# 9. Kaleidoscope: Adding Debug Information

## 9.1. Chapter 9 Introduction

Welcome to Chapter 9 of the “Implementing a language with LLVM” tutorial. In chapters 1 through 8, we’ve built a decent little programming language with functions and variables. What happens if something goes wrong though, how do you debug your program?

Source level debugging uses formatted data that helps a debugger translate from binary and the state of the machine back to the source that the programmer wrote. In LLVM we generally use a format called DWARF. DWARF is a compact encoding that represents types, source locations, and variable locations.

The short summary of this chapter is that we’ll go through the various things you have to add to a programming language to support debug info, and how you translate that into DWARF.

Caveat: For now we can’t debug via the JIT, so we’ll need to compile our program down to something small and standalone. As part of this we’ll make a few modifications to the running of the language and how programs are compiled. This means that we’ll have a source file with a simple program written in Kaleidoscope rather than the interactive JIT. It does involve a limitation that we can only have one “top level” command at a time to reduce the number of changes necessary.

Here’s the sample program we’ll be compiling:

    def fib(x)
      if x < 3 then
        1
      else
        fib(x-1)+fib(x-2);

    fib(10)

## 9.2. Why is this a hard problem?

Debug information is a hard problem for a few different reasons - mostly centered around optimized code. First, optimization makes keeping source locations more difficult. In LLVM IR we keep the original source location for each IR level instruction on the instruction. Optimization passes should keep the source locations for newly created instructions, but merged instructions only get to keep a single location - this can cause jumping around when stepping through optimized programs. Secondly, optimization can move variables in ways that are either optimized out, shared in memory with other variables, or difficult to track. For the purposes of this tutorial we’re going to avoid optimization (as you’ll see with one of the next sets of patches).

## 9.3. Ahead-of-Time Compilation Mode

To highlight only the aspects of adding debug information to a source language without needing to worry about the complexities of JIT debugging we’re going to make a few changes to Kaleidoscope to support compiling the IR emitted by the front end into a simple standalone program that you can execute, debug, and see results.

First we make our anonymous function that contains our top level statement be our “main”:

    -    auto Proto = std::make_unique<PrototypeAST>("", std::vector<std::string>());
    +    auto Proto = std::make_unique<PrototypeAST>("main", std::vector<std::string>());

just with the simple change of giving it a name.

Then we’re going to remove the command line code wherever it exists:

    @@ -1129,7 +1129,6 @@ static void HandleTopLevelExpression() {
     /// top ::= definition | external | expression | ';'
     static void MainLoop() {
       while (true) {
    -    fprintf(stderr, "ready> ");
         switch (CurTok) {
         case tok_eof:
           return;
    @@ -1184,7 +1183,6 @@ int main() {
       BinopPrecedence['*'] = 40; // highest.

       // Prime the first token.
    -  fprintf(stderr, "ready> ");
       getNextToken();

Lastly we’re going to disable all of the optimization passes and the JIT so that the only thing that happens after we’re done parsing and generating code is that the LLVM IR goes to standard error:

    @@ -1108,17 +1108,8 @@ static void HandleExtern() {
     static void HandleTopLevelExpression() {
       // Evaluate a top-level expression into an anonymous function.
       if (auto FnAST = ParseTopLevelExpr()) {
    -    if (auto *FnIR = FnAST->codegen()) {
    -      // We're just doing this to make sure it executes.
    -      TheExecutionEngine->finalizeObject();
    -      // JIT the function, returning a function pointer.
    -      void *FPtr = TheExecutionEngine->getPointerToFunction(FnIR);
    -
    -      // 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;
    -      // Ignore the return value for this.
    -      (void)FP;
    +    if (!FnAST->codegen()) {
    +      fprintf(stderr, "Error generating code for top level expr");
         }
       } else {
         // Skip token for error recovery.
    @@ -1439,11 +1459,11 @@ int main() {
       // target lays out data structures.
       TheModule->setDataLayout(TheExecutionEngine->getDataLayout());
       OurFPM.add(new DataLayoutPass());
    +#if 0
       OurFPM.add(createBasicAliasAnalysisPass());
       // Promote allocas to registers.
       OurFPM.add(createPromoteMemoryToRegisterPass());
    @@ -1218,7 +1210,7 @@ int main() {
       OurFPM.add(createGVNPass());
       // Simplify the control flow graph (deleting unreachable blocks, etc).
       OurFPM.add(createCFGSimplificationPass());
    -
    +  #endif
       OurFPM.doInitialization();

       // Set the global so the code gen can use this.

This relatively small set of changes get us to the point that we can compile our piece of Kaleidoscope language down to an executable program via this command line:

    Kaleidoscope-Ch9 < fib.ks | & clang -x ir -

which gives an a.out/a.exe in the current working directory.

## 9.4. Compile Unit

The top level container for a section of code in DWARF is a compile unit. This contains the type and function data for an individual translation unit (read: one file of source code). So the first thing we need to do is construct one for our fib.ks file.

## 9.5. DWARF Emission Setup

Similar to the `IRBuilder` class we have a DIBuilder class that helps in constructing debug metadata for an LLVM IR file. It corresponds 1:1 similarly to `IRBuilder` and LLVM IR, but with nicer names. Using it does require that you be more familiar with DWARF terminology than you needed to be with `IRBuilder` and `Instruction` names, but if you read through the general documentation on the Metadata Format it should be a little more clear. We’ll be using this class to construct all of our IR level descriptions. Construction for it takes a module so we need to construct it shortly after we construct our module. We’ve left it as a global static variable to make it a bit easier to use.

Next we’re going to create a small container to cache some of our frequent data. The first will be our compile unit, but we’ll also write a bit of code for our one type since we won’t have to worry about multiple typed expressions:

    static std::unique_ptr<DIBuilder> DBuilder;

    struct DebugInfo {
      DICompileUnit *TheCU;
      DIType *DblTy;

      DIType *getDoubleTy();
    } KSDbgInfo;

    DIType *DebugInfo::getDoubleTy() {
      if (DblTy)
        return DblTy;

      DblTy = DBuilder->createBasicType("double", 64, dwarf::DW_ATE_float);
      return DblTy;
    }

And then later on in `main` when we’re constructing our module:

    DBuilder = std::make_unique<DIBuilder>(*TheModule);

    KSDbgInfo.TheCU = DBuilder->createCompileUnit(
        dwarf::DW_LANG_C, DBuilder->createFile("fib.ks", "."),
        "Kaleidoscope Compiler", false, "", 0);

There are a couple of things to note here. First, while we’re producing a compile unit for a language called Kaleidoscope we used the language constant for C. This is because a debugger wouldn’t necessarily understand the calling conventions or default ABI for a language it doesn’t recognize and we follow the C ABI in our LLVM code generation so it’s the closest thing to accurate. This ensures we can actually call functions from the debugger and have them execute. Secondly, you’ll see the “fib.ks” in the call to `createCompileUnit`. This is a default hard coded value since we’re using shell redirection to put our source into the Kaleidoscope compiler. In a usual front end you’d have an input file name and it would go there.

One last thing as part of emitting debug information via DIBuilder is that we need to “finalize” the debug information. The reasons are part of the underlying API for DIBuilder, but make sure you do this near the end of main:

    DBuilder->finalize();

before you dump out the module.

## 9.6. Functions

Now that we have our `Compile Unit` and our source locations, we can add function definitions to the debug info. So in `FunctionAST::codegen()` we add a few lines of code to describe a context for our subprogram, in this case the “File”, and the actual definition of the function itself.

So the context:

    DIFile *Unit = DBuilder->createFile(KSDbgInfo.TheCU->getFilename(),
                                        KSDbgInfo.TheCU->getDirectory());

giving us an DIFile and asking the `Compile Unit` we created above for the directory and filename where we are currently. Then, for now, we use some source locations of 0 (since our AST doesn’t currently have source location information) and construct our function definition:

    DIScope *FContext = Unit;
    unsigned LineNo = 0;
    unsigned ScopeLine = 0;
    DISubprogram *SP = DBuilder->createFunction(
        FContext, P.getName(), StringRef(), Unit, LineNo,
        CreateFunctionType(TheFunction->arg_size()),
        ScopeLine,
        DINode::FlagPrototyped,
        DISubprogram::SPFlagDefinition);
    TheFunction->setSubprogram(SP);

and we now have an DISubprogram that contains a reference to all of our metadata for the function.

## 9.7. Source Locations

The most important thing for debug information is accurate source location - this makes it possible to map your source code back. We have a problem though, Kaleidoscope really doesn’t have any source location information in the lexer or parser so we’ll need to add it.

    struct SourceLocation {
      int Line;
      int Col;
    };
    static SourceLocation CurLoc;
    static SourceLocation LexLoc = {1, 0};

    static int advance() {
      int LastChar = getchar();

      if (LastChar == '\n' || LastChar == '\r') {
        LexLoc.Line++;
        LexLoc.Col = 0;
      } else
        LexLoc.Col++;
      return LastChar;
    }

In this set of code we’ve added some functionality on how to keep track of the line and column of the “source file”. As we lex every token we set our current current “lexical location” to the assorted line and column for the beginning of the token. We do this by overriding all of the previous calls to `getchar()` with our new `advance()` that keeps track of the information and then we have added to all of our AST classes a source location:

    class ExprAST {
      SourceLocation Loc;

      public:
        ExprAST(SourceLocation Loc = CurLoc) : Loc(Loc) {}
        virtual ~ExprAST() {}
        virtual Value* codegen() = 0;
        int getLine() const { return Loc.Line; }
        int getCol() const { return Loc.Col; }
        virtual raw_ostream &dump(raw_ostream &out, int ind) {
          return out << ':' << getLine() << ':' << getCol() << '\n';
        }

that we pass down through when we create a new expression:

    LHS = std::make_unique<BinaryExprAST>(BinLoc, BinOp, std::move(LHS),
                                           std::move(RHS));

giving us locations for each of our expressions and variables.

To make sure that every instruction gets proper source location information, we have to tell `Builder` whenever we’re at a new source location. We use a small helper function for this:

    void DebugInfo::emitLocation(ExprAST *AST) {
      if (!AST)
        return Builder->SetCurrentDebugLocation(DebugLoc());
      DIScope *Scope;
      if (LexicalBlocks.empty())
        Scope = TheCU;
      else
        Scope = LexicalBlocks.back();
      Builder->SetCurrentDebugLocation(
          DILocation::get(Scope->getContext(), AST->getLine(), AST->getCol(), Scope));
    }

This both tells the main `IRBuilder` where we are, but also what scope we’re in. The scope can either be on compile-unit level or be the nearest enclosing lexical block like the current function. To represent this we create a stack of scopes in `DebugInfo`:

    std::vector<DIScope *> LexicalBlocks;

and push the scope (function) to the top of the stack when we start generating the code for each function:

    KSDbgInfo.LexicalBlocks.push_back(SP);

Also, we may not forget to pop the scope back off of the scope stack at the end of the code generation for the function:

    // Pop off the lexical block for the function since we added it
    // unconditionally.
    KSDbgInfo.LexicalBlocks.pop_back();

Then we make sure to emit the location every time we start to generate code for a new AST object:

    KSDbgInfo.emitLocation(this);

## 9.8. Variables

Now that we have functions, we need to be able to print out the variables we have in scope. Let’s get our function arguments set up so we can get decent backtraces and see how our functions are being called. It isn’t a lot of code, and we generally handle it when we’re creating the argument allocas in `FunctionAST::codegen`.

    // Record the function arguments in the NamedValues map.
    NamedValues.clear();
    unsigned ArgIdx = 0;
    for (auto &Arg : TheFunction->args()) {
      // Create an alloca for this variable.
      AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, Arg.getName());

      // Create a debug descriptor for the variable.
      DILocalVariable *D = DBuilder->createParameterVariable(
          SP, Arg.getName(), ++ArgIdx, Unit, LineNo, KSDbgInfo.getDoubleTy(),
          true);

      DBuilder->insertDeclare(Alloca, D, DBuilder->createExpression(),
                              DILocation::get(SP->getContext(), LineNo, 0, SP),
                              Builder->GetInsertBlock());

      // Store the initial value into the alloca.
      Builder->CreateStore(&Arg, Alloca);

      // Add arguments to variable symbol table.
      NamedValues[std::string(Arg.getName())] = Alloca;
    }

Here we’re first creating the variable, giving it the scope (`SP`), the name, source location, type, and since it’s an argument, the argument index. Next, we create an `lvm.dbg.declare` call to indicate at the IR level that we’ve got a variable in an alloca (and it gives a starting location for the variable), and setting a source location for the beginning of the scope on the declare.

One interesting thing to note at this point is that various debuggers have assumptions based on how code and debug information was generated for them in the past. In this case we need to do a little bit of a hack to avoid generating line information for the function prologue so that the debugger knows to skip over those instructions when setting a breakpoint. So in `FunctionAST::CodeGen` we add some more lines:

    // Unset the location for the prologue emission (leading instructions with no
    // location in a function are considered part of the prologue and the debugger
    // will run past them when breaking on a function)
    KSDbgInfo.emitLocation(nullptr);

and then emit a new location when we actually start generating code for the body of the function:

    KSDbgInfo.emitLocation(Body.get());

With this we have enough debug information to set breakpoints in functions, print out argument variables, and call functions. Not too bad for just a few simple lines of code!

## 9.9. Full Code Listing

Here is the complete code listing for our running example, enhanced with debug information. To build this example, use:

    # Compile
    clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core orcjit native` -O3 -o toy
    # Run
    ./toy

Here is the code:

    #include "../include/KaleidoscopeJIT.h"
    #include "llvm/ADT/STLExtras.h"
    #include "llvm/Analysis/BasicAliasAnalysis.h"
    #include "llvm/Analysis/Passes.h"
    #include "llvm/IR/DIBuilder.h"
    #include "llvm/IR/IRBuilder.h"
    #include "llvm/IR/LLVMContext.h"
    #include "llvm/IR/LegacyPassManager.h"
    #include "llvm/IR/Module.h"
    #include "llvm/IR/Verifier.h"
    #include "llvm/Support/TargetSelect.h"
    #include "llvm/TargetParser/Host.h"
    #include "llvm/Transforms/Scalar.h"
    #include <cctype>
    #include <cstdio>
    #include <map>
    #include <string>
    #include <vector>

    using namespace llvm;
    using namespace llvm::orc;

    //===----------------------------------------------------------------------===//
    // 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
    };

    std::string getTokName(int Tok) {
      switch (Tok) {
      case tok_eof:
        return "eof";
      case tok_def:
        return "def";
      case tok_extern:
        return "extern";
      case tok_identifier:
        return "identifier";
      case tok_number:
        return "number";
      case tok_if:
        return "if";
      case tok_then:
        return "then";
      case tok_else:
        return "else";
      case tok_for:
        return "for";
      case tok_in:
        return "in";
      case tok_binary:
        return "binary";
      case tok_unary:
        return "unary";
      case tok_var:
        return "var";
      }
      return std::string(1, (char)Tok);
    }

    namespace {
    class PrototypeAST;
    class ExprAST;
    }

    struct DebugInfo {
      DICompileUnit *TheCU;
      DIType *DblTy;
      std::vector<DIScope *> LexicalBlocks;

      void emitLocation(ExprAST *AST);
      DIType *getDoubleTy();
    } KSDbgInfo;

    struct SourceLocation {
      int Line;
      int Col;
    };
    static SourceLocation CurLoc;
    static SourceLocation LexLoc = {1, 0};

    static int advance() {
      int LastChar = getchar();

      if (LastChar == '\n' || LastChar == '\r') {
        LexLoc.Line++;
        LexLoc.Col = 0;
      } else
        LexLoc.Col++;
      return LastChar;
    }

    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 = advance();

      CurLoc = LexLoc;

      if (isalpha(LastChar)) { // identifier: [a-zA-Z][a-zA-Z0-9]*
        IdentifierStr = LastChar;
        while (isalnum((LastChar = advance())))
          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 = advance();
        } while (isdigit(LastChar) || LastChar == '.');

        NumVal = strtod(NumStr.c_str(), nullptr);
        return tok_number;
      }

      if (LastChar == '#') {
        // Comment until end of line.
        do
          LastChar = advance();
        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 = advance();
      return ThisChar;
    }

    //===----------------------------------------------------------------------===//
    // Abstract Syntax Tree (aka Parse Tree)
    //===----------------------------------------------------------------------===//
    namespace {

    raw_ostream &indent(raw_ostream &O, int size) {
      return O << std::string(size, ' ');
    }

    /// ExprAST - Base class for all expression nodes.
    class ExprAST {
      SourceLocation Loc;

    public:
      ExprAST(SourceLocation Loc = CurLoc) : Loc(Loc) {}
      virtual ~ExprAST() {}
      virtual Value *codegen() = 0;
      int getLine() const { return Loc.Line; }
      int getCol() const { return Loc.Col; }
      virtual raw_ostream &dump(raw_ostream &out, int ind) {
        return out << ':' << getLine() << ':' << getCol() << '\n';
      }
    };

    /// NumberExprAST - Expression class for numeric literals like "1.0".
    class NumberExprAST : public ExprAST {
      double Val;

    public:
      NumberExprAST(double Val) : Val(Val) {}
      raw_ostream &dump(raw_ostream &out, int ind) override {
        return ExprAST::dump(out << Val, ind);
      }
      Value *codegen() override;
    };

    /// VariableExprAST - Expression class for referencing a variable, like "a".
    class VariableExprAST : public ExprAST {
      std::string Name;

    public:
      VariableExprAST(SourceLocation Loc, const std::string &Name)
          : ExprAST(Loc), Name(Name) {}
      const std::string &getName() const { return Name; }
      Value *codegen() override;
      raw_ostream &dump(raw_ostream &out, int ind) override {
        return ExprAST::dump(out << Name, ind);
      }
    };

    /// UnaryExprAST - Expression class for a unary operator.
    class UnaryExprAST : public ExprAST {
      char Opcode;
      std::unique_ptr<ExprAST> Operand;

    public:
      UnaryExprAST(char Opcode, std::unique_ptr<ExprAST> Operand)
          : Opcode(Opcode), Operand(std::move(Operand)) {}
      Value *codegen() override;
      raw_ostream &dump(raw_ostream &out, int ind) override {
        ExprAST::dump(out << "unary" << Opcode, ind);
        Operand->dump(out, ind + 1);
        return out;
      }
    };

    /// BinaryExprAST - Expression class for a binary operator.
    class BinaryExprAST : public ExprAST {
      char Op;
      std::unique_ptr<ExprAST> LHS, RHS;

    public:
      BinaryExprAST(SourceLocation Loc, char Op, std::unique_ptr<ExprAST> LHS,
                    std::unique_ptr<ExprAST> RHS)
          : ExprAST(Loc), Op(Op), LHS(std::move(LHS)), RHS(std::move(RHS)) {}
      Value *codegen() override;
      raw_ostream &dump(raw_ostream &out, int ind) override {
        ExprAST::dump(out << "binary" << Op, ind);
        LHS->dump(indent(out, ind) << "LHS:", ind + 1);
        RHS->dump(indent(out, ind) << "RHS:", ind + 1);
        return out;
      }
    };

    /// CallExprAST - Expression class for function calls.
    class CallExprAST : public ExprAST {
      std::string Callee;
      std::vector<std::unique_ptr<ExprAST>> Args;

    public:
      CallExprAST(SourceLocation Loc, const std::string &Callee,
                  std::vector<std::unique_ptr<ExprAST>> Args)
          : ExprAST(Loc), Callee(Callee), Args(std::move(Args)) {}
      Value *codegen() override;
      raw_ostream &dump(raw_ostream &out, int ind) override {
        ExprAST::dump(out << "call " << Callee, ind);
        for (const auto &Arg : Args)
          Arg->dump(indent(out, ind + 1), ind + 1);
        return out;
      }
    };

    /// IfExprAST - Expression class for if/then/else.
    class IfExprAST : public ExprAST {
      std::unique_ptr<ExprAST> Cond, Then, Else;

    public:
      IfExprAST(SourceLocation Loc, std::unique_ptr<ExprAST> Cond,
                std::unique_ptr<ExprAST> Then, std::unique_ptr<ExprAST> Else)
          : ExprAST(Loc), Cond(std::move(Cond)), Then(std::move(Then)),
            Else(std::move(Else)) {}
      Value *codegen() override;
      raw_ostream &dump(raw_ostream &out, int ind) override {
        ExprAST::dump(out << "if", ind);
        Cond->dump(indent(out, ind) << "Cond:", ind + 1);
        Then->dump(indent(out, ind) << "Then:", ind + 1);
        Else->dump(indent(out, ind) << "Else:", ind + 1);
        return out;
      }
    };

    /// ForExprAST - Expression class for for/in.
    class ForExprAST : public ExprAST {
      std::string VarName;
      std::unique_ptr<ExprAST> Start, End, Step, Body;

    public:
      ForExprAST(const std::string &VarName, std::unique_ptr<ExprAST> Start,
                 std::unique_ptr<ExprAST> End, std::unique_ptr<ExprAST> Step,
                 std::unique_ptr<ExprAST> Body)
          : VarName(VarName), Start(std::move(Start)), End(std::move(End)),
            Step(std::move(Step)), Body(std::move(Body)) {}
      Value *codegen() override;
      raw_ostream &dump(raw_ostream &out, int ind) override {
        ExprAST::dump(out << "for", ind);
        Start->dump(indent(out, ind) << "Cond:", ind + 1);
        End->dump(indent(out, ind) << "End:", ind + 1);
        Step->dump(indent(out, ind) << "Step:", ind + 1);
        Body->dump(indent(out, ind) << "Body:", ind + 1);
        return out;
      }
    };

    /// VarExprAST - Expression class for var/in
    class VarExprAST : public ExprAST {
      std::vector<std::pair<std::string, std::unique_ptr<ExprAST>>> VarNames;
      std::unique_ptr<ExprAST> Body;

    public:
      VarExprAST(
          std::vector<std::pair<std::string, std::unique_ptr<ExprAST>>> VarNames,
          std::unique_ptr<ExprAST> Body)
          : VarNames(std::move(VarNames)), Body(std::move(Body)) {}
      Value *codegen() override;
      raw_ostream &dump(raw_ostream &out, int ind) override {
        ExprAST::dump(out << "var", ind);
        for (const auto &NamedVar : VarNames)
          NamedVar.second->dump(indent(out, ind) << NamedVar.first << ':', ind + 1);
        Body->dump(indent(out, ind) << "Body:", ind + 1);
        return out;
      }
    };

    /// PrototypeAST - This class represents the "prototype" for a function,
    /// which captures its name, and its argument names (thus implicitly the number
    /// of arguments the function takes), as well as if it is an operator.
    class PrototypeAST {
      std::string Name;
      std::vector<std::string> Args;
      bool IsOperator;
      unsigned Precedence; // Precedence if a binary op.
      int Line;

    public:
      PrototypeAST(SourceLocation Loc, const std::string &Name,
                   std::vector<std::string> Args, bool IsOperator = false,
                   unsigned Prec = 0)
          : Name(Name), Args(std::move(Args)), IsOperator(IsOperator),
            Precedence(Prec), Line(Loc.Line) {}
      Function *codegen();
      const std::string &getName() const { return Name; }

      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; }
      int getLine() const { return Line; }
    };

    /// FunctionAST - This class represents a function definition itself.
    class FunctionAST {
      std::unique_ptr<PrototypeAST> Proto;
      std::unique_ptr<ExprAST> Body;

    public:
      FunctionAST(std::unique_ptr<PrototypeAST> Proto,
                  std::unique_ptr<ExprAST> Body)
          : Proto(std::move(Proto)), Body(std::move(Body)) {}
      Function *codegen();
      raw_ostream &dump(raw_ostream &out, int ind) {
        indent(out, ind) << "FunctionAST\n";
        ++ind;
        indent(out, ind) << "Body:";
        return Body ? Body->dump(out, ind) : out << "null\n";
      }
    };
    } // end anonymous namespace

    //===----------------------------------------------------------------------===//
    // Parser
    //===----------------------------------------------------------------------===//

    /// CurTok/getNextToken - Provide a simple token buffer.  CurTok is the current
    /// token the parser is looking at.  getNextToken reads another token from the
    /// lexer and updates CurTok with its results.
    static int CurTok;
    static int getNextToken() { return CurTok = gettok(); }

    /// BinopPrecedence - This holds the precedence for each binary operator that is
    /// defined.
    static std::map<char, int> BinopPrecedence;

    /// GetTokPrecedence - Get the precedence of the pending binary operator token.
    static int GetTokPrecedence() {
      if (!isascii(CurTok))
        return -1;

      // Make sure it's a declared binop.
      int TokPrec = BinopPrecedence[CurTok];
      if (TokPrec <= 0)
        return -1;
      return TokPrec;
    }

    /// LogError* - These are little helper functions for error handling.
    std::unique_ptr<ExprAST> LogError(const char *Str) {
      fprintf(stderr, "Error: %s\n", Str);
      return nullptr;
    }

    std::unique_ptr<PrototypeAST> LogErrorP(const char *Str) {
      LogError(Str);
      return nullptr;
    }

    static std::unique_ptr<ExprAST> ParseExpression();

    /// numberexpr ::= number
    static std::unique_ptr<ExprAST> ParseNumberExpr() {
      auto Result = std::make_unique<NumberExprAST>(NumVal);
      getNextToken(); // consume the number
      return std::move(Result);
    }

    /// parenexpr ::= '(' expression ')'
    static std::unique_ptr<ExprAST> ParseParenExpr() {
      getNextToken(); // eat (.
      auto V = ParseExpression();
      if (!V)
        return nullptr;

      if (CurTok != ')')
        return LogError("expected ')'");
      getNextToken(); // eat ).
      return V;
    }

    /// identifierexpr
    ///   ::= identifier
    ///   ::= identifier '(' expression* ')'
    static std::unique_ptr<ExprAST> ParseIdentifierExpr() {
      std::string IdName = IdentifierStr;

      SourceLocation LitLoc = CurLoc;

      getNextToken(); // eat identifier.

      if (CurTok != '(') // Simple variable ref.
        return std::make_unique<VariableExprAST>(LitLoc, IdName);

      // Call.
      getNextToken(); // eat (
      std::vector<std::unique_ptr<ExprAST>> Args;
      if (CurTok != ')') {
        while (true) {
          if (auto Arg = ParseExpression())
            Args.push_back(std::move(Arg));
          else
            return nullptr;

          if (CurTok == ')')
            break;

          if (CurTok != ',')
            return LogError("Expected ')' or ',' in argument list");
          getNextToken();
        }
      }

      // Eat the ')'.
      getNextToken();

      return std::make_unique<CallExprAST>(LitLoc, IdName, std::move(Args));
    }

    /// ifexpr ::= 'if' expression 'then' expression 'else' expression
    static std::unique_ptr<ExprAST> ParseIfExpr() {
      SourceLocation IfLoc = CurLoc;

      getNextToken(); // eat the if.

      // condition.
      auto Cond = ParseExpression();
      if (!Cond)
        return nullptr;

      if (CurTok != tok_then)
        return LogError("expected then");
      getNextToken(); // eat the then

      auto Then = ParseExpression();
      if (!Then)
        return nullptr;

      if (CurTok != tok_else)
        return LogError("expected else");

      getNextToken();

      auto Else = ParseExpression();
      if (!Else)
        return nullptr;

      return std::make_unique<IfExprAST>(IfLoc, std::move(Cond), std::move(Then),
                                          std::move(Else));
    }

    /// forexpr ::= 'for' identifier '=' expr ',' expr (',' expr)? 'in' expression
    static std::unique_ptr<ExprAST> ParseForExpr() {
      getNextToken(); // eat the for.

      if (CurTok != tok_identifier)
        return LogError("expected identifier after for");

      std::string IdName = IdentifierStr;
      getNextToken(); // eat identifier.

      if (CurTok != '=')
        return LogError("expected '=' after for");
      getNextToken(); // eat '='.

      auto Start = ParseExpression();
      if (!Start)
        return nullptr;
      if (CurTok != ',')
        return LogError("expected ',' after for start value");
      getNextToken();

      auto End = ParseExpression();
      if (!End)
        return nullptr;

      // The step value is optional.
      std::unique_ptr<ExprAST> Step;
      if (CurTok == ',') {
        getNextToken();
        Step = ParseExpression();
        if (!Step)
          return nullptr;
      }

      if (CurTok != tok_in)
        return LogError("expected 'in' after for");
      getNextToken(); // eat 'in'.

      auto Body = ParseExpression();
      if (!Body)
        return nullptr;

      return std::make_unique<ForExprAST>(IdName, std::move(Start), std::move(End),
                                           std::move(Step), std::move(Body));
    }

    /// varexpr ::= 'var' identifier ('=' expression)?
    //                    (',' identifier ('=' expression)?)* 'in' expression
    static std::unique_ptr<ExprAST> ParseVarExpr() {
      getNextToken(); // eat the var.

      std::vector<std::pair<std::string, std::unique_ptr<ExprAST>>> VarNames;

      // At least one variable name is required.
      if (CurTok != tok_identifier)
        return LogError("expected identifier after var");

      while (true) {
        std::string Name = IdentifierStr;
        getNextToken(); // eat identifier.

        // Read the optional initializer.
        std::unique_ptr<ExprAST> Init = nullptr;
        if (CurTok == '=') {
          getNextToken(); // eat the '='.

          Init = ParseExpression();
          if (!Init)
            return nullptr;
        }

        VarNames.push_back(std::make_pair(Name, std::move(Init)));

        // End of var list, exit loop.
        if (CurTok != ',')
          break;
        getNextToken(); // eat the ','.

        if (CurTok != tok_identifier)
          return LogError("expected identifier list after var");
      }

      // At this point, we have to have 'in'.
      if (CurTok != tok_in)
        return LogError("expected 'in' keyword after 'var'");
      getNextToken(); // eat 'in'.

      auto Body = ParseExpression();
      if (!Body)
        return nullptr;

      return std::make_unique<VarExprAST>(std::move(VarNames), std::move(Body));
    }

    /// primary
    ///   ::= identifierexpr
    ///   ::= numberexpr
    ///   ::= parenexpr
    ///   ::= ifexpr
    ///   ::= forexpr
    ///   ::= varexpr
    static std::unique_ptr<ExprAST> ParsePrimary() {
      switch (CurTok) {
      default:
        return LogError("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 std::unique_ptr<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 (auto Operand = ParseUnary())
        return std::make_unique<UnaryExprAST>(Opc, std::move(Operand));
      return nullptr;
    }

    /// binoprhs
    ///   ::= ('+' unary)*
    static std::unique_ptr<ExprAST> ParseBinOpRHS(int ExprPrec,
                                                  std::unique_ptr<ExprAST> LHS) {
      // If this is a binop, find its precedence.
      while (true) {
        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;
        SourceLocation BinLoc = CurLoc;
        getNextToken(); // eat binop

        // Parse the unary expression after the binary operator.
        auto RHS = ParseUnary();
        if (!RHS)
          return nullptr;

        // 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, std::move(RHS));
          if (!RHS)
            return nullptr;
        }

        // Merge LHS/RHS.
        LHS = std::make_unique<BinaryExprAST>(BinLoc, BinOp, std::move(LHS),
                                               std::move(RHS));
      }
    }

    /// expression
    ///   ::= unary binoprhs
    ///
    static std::unique_ptr<ExprAST> ParseExpression() {
      auto LHS = ParseUnary();
      if (!LHS)
        return nullptr;

      return ParseBinOpRHS(0, std::move(LHS));
    }

    /// prototype
    ///   ::= id '(' id* ')'
    ///   ::= binary LETTER number? (id, id)
    ///   ::= unary LETTER (id)
    static std::unique_ptr<PrototypeAST> ParsePrototype() {
      std::string FnName;

      SourceLocation FnLoc = CurLoc;

      unsigned Kind = 0; // 0 = identifier, 1 = unary, 2 = binary.
      unsigned BinaryPrecedence = 30;

      switch (CurTok) {
      default:
        return LogErrorP("Expected function name in prototype");
      case tok_identifier:
        FnName = IdentifierStr;
        Kind = 0;
        getNextToken();
        break;
      case tok_unary:
        getNextToken();
        if (!isascii(CurTok))
          return LogErrorP("Expected unary operator");
        FnName = "unary";
        FnName += (char)CurTok;
        Kind = 1;
        getNextToken();
        break;
      case tok_binary:
        getNextToken();
        if (!isascii(CurTok))
          return LogErrorP("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 LogErrorP("Invalid precedence: must be 1..100");
          BinaryPrecedence = (unsigned)NumVal;
          getNextToken();
        }
        break;
      }

      if (CurTok != '(')
        return LogErrorP("Expected '(' in prototype");

      std::vector<std::string> ArgNames;
      while (getNextToken() == tok_identifier)
        ArgNames.push_back(IdentifierStr);
      if (CurTok != ')')
        return LogErrorP("Expected ')' in prototype");

      // success.
      getNextToken(); // eat ')'.

      // Verify right number of names for operator.
      if (Kind && ArgNames.size() != Kind)
        return LogErrorP("Invalid number of operands for operator");

      return std::make_unique<PrototypeAST>(FnLoc, FnName, ArgNames, Kind != 0,
                                             BinaryPrecedence);
    }

    /// definition ::= 'def' prototype expression
    static std::unique_ptr<FunctionAST> ParseDefinition() {
      getNextToken(); // eat def.
      auto Proto = ParsePrototype();
      if (!Proto)
        return nullptr;

      if (auto E = ParseExpression())
        return std::make_unique<FunctionAST>(std::move(Proto), std::move(E));
      return nullptr;
    }

    /// toplevelexpr ::= expression
    static std::unique_ptr<FunctionAST> ParseTopLevelExpr() {
      SourceLocation FnLoc = CurLoc;
      if (auto E = ParseExpression()) {
        // Make the top-level expression be our "main" function.
        auto Proto = std::make_unique<PrototypeAST>(FnLoc, "main",
                                                     std::vector<std::string>());
        return std::make_unique<FunctionAST>(std::move(Proto), std::move(E));
      }
      return nullptr;
    }

    /// external ::= 'extern' prototype
    static std::unique_ptr<PrototypeAST> ParseExtern() {
      getNextToken(); // eat extern.
      return ParsePrototype();
    }

    //===----------------------------------------------------------------------===//
    // Code Generation Globals
    //===----------------------------------------------------------------------===//

    static std::unique_ptr<LLVMContext> TheContext;
    static std::unique_ptr<Module> TheModule;
    static std::unique_ptr<IRBuilder<>> Builder;
    static ExitOnError ExitOnErr;

    static std::map<std::string, AllocaInst *> NamedValues;
    static std::unique_ptr<KaleidoscopeJIT> TheJIT;
    static std::map<std::string, std::unique_ptr<PrototypeAST>> FunctionProtos;

    //===----------------------------------------------------------------------===//
    // Debug Info Support
    //===----------------------------------------------------------------------===//

    static std::unique_ptr<DIBuilder> DBuilder;

    DIType *DebugInfo::getDoubleTy() {
      if (DblTy)
        return DblTy;

      DblTy = DBuilder->createBasicType("double", 64, dwarf::DW_ATE_float);
      return DblTy;
    }

    void DebugInfo::emitLocation(ExprAST *AST) {
      if (!AST)
        return Builder->SetCurrentDebugLocation(DebugLoc());
      DIScope *Scope;
      if (LexicalBlocks.empty())
        Scope = TheCU;
      else
        Scope = LexicalBlocks.back();
      Builder->SetCurrentDebugLocation(DILocation::get(
          Scope->getContext(), AST->getLine(), AST->getCol(), Scope));
    }

    static DISubroutineType *CreateFunctionType(unsigned NumArgs) {
      SmallVector<Metadata *, 8> EltTys;
      DIType *DblTy = KSDbgInfo.getDoubleTy();

      // Add the result type.
      EltTys.push_back(DblTy);

      for (unsigned i = 0, e = NumArgs; i != e; ++i)
        EltTys.push_back(DblTy);

      return DBuilder->createSubroutineType(DBuilder->getOrCreateTypeArray(EltTys));
    }

    //===----------------------------------------------------------------------===//
    // Code Generation
    //===----------------------------------------------------------------------===//

    Value *LogErrorV(const char *Str) {
      LogError(Str);
      return nullptr;
    }

    Function *getFunction(std::string Name) {
      // First, see if the function has already been added to the current module.
      if (auto *F = TheModule->getFunction(Name))
        return F;

      // If not, check whether we can codegen the declaration from some existing
      // prototype.
      auto FI = FunctionProtos.find(Name);
      if (FI != FunctionProtos.end())
        return FI->second->codegen();

      // If no existing prototype exists, return null.
      return nullptr;
    }

    /// CreateEntryBlockAlloca - Create an alloca instruction in the entry block of
    /// the function.  This is used for mutable variables etc.
    static AllocaInst *CreateEntryBlockAlloca(Function *TheFunction,
                                              StringRef VarName) {
      IRBuilder<> TmpB(&TheFunction->getEntryBlock(),
                       TheFunction->getEntryBlock().begin());
      return TmpB.CreateAlloca(Type::getDoubleTy(*TheContext), nullptr, VarName);
    }

    Value *NumberExprAST::codegen() {
      KSDbgInfo.emitLocation(this);
      return ConstantFP::get(*TheContext, APFloat(Val));
    }

    Value *VariableExprAST::codegen() {
      // Look this variable up in the function.
      Value *V = NamedValues[Name];
      if (!V)
        return LogErrorV("Unknown variable name");

      KSDbgInfo.emitLocation(this);
      // Load the value.
      return Builder->CreateLoad(Type::getDoubleTy(*TheContext), V, Name.c_str());
    }

    Value *UnaryExprAST::codegen() {
      Value *OperandV = Operand->codegen();
      if (!OperandV)
        return nullptr;

      Function *F = getFunction(std::string("unary") + Opcode);
      if (!F)
        return LogErrorV("Unknown unary operator");

      KSDbgInfo.emitLocation(this);
      return Builder->CreateCall(F, OperandV, "unop");
    }

    Value *BinaryExprAST::codegen() {
      KSDbgInfo.emitLocation(this);

      // Special case '=' because we don't want to emit the LHS as an expression.
      if (Op == '=') {
        // Assignment requires the LHS to be an identifier.
        // This assume we're building without RTTI because LLVM builds that way by
        // default.  If you build LLVM with RTTI this can be changed to a
        // dynamic_cast for automatic error checking.
        VariableExprAST *LHSE = static_cast<VariableExprAST *>(LHS.get());
        if (!LHSE)
          return LogErrorV("destination of '=' must be a variable");
        // Codegen the RHS.
        Value *Val = RHS->codegen();
        if (!Val)
          return nullptr;

        // Look up the name.
        Value *Variable = NamedValues[LHSE->getName()];
        if (!Variable)
          return LogErrorV("Unknown variable name");

        Builder->CreateStore(Val, Variable);
        return Val;
      }

      Value *L = LHS->codegen();
      Value *R = RHS->codegen();
      if (!L || !R)
        return nullptr;

      switch (Op) {
      case '+':
        return Builder->CreateFAdd(L, R, "addtmp");
      case '-':
        return Builder->CreateFSub(L, R, "subtmp");
      case '*':
        return Builder->CreateFMul(L, R, "multmp");
      case '<':
        L = Builder->CreateFCmpULT(L, R, "cmptmp");
        // Convert bool 0/1 to double 0.0 or 1.0
        return Builder->CreateUIToFP(L, Type::getDoubleTy(*TheContext), "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 = getFunction(std::string("binary") + Op);
      assert(F && "binary operator not found!");

      Value *Ops[] = {L, R};
      return Builder->CreateCall(F, Ops, "binop");
    }

    Value *CallExprAST::codegen() {
      KSDbgInfo.emitLocation(this);

      // Look up the name in the global module table.
      Function *CalleeF = getFunction(Callee);
      if (!CalleeF)
        return LogErrorV("Unknown function referenced");

      // If argument mismatch error.
      if (CalleeF->arg_size() != Args.size())
        return LogErrorV("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())
          return nullptr;
      }

      return Builder->CreateCall(CalleeF, ArgsV, "calltmp");
    }

    Value *IfExprAST::codegen() {
      KSDbgInfo.emitLocation(this);

      Value *CondV = Cond->codegen();
      if (!CondV)
        return nullptr;

      // Convert condition to a bool by comparing non-equal to 0.0.
      CondV = Builder->CreateFCmpONE(
          CondV, ConstantFP::get(*TheContext, 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(*TheContext, "then", TheFunction);
      BasicBlock *ElseBB = BasicBlock::Create(*TheContext, "else");
      BasicBlock *MergeBB = BasicBlock::Create(*TheContext, "ifcont");

      Builder->CreateCondBr(CondV, ThenBB, ElseBB);

      // Emit then value.
      Builder->SetInsertPoint(ThenBB);

      Value *ThenV = Then->codegen();
      if (!ThenV)
        return nullptr;

      Builder->CreateBr(MergeBB);
      // Codegen of 'Then' can change the current block, update ThenBB for the PHI.
      ThenBB = Builder->GetInsertBlock();

      // Emit else block.
      TheFunction->insert(TheFunction->end(), ElseBB);
      Builder->SetInsertPoint(ElseBB);

      Value *ElseV = Else->codegen();
      if (!ElseV)
        return nullptr;

      Builder->CreateBr(MergeBB);
      // Codegen of 'Else' can change the current block, update ElseBB for the PHI.
      ElseBB = Builder->GetInsertBlock();

      // Emit merge block.
      TheFunction->insert(TheFunction->end(), MergeBB);
      Builder->SetInsertPoint(MergeBB);
      PHINode *PN = Builder->CreatePHI(Type::getDoubleTy(*TheContext), 2, "iftmp");

      PN->addIncoming(ThenV, ThenBB);
      PN->addIncoming(ElseV, ElseBB);
      return PN;
    }

    // Output for-loop 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:
    Value *ForExprAST::codegen() {
      Function *TheFunction = Builder->GetInsertBlock()->getParent();

      // Create an alloca for the variable in the entry block.
      AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, VarName);

      KSDbgInfo.emitLocation(this);

      // Emit the start code first, without 'variable' in scope.
      Value *StartVal = Start->codegen();
      if (!StartVal)
        return nullptr;

      // 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(*TheContext, "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())
        return nullptr;

      // Emit the step value.
      Value *StepVal = nullptr;
      if (Step) {
        StepVal = Step->codegen();
        if (!StepVal)
          return nullptr;
      } else {
        // If not specified, use 1.0.
        StepVal = ConstantFP::get(*TheContext, APFloat(1.0));
      }

      // Compute the end condition.
      Value *EndCond = End->codegen();
      if (!EndCond)
        return nullptr;

      // Reload, increment, and restore the alloca.  This handles the case where
      // the body of the loop mutates the variable.
      Value *CurVar = Builder->CreateLoad(Type::getDoubleTy(*TheContext), Alloca,
                                          VarName.c_str());
      Value *NextVar = Builder->CreateFAdd(CurVar, StepVal, "nextvar");
      Builder->CreateStore(NextVar, Alloca);

      // Convert condition to a bool by comparing non-equal to 0.0.
      EndCond = Builder->CreateFCmpONE(
          EndCond, ConstantFP::get(*TheContext, APFloat(0.0)), "loopcond");

      // Create the "after loop" block and insert it.
      BasicBlock *AfterBB =
          BasicBlock::Create(*TheContext, "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(*TheContext));
    }

    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.get();

        // 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)
            return nullptr;
        } else { // If not specified, use 0.0.
          InitVal = ConstantFP::get(*TheContext, 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;
      }

      KSDbgInfo.emitLocation(this);

      // Codegen the body, now that all vars are in scope.
      Value *BodyVal = Body->codegen();
      if (!BodyVal)
        return nullptr;

      // Pop all our variables from scope.
      for (unsigned i = 0, e = VarNames.size(); i != e; ++i)
        NamedValues[VarNames[i].first] = OldBindings[i];

      // Return the body computation.
      return BodyVal;
    }

    Function *PrototypeAST::codegen() {
      // Make the function type:  double(double,double) etc.
      std::vector<Type *> Doubles(Args.size(), Type::getDoubleTy(*TheContext));
      FunctionType *FT =
          FunctionType::get(Type::getDoubleTy(*TheContext), Doubles, false);

      Function *F =
          Function::Create(FT, Function::ExternalLinkage, Name, TheModule.get());

      // Set names for all arguments.
      unsigned Idx = 0;
      for (auto &Arg : F->args())
        Arg.setName(Args[Idx++]);

      return F;
    }

    Function *FunctionAST::codegen() {
      // Transfer ownership of the prototype to the FunctionProtos map, but keep a
      // reference to it for use below.
      auto &P = *Proto;
      FunctionProtos[Proto->getName()] = std::move(Proto);
      Function *TheFunction = getFunction(P.getName());
      if (!TheFunction)
        return nullptr;

      // If this is an operator, install it.
      if (P.isBinaryOp())
        BinopPrecedence[P.getOperatorName()] = P.getBinaryPrecedence();

      // Create a new basic block to start insertion into.
      BasicBlock *BB = BasicBlock::Create(*TheContext, "entry", TheFunction);
      Builder->SetInsertPoint(BB);

      // Create a subprogram DIE for this function.
      DIFile *Unit = DBuilder->createFile(KSDbgInfo.TheCU->getFilename(),
                                          KSDbgInfo.TheCU->getDirectory());
      DIScope *FContext = Unit;
      unsigned LineNo = P.getLine();
      unsigned ScopeLine = LineNo;
      DISubprogram *SP = DBuilder->createFunction(
          FContext, P.getName(), StringRef(), Unit, LineNo,
          CreateFunctionType(TheFunction->arg_size()), ScopeLine,
          DINode::FlagPrototyped, DISubprogram::SPFlagDefinition);
      TheFunction->setSubprogram(SP);

      // Push the current scope.
      KSDbgInfo.LexicalBlocks.push_back(SP);

      // Unset the location for the prologue emission (leading instructions with no
      // location in a function are considered part of the prologue and the debugger
      // will run past them when breaking on a function)
      KSDbgInfo.emitLocation(nullptr);

      // Record the function arguments in the NamedValues map.
      NamedValues.clear();
      unsigned ArgIdx = 0;
      for (auto &Arg : TheFunction->args()) {
        // Create an alloca for this variable.
        AllocaInst *Alloca = CreateEntryBlockAlloca(TheFunction, Arg.getName());

        // Create a debug descriptor for the variable.
        DILocalVariable *D = DBuilder->createParameterVariable(
            SP, Arg.getName(), ++ArgIdx, Unit, LineNo, KSDbgInfo.getDoubleTy(),
            true);

        DBuilder->insertDeclare(Alloca, D, DBuilder->createExpression(),
                                DILocation::get(SP->getContext(), LineNo, 0, SP),
                                Builder->GetInsertBlock());

        // Store the initial value into the alloca.
        Builder->CreateStore(&Arg, Alloca);

        // Add arguments to variable symbol table.
        NamedValues[std::string(Arg.getName())] = Alloca;
      }

      KSDbgInfo.emitLocation(Body.get());

      if (Value *RetVal = Body->codegen()) {
        // Finish off the function.
        Builder->CreateRet(RetVal);

        // Pop off the lexical block for the function.
        KSDbgInfo.LexicalBlocks.pop_back();

        // Validate the generated code, checking for consistency.
        verifyFunction(*TheFunction);

        return TheFunction;
      }

      // Error reading body, remove function.
      TheFunction->eraseFromParent();

      if (P.isBinaryOp())
        BinopPrecedence.erase(Proto->getOperatorName());

      // Pop off the lexical block for the function since we added it
      // unconditionally.
      KSDbgInfo.LexicalBlocks.pop_back();

      return nullptr;
    }

    //===----------------------------------------------------------------------===//
    // Top-Level parsing and JIT Driver
    //===----------------------------------------------------------------------===//

    static void InitializeModule() {
      // Open a new module.
      TheContext = std::make_unique<LLVMContext>();
      TheModule = std::make_unique<Module>("my cool jit", *TheContext);
      TheModule->setDataLayout(TheJIT->getDataLayout());

      Builder = std::make_unique<IRBuilder<>>(*TheContext);
    }

    static void HandleDefinition() {
      if (auto FnAST = ParseDefinition()) {
        if (!FnAST->codegen())
          fprintf(stderr, "Error reading function definition:");
      } else {
        // Skip token for error recovery.
        getNextToken();
      }
    }

    static void HandleExtern() {
      if (auto ProtoAST = ParseExtern()) {
        if (!ProtoAST->codegen())
          fprintf(stderr, "Error reading extern");
        else
          FunctionProtos[ProtoAST->getName()] = std::move(ProtoAST);
      } else {
        // Skip token for error recovery.
        getNextToken();
      }
    }

    static void HandleTopLevelExpression() {
      // Evaluate a top-level expression into an anonymous function.
      if (auto FnAST = ParseTopLevelExpr()) {
        if (!FnAST->codegen()) {
          fprintf(stderr, "Error generating code for top level expr");
        }
      } else {
        // Skip token for error recovery.
        getNextToken();
      }
    }

    /// top ::= definition | external | expression | ';'
    static void MainLoop() {
      while (true) {
        switch (CurTok) {
        case tok_eof:
          return;
        case ';': // ignore top-level semicolons.
          getNextToken();
          break;
        case tok_def:
          HandleDefinition();
          break;
        case tok_extern:
          HandleExtern();
          break;
        default:
          HandleTopLevelExpression();
          break;
        }
      }
    }

    //===----------------------------------------------------------------------===//
    // "Library" functions that can be "extern'd" from user code.
    //===----------------------------------------------------------------------===//

    #ifdef _WIN32
    #define DLLEXPORT __declspec(dllexport)
    #else
    #define DLLEXPORT
    #endif

    /// putchard - putchar that takes a double and returns 0.
    extern "C" DLLEXPORT double putchard(double X) {
      fputc((char)X, stderr);
      return 0;
    }

    /// printd - printf that takes a double prints it as "%f\n", returning 0.
    extern "C" DLLEXPORT double printd(double X) {
      fprintf(stderr, "%f\n", X);
      return 0;
    }

    //===----------------------------------------------------------------------===//
    // Main driver code.
    //===----------------------------------------------------------------------===//

    int main() {
      InitializeNativeTarget();
      InitializeNativeTargetAsmPrinter();
      InitializeNativeTargetAsmParser();

      // Install standard binary operators.
      // 1 is lowest precedence.
      BinopPrecedence['='] = 2;
      BinopPrecedence['<'] = 10;
      BinopPrecedence['+'] = 20;
      BinopPrecedence['-'] = 20;
      BinopPrecedence['*'] = 40; // highest.

      // Prime the first token.
      getNextToken();

      TheJIT = ExitOnErr(KaleidoscopeJIT::Create());

      InitializeModule();

      // Add the current debug info version into the module.
      TheModule->addModuleFlag(Module::Warning, "Debug Info Version",
                               DEBUG_METADATA_VERSION);

      // Darwin only supports dwarf2.
      if (Triple(sys::getProcessTriple()).isOSDarwin())
        TheModule->addModuleFlag(llvm::Module::Warning, "Dwarf Version", 2);

      // Construct the DIBuilder, we do this here because we need the module.
      DBuilder = std::make_unique<DIBuilder>(*TheModule);

      // Create the compile unit for the module.
      // Currently down as "fib.ks" as a filename since we're redirecting stdin
      // but we'd like actual source locations.
      KSDbgInfo.TheCU = DBuilder->createCompileUnit(
          dwarf::DW_LANG_C, DBuilder->createFile("fib.ks", "."),
          "Kaleidoscope Compiler", false, "", 0);

      // Run the main "interpreter loop" now.
      MainLoop();

      // Finalize the debug info.
      DBuilder->finalize();

      // Print out all of the generated code.
      TheModule->print(errs(), nullptr);

      return 0;
    }

Next: Conclusion and other useful LLVM tidbits