+}
+</pre>
+</div>
+
+<p>Here there is only one variable (x, the input argument) but you can still
+see the extremely simple-minded code generation strategy we are using.  In the
+entry block, an alloca is created, and the initial input value is stored into
+it.  Each reference to the variable does a reload from the stack.  Also, note
+that we didn't modify the if/then/else expression, so it still inserts a PHI
+node.  While we could make an alloca for it, it is actually easier to create a 
+PHI node for it, so we still just make the PHI.</p>
+
+<p>Here is the code after the mem2reg pass runs:</p>
+
+<div class="doc_code">
+<pre>
+define double @fib(double %x) {
+entry:
+	%cmptmp = fcmp ult double <b>%x</b>, 3.000000e+00
+	%booltmp = uitofp i1 %cmptmp to double
+	%ifcond = fcmp one double %booltmp, 0.000000e+00
+	br i1 %ifcond, label %then, label %else
+
+then:
+	br label %ifcont
+
+else:
+	%subtmp = fsub double <b>%x</b>, 1.000000e+00
+	%calltmp = call double @fib( double %subtmp )
+	%subtmp5 = fsub double <b>%x</b>, 2.000000e+00
+	%calltmp6 = call double @fib( double %subtmp5 )
+	%addtmp = fadd double %calltmp, %calltmp6
+	br label %ifcont
+
+ifcont:		; preds = %else, %then
+	%iftmp = phi double [ 1.000000e+00, %then ], [ %addtmp, %else ]
+	ret double %iftmp
+}
+</pre>
+</div>
+
+<p>This is a trivial case for mem2reg, since there are no redefinitions of the
+variable.  The point of showing this is to calm your tension about inserting
+such blatent inefficiencies :).</p>
+
+<p>After the rest of the optimizers run, we get:</p>
+
+<div class="doc_code">
+<pre>
+define double @fib(double %x) {
+entry:
+	%cmptmp = fcmp ult double %x, 3.000000e+00
+	%booltmp = uitofp i1 %cmptmp to double
+	%ifcond = fcmp ueq double %booltmp, 0.000000e+00
+	br i1 %ifcond, label %else, label %ifcont
+
+else:
+	%subtmp = fsub double %x, 1.000000e+00
+	%calltmp = call double @fib( double %subtmp )
+	%subtmp5 = fsub double %x, 2.000000e+00
+	%calltmp6 = call double @fib( double %subtmp5 )
+	%addtmp = fadd double %calltmp, %calltmp6
+	ret double %addtmp
+
+ifcont:
+	ret double 1.000000e+00
+}
+</pre>
+</div>
+
+<p>Here we see that the simplifycfg pass decided to clone the return instruction
+into the end of the 'else' block.  This allowed it to eliminate some branches
+and the PHI node.</p>
+
+<p>Now that all symbol table references are updated to use stack variables, 
+we'll add the assignment operator.</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="assignment">New Assignment Operator</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>With our current framework, adding a new assignment operator is really
+simple.  We will parse it just like any other binary operator, but handle it
+internally (instead of allowing the user to define it).  The first step is to
+set a precedence:</p>
+
+<div class="doc_code">
+<pre>
+ int main() {
+   // Install standard binary operators.
+   // 1 is lowest precedence.
+   <b>BinopPrecedence['='] = 2;</b>
+   BinopPrecedence['&lt;'] = 10;
+   BinopPrecedence['+'] = 20;
+   BinopPrecedence['-'] = 20;
+</pre>
+</div>
+
+<p>Now that the parser knows the precedence of the binary operator, it takes
+care of all the parsing and AST generation.  We just need to implement codegen
+for the assignment operator.  This looks like:</p> 
+
+<div class="doc_code">
+<pre>
+Value *BinaryExprAST::Codegen() {
+  // Special case '=' because we don't want to emit the LHS as an expression.
+  if (Op == '=') {
+    // Assignment requires the LHS to be an identifier.
+    VariableExprAST *LHSE = dynamic_cast&lt;VariableExprAST*&gt;(LHS);
+    if (!LHSE)
+      return ErrorV("destination of '=' must be a variable");
+</pre>
+</div>
+
+<p>Unlike the rest of the binary operators, our assignment operator doesn't
+follow the "emit LHS, emit RHS, do computation" model.  As such, it is handled
+as a special case before the other binary operators are handled.  The other 
+strange thing is that it requires the LHS to be a variable.  It is invalid to
+have "(x+1) = expr" - only things like "x = expr" are allowed.
+</p>
+
+<div class="doc_code">
+<pre>
+    // Codegen the RHS.
+    Value *Val = RHS-&gt;Codegen();
+    if (Val == 0) return 0;
+
+    // Look up the name.
+    Value *Variable = NamedValues[LHSE-&gt;getName()];
+    if (Variable == 0) return ErrorV("Unknown variable name");
+
+    Builder.CreateStore(Val, Variable);
+    return Val;
+  }
+  ...  
+</pre>
+</div>
+
+<p>Once we have the variable, codegen'ing the assignment is straightforward:
+we emit the RHS of the assignment, create a store, and return the computed
+value.  Returning a value allows for chained assignments like "X = (Y = Z)".</p>
+
+<p>Now that we have an assignment operator, we can mutate loop variables and
+arguments.  For example, we can now run code like this:</p>
+
+<div class="doc_code">
+<pre>
+# Function to print a double.
+extern printd(x);
+
+# Define ':' for sequencing: as a low-precedence operator that ignores operands
+# and just returns the RHS.
+def binary : 1 (x y) y;
+
+def test(x)
+  printd(x) :
+  x = 4 :
+  printd(x);
+
+test(123);
+</pre>
+</div>
+
+<p>When run, this example prints "123" and then "4", showing that we did
+actually mutate the value!  Okay, we have now officially implemented our goal:
+getting this to work requires SSA construction in the general case.  However,
+to be really useful, we want the ability to define our own local variables, lets
+add this next! 
