本文为LLVM教程的笔记(三)。
1.准备工作
先在每个AST类添加一个虚函数Codegen(code generation),实现代码生成。
Codegen()方法负责生成该类型AST节点的IR代码及其他必要信息,生成的内容会以LLVM对象的形式返回。
LLVM用value类表示静态一次性赋值寄存器或者说SSA值(static single assignment),SSA值由对应的指令运算得出之后就会固定下来,是不变的,而且知道该指令再次执行之前,SSA值都不可修改。
“这个概念并不难,习惯了就好。”
/// ExprAST - Base class for all expression nodes.
class ExprAST {
public:
virtual ~ExprAST() {}
virtual Value *Codegen() = 0;
};
/// NumberExprAST - Expression class for numeric literals like "1.0".
class NumberExprAST : public ExprAST {
double Val;
public:
NumberExprAST(double val) : Val(val) {}
virtual Value *Codegen();
};
...
除了在ExprAST类中添加虚方法,还可以利用visitor模式等其他方法实现代码生成,“但就当前需求,添加虚函数是最好的方法”。
此外,还需要Error方法,就像语法解析器里的报错函数,它用来报告代码生成过程中发生的错误,比如引用未经声明的参数:
Value *ErrorV(const char *Str) { Error(Str); return 0; }
// TheModule是LLVM中用于存放代码段中所有函数和全局变量的结构。从某种意义上讲,可以把它当作LLVM IR代码的顶层容器。
static Module *TheModule;
// Builder是用于简化LLVM指令生成的辅助对象,IRBuilder类模板的实例可用于跟踪当前插入指令的位置,同时还带有用于生成新指令的方法。
static IRBuilder<> Builder(getGlobalContext());
// NamedValues映射表用于记录定义于当前作用域内的变量及与之相对应的LLVM表示(换言之,也就是代码的符号表)。
static std::map<std::string, Value*> NamedValues;
在kaleidoscope中,可引用的变量只有函数的参数,会存在 NamedValues表中。
生成代码之前比如设置Builder对象,指明写入代码的位置。
2.表达式代码生成
- 数值常量
LLVM IR中的数值常量由ConstantFP表示,在其内部又具体由APFloat表示。
Value *NumberExprAST::Codegen() {
return ConstantFP::get(getGlobalContext(), APFloat(Val));
}
- 引用变量
假设引用的变量已经在某处定义赋值,NamedValues映射表中的变量只可能是函数的调用参数。
这段代码首先确认给定的变量名是否存在于映射表中(如果不存在,就说明引用了未定义的变量)然后返回该变量的值。
Value *VariableExprAST::Codegen() {
// Look this variable up in the function.
Value *V = NamedValues[Name];
return V ? V : ErrorV("Unknown variable name");
}
- 二元运算符
递归地生成代码,先处理表达式的左侧,再处理表达式的右侧,最后计算整个二元表达式的值。
Value *BinaryExprAST::Codegen() {
Value *L = LHS->Codegen();
Value *R = RHS->Codegen();
if (L == 0 || R == 0) return 0;
// opcode的取值用了一个简单的switch语句,从而为各种二元运算符创建出相应的LLVM指令。
switch (Op) {
// 只需想清楚该用哪些操作数(即此处的L和R)生成哪条指令(通过调用CreateFAdd等方法)即可,至于新指令该插入到什么位置,交给IRBuilder就可以了
// 此处的指令名只是一个提示。假设代码生成了多条“addtmp”指令,LLVM会自动给每条指令的名字追加一个自增的唯一数字后缀。
case '+': return Builder.CreateFAdd(L, R, "addtmp");
case '-': return Builder.CreateFSub(L, R, "subtmp");
case '*': return Builder.CreateFMul(L, R, "multmp");
case '<':
// add指令的Left、Right操作数必须同属一个类型,结果的类型则必须与操作数的类型相容。
// fcmp指令的返回值类型必须是‘i1’(单比特整数),而Kaleidoscope只能接受0.0或1.0。为了弥合语义上的差异,给fcmp指令配上一条uitofp指令。
// 这条指令会将输入的整数视作无符号数,并将之转换成浮点数。
// 如果用的是sitofp指令,Kaleidoscope的‘<’运算符将视输入的不同而返回0.0或-1.0。
L = Builder.CreateFCmpULT(L, R, "cmptmp");
// Convert bool 0/1 to double 0.0 or 1.0
return Builder.CreateUIToFP(L, Type::getDoubleTy(getGlobalContext()),
"booltmp");
default: return ErrorV("invalid binary operator");
}
}
- 函数调用
代码开头的几行将在LLVM Module的符号表中查找函数名,LLVM Module是个容器,待处理的函数全都在里面。
只要函数的名字与用户指定的函数名一致,LLVM的符号表就可以替我们完成函数名的解析。
等取到了该调用的函数,就能递归地生成传入的各参数代码,并创建一条LLVM call指令。
Value *CallExprAST::Codegen() {
// Look up the name in the global module table.
Function *CalleeF = TheModule->getFunction(Callee);
if (CalleeF == 0)
return ErrorV("Unknown function referenced");
// If argument mismatch error.
if (CalleeF->arg_size() != Args.size())
return ErrorV("Incorrect # arguments passed");
std::vector<Value*> ArgsV;
for (unsigned i = 0, e = Args.size(); i != e; ++i) {
ArgsV.push_back(Args[i]->Codegen());
if (ArgsV.back() == 0) return 0;
}
return Builder.CreateCall(CalleeF, ArgsV, "calltmp");
}
3.函数的代码生成
- 函数原型的代码生成过程
函数的返回值类型将是Function *而不是Value *,因为函数原型描述的是函数的对外接口而不是表达式计算出的值。
Function *PrototypeAST::Codegen() {
// Make the function type: double(double,double) etc.
// 创建了double的vector
std::vector<Type*> Doubles(Args.size(),
Type::getDoubleTy(getGlobalContext()));
// FunctionType::get调用用于为给定的函数原型创建对应的FunctionType对象,创建出一个参数不可变的函数类型(false)。
FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
Doubles, false);
// 创建与该函数原型相对应的函数,包含了类型、链接方式和函数名等,并指定该函数待插入的模块。
// “ExternalLinkage”表示该函数可能定义于当前模块之外,且/或可以被当前模块之外的函数调用。
// Name是用户指定的函数名.
// 函数定义在“TheModule”内,函数名自然也注册在“TheModule”的符号表内。
Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
- 检查函数是否被定义过
处理名称冲突时,Module的符号表与Function的符号表类似:在模块中添加新函数时,如果发现函数名与符号表中现有的名称重复,新函数会被默默地重命名。
“对于Kaleidoscope,在两种情况下允许重定义函数:第一,允许对同一个函数进行多次extern声明,前提是所有声明中的函数原型保持一致(由于只有一种参数类型,我们只需要检查参数的个数是否匹配即可)。第二,允许先对函数进行extern声明,再定义函数体。这样一来,才能定义出相互递归调用的函数。”
// If F conflicted, there was already something named 'Name'. If it has a
// body, don't allow redefinition or reextern.
// 如果有函数名冲突,(调用eraseFunctionParent)将刚刚创建的函数对象删除
if (F->getName() != Name) {
// Delete the one we just made and get the existing one.
F->eraseFromParent();
// 然后调用getFunction获取与函数名相对应的函数对象。
F = TheModule->getFunction(Name);
- 检查函数基本块与参数个数
“看看之前定义的函数对象是否为“空”。换言之,也就是看看该函数有没有定义基本块。没有基本块就意味着该函数尚未定义函数体,只是一个前导声明。如果已经定义了函数体,就不能继续下去了,抛出错误予以拒绝。”
“如果之前的函数对象只是个“extern”声明,则检查该函数的参数个数是否与当前的参数个数相符。如果不符,抛出错误。”
// If F already has a body, reject this.
if (!F->empty()) {
ErrorF("redefinition of function");
return 0;
}
// If F took a different number of args, reject.
if (F->arg_size() != Args.size()) {
ErrorF("redefinition of function with different # args");
return 0;
}
- 设置参数名
接着遍历函数原型所有参数,为这些LLVM Argument对象设置参数名,并把这些参数注册到NameValues映射表,最后返回Function对象。
// Set names for all arguments.
unsigned Idx = 0;
for (Function::arg_iterator AI = F->arg_begin(); Idx != Args.size();
++AI, ++Idx) {
AI->setName(Args[Idx]);
// Add arguments to variable symbol table.
