# 8. Kaleidoscope: Compiling to Object Code

- Chapter 8 Introduction
- Choosing a target
- Target Machine
- Configuring the Module
- Emit Object Code
- Putting It All Together
- Full Code Listing

## 8.1. Chapter 8 Introduction

Welcome to Chapter 8 of the “Implementing a language with LLVM” tutorial. This chapter describes how to compile our language down to object files.

## 8.2. Choosing a target

LLVM has native support for cross-compilation. You can compile to the architecture of your current machine, or just as easily compile for other architectures. In this tutorial, we’ll target the current machine.

To specify the architecture that you want to target, we use a string called a “target triple”. This takes the form `<arch><sub>-<vendor>-<sys>-<abi>` (see the cross compilation docs).

As an example, we can see what clang thinks is our current target triple:

    $ clang --version | grep Target
    Target: x86_64-unknown-linux-gnu

Running this command may show something different on your machine as you might be using a different architecture or operating system to me.

Fortunately, we don’t need to hard-code a target triple to target the current machine. LLVM provides `sys::getDefaultTargetTriple`, which returns the target triple of the current machine.

    auto TargetTriple = sys::getDefaultTargetTriple();

LLVM doesn’t require us to link in all the target functionality. For example, if we’re just using the JIT, we don’t need the assembly printers. Similarly, if we’re only targeting certain architectures, we can only link in the functionality for those architectures.

For this example, we’ll initialize all the targets for emitting object code.

    InitializeAllTargetInfos();
    InitializeAllTargets();
    InitializeAllTargetMCs();
    InitializeAllAsmParsers();
    InitializeAllAsmPrinters();

We can now use our target triple to get a `Target`:

    std::string Error;
    auto Target = TargetRegistry::lookupTarget(TargetTriple, Error);

    // Print an error and exit if we couldn't find the requested target.
    // This generally occurs if we've forgotten to initialise the
    // TargetRegistry or we have a bogus target triple.
    if (!Target) {
      errs() << Error;
      return 1;
    }

## 8.3. Target Machine

We will also need a `TargetMachine`. This class provides a complete machine description of the machine we’re targeting. If we want to target a specific feature (such as SSE) or a specific CPU (such as Intel’s Sandylake), we do so now.

To see which features and CPUs that LLVM knows about, we can use `llc`. For example, let’s look at x86:

    $ llvm-as < /dev/null | llc -march=x86 -mattr=help
    Available CPUs for this target:

      amdfam10      - Select the amdfam10 processor.
      athlon        - Select the athlon processor.
      athlon-4      - Select the athlon-4 processor.
      ...

    Available features for this target:

      16bit-mode            - 16-bit mode (i8086).
      32bit-mode            - 32-bit mode (80386).
      3dnow                 - Enable 3DNow! instructions.
      3dnowa                - Enable 3DNow! Athlon instructions.
      ...

For our example, we’ll use the generic CPU without any additional features, options or relocation model.

    auto CPU = "generic";
    auto Features = "";

    TargetOptions opt;
    auto RM = Optional<Reloc::Model>();
    auto TargetMachine = Target->createTargetMachine(TargetTriple, CPU, Features, opt, RM);

## 8.4. Configuring the Module

We’re now ready to configure our module, to specify the target and data layout. This isn’t strictly necessary, but the frontend performance guide recommends this. Optimizations benefit from knowing about the target and data layout.

    TheModule->setDataLayout(TargetMachine->createDataLayout());
    TheModule->setTargetTriple(TargetTriple);

## 8.5. Emit Object Code

We’re ready to emit object code! Let’s define where we want to write our file to:

    auto Filename = "output.o";
    std::error_code EC;
    raw_fd_ostream dest(Filename, EC, sys::fs::F_None);

    if (EC) {
      errs() << "Could not open file: " << EC.message();
      return 1;
    }

Finally, we define a pass that emits object code, then we run that pass:

    legacy::PassManager pass;
    auto FileType = TargetMachine::CGFT_ObjectFile;

    if (TargetMachine->addPassesToEmitFile(pass, dest, FileType)) {
      errs() << "TargetMachine can't emit a file of this type";
      return 1;
    }

    pass.run(*TheModule);
    dest.flush();

## 8.6. Putting It All Together

Does it work? Let’s give it a try. We need to compile our code, but note that the arguments to `llvm-config` are different to the previous chapters.

    $ clang++ -g -O3 toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs all` -o toy

Let’s run it, and define a simple `average` function. Press Ctrl-D when you’re done.

    $ ./toy
    ready> def average(x y) (x + y) * 0.5;
    ^D
    Wrote output.o

We have an object file! To test it, let’s write a simple program and link it with our output. Here’s the source code:

    #include <iostream>

    extern "C" {
        double average(double, double);
    }

    int main() {
        std::cout << "average of 3.0 and 4.0: " << average(3.0, 4.0) << std::endl;
    }

We link our program to output.o and check the result is what we expected:

    $ clang++ main.cpp output.o -o main
    $ ./main
    average of 3.0 and 4.0: 3.5

## 8.7. Full Code Listing

    #include "llvm/ADT/APFloat.h"
    #include "llvm/ADT/Optional.h"
    #include "llvm/ADT/STLExtras.h"
    #include "llvm/IR/BasicBlock.h"
    #include "llvm/IR/Constants.h"
    #include "llvm/IR/DerivedTypes.h"
    #include "llvm/IR/Function.h"
    #include "llvm/IR/Instructions.h"
    #include "llvm/IR/IRBuilder.h"
    #include "llvm/IR/LLVMContext.h"
    #include "llvm/IR/LegacyPassManager.h"
    #include "llvm/IR/Module.h"
    #include "llvm/IR/Type.h"
    #include "llvm/IR/Verifier.h"
    #include "llvm/Support/FileSystem.h"
    #include "llvm/Support/Host.h"
    #include "llvm/Support/raw_ostream.h"
    #include "llvm/Support/TargetRegistry.h"
    #include "llvm/Support/TargetSelect.h"
    #include "llvm/Target/TargetMachine.h"
    #include "llvm/Target/TargetOptions.h"
    #include <algorithm>
    #include <cassert>
    #include <cctype>
    #include <cstdio>
    #include <cstdlib>
    #include <map>
    #include <memory>
    #include <string>
    #include <system_error>
    #include <utility>
    #include <vector>

    using namespace llvm;
    using namespace llvm::sys;

    //===----------------------------------------------------------------------===//
    // 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(), nullptr);
        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)
    //===----------------------------------------------------------------------===//

    namespace {

    /// ExprAST - Base class for all expression nodes.
    class ExprAST {
    public:
      virtual ~ExprAST() = default;

      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) {}

      Value *codegen() override;
    };

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

    public:
      VariableExprAST(const std::string &Name) : Name(Name) {}

      Value *codegen() override;
      const std::string &getName() const { return Name; }
    };

    /// 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;
    };

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

    public:
      BinaryExprAST(char Op, std::unique_ptr<ExprAST> LHS,
                    std::unique_ptr<ExprAST> RHS)
          : Op(Op), LHS(std::move(LHS)), RHS(std::move(RHS)) {}

      Value *codegen() override;
    };

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

    public:
      CallExprAST(const std::string &Callee,
                  std::vector<std::unique_ptr<ExprAST>> Args)
          : Callee(Callee), Args(std::move(Args)) {}

      Value *codegen() override;
    };

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

    public:
      IfExprAST(std::unique_ptr<ExprAST> Cond, std::unique_ptr<ExprAST> Then,
                std::unique_ptr<ExprAST> Else)
          : Cond(std::move(Cond)), Then(std::move(Then)), Else(std::move(Else)) {}

      Value *codegen() override;
    };

    /// 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;
    };

    /// 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;
    };

    /// 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.

    public:
      PrototypeAST(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) {}

      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; }
    };

    /// 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();
    };

    } // 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 = llvm::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;

      getNextToken(); // eat identifier.

      if (CurTok != '(') // Simple variable ref.
        return llvm::make_unique<VariableExprAST>(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 llvm::make_unique<CallExprAST>(IdName, std::move(Args));
    }

    /// ifexpr ::= 'if' expression 'then' expression 'else' expression
    static std::unique_ptr<ExprAST> ParseIfExpr() {
      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 llvm::make_unique<IfExprAST>(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 llvm::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 llvm::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 llvm::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;
        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 =
            llvm::make_unique<BinaryExprAST>(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;

      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 llvm::make_unique<PrototypeAST>(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 llvm::make_unique<FunctionAST>(std::move(Proto), std::move(E));
      return nullptr;
    }

    /// toplevelexpr ::= expression
    static std::unique_ptr<FunctionAST> ParseTopLevelExpr() {
      if (auto E = ParseExpression()) {
        // Make an anonymous proto.
        auto Proto = llvm::make_unique<PrototypeAST>("__anon_expr",
                                                     std::vector<std::string>());
        return llvm::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
    //===----------------------------------------------------------------------===//

    static LLVMContext TheContext;
    static IRBuilder<> Builder(TheContext);
    static std::unique_ptr<Module> TheModule;
    static std::map<std::string, AllocaInst *> NamedValues;
    static std::map<std::string, std::unique_ptr<PrototypeAST>> FunctionProtos;

    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,
                                              const std::string &VarName) {
      IRBuilder<> TmpB(&TheFunction->getEntryBlock(),
                       TheFunction->getEntryBlock().begin());
      return TmpB.CreateAlloca(Type::getDoubleTy(TheContext), nullptr, VarName);
    }

    Value *NumberExprAST::codegen() {
      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");

      // Load the value.
      return Builder.CreateLoad(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");

      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.
        // 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() {
      // 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() {
      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->getBasicBlockList().push_back(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->getBasicBlockList().push_back(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);

      // 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(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;
      }

      // 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);

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

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

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

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

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

        return TheFunction;
      }

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

      if (P.isBinaryOp())
        BinopPrecedence.erase(P.getOperatorName());
      return nullptr;
    }

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

    static void InitializeModuleAndPassManager() {
      // Open a new module.
      TheModule = llvm::make_unique<Module>("my cool jit", TheContext);
    }

    static void HandleDefinition() {
      if (auto FnAST = ParseDefinition()) {
        if (auto *FnIR = FnAST->codegen()) {
          fprintf(stderr, "Read function definition:");
          FnIR->print(errs());
          fprintf(stderr, "\n");
        }
      } else {
        // Skip token for error recovery.
        getNextToken();
      }
    }

    static void HandleExtern() {
      if (auto ProtoAST = ParseExtern()) {
        if (auto *FnIR = ProtoAST->codegen()) {
          fprintf(stderr, "Read extern: ");
          FnIR->print(errs());
          fprintf(stderr, "\n");
          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()) {
        FnAST->codegen();
      } 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() {
      // Install standard binary operators.
      // 1 is lowest precedence.
      BinopPrecedence['<'] = 10;
      BinopPrecedence['+'] = 20;
      BinopPrecedence['-'] = 20;
      BinopPrecedence['*'] = 40; // highest.

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

      InitializeModuleAndPassManager();

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

      // Initialize the target registry etc.
      InitializeAllTargetInfos();
      InitializeAllTargets();
      InitializeAllTargetMCs();
      InitializeAllAsmParsers();
      InitializeAllAsmPrinters();

      auto TargetTriple = sys::getDefaultTargetTriple();
      TheModule->setTargetTriple(TargetTriple);

      std::string Error;
      auto Target = TargetRegistry::lookupTarget(TargetTriple, Error);

      // Print an error and exit if we couldn't find the requested target.
      // This generally occurs if we've forgotten to initialise the
      // TargetRegistry or we have a bogus target triple.
      if (!Target) {
        errs() << Error;
        return 1;
      }

      auto CPU = "generic";
      auto Features = "";

      TargetOptions opt;
      auto RM = Optional<Reloc::Model>();
      auto TheTargetMachine =
          Target->createTargetMachine(TargetTriple, CPU, Features, opt, RM);

      TheModule->setDataLayout(TheTargetMachine->createDataLayout());

      auto Filename = "output.o";
      std::error_code EC;
      raw_fd_ostream dest(Filename, EC, sys::fs::F_None);

      if (EC) {
        errs() << "Could not open file: " << EC.message();
        return 1;
      }

      legacy::PassManager pass;
      auto FileType = TargetMachine::CGFT_ObjectFile;

      if (TheTargetMachine->addPassesToEmitFile(pass, dest, nullptr, FileType)) {
        errs() << "TheTargetMachine can't emit a file of this type";
        return 1;
      }

      pass.run(*TheModule);
      dest.flush();

      outs() << "Wrote " << Filename << "\n";

      return 0;
    }

Next: Adding Debug Information