+</p>
+
+</div>
+
+<!-- *********************************************************************** -->
+<div class="doc_section"><a name="localvars">User-defined Local 
+Variables</a></div>
+<!-- *********************************************************************** -->
+
+<div class="doc_text">
+
+<p>Adding var/in is just like any other other extensions we made to 
+Kaleidoscope: we extend the lexer, the parser, the AST and the code generator.
+The first step for adding our new 'var/in' construct is to extend the lexer.
+As before, this is pretty trivial, the code looks like this:</p>
+
+<div class="doc_code">
+<pre>
+enum Token {
+  ...
+  <b>// var definition
+  tok_var = -13</b>
+...
+}
+...
+static int gettok() {
+...
+    if (IdentifierStr == "in") return tok_in;
+    if (IdentifierStr == "binary") return tok_binary;
+    if (IdentifierStr == "unary") return tok_unary;
+    <b>if (IdentifierStr == "var") return tok_var;</b>
+    return tok_identifier;
+...
+</pre>
+</div>
+
+<p>The next step is to define the AST node that we will construct.  For var/in,
+it looks like this:</p>
+
+<div class="doc_code">
+<pre>
+/// VarExprAST - Expression class for var/in
+class VarExprAST : public ExprAST {
+  std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; VarNames;
+  ExprAST *Body;
+public:
+  VarExprAST(const std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; &amp;varnames,
+             ExprAST *body)
+  : VarNames(varnames), Body(body) {}
+  
+  virtual Value *Codegen();
+};
+</pre>
+</div>
+
+<p>var/in allows a list of names to be defined all at once, and each name can
+optionally have an initializer value.  As such, we capture this information in
+the VarNames vector.  Also, var/in has a body, this body is allowed to access
+the variables defined by the var/in.</p>
+
+<p>With this in place, we can define the parser pieces.  The first thing we do is add
+it as a primary expression:</p>
+
+<div class="doc_code">
+<pre>
+/// primary
+///   ::= identifierexpr
+///   ::= numberexpr
+///   ::= parenexpr
+///   ::= ifexpr
+///   ::= forexpr
+<b>///   ::= varexpr</b>
+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();
+  <b>case tok_var:        return ParseVarExpr();</b>
+  }
+}
+</pre>
+</div>
+
+<p>Next we define ParseVarExpr:</p>
+
+<div class="doc_code">
+<pre>
+/// varexpr ::= 'var' identifier ('=' expression)? 
+//                    (',' identifier ('=' expression)?)* 'in' expression
+static ExprAST *ParseVarExpr() {
+  getNextToken();  // eat the var.
+
+  std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; VarNames;
+
+  // At least one variable name is required.
+  if (CurTok != tok_identifier)
+    return Error("expected identifier after var");
+</pre>
+</div>
+
+<p>The first part of this code parses the list of identifier/expr pairs into the
+local <tt>VarNames</tt> vector.  
+
+<div class="doc_code">
+<pre>
+  while (1) {
+    std::string Name = IdentifierStr;
+    getNextToken();  // eat identifier.
+
+    // Read the optional initializer.
+    ExprAST *Init = 0;
+    if (CurTok == '=') {
+      getNextToken(); // eat the '='.
+      
+      Init = ParseExpression();
+      if (Init == 0) return 0;
+    }
+    
+    VarNames.push_back(std::make_pair(Name, Init));
+    
+    // End of var list, exit loop.
+    if (CurTok != ',') break;
+    getNextToken(); // eat the ','.
+    
+    if (CurTok != tok_identifier)
+      return Error("expected identifier list after var");
+  }
+</pre>
+</div>
+
+<p>Once all the variables are parsed, we then parse the body and create the
+AST node:</p>
+
+<div class="doc_code">
+<pre>
+  // At this point, we have to have 'in'.
+  if (CurTok != tok_in)
+    return Error("expected 'in' keyword after 'var'");
+  getNextToken();  // eat 'in'.
+  
+  ExprAST *Body = ParseExpression();
+  if (Body == 0) return 0;
+  
+  return new VarExprAST(VarNames, Body);
+}
+</pre>
+</div>
+
+<p>Now that we can parse and represent the code, we need to support emission of
+LLVM IR for it.  This code starts out with:</p>
+
+<div class="doc_code">
+<pre>
+Value *VarExprAST::Codegen() {
+  std::vector&lt;AllocaInst *&gt; OldBindings;
+  
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+
+  // Register all variables and emit their initializer.
+  for (unsigned i = 0, e = VarNames.size(); i != e; ++i) {
+    const std::string &amp;VarName = VarNames[i].first;
+    ExprAST *Init = VarNames[i].second;
+</pre>
+</div>
+
+<p>Basically it loops over all the variables, installing them one at a time.
+For each variable we put into the symbol table, we remember the previous value
+that we replace in OldBindings.</p>
+
+<div class="doc_code">
+<pre>
+    // Emit the initializer before adding the variable to scope, this prevents
+    // the initializer from referencing the variable itself, and permits stuff
+    // like this:
+    //  var a = 1 in
+    //    var a = a in ...   # refers to outer 'a'.
+    Value *InitVal;
+    if (Init) {
+      InitVal = Init-&gt;Codegen();
+      if (InitVal == 0) return 0;
+    } else { // If not specified, use 0.0.
+      InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0));
+    }
+    
+    AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+    Builder.CreateStore(InitVal, Alloca);
+
+    // Remember the old variable binding so that we can restore the binding when
+    // we unrecurse.
+    OldBindings.push_back(NamedValues[VarName]);
+    
+    // Remember this binding.
+    NamedValues[VarName] = Alloca;
+  }
+</pre>
+</div>
+
+<p>There are more comments here than code.  The basic idea is that we emit the
+initializer, create the alloca, then update the symbol table to point to it.
+Once all the variables are installed in the symbol table, we evaluate the body
+of the var/in expression:</p>
+
+<div class="doc_code">
+<pre>
+  // Codegen the body, now that all vars are in scope.
+  Value *BodyVal = Body-&gt;Codegen();
+  if (BodyVal == 0) return 0;
+</pre>
+</div>
+
+<p>Finally, before returning, we restore the previous variable bindings:</p>
+
+<div class="doc_code">
+<pre>
+  // Pop all our variables from scope.
+  for (unsigned i = 0, e = VarNames.size(); i != e; ++i)
+    NamedValues[VarNames[i].first] = OldBindings[i];
+
+  // Return the body computation.
+  return BodyVal;
+}
+</pre>
+</div>
+
+<p>The end result of all of this is that we get properly scoped variable 
+definitions, and we even (trivially) allow mutation of them :).</p>
+
+<p>With this, we completed what we set out to do.  Our nice iterative fib
+example from the intro compiles and runs just fine.  The mem2reg pass optimizes
+all of our stack variables into SSA registers, inserting PHI nodes where needed,
+and our front-end remains simple: no "iterated dominance frontier" computation
+anywhere in sight.</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 mutable
+variables and var/in support.  To build this example, use:
+</p>
+
+<div class="doc_code">
+<pre>
+   # Compile
+   g++ -g toy.cpp `llvm-config --cppflags --ldflags --libs core jit native` -O3 -o toy
+   # Run
+   ./toy
+</pre>
+</div>
+
+<p>Here is the code:</p>
+
+<div class="doc_code">
+<pre>
+#include "llvm/DerivedTypes.h"
+#include "llvm/ExecutionEngine/ExecutionEngine.h"
+#include "llvm/ExecutionEngine/JIT.h"
+#include "llvm/LLVMContext.h"
+#include "llvm/Module.h"
+#include "llvm/PassManager.h"
+#include "llvm/Analysis/Verifier.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Target/TargetSelect.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Support/IRBuilder.h"
+#include &lt;cstdio&gt;
+#include &lt;string&gt;
+#include &lt;map&gt;
+#include &lt;vector&gt;
+using namespace llvm;
+
+//===----------------------------------------------------------------------===//
+// Lexer
+//===----------------------------------------------------------------------===//
+
+// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
+// of these for known things.
+enum Token {
+  tok_eof = -1,
+
+  // commands
+  tok_def = -2, tok_extern = -3,
+
+  // primary
+  tok_identifier = -4, tok_number = -5,
+  
+  // control
+  tok_if = -6, tok_then = -7, tok_else = -8,
+  tok_for = -9, tok_in = -10,
+  
+  // operators
+  tok_binary = -11, tok_unary = -12,
+  
+  // var definition
+  tok_var = -13
+};
+
+static std::string IdentifierStr;  // Filled in if tok_identifier
+static double NumVal;              // Filled in if tok_number
+
+/// gettok - Return the next token from standard input.
+static int gettok() {
+  static int LastChar = ' ';
+
+  // Skip any whitespace.
+  while (isspace(LastChar))
+    LastChar = getchar();
+
+  if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
+    IdentifierStr = LastChar;
+    while (isalnum((LastChar = getchar())))
+      IdentifierStr += LastChar;
+
+    if (IdentifierStr == "def") return tok_def;
+    if (IdentifierStr == "extern") return tok_extern;
+    if (IdentifierStr == "if") return tok_if;
+    if (IdentifierStr == "then") return tok_then;
+    if (IdentifierStr == "else") return tok_else;
+    if (IdentifierStr == "for") return tok_for;
+    if (IdentifierStr == "in") return tok_in;
+    if (IdentifierStr == "binary") return tok_binary;
+    if (IdentifierStr == "unary") return tok_unary;
+    if (IdentifierStr == "var") return tok_var;
+    return tok_identifier;
+  }
+
+  if (isdigit(LastChar) || LastChar == '.') {   // Number: [0-9.]+
+    std::string NumStr;
+    do {
+      NumStr += LastChar;
+      LastChar = getchar();
+    } while (isdigit(LastChar) || LastChar == '.');
+
+    NumVal = strtod(NumStr.c_str(), 0);
+    return tok_number;
+  }
+
+  if (LastChar == '#') {
+    // Comment until end of line.
+    do LastChar = getchar();
+    while (LastChar != EOF &amp;&amp; LastChar != '\n' &amp;&amp; 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 &amp;name) : Name(name) {}
+  const std::string &amp;getName() const { return Name; }
+  virtual Value *Codegen();
+};
+
+/// UnaryExprAST - Expression class for a unary operator.
+class UnaryExprAST : public ExprAST {
+  char Opcode;
+  ExprAST *Operand;
+public:
+  UnaryExprAST(char opcode, ExprAST *operand) 
+    : Opcode(opcode), Operand(operand) {}
+  virtual Value *Codegen();
+};
+
+/// BinaryExprAST - Expression class for a binary operator.
+class BinaryExprAST : public ExprAST {
+  char Op;
+  ExprAST *LHS, *RHS;
+public:
+  BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs) 
+    : Op(op), LHS(lhs), RHS(rhs) {}
+  virtual Value *Codegen();
+};
+
+/// CallExprAST - Expression class for function calls.
+class CallExprAST : public ExprAST {
+  std::string Callee;
+  std::vector&lt;ExprAST*&gt; Args;
+public:
+  CallExprAST(const std::string &amp;callee, std::vector&lt;ExprAST*&gt; &amp;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 &amp;varname, ExprAST *start, ExprAST *end,
+             ExprAST *step, ExprAST *body)
+    : VarName(varname), Start(start), End(end), Step(step), Body(body) {}
+  virtual Value *Codegen();
+};
+
+/// VarExprAST - Expression class for var/in
+class VarExprAST : public ExprAST {
+  std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; VarNames;
+  ExprAST *Body;
+public:
+  VarExprAST(const std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; &amp;varnames,
+             ExprAST *body)
+  : VarNames(varnames), Body(body) {}
+  
+  virtual Value *Codegen();
+};
+
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its name, and its argument names (thus implicitly the number
+/// of arguments the function takes), as well as if it is an operator.
+class PrototypeAST {
+  std::string Name;
+  std::vector&lt;std::string&gt; Args;
+  bool isOperator;
+  unsigned Precedence;  // Precedence if a binary op.
+public:
+  PrototypeAST(const std::string &amp;name, const std::vector&lt;std::string&gt; &amp;args,
+               bool isoperator = false, unsigned prec = 0)
+  : Name(name), Args(args), isOperator(isoperator), Precedence(prec) {}
+  
+  bool isUnaryOp() const { return isOperator &amp;&amp; Args.size() == 1; }
+  bool isBinaryOp() const { return isOperator &amp;&amp; Args.size() == 2; }
+  
+  char getOperatorName() const {
+    assert(isUnaryOp() || isBinaryOp());
+    return Name[Name.size()-1];
+  }
+  
+  unsigned getBinaryPrecedence() const { return Precedence; }
+  
+  Function *Codegen();
+  
+  void CreateArgumentAllocas(Function *F);
+};
+
+/// FunctionAST - This class represents a function definition itself.
+class FunctionAST {
+  PrototypeAST *Proto;
+  ExprAST *Body;
+public:
+  FunctionAST(PrototypeAST *proto, ExprAST *body)
+    : Proto(proto), Body(body) {}
+  
+  Function *Codegen();
+};
+
+//===----------------------------------------------------------------------===//
+// Parser
+//===----------------------------------------------------------------------===//
+
+/// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
+/// token the parser is looking at.  getNextToken reads another token from the
+/// lexer and updates CurTok with its results.
+static int CurTok;
+static int getNextToken() {
+  return CurTok = gettok();
+}
+
+/// BinopPrecedence - This holds the precedence for each binary operator that is
+/// defined.
+static std::map&lt;char, int&gt; 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 &lt;= 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&lt;ExprAST*&gt; Args;
+  if (CurTok != ')') {
+    while (1) {
+      ExprAST *Arg = ParseExpression();
+      if (!Arg) return 0;
+      Args.push_back(Arg);
+
+      if (CurTok == ')') break;
+
+      if (CurTok != ',')
+        return Error("Expected ')' or ',' in argument list");
+      getNextToken();
+    }
+  }
+
+  // Eat the ')'.
+  getNextToken();
+  
+  return new CallExprAST(IdName, Args);
+}
+
+/// numberexpr ::= number
+static ExprAST *ParseNumberExpr() {
+  ExprAST *Result = new NumberExprAST(NumVal);
+  getNextToken(); // consume the number
+  return Result;
+}
+
+/// parenexpr ::= '(' expression ')'
+static ExprAST *ParseParenExpr() {
+  getNextToken();  // eat (.
+  ExprAST *V = ParseExpression();
+  if (!V) return 0;
+  
+  if (CurTok != ')')
+    return Error("expected ')'");
+  getNextToken();  // eat ).
+  return V;
+}
+
+/// ifexpr ::= 'if' expression 'then' expression 'else' expression
+static ExprAST *ParseIfExpr() {
+  getNextToken();  // eat the if.
+  
+  // condition.
+  ExprAST *Cond = ParseExpression();
+  if (!Cond) return 0;
+  
+  if (CurTok != tok_then)
+    return Error("expected then");
+  getNextToken();  // eat the then
+  
+  ExprAST *Then = ParseExpression();
+  if (Then == 0) return 0;
+  
+  if (CurTok != tok_else)
+    return Error("expected else");
+  
+  getNextToken();
+  
+  ExprAST *Else = ParseExpression();
+  if (!Else) return 0;
+  
+  return new IfExprAST(Cond, Then, Else);
+}
+
+/// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
+static ExprAST *ParseForExpr() {
+  getNextToken();  // eat the for.
+
+  if (CurTok != tok_identifier)
+    return Error("expected identifier after for");
+  
+  std::string IdName = IdentifierStr;
+  getNextToken();  // eat identifier.
+  
+  if (CurTok != '=')
+    return Error("expected '=' after for");
+  getNextToken();  // eat '='.
+  
+  
+  ExprAST *Start = ParseExpression();
+  if (Start == 0) return 0;
+  if (CurTok != ',')
+    return Error("expected ',' after for start value");
+  getNextToken();
+  
+  ExprAST *End = ParseExpression();
+  if (End == 0) return 0;
+  
+  // The step value is optional.
+  ExprAST *Step = 0;
+  if (CurTok == ',') {
+    getNextToken();
+    Step = ParseExpression();
+    if (Step == 0) return 0;
+  }
+  
+  if (CurTok != tok_in)
+    return Error("expected 'in' after for");
+  getNextToken();  // eat 'in'.
+  
+  ExprAST *Body = ParseExpression();
+  if (Body == 0) return 0;
+
+  return new ForExprAST(IdName, Start, End, Step, Body);
+}
+
+/// varexpr ::= 'var' identifier ('=' expression)? 
+//                    (',' identifier ('=' expression)?)* 'in' expression
+static ExprAST *ParseVarExpr() {
+  getNextToken();  // eat the var.
+
+  std::vector&lt;std::pair&lt;std::string, ExprAST*&gt; &gt; VarNames;
+
+  // At least one variable name is required.
+  if (CurTok != tok_identifier)
+    return Error("expected identifier after var");
+  
+  while (1) {
+    std::string Name = IdentifierStr;
+    getNextToken();  // eat identifier.
+
+    // Read the optional initializer.
+    ExprAST *Init = 0;
+    if (CurTok == '=') {
+      getNextToken(); // eat the '='.
+      
+      Init = ParseExpression();
+      if (Init == 0) return 0;
+    }
+    
+    VarNames.push_back(std::make_pair(Name, Init));
+    
+    // End of var list, exit loop.
+    if (CurTok != ',') break;
+    getNextToken(); // eat the ','.
+    
+    if (CurTok != tok_identifier)
+      return Error("expected identifier list after var");
+  }
+  
+  // At this point, we have to have 'in'.
+  if (CurTok != tok_in)
+    return Error("expected 'in' keyword after 'var'");
+  getNextToken();  // eat 'in'.
+  
+  ExprAST *Body = ParseExpression();
+  if (Body == 0) return 0;
+  
+  return new VarExprAST(VarNames, Body);
+}
+
+/// primary
+///   ::= identifierexpr
+///   ::= numberexpr
+///   ::= parenexpr
+///   ::= ifexpr
+///   ::= forexpr
+///   ::= varexpr
+static ExprAST *ParsePrimary() {
+  switch (CurTok) {
+  default: return Error("unknown token when expecting an expression");
+  case tok_identifier: return ParseIdentifierExpr();
+  case tok_number:     return ParseNumberExpr();
+  case '(':            return ParseParenExpr();
+  case tok_if:         return ParseIfExpr();
+  case tok_for:        return ParseForExpr();
+  case tok_var:        return ParseVarExpr();
+  }
+}
+
+/// unary
+///   ::= primary
+///   ::= '!' unary
+static ExprAST *ParseUnary() {
+  // If the current token is not an operator, it must be a primary expr.
+  if (!isascii(CurTok) || CurTok == '(' || CurTok == ',')
+    return ParsePrimary();
+  
+  // If this is a unary operator, read it.
+  int Opc = CurTok;
+  getNextToken();
+  if (ExprAST *Operand = ParseUnary())
+    return new UnaryExprAST(Opc, Operand);
+  return 0;
+}
+
+/// binoprhs
+///   ::= ('+' unary)*
+static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+  // If this is a binop, find its precedence.
+  while (1) {
+    int TokPrec = GetTokPrecedence();
+    
+    // If this is a binop that binds at least as tightly as the current binop,
+    // consume it, otherwise we are done.
+    if (TokPrec &lt; ExprPrec)
+      return LHS;
+    
+    // Okay, we know this is a binop.
+    int BinOp = CurTok;
+    getNextToken();  // eat binop
+    
+    // Parse the unary expression after the binary operator.
+    ExprAST *RHS = ParseUnary();
+    if (!RHS) return 0;
+    
+    // If BinOp binds less tightly with RHS than the operator after RHS, let
+    // the pending operator take RHS as its LHS.
+    int NextPrec = GetTokPrecedence();
+    if (TokPrec &lt; NextPrec) {
+      RHS = ParseBinOpRHS(TokPrec+1, RHS);
+      if (RHS == 0) return 0;
+    }
+    
+    // Merge LHS/RHS.
+    LHS = new BinaryExprAST(BinOp, LHS, RHS);
+  }
+}
+
+/// expression
+///   ::= unary binoprhs
+///
+static ExprAST *ParseExpression() {
+  ExprAST *LHS = ParseUnary();
+  if (!LHS) return 0;
+  
+  return ParseBinOpRHS(0, LHS);
+}
+
+/// prototype
+///   ::= id '(' id* ')'
+///   ::= binary LETTER number? (id, id)
+///   ::= unary LETTER (id)
+static PrototypeAST *ParsePrototype() {
+  std::string FnName;
+  
+  unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary.
+  unsigned BinaryPrecedence = 30;
+  
+  switch (CurTok) {
+  default:
+    return ErrorP("Expected function name in prototype");
+  case tok_identifier:
+    FnName = IdentifierStr;
+    Kind = 0;
+    getNextToken();
+    break;
+  case tok_unary:
+    getNextToken();
+    if (!isascii(CurTok))
+      return ErrorP("Expected unary operator");
+    FnName = "unary";
+    FnName += (char)CurTok;
+    Kind = 1;
+    getNextToken();
+    break;
+  case tok_binary:
+    getNextToken();
+    if (!isascii(CurTok))
+      return ErrorP("Expected binary operator");
+    FnName = "binary";
+    FnName += (char)CurTok;
+    Kind = 2;
+    getNextToken();
+    
+    // Read the precedence if present.
+    if (CurTok == tok_number) {
+      if (NumVal &lt; 1 || NumVal &gt; 100)
+        return ErrorP("Invalid precedecnce: must be 1..100");
+      BinaryPrecedence = (unsigned)NumVal;
+      getNextToken();
+    }
+    break;
+  }
+  
+  if (CurTok != '(')
+    return ErrorP("Expected '(' in prototype");
+  
+  std::vector&lt;std::string&gt; ArgNames;
+  while (getNextToken() == tok_identifier)
+    ArgNames.push_back(IdentifierStr);
+  if (CurTok != ')')
+    return ErrorP("Expected ')' in prototype");
+  
+  // success.
+  getNextToken();  // eat ')'.
+  
+  // Verify right number of names for operator.
+  if (Kind &amp;&amp; ArgNames.size() != Kind)
+    return ErrorP("Invalid number of operands for operator");
+  
+  return new PrototypeAST(FnName, ArgNames, Kind != 0, BinaryPrecedence);
+}
+
+/// definition ::= 'def' prototype expression
+static FunctionAST *ParseDefinition() {
+  getNextToken();  // eat def.
+  PrototypeAST *Proto = ParsePrototype();
+  if (Proto == 0) return 0;
+
+  if (ExprAST *E = ParseExpression())
+    return new FunctionAST(Proto, E);
+  return 0;
+}
+
+/// toplevelexpr ::= expression
+static FunctionAST *ParseTopLevelExpr() {
+  if (ExprAST *E = ParseExpression()) {
+    // Make an anonymous proto.
+    PrototypeAST *Proto = new PrototypeAST("", std::vector&lt;std::string&gt;());
+    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&lt;&gt; Builder(getGlobalContext());
+static std::map&lt;std::string, AllocaInst*&gt; NamedValues;
+static FunctionPassManager *TheFPM;
+
+Value *ErrorV(const char *Str) { Error(Str); return 0; }
+
+/// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of
+/// the function.  This is used for mutable variables etc.
+static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
+                                          const std::string &amp;VarName) {
+  IRBuilder&lt;&gt; TmpB(&amp;TheFunction-&gt;getEntryBlock(),
+                 TheFunction-&gt;getEntryBlock().begin());
+  return TmpB.CreateAlloca(Type::getDoubleTy(getGlobalContext()), 0,
+                           VarName.c_str());
+}
+
+Value *NumberExprAST::Codegen() {
+  return ConstantFP::get(getGlobalContext(), APFloat(Val));
+}
+
+Value *VariableExprAST::Codegen() {
+  // Look this variable up in the function.
+  Value *V = NamedValues[Name];
+  if (V == 0) return ErrorV("Unknown variable name");
+
+  // Load the value.
+  return Builder.CreateLoad(V, Name.c_str());
+}
+
+Value *UnaryExprAST::Codegen() {
+  Value *OperandV = Operand-&gt;Codegen();
+  if (OperandV == 0) return 0;
+  
+  Function *F = TheModule-&gt;getFunction(std::string("unary")+Opcode);
+  if (F == 0)
+    return ErrorV("Unknown unary operator");
+  
+  return Builder.CreateCall(F, OperandV, "unop");
+}
+
+Value *BinaryExprAST::Codegen() {
+  // Special case '=' because we don't want to emit the LHS as an expression.
+  if (Op == '=') {
+    // Assignment requires the LHS to be an identifier.
+    VariableExprAST *LHSE = dynamic_cast&lt;VariableExprAST*&gt;(LHS);
+    if (!LHSE)
+      return ErrorV("destination of '=' must be a variable");
+    // Codegen the RHS.
+    Value *Val = RHS-&gt;Codegen();
+    if (Val == 0) return 0;
+
+    // Look up the name.
+    Value *Variable = NamedValues[LHSE-&gt;getName()];
+    if (Variable == 0) return ErrorV("Unknown variable name");
+
+    Builder.CreateStore(Val, Variable);
+    return Val;
+  }
+  
+  Value *L = LHS-&gt;Codegen();
+  Value *R = RHS-&gt;Codegen();
+  if (L == 0 || R == 0) return 0;
+  
+  switch (Op) {
+  case '+': return Builder.CreateAdd(L, R, "addtmp");
+  case '-': return Builder.CreateSub(L, R, "subtmp");
+  case '*': return Builder.CreateMul(L, R, "multmp");
+  case '&lt;':
+    L = Builder.CreateFCmpULT(L, R, "cmptmp");
+    // Convert bool 0/1 to double 0.0 or 1.0
+    return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
+                                "booltmp");
+  default: break;
+  }
+  
+  // If it wasn't a builtin binary operator, it must be a user defined one. Emit
+  // a call to it.
+  Function *F = TheModule-&gt;getFunction(std::string("binary")+Op);
+  assert(F &amp;&amp; "binary operator not found!");
+  
+  Value *Ops[] = { L, R };
+  return Builder.CreateCall(F, Ops, Ops+2, "binop");
+}
+
+Value *CallExprAST::Codegen() {
+  // Look up the name in the global module table.
+  Function *CalleeF = TheModule-&gt;getFunction(Callee);
+  if (CalleeF == 0)
+    return ErrorV("Unknown function referenced");
+  
+  // If argument mismatch error.
+  if (CalleeF-&gt;arg_size() != Args.size())
+    return ErrorV("Incorrect # arguments passed");
+
+  std::vector&lt;Value*&gt; ArgsV;
+  for (unsigned i = 0, e = Args.size(); i != e; ++i) {
+    ArgsV.push_back(Args[i]-&gt;Codegen());
+    if (ArgsV.back() == 0) return 0;
+  }
+  
+  return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp");
+}
+
+Value *IfExprAST::Codegen() {
+  Value *CondV = Cond-&gt;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()-&gt;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-&gt;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-&gt;getBasicBlockList().push_back(ElseBB);
+  Builder.SetInsertPoint(ElseBB);
+  
+  Value *ElseV = Else-&gt;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-&gt;getBasicBlockList().push_back(MergeBB);
+  Builder.SetInsertPoint(MergeBB);
+  PHINode *PN = Builder.CreatePHI(Type::getDoubleTy(getGlobalContext()),
+                                  "iftmp");
+  
+  PN-&gt;addIncoming(ThenV, ThenBB);
+  PN-&gt;addIncoming(ElseV, ElseBB);
+  return PN;
+}
+
+Value *ForExprAST::Codegen() {
+  // Output this as:
+  //   var = alloca double
+  //   ...
+  //   start = startexpr
+  //   store start -&gt; var
+  //   goto loop
+  // loop: 
+  //   ...
+  //   bodyexpr
+  //   ...
+  // loopend:
+  //   step = stepexpr
+  //   endcond = endexpr
+  //
+  //   curvar = load var
+  //   nextvar = curvar + step
+  //   store nextvar -&gt; var
+  //   br endcond, loop, endloop
+  // outloop:
+  
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+
+  // Create an alloca for the variable in the entry block.
+  AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+  
+  // Emit the start code first, without 'variable' in scope.
+  Value *StartVal = Start-&gt;Codegen();
+  if (StartVal == 0) return 0;
+  
+  // Store the value into the alloca.
+  Builder.CreateStore(StartVal, Alloca);
+  
+  // Make the new basic block for the loop header, inserting after current
+  // block.
+  BasicBlock *LoopBB = BasicBlock::Create(getGlobalContext(), "loop", TheFunction);
+  
+  // Insert an explicit fall through from the current block to the LoopBB.
+  Builder.CreateBr(LoopBB);
+
+  // Start insertion in LoopBB.
+  Builder.SetInsertPoint(LoopBB);
+  
+  // Within the loop, the variable is defined equal to the PHI node.  If it
+  // shadows an existing variable, we have to restore it, so save it now.
+  AllocaInst *OldVal = NamedValues[VarName];
+  NamedValues[VarName] = Alloca;
+  
+  // Emit the body of the loop.  This, like any other expr, can change the
+  // current BB.  Note that we ignore the value computed by the body, but don't
+  // allow an error.
+  if (Body-&gt;Codegen() == 0)
+    return 0;
+  
+  // Emit the step value.
+  Value *StepVal;
+  if (Step) {
+    StepVal = Step-&gt;Codegen();
+    if (StepVal == 0) return 0;
+  } else {
+    // If not specified, use 1.0.
+    StepVal = ConstantFP::get(getGlobalContext(), APFloat(1.0));
+  }
+  
+  // Compute the end condition.
+  Value *EndCond = End-&gt;Codegen();
+  if (EndCond == 0) return EndCond;
+  
+  // Reload, increment, and restore the alloca.  This handles the case where
+  // the body of the loop mutates the variable.
+  Value *CurVar = Builder.CreateLoad(Alloca, VarName.c_str());
+  Value *NextVar = Builder.CreateAdd(CurVar, StepVal, "nextvar");
+  Builder.CreateStore(NextVar, Alloca);
+  
+  // Convert condition to a bool by comparing equal to 0.0.
+  EndCond = Builder.CreateFCmpONE(EndCond, 
+                              ConstantFP::get(getGlobalContext(), APFloat(0.0)),
+                                  "loopcond");
+  
+  // Create the "after loop" block and insert it.
+  BasicBlock *AfterBB = BasicBlock::Create(getGlobalContext(), "afterloop", TheFunction);
+  
+  // Insert the conditional branch into the end of LoopEndBB.
+  Builder.CreateCondBr(EndCond, LoopBB, AfterBB);
+  
+  // Any new code will be inserted in AfterBB.
+  Builder.SetInsertPoint(AfterBB);
+  
+  // Restore the unshadowed variable.
+  if (OldVal)
+    NamedValues[VarName] = OldVal;
+  else
+    NamedValues.erase(VarName);
+
+  
+  // for expr always returns 0.0.
+  return Constant::getNullValue(Type::getDoubleTy(getGlobalContext()));
+}
+
+Value *VarExprAST::Codegen() {
+  std::vector&lt;AllocaInst *&gt; OldBindings;
+  
+  Function *TheFunction = Builder.GetInsertBlock()-&gt;getParent();
+
+  // Register all variables and emit their initializer.
+  for (unsigned i = 0, e = VarNames.size(); i != e; ++i) {
+    const std::string &amp;VarName = VarNames[i].first;
+    ExprAST *Init = VarNames[i].second;
+    
+    // Emit the initializer before adding the variable to scope, this prevents
+    // the initializer from referencing the variable itself, and permits stuff
+    // like this:
+    //  var a = 1 in
+    //    var a = a in ...   # refers to outer 'a'.
+    Value *InitVal;
+    if (Init) {
+      InitVal = Init-&gt;Codegen();
+      if (InitVal == 0) return 0;
+    } else { // If not specified, use 0.0.
+      InitVal = ConstantFP::get(getGlobalContext(), APFloat(0.0));
+    }
+    
+    AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+    Builder.CreateStore(InitVal, Alloca);
+
+    // Remember the old variable binding so that we can restore the binding when
+    // we unrecurse.
+    OldBindings.push_back(NamedValues[VarName]);
+    
+    // Remember this binding.
+    NamedValues[VarName] = Alloca;
+  }
+  
+  // Codegen the body, now that all vars are in scope.
+  Value *BodyVal = Body-&gt;Codegen();
+  if (BodyVal == 0) return 0;
+  
+  // Pop all our variables from scope.
+  for (unsigned i = 0, e = VarNames.size(); i != e; ++i)
+    NamedValues[VarNames[i].first] = OldBindings[i];
+
+  // Return the body computation.
+  return BodyVal;
+}
+
+Function *PrototypeAST::Codegen() {
+  // Make the function type:  double(double,double) etc.
+  std::vector&lt;const Type*&gt; 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-&gt;getName() != Name) {
+    // Delete the one we just made and get the existing one.
+    F-&gt;eraseFromParent();
+    F = TheModule-&gt;getFunction(Name);
+    
+    // If F already has a body, reject this.
+    if (!F-&gt;empty()) {
+      ErrorF("redefinition of function");
+      return 0;
+    }
+    
+    // If F took a different number of args, reject.
+    if (F-&gt;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-&gt;arg_begin(); Idx != Args.size();
+       ++AI, ++Idx)
+    AI-&gt;setName(Args[Idx]);
+    
+  return F;
+}
+
+/// CreateArgumentAllocas - Create an alloca for each argument and register the
+/// argument in the symbol table so that references to it will succeed.
+void PrototypeAST::CreateArgumentAllocas(Function *F) {
+  Function::arg_iterator AI = F-&gt;arg_begin();
+  for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) {
+    // Create an alloca for this variable.
+    AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]);
+
+    // Store the initial value into the alloca.
+    Builder.CreateStore(AI, Alloca);
+
+    // Add arguments to variable symbol table.
+    NamedValues[Args[Idx]] = Alloca;
+  }
+}
+
+Function *FunctionAST::Codegen() {
+  NamedValues.clear();
+  
+  Function *TheFunction = Proto-&gt;Codegen();
+  if (TheFunction == 0)
+    return 0;
+  
+  // If this is an operator, install it.
+  if (Proto-&gt;isBinaryOp())
+    BinopPrecedence[Proto-&gt;getOperatorName()] = Proto-&gt;getBinaryPrecedence();
+  
+  // Create a new basic block to start insertion into.
+  BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
+  Builder.SetInsertPoint(BB);
+  
+  // Add all arguments to the symbol table and create their allocas.
+  Proto-&gt;CreateArgumentAllocas(TheFunction);
+
+  if (Value *RetVal = Body-&gt;Codegen()) {
+    // Finish off the function.
+    Builder.CreateRet(RetVal);
+
+    // Validate the generated code, checking for consistency.
+    verifyFunction(*TheFunction);
+
+    // Optimize the function.
+    TheFPM-&gt;run(*TheFunction);
+    
+    return TheFunction;
+  }
+  
+  // Error reading body, remove function.
+  TheFunction-&gt;eraseFromParent();
+
+  if (Proto-&gt;isBinaryOp())
+    BinopPrecedence.erase(Proto-&gt;getOperatorName());
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Top-Level parsing and JIT Driver
+//===----------------------------------------------------------------------===//
+
+static ExecutionEngine *TheExecutionEngine;
+
+static void HandleDefinition() {
+  if (FunctionAST *F = ParseDefinition()) {
+    if (Function *LF = F-&gt;Codegen()) {
+      fprintf(stderr, "Read function definition:");
+      LF-&gt;dump();
+    }
+  } else {
+    // Skip token for error recovery.
+    getNextToken();
+  }
+}
+
+static void HandleExtern() {
+  if (PrototypeAST *P = ParseExtern()) {
+    if (Function *F = P-&gt;Codegen()) {
+      fprintf(stderr, "Read extern: ");
+      F-&gt;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-&gt;Codegen()) {
+      // JIT the function, returning a function pointer.
+      void *FPtr = TheExecutionEngine-&gt;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&gt; ");
+    switch (CurTok) {
+    case tok_eof:    return;
+    case ';':        getNextToken(); break;  // ignore top-level semicolons.
+    case tok_def:    HandleDefinition(); break;
+    case tok_extern: HandleExtern(); break;
+    default:         HandleTopLevelExpression(); break;
+    }
+  }
+}
+
+//===----------------------------------------------------------------------===//
+// "Library" functions that can be "extern'd" from user code.
+//===----------------------------------------------------------------------===//
+
+/// putchard - putchar that takes a double and returns 0.
+extern "C" 
+double putchard(double X) {
+  putchar((char)X);
+  return 0;
+}
+
+/// printd - printf that takes a double prints it as "%f\n", returning 0.
+extern "C" 
+double printd(double X) {
+  printf("%f\n", X);
+  return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Main driver code.
+//===----------------------------------------------------------------------===//
+
+int main() {
+  InitializeNativeTarget();
+  LLVMContext &amp;Context = getGlobalContext();
+
+  // Install standard binary operators.
+  // 1 is lowest precedence.
+  BinopPrecedence['='] = 2;
+  BinopPrecedence['&lt;'] = 10;
+  BinopPrecedence['+'] = 20;
+  BinopPrecedence['-'] = 20;
+  BinopPrecedence['*'] = 40;  // highest.
+
+  // Prime the first token.
+  fprintf(stderr, "ready&gt; ");
+  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(&amp;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 TargetData(*TheExecutionEngine-&gt;getTargetData()));
+  // Promote allocas to registers.
+  OurFPM.add(createPromoteMemoryToRegisterPass());
+  // Do simple "peephole" optimizations and bit-twiddling optzns.
+  OurFPM.add(createInstructionCombiningPass());
+  // Reassociate expressions.
+  OurFPM.add(createReassociatePass());
+  // Eliminate Common SubExpressions.
+  OurFPM.add(createGVNPass());
+  // Simplify the control flow graph (deleting unreachable blocks, etc).
+  OurFPM.add(createCFGSimplificationPass());
+
+  OurFPM.doInitialization();
+
+  // Set the global so the code gen can use this.
+  TheFPM = &amp;OurFPM;
+
+  // Run the main "interpreter loop" now.
+  MainLoop();
+
+  TheFPM = 0;
+
+  // Print out all of the generated code.
+  TheModule-&gt;dump();
+
+  return 0;
+}
+</pre>
+</div>
+
+<a href="LangImpl8.html">Next: Conclusion and other useful LLVM tidbits</a>
+</div>
+
+<!-- *********************************************************************** -->
+<hr>
+<address>
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+
+  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
+  <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
+  Last modified: $Date$
+</address>
+</body>
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author	mike-m <mikem.llvm@gmail.com>	2010-05-07 00:28:04 +0000
committer	mike-m <mikem.llvm@gmail.com>	2010-05-07 00:28:04 +0000
commit	e2c3a49c8029ebd9ef530101cc24c66562e3dff5 (patch)
tree	91bf9600cc8df90cf99751a8f8bafc317cffc91e /docs/tutorial/LangImpl7.html
parent	c10b5afbe8138b0fdf3af4ed3e1ddf96cf3cb4cb (diff)
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