NamedValues[Args[Idx]] = AI;
}
- 函数定义的代码生成过程
生成函数原型(proto)的代码并且校验,清空NamedValues映射表,确保之前生成代码过程产生的内容不会残留。
Function *FunctionAST::Codegen() {
NamedValues.clear();
Function *TheFunction = Proto->Codegen();
if (TheFunction == 0)
return 0;
-
设置Builder对象
// Create a new basic block to start insertion into. // 新建entry基本块,将该对象插入TheFunction BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction); // 告诉Builder后续指令插至新建的基本块末尾 Builder.SetInsertPoint(BB);
LLVM基本块是用于定义控制流图(Control Flow Graph)的重要部件。当前我们还不涉及到控制流,所以所有的函数都只有一个基本块。
-
调用函数主表达式的CodegGen方法
if (Value *RetVal = Body->Codegen()) { // Finish off the function. Builder.CreateRet(RetVal); // Validate the generated code, checking for consistency. verifyFunction(*TheFunction); return TheFunction; }
4.完整代码
// To build this:
// See example below.
#include "llvm/DerivedTypes.h"
#include "llvm/IRBuilder.h"
#include "llvm/LLVMContext.h"
#include "llvm/Module.h"
#include "llvm/Analysis/Verifier.h"
#include <cstdio>
#include <string>
#include <map>
#include <vector>
using namespace llvm;
//===----------------------------------------------------------------------===//
// Lexer
//===----------------------------------------------------------------------===//
// The lexer returns tokens [0-255] if it is an unknown character, otherwise one
// of these for known things.
enum Token {
tok_eof = -1,
// commands
tok_def = -2, tok_extern = -3,
// primary
tok_identifier = -4, tok_number = -5
};
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;
return tok_identifier;
}
if (isdigit(LastChar) || LastChar == '.') { // Number: [0-9.]+
std::string NumStr;
do {
NumStr += LastChar;
LastChar = getchar();
} while (isdigit(LastChar) || LastChar == '.');
NumVal = strtod(NumStr.c_str(), 0);
return tok_number;
}
if (LastChar == '#') {
// Comment until end of line.
do LastChar = getchar();
while (LastChar != EOF && LastChar != '\n' && LastChar != '\r');
if (LastChar != EOF)
return gettok();
}
// Check for end of file. Don't eat the EOF.
if (LastChar == EOF)
return tok_eof;
// Otherwise, just return the character as its ascii value.
int ThisChar = LastChar;
LastChar = getchar();
return ThisChar;
}
//===----------------------------------------------------------------------===//
// Abstract Syntax Tree (aka Parse Tree)
//===----------------------------------------------------------------------===//
/// ExprAST - Base class for all expression nodes.
class ExprAST {
public:
virtual ~ExprAST() {}
virtual Value *Codegen() = 0;
};
/// NumberExprAST - Expression class for numeric literals like "1.0".
class NumberExprAST : public ExprAST {
double Val;
public:
NumberExprAST(double val) : Val(val) {}
virtual Value *Codegen();
};
/// VariableExprAST - Expression class for referencing a variable, like "a".
class VariableExprAST : public ExprAST {
std::string Name;
public:
VariableExprAST(const std::string &name) : Name(name) {}
virtual Value *Codegen();
};
/// BinaryExprAST - Expression class for a binary operator.
class BinaryExprAST : public ExprAST {
char Op;
ExprAST *LHS, *RHS;
public:
BinaryExprAST(char op, ExprAST *lhs, ExprAