<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">Part 7 Introduction</a></div>
+<div class="doc_section"><a name="intro">Chapter 7 Introduction</a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
-<p>Welcome to Part 7 of the "<a href="index.html">Implementing a language with
-LLVM</a>" tutorial. In parts 1 through 6, we've built a very respectable,
-albeit simple, <a
+<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 and JIT compile it.</p>
+the resultant code as well as JIT compile it.</p>
-<p>While Kaleidoscope is interesting as a functional language, this 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
+<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
(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_tree, it 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 phi nodes when lowering
+<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,
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 does try to version memory objects. In LLVM, instead of
+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>
<p>In LLVM, all memory accesses are explicit with load/store instructions, and
-it is carefully designed to not have (or need) an "address-of" operator. Notice
+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, but instead of being
-declared with global variable definitions, they are declared with 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 @test(i1 %Condition) {
+define i32 @example() {
entry:
%X = alloca i32 ; type of %X is i32*.
...
</pre>
</div>
-<p>The mem2reg pass implements the standard "iterated dominator frontier"
+<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. mem2reg really is the answer for dealing with
-mutable variables, and we highly recommend that you depend on it. Note that
+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>
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 scalar values, and only if the array size
+<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>
<p>
All of these properties are easy to satisfy for most imperative languages, and
-we'll illustrated this below with Kaleidoscope. The final question you may be
+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 use you this technique
+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>
<li>Needed for debug info generation: <a href="../SourceLevelDebugging.html">
Debug information in LLVM</a> relies on having the address of the variable
-exposed to attach debug info to it. This technique dovetails very naturally
-with this style of debug info.</li>
+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 < 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 < 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<std::string, AllocaInst*> 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 &VarName) {
+ IRBuilder<> TmpB(&TheFunction->getEntryBlock(),
+ TheFunction->getEntryBlock().begin());
+ return TmpB.CreateAlloca(Type::DoubleTy, 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->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->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->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->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 = sub double %x3, 1.000000e+00
+ %calltmp = call double @fib( double %subtmp )
+ <b>%x4 = load double* %x1</b>
+ %subtmp5 = sub double %x4, 2.000000e+00
+ %calltmp6 = call double @fib( double %subtmp5 )
+ %addtmp = add 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 = sub double <b>%x</b>, 1.000000e+00
+ %calltmp = call double @fib( double %subtmp )
+ %subtmp5 = sub double <b>%x</b>, 2.000000e+00
+ %calltmp6 = call double @fib( double %subtmp5 )
+ %addtmp = add 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 = sub double %x, 1.000000e+00
+ %calltmp = call double @fib( double %subtmp )
+ %subtmp5 = sub double %x, 2.000000e+00
+ %calltmp6 = call double @fib( double %subtmp5 )
+ %addtmp = add 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['<'] = 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<VariableExprAST*>(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->Codegen();
+ if (Val == 0) return 0;
+
+ // Look up the name.
+ Value *Variable = NamedValues[LHSE->getName()];
+ if (Variable == 0) return ErrorV("Unknown variable name");
+
+ Builder.CreateStore(Val, Variable);
+ return Val;
+ }
+ ...
+</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<std::pair<std::string, ExprAST*> > VarNames;
+ ExprAST *Body;
+public:
+ VarExprAST(const std::vector<std::pair<std::string, ExprAST*> > &varnames,
+ ExprAST *body)
+ : VarNames(varnames), Body(body) {}
+
+ virtual Value *Codegen();
+};
+</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<std::pair<std::string, ExprAST*> > 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<AllocaInst *> OldBindings;
+
+ Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+ // Register all variables and emit their initializer.
+ for (unsigned i = 0, e = VarNames.size(); i != e; ++i) {
+ const std::string &VarName = VarNames[i].first;
+ ExprAST *Init = VarNames[i].second;
+</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->Codegen();
+ if (InitVal == 0) return 0;
+ } else { // If not specified, use 0.0.
+ InitVal = ConstantFP::get(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->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 the
-if/then/else and for expressions.. To build this example, use:
+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">
<div class="doc_code">
<pre>
+#include "llvm/DerivedTypes.h"
+#include "llvm/ExecutionEngine/ExecutionEngine.h"
+#include "llvm/Module.h"
+#include "llvm/ModuleProvider.h"
+#include "llvm/PassManager.h"
+#include "llvm/Analysis/Verifier.h"
+#include "llvm/Target/TargetData.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Support/IRBuilder.h"
+#include <cstdio>
+#include <string>
+#include <map>
+#include <vector>
+using namespace llvm;
+
+//===----------------------------------------------------------------------===//
+// Lexer
+//===----------------------------------------------------------------------===//
+
+// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
+// of these for known things.
+enum Token {
+ tok_eof = -1,
+
+ // commands
+ tok_def = -2, tok_extern = -3,
+
+ // primary
+ tok_identifier = -4, tok_number = -5,
+
+ // control
+ tok_if = -6, tok_then = -7, tok_else = -8,
+ tok_for = -9, tok_in = -10,
+
+ // operators
+ tok_binary = -11, tok_unary = -12,
+
+ // var definition
+ tok_var = -13
+};
+
+static std::string IdentifierStr; // Filled in if tok_identifier
+static double NumVal; // Filled in if tok_number
+
+/// gettok - Return the next token from standard input.
+static int gettok() {
+ static int LastChar = ' ';
+
+ // Skip any whitespace.
+ while (isspace(LastChar))
+ LastChar = getchar();
+
+ if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
+ IdentifierStr = LastChar;
+ while (isalnum((LastChar = getchar())))
+ IdentifierStr += LastChar;
+
+ if (IdentifierStr == "def") return tok_def;
+ if (IdentifierStr == "extern") return tok_extern;
+ if (IdentifierStr == "if") return tok_if;
+ if (IdentifierStr == "then") return tok_then;
+ if (IdentifierStr == "else") return tok_else;
+ if (IdentifierStr == "for") return tok_for;
+ if (IdentifierStr == "in") return tok_in;
+ if (IdentifierStr == "binary") return tok_binary;
+ if (IdentifierStr == "unary") return tok_unary;
+ if (IdentifierStr == "var") return tok_var;
+ return tok_identifier;
+ }
+
+ if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
+ std::string NumStr;
+ do {
+ NumStr += LastChar;
+ LastChar = getchar();
+ } while (isdigit(LastChar) || LastChar == '.');
+
+ NumVal = strtod(NumStr.c_str(), 0);
+ return tok_number;
+ }
+
+ if (LastChar == '#') {
+ // Comment until end of line.
+ do LastChar = getchar();
+ while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
+
+ if (LastChar != EOF)
+ return gettok();
+ }
+
+ // Check for end of file. Don't eat the EOF.
+ if (LastChar == EOF)
+ return tok_eof;
+
+ // Otherwise, just return the character as its ascii value.
+ int ThisChar = LastChar;
+ LastChar = getchar();
+ return ThisChar;
+}
+
+//===----------------------------------------------------------------------===//
+// Abstract Syntax Tree (aka Parse Tree)
+//===----------------------------------------------------------------------===//
+
+/// ExprAST - Base class for all expression nodes.
+class ExprAST {
+public:
+ virtual ~ExprAST() {}
+ virtual Value *Codegen() = 0;
+};
+
+/// NumberExprAST - Expression class for numeric literals like "1.0".
+class NumberExprAST : public ExprAST {
+ double Val;
+public:
+ NumberExprAST(double val) : Val(val) {}
+ virtual Value *Codegen();
+};
+
+/// VariableExprAST - Expression class for referencing a variable, like "a".
+class VariableExprAST : public ExprAST {
+ std::string Name;
+public:
+ VariableExprAST(const std::string &name) : Name(name) {}
+ const std::string &getName() const { return Name; }
+ virtual Value *Codegen();
+};
+
+/// UnaryExprAST - Expression class for a unary operator.
+class UnaryExprAST : public ExprAST {
+ char Opcode;
+ ExprAST *Operand;
+public:
+ UnaryExprAST(char opcode, ExprAST *operand)
+ : Opcode(opcode), Operand(operand) {}
+ virtual Value *Codegen();
+};
+
+/// BinaryExprAST - Expression class for a binary operator.
+class BinaryExprAST : public ExprAST {
+ char Op;
+ ExprAST *LHS, *RHS;
+public:
+ BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs)
+ : Op(op), LHS(lhs), RHS(rhs) {}
+ virtual Value *Codegen();
+};
+
+/// CallExprAST - Expression class for function calls.
+class CallExprAST : public ExprAST {
+ std::string Callee;
+ std::vector<ExprAST*> Args;
+public:
+ CallExprAST(const std::string &callee, std::vector<ExprAST*> &args)
+ : Callee(callee), Args(args) {}
+ virtual Value *Codegen();
+};
+
+/// IfExprAST - Expression class for if/then/else.
+class IfExprAST : public ExprAST {
+ ExprAST *Cond, *Then, *Else;
+public:
+ IfExprAST(ExprAST *cond, ExprAST *then, ExprAST *_else)
+ : Cond(cond), Then(then), Else(_else) {}
+ virtual Value *Codegen();
+};
+
+/// ForExprAST - Expression class for for/in.
+class ForExprAST : public ExprAST {
+ std::string VarName;
+ ExprAST *Start, *End, *Step, *Body;
+public:
+ ForExprAST(const std::string &varname, ExprAST *start, ExprAST *end,
+ ExprAST *step, ExprAST *body)
+ : VarName(varname), Start(start), End(end), Step(step), Body(body) {}
+ virtual Value *Codegen();
+};
+
+/// VarExprAST - Expression class for var/in
+class VarExprAST : public ExprAST {
+ std::vector<std::pair<std::string, ExprAST*> > VarNames;
+ ExprAST *Body;
+public:
+ VarExprAST(const std::vector<std::pair<std::string, ExprAST*> > &varnames,
+ ExprAST *body)
+ : VarNames(varnames), Body(body) {}
+
+ virtual Value *Codegen();
+};
+
+/// PrototypeAST - This class represents the "prototype" for a function,
+/// which captures its argument names as well as if it is an operator.
+class PrototypeAST {
+ std::string Name;
+ std::vector<std::string> Args;
+ bool isOperator;
+ unsigned Precedence; // Precedence if a binary op.
+public:
+ PrototypeAST(const std::string &name, const std::vector<std::string> &args,
+ bool isoperator = false, unsigned prec = 0)
+ : Name(name), Args(args), isOperator(isoperator), Precedence(prec) {}
+
+ bool isUnaryOp() const { return isOperator && Args.size() == 1; }
+ bool isBinaryOp() const { return isOperator && Args.size() == 2; }
+
+ char getOperatorName() const {
+ assert(isUnaryOp() || isBinaryOp());
+ return Name[Name.size()-1];
+ }
+
+ unsigned getBinaryPrecedence() const { return Precedence; }
+
+ Function *Codegen();
+
+ void CreateArgumentAllocas(Function *F);
+};
+
+/// FunctionAST - This class represents a function definition itself.
+class FunctionAST {
+ PrototypeAST *Proto;
+ ExprAST *Body;
+public:
+ FunctionAST(PrototypeAST *proto, ExprAST *body)
+ : Proto(proto), Body(body) {}
+
+ Function *Codegen();
+};
+
+//===----------------------------------------------------------------------===//
+// Parser
+//===----------------------------------------------------------------------===//
+
+/// CurTok/getNextToken - Provide a simple token buffer. CurTok is the current
+/// token the parser it looking at. getNextToken reads another token from the
+/// lexer and updates CurTok with its results.
+static int CurTok;
+static int getNextToken() {
+ return CurTok = gettok();
+}
+
+/// BinopPrecedence - This holds the precedence for each binary operator that is
+/// defined.
+static std::map<char, int> BinopPrecedence;
+
+/// GetTokPrecedence - Get the precedence of the pending binary operator token.
+static int GetTokPrecedence() {
+ if (!isascii(CurTok))
+ return -1;
+
+ // Make sure it's a declared binop.
+ int TokPrec = BinopPrecedence[CurTok];
+ if (TokPrec <= 0) return -1;
+ return TokPrec;
+}
+
+/// Error* - These are little helper functions for error handling.
+ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
+PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
+FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
+
+static ExprAST *ParseExpression();
+
+/// identifierexpr
+/// ::= identifier
+/// ::= identifier '(' expression* ')'
+static ExprAST *ParseIdentifierExpr() {
+ std::string IdName = IdentifierStr;
+
+ getNextToken(); // eat identifier.
+
+ if (CurTok != '(') // Simple variable ref.
+ return new VariableExprAST(IdName);
+
+ // Call.
+ getNextToken(); // eat (
+ std::vector<ExprAST*> Args;
+ if (CurTok != ')') {
+ while (1) {
+ ExprAST *Arg = ParseExpression();
+ if (!Arg) return 0;
+ Args.push_back(Arg);
+
+ if (CurTok == ')') break;
+
+ if (CurTok != ',')
+ return Error("Expected ')' or ',' in argument list");
+ getNextToken();
+ }
+ }
+
+ // Eat the ')'.
+ getNextToken();
+
+ return new CallExprAST(IdName, Args);
+}
+
+/// numberexpr ::= number
+static ExprAST *ParseNumberExpr() {
+ ExprAST *Result = new NumberExprAST(NumVal);
+ getNextToken(); // consume the number
+ return Result;
+}
+
+/// parenexpr ::= '(' expression ')'
+static ExprAST *ParseParenExpr() {
+ getNextToken(); // eat (.
+ ExprAST *V = ParseExpression();
+ if (!V) return 0;
+
+ if (CurTok != ')')
+ return Error("expected ')'");
+ getNextToken(); // eat ).
+ return V;
+}
+
+/// ifexpr ::= 'if' expression 'then' expression 'else' expression
+static ExprAST *ParseIfExpr() {
+ getNextToken(); // eat the if.
+
+ // condition.
+ ExprAST *Cond = ParseExpression();
+ if (!Cond) return 0;
+
+ if (CurTok != tok_then)
+ return Error("expected then");
+ getNextToken(); // eat the then
+
+ ExprAST *Then = ParseExpression();
+ if (Then == 0) return 0;
+
+ if (CurTok != tok_else)
+ return Error("expected else");
+
+ getNextToken();
+
+ ExprAST *Else = ParseExpression();
+ if (!Else) return 0;
+
+ return new IfExprAST(Cond, Then, Else);
+}
+
+/// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
+static ExprAST *ParseForExpr() {
+ getNextToken(); // eat the for.
+
+ if (CurTok != tok_identifier)
+ return Error("expected identifier after for");
+
+ std::string IdName = IdentifierStr;
+ getNextToken(); // eat identifier.
+
+ if (CurTok != '=')
+ return Error("expected '=' after for");
+ getNextToken(); // eat '='.
+
+
+ ExprAST *Start = ParseExpression();
+ if (Start == 0) return 0;
+ if (CurTok != ',')
+ return Error("expected ',' after for start value");
+ getNextToken();
+
+ ExprAST *End = ParseExpression();
+ if (End == 0) return 0;
+
+ // The step value is optional.
+ ExprAST *Step = 0;
+ if (CurTok == ',') {
+ getNextToken();
+ Step = ParseExpression();
+ if (Step == 0) return 0;
+ }
+
+ if (CurTok != tok_in)
+ return Error("expected 'in' after for");
+ getNextToken(); // eat 'in'.
+
+ ExprAST *Body = ParseExpression();
+ if (Body == 0) return 0;
+
+ return new ForExprAST(IdName, Start, End, Step, Body);
+}
+
+/// varexpr ::= 'var' identifier ('=' expression)?
+// (',' identifier ('=' expression)?)* 'in' expression
+static ExprAST *ParseVarExpr() {
+ getNextToken(); // eat the var.
+
+ std::vector<std::pair<std::string, ExprAST*> > VarNames;
+
+ // At least one variable name is required.
+ if (CurTok != tok_identifier)
+ return Error("expected identifier after var");
+
+ while (1) {
+ std::string Name = IdentifierStr;
+ getNextToken(); // eat identifier.
+
+ // Read the optional initializer.
+ ExprAST *Init = 0;
+ if (CurTok == '=') {
+ getNextToken(); // eat the '='.
+
+ Init = ParseExpression();
+ if (Init == 0) return 0;
+ }
+
+ VarNames.push_back(std::make_pair(Name, Init));
+
+ // End of var list, exit loop.
+ if (CurTok != ',') break;
+ getNextToken(); // eat the ','.
+
+ if (CurTok != tok_identifier)
+ return Error("expected identifier list after var");
+ }
+
+ // At this point, we have to have 'in'.
+ if (CurTok != tok_in)
+ return Error("expected 'in' keyword after 'var'");
+ getNextToken(); // eat 'in'.
+
+ ExprAST *Body = ParseExpression();
+ if (Body == 0) return 0;
+
+ return new VarExprAST(VarNames, Body);
+}
+
+
+/// primary
+/// ::= identifierexpr
+/// ::= numberexpr
+/// ::= parenexpr
+/// ::= ifexpr
+/// ::= forexpr
+/// ::= varexpr
+static ExprAST *ParsePrimary() {
+ switch (CurTok) {
+ default: return Error("unknown token when expecting an expression");
+ case tok_identifier: return ParseIdentifierExpr();
+ case tok_number: return ParseNumberExpr();
+ case '(': return ParseParenExpr();
+ case tok_if: return ParseIfExpr();
+ case tok_for: return ParseForExpr();
+ case tok_var: return ParseVarExpr();
+ }
+}
+
+/// unary
+/// ::= primary
+/// ::= '!' unary
+static ExprAST *ParseUnary() {
+ // If the current token is not an operator, it must be a primary expr.
+ if (!isascii(CurTok) || CurTok == '(' || CurTok == ',')
+ return ParsePrimary();
+
+ // If this is a unary operator, read it.
+ int Opc = CurTok;
+ getNextToken();
+ if (ExprAST *Operand = ParseUnary())
+ return new UnaryExprAST(Opc, Operand);
+ return 0;
+}
+
+/// binoprhs
+/// ::= ('+' unary)*
+static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
+ // If this is a binop, find its precedence.
+ while (1) {
+ int TokPrec = GetTokPrecedence();
+
+ // If this is a binop that binds at least as tightly as the current binop,
+ // consume it, otherwise we are done.
+ if (TokPrec < ExprPrec)
+ return LHS;
+
+ // Okay, we know this is a binop.
+ int BinOp = CurTok;
+ getNextToken(); // eat binop
+
+ // Parse the unary expression after the binary operator.
+ ExprAST *RHS = ParseUnary();
+ if (!RHS) return 0;
+
+ // If BinOp binds less tightly with RHS than the operator after RHS, let
+ // the pending operator take RHS as its LHS.
+ int NextPrec = GetTokPrecedence();
+ if (TokPrec < NextPrec) {
+ RHS = ParseBinOpRHS(TokPrec+1, RHS);
+ if (RHS == 0) return 0;
+ }
+
+ // Merge LHS/RHS.
+ LHS = new BinaryExprAST(BinOp, LHS, RHS);
+ }
+}
+
+/// expression
+/// ::= unary binoprhs
+///
+static ExprAST *ParseExpression() {
+ ExprAST *LHS = ParseUnary();
+ if (!LHS) return 0;
+
+ return ParseBinOpRHS(0, LHS);
+}
+
+/// prototype
+/// ::= id '(' id* ')'
+/// ::= binary LETTER number? (id, id)
+/// ::= unary LETTER (id)
+static PrototypeAST *ParsePrototype() {
+ std::string FnName;
+
+ int Kind = 0; // 0 = identifier, 1 = unary, 2 = binary.
+ unsigned BinaryPrecedence = 30;
+
+ switch (CurTok) {
+ default:
+ return ErrorP("Expected function name in prototype");
+ case tok_identifier:
+ FnName = IdentifierStr;
+ Kind = 0;
+ getNextToken();
+ break;
+ case tok_unary:
+ getNextToken();
+ if (!isascii(CurTok))
+ return ErrorP("Expected unary operator");
+ FnName = "unary";
+ FnName += (char)CurTok;
+ Kind = 1;
+ getNextToken();
+ break;
+ case tok_binary:
+ getNextToken();
+ if (!isascii(CurTok))
+ return ErrorP("Expected binary operator");
+ FnName = "binary";
+ FnName += (char)CurTok;
+ Kind = 2;
+ getNextToken();
+
+ // Read the precedence if present.
+ if (CurTok == tok_number) {
+ if (NumVal < 1 || NumVal > 100)
+ return ErrorP("Invalid precedecnce: must be 1..100");
+ BinaryPrecedence = (unsigned)NumVal;
+ getNextToken();
+ }
+ break;
+ }
+
+ if (CurTok != '(')
+ return ErrorP("Expected '(' in prototype");
+
+ std::vector<std::string> ArgNames;
+ while (getNextToken() == tok_identifier)
+ ArgNames.push_back(IdentifierStr);
+ if (CurTok != ')')
+ return ErrorP("Expected ')' in prototype");
+
+ // success.
+ getNextToken(); // eat ')'.
+
+ // Verify right number of names for operator.
+ if (Kind && ArgNames.size() != Kind)
+ return ErrorP("Invalid number of operands for operator");
+
+ return new PrototypeAST(FnName, ArgNames, Kind != 0, BinaryPrecedence);
+}
+
+/// definition ::= 'def' prototype expression
+static FunctionAST *ParseDefinition() {
+ getNextToken(); // eat def.
+ PrototypeAST *Proto = ParsePrototype();
+ if (Proto == 0) return 0;
+
+ if (ExprAST *E = ParseExpression())
+ return new FunctionAST(Proto, E);
+ return 0;
+}
+
+/// toplevelexpr ::= expression
+static FunctionAST *ParseTopLevelExpr() {
+ if (ExprAST *E = ParseExpression()) {
+ // Make an anonymous proto.
+ PrototypeAST *Proto = new PrototypeAST("", std::vector<std::string>());
+ return new FunctionAST(Proto, E);
+ }
+ return 0;
+}
+
+/// external ::= 'extern' prototype
+static PrototypeAST *ParseExtern() {
+ getNextToken(); // eat extern.
+ return ParsePrototype();
+}
+
+//===----------------------------------------------------------------------===//
+// Code Generation
+//===----------------------------------------------------------------------===//
+
+static Module *TheModule;
+static IRBuilder<> Builder;
+static std::map<std::string, AllocaInst*> NamedValues;
+static FunctionPassManager *TheFPM;
+
+Value *ErrorV(const char *Str) { Error(Str); return 0; }
+
+/// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of
+/// the function. This is used for mutable variables etc.
+static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
+ const std::string &VarName) {
+ IRBuilder<> TmpB(&TheFunction->getEntryBlock(),
+ TheFunction->getEntryBlock().begin());
+ return TmpB.CreateAlloca(Type::DoubleTy, 0, VarName.c_str());
+}
+
+
+Value *NumberExprAST::Codegen() {
+ return ConstantFP::get(APFloat(Val));
+}
+
+Value *VariableExprAST::Codegen() {
+ // Look this variable up in the function.
+ Value *V = NamedValues[Name];
+ if (V == 0) return ErrorV("Unknown variable name");
+
+ // Load the value.
+ return Builder.CreateLoad(V, Name.c_str());
+}
+
+Value *UnaryExprAST::Codegen() {
+ Value *OperandV = Operand->Codegen();
+ if (OperandV == 0) return 0;
+
+ Function *F = TheModule->getFunction(std::string("unary")+Opcode);
+ if (F == 0)
+ return ErrorV("Unknown unary operator");
+
+ return Builder.CreateCall(F, OperandV, "unop");
+}
+
+
+Value *BinaryExprAST::Codegen() {
+ // Special case '=' because we don't want to emit the LHS as an expression.
+ if (Op == '=') {
+ // Assignment requires the LHS to be an identifier.
+ VariableExprAST *LHSE = dynamic_cast<VariableExprAST*>(LHS);
+ if (!LHSE)
+ return ErrorV("destination of '=' must be a variable");
+ // Codegen the RHS.
+ Value *Val = RHS->Codegen();
+ if (Val == 0) return 0;
+
+ // Look up the name.
+ Value *Variable = NamedValues[LHSE->getName()];
+ if (Variable == 0) return ErrorV("Unknown variable name");
+
+ Builder.CreateStore(Val, Variable);
+ return Val;
+ }
+
+
+ Value *L = LHS->Codegen();
+ Value *R = RHS->Codegen();
+ if (L == 0 || R == 0) return 0;
+
+ switch (Op) {
+ case '+': return Builder.CreateAdd(L, R, "addtmp");
+ case '-': return Builder.CreateSub(L, R, "subtmp");
+ case '*': return Builder.CreateMul(L, R, "multmp");
+ case '<':
+ L = Builder.CreateFCmpULT(L, R, "cmptmp");
+ // Convert bool 0/1 to double 0.0 or 1.0
+ return Builder.CreateUIToFP(L, Type::DoubleTy, "booltmp");
+ default: break;
+ }
+
+ // If it wasn't a builtin binary operator, it must be a user defined one. Emit
+ // a call to it.
+ Function *F = TheModule->getFunction(std::string("binary")+Op);
+ assert(F && "binary operator not found!");
+
+ Value *Ops[] = { 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->getFunction(Callee);
+ if (CalleeF == 0)
+ return ErrorV("Unknown function referenced");
+
+ // If argument mismatch error.
+ if (CalleeF->arg_size() != Args.size())
+ return ErrorV("Incorrect # arguments passed");
+
+ std::vector<Value*> ArgsV;
+ for (unsigned i = 0, e = Args.size(); i != e; ++i) {
+ ArgsV.push_back(Args[i]->Codegen());
+ if (ArgsV.back() == 0) return 0;
+ }
+
+ return Builder.CreateCall(CalleeF, ArgsV.begin(), ArgsV.end(), "calltmp");
+}
+
+Value *IfExprAST::Codegen() {
+ Value *CondV = Cond->Codegen();
+ if (CondV == 0) return 0;
+
+ // Convert condition to a bool by comparing equal to 0.0.
+ CondV = Builder.CreateFCmpONE(CondV,
+ ConstantFP::get(APFloat(0.0)),
+ "ifcond");
+
+ Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+ // Create blocks for the then and else cases. Insert the 'then' block at the
+ // end of the function.
+ BasicBlock *ThenBB = BasicBlock::Create("then", TheFunction);
+ BasicBlock *ElseBB = BasicBlock::Create("else");
+ BasicBlock *MergeBB = BasicBlock::Create("ifcont");
+
+ Builder.CreateCondBr(CondV, ThenBB, ElseBB);
+
+ // Emit then value.
+ Builder.SetInsertPoint(ThenBB);
+
+ Value *ThenV = Then->Codegen();
+ if (ThenV == 0) return 0;
+
+ Builder.CreateBr(MergeBB);
+ // Codegen of 'Then' can change the current block, update ThenBB for the PHI.
+ ThenBB = Builder.GetInsertBlock();
+
+ // Emit else block.
+ TheFunction->getBasicBlockList().push_back(ElseBB);
+ Builder.SetInsertPoint(ElseBB);
+
+ Value *ElseV = Else->Codegen();
+ if (ElseV == 0) return 0;
+
+ Builder.CreateBr(MergeBB);
+ // Codegen of 'Else' can change the current block, update ElseBB for the PHI.
+ ElseBB = Builder.GetInsertBlock();
+
+ // Emit merge block.
+ TheFunction->getBasicBlockList().push_back(MergeBB);
+ Builder.SetInsertPoint(MergeBB);
+ PHINode *PN = Builder.CreatePHI(Type::DoubleTy, "iftmp");
+
+ PN->addIncoming(ThenV, ThenBB);
+ PN->addIncoming(ElseV, ElseBB);
+ return PN;
+}
+
+Value *ForExprAST::Codegen() {
+ // Output this as:
+ // var = alloca double
+ // ...
+ // start = startexpr
+ // store start -> var
+ // goto loop
+ // loop:
+ // ...
+ // bodyexpr
+ // ...
+ // loopend:
+ // step = stepexpr
+ // endcond = endexpr
+ //
+ // curvar = load var
+ // nextvar = curvar + step
+ // store nextvar -> var
+ // br endcond, loop, endloop
+ // outloop:
+
+ Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+ // Create an alloca for the variable in the entry block.
+ AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+
+ // Emit the start code first, without 'variable' in scope.
+ Value *StartVal = Start->Codegen();
+ if (StartVal == 0) return 0;
+
+ // Store the value into the alloca.
+ Builder.CreateStore(StartVal, Alloca);
+
+ // Make the new basic block for the loop header, inserting after current
+ // block.
+ BasicBlock *PreheaderBB = Builder.GetInsertBlock();
+ BasicBlock *LoopBB = BasicBlock::Create("loop", TheFunction);
+
+ // Insert an explicit fall through from the current block to the LoopBB.
+ Builder.CreateBr(LoopBB);
+
+ // Start insertion in LoopBB.
+ Builder.SetInsertPoint(LoopBB);
+
+ // Within the loop, the variable is defined equal to the PHI node. If it
+ // shadows an existing variable, we have to restore it, so save it now.
+ AllocaInst *OldVal = NamedValues[VarName];
+ NamedValues[VarName] = Alloca;
+
+ // Emit the body of the loop. This, like any other expr, can change the
+ // current BB. Note that we ignore the value computed by the body, but don't
+ // allow an error.
+ if (Body->Codegen() == 0)
+ return 0;
+
+ // Emit the step value.
+ Value *StepVal;
+ if (Step) {
+ StepVal = Step->Codegen();
+ if (StepVal == 0) return 0;
+ } else {
+ // If not specified, use 1.0.
+ StepVal = ConstantFP::get(APFloat(1.0));
+ }
+
+ // Compute the end condition.
+ Value *EndCond = End->Codegen();
+ if (EndCond == 0) return EndCond;
+
+ // Reload, increment, and restore the alloca. This handles the case where
+ // the body of the loop mutates the variable.
+ Value *CurVar = Builder.CreateLoad(Alloca, VarName.c_str());
+ Value *NextVar = Builder.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(APFloat(0.0)),
+ "loopcond");
+
+ // Create the "after loop" block and insert it.
+ BasicBlock *LoopEndBB = Builder.GetInsertBlock();
+ BasicBlock *AfterBB = BasicBlock::Create("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::DoubleTy);
+}
+
+Value *VarExprAST::Codegen() {
+ std::vector<AllocaInst *> OldBindings;
+
+ Function *TheFunction = Builder.GetInsertBlock()->getParent();
+
+ // Register all variables and emit their initializer.
+ for (unsigned i = 0, e = VarNames.size(); i != e; ++i) {
+ const std::string &VarName = VarNames[i].first;
+ ExprAST *Init = VarNames[i].second;
+
+ // Emit the initializer before adding the variable to scope, this prevents
+ // the initializer from referencing the variable itself, and permits stuff
+ // like this:
+ // var a = 1 in
+ // var a = a in ... # refers to outer 'a'.
+ Value *InitVal;
+ if (Init) {
+ InitVal = Init->Codegen();
+ if (InitVal == 0) return 0;
+ } else { // If not specified, use 0.0.
+ InitVal = ConstantFP::get(APFloat(0.0));
+ }
+
+ AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);
+ Builder.CreateStore(InitVal, Alloca);
+
+ // Remember the old variable binding so that we can restore the binding when
+ // we unrecurse.
+ OldBindings.push_back(NamedValues[VarName]);
+
+ // Remember this binding.
+ NamedValues[VarName] = Alloca;
+ }
+
+ // Codegen the body, now that all vars are in scope.
+ Value *BodyVal = Body->Codegen();
+ if (BodyVal == 0) return 0;
+
+ // Pop all our variables from scope.
+ for (unsigned i = 0, e = VarNames.size(); i != e; ++i)
+ NamedValues[VarNames[i].first] = OldBindings[i];
+
+ // Return the body computation.
+ return BodyVal;
+}
+
+
+Function *PrototypeAST::Codegen() {
+ // Make the function type: double(double,double) etc.
+ std::vector<const Type*> Doubles(Args.size(), Type::DoubleTy);
+ FunctionType *FT = FunctionType::get(Type::DoubleTy, Doubles, false);
+
+ Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
+
+ // If F conflicted, there was already something named 'Name'. If it has a
+ // body, don't allow redefinition or reextern.
+ if (F->getName() != Name) {
+ // Delete the one we just made and get the existing one.
+ F->eraseFromParent();
+ F = TheModule->getFunction(Name);
+
+ // If F already has a body, reject this.
+ if (!F->empty()) {
+ ErrorF("redefinition of function");
+ return 0;
+ }
+
+ // If F took a different number of args, reject.
+ if (F->arg_size() != Args.size()) {
+ ErrorF("redefinition of function with different # args");
+ return 0;
+ }
+ }
+
+ // Set names for all arguments.
+ unsigned Idx = 0;
+ for (Function::arg_iterator AI = F->arg_begin(); Idx != Args.size();
+ ++AI, ++Idx)
+ AI->setName(Args[Idx]);
+
+ return F;
+}
+
+/// CreateArgumentAllocas - Create an alloca for each argument and register the
+/// argument in the symbol table so that references to it will succeed.
+void PrototypeAST::CreateArgumentAllocas(Function *F) {
+ Function::arg_iterator AI = F->arg_begin();
+ for (unsigned Idx = 0, e = Args.size(); Idx != e; ++Idx, ++AI) {
+ // Create an alloca for this variable.
+ AllocaInst *Alloca = CreateEntryBlockAlloca(F, Args[Idx]);
+
+ // Store the initial value into the alloca.
+ Builder.CreateStore(AI, Alloca);
+
+ // Add arguments to variable symbol table.
+ NamedValues[Args[Idx]] = Alloca;
+ }
+}
+
+
+Function *FunctionAST::Codegen() {
+ NamedValues.clear();
+
+ Function *TheFunction = Proto->Codegen();
+ if (TheFunction == 0)
+ return 0;
+
+ // If this is an operator, install it.
+ if (Proto->isBinaryOp())
+ BinopPrecedence[Proto->getOperatorName()] = Proto->getBinaryPrecedence();
+
+ // Create a new basic block to start insertion into.
+ BasicBlock *BB = BasicBlock::Create("entry", TheFunction);
+ Builder.SetInsertPoint(BB);
+
+ // Add all arguments to the symbol table and create their allocas.
+ Proto->CreateArgumentAllocas(TheFunction);
+
+ if (Value *RetVal = Body->Codegen()) {
+ // Finish off the function.
+ Builder.CreateRet(RetVal);
+
+ // Validate the generated code, checking for consistency.
+ verifyFunction(*TheFunction);
+
+ // Optimize the function.
+ TheFPM->run(*TheFunction);
+
+ return TheFunction;
+ }
+
+ // Error reading body, remove function.
+ TheFunction->eraseFromParent();
+
+ if (Proto->isBinaryOp())
+ BinopPrecedence.erase(Proto->getOperatorName());
+ return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Top-Level parsing and JIT Driver
+//===----------------------------------------------------------------------===//
+
+static ExecutionEngine *TheExecutionEngine;
+
+static void HandleDefinition() {
+ if (FunctionAST *F = ParseDefinition()) {
+ if (Function *LF = F->Codegen()) {
+ fprintf(stderr, "Read function definition:");
+ LF->dump();
+ }
+ } else {
+ // Skip token for error recovery.
+ getNextToken();
+ }
+}
+
+static void HandleExtern() {
+ if (PrototypeAST *P = ParseExtern()) {
+ if (Function *F = P->Codegen()) {
+ fprintf(stderr, "Read extern: ");
+ F->dump();
+ }
+ } else {
+ // Skip token for error recovery.
+ getNextToken();
+ }
+}
+
+static void HandleTopLevelExpression() {
+ // Evaluate a top level expression into an anonymous function.
+ if (FunctionAST *F = ParseTopLevelExpr()) {
+ if (Function *LF = F->Codegen()) {
+ // JIT the function, returning a function pointer.
+ void *FPtr = TheExecutionEngine->getPointerToFunction(LF);
+
+ // Cast it to the right type (takes no arguments, returns a double) so we
+ // can call it as a native function.
+ double (*FP)() = (double (*)())FPtr;
+ fprintf(stderr, "Evaluated to %f\n", FP());
+ }
+ } else {
+ // Skip token for error recovery.
+ getNextToken();
+ }
+}
+
+/// top ::= definition | external | expression | ';'
+static void MainLoop() {
+ while (1) {
+ fprintf(stderr, "ready> ");
+ switch (CurTok) {
+ case tok_eof: return;
+ case ';': getNextToken(); break; // ignore top level semicolons.
+ case tok_def: HandleDefinition(); break;
+ case tok_extern: HandleExtern(); break;
+ default: HandleTopLevelExpression(); break;
+ }
+ }
+}
+
+
+
+//===----------------------------------------------------------------------===//
+// "Library" functions that can be "extern'd" from user code.
+//===----------------------------------------------------------------------===//
+
+/// putchard - putchar that takes a double and returns 0.
+extern "C"
+double putchard(double X) {
+ putchar((char)X);
+ return 0;
+}
+
+/// printd - printf that takes a double prints it as "%f\n", returning 0.
+extern "C"
+double printd(double X) {
+ printf("%f\n", X);
+ return 0;
+}
+
+//===----------------------------------------------------------------------===//
+// Main driver code.
+//===----------------------------------------------------------------------===//
+
+int main() {
+ // Install standard binary operators.
+ // 1 is lowest precedence.
+ BinopPrecedence['='] = 2;
+ BinopPrecedence['<'] = 10;
+ BinopPrecedence['+'] = 20;
+ BinopPrecedence['-'] = 20;
+ BinopPrecedence['*'] = 40; // highest.
+
+ // Prime the first token.
+ fprintf(stderr, "ready> ");
+ getNextToken();
+
+ // Make the module, which holds all the code.
+ TheModule = new Module("my cool jit");
+
+ // Create the JIT.
+ TheExecutionEngine = ExecutionEngine::create(TheModule);
+
+ {
+ ExistingModuleProvider OurModuleProvider(TheModule);
+ FunctionPassManager OurFPM(&OurModuleProvider);
+
+ // Set up the optimizer pipeline. Start with registering info about how the
+ // target lays out data structures.
+ OurFPM.add(new TargetData(*TheExecutionEngine->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());
+
+ // Set the global so the code gen can use this.
+ TheFPM = &OurFPM;
+
+ // Run the main "interpreter loop" now.
+ MainLoop();
+
+ TheFPM = 0;
+
+ // Print out all of the generated code.
+ TheModule->dump();
+
+ } // Free module provider (and thus the module) and pass manager.
+
+ return 0;
+}
</pre>
</div>
+<a href="LangImpl8.html">Next: Conclusion and other useful LLVM tidbits</a>
</div>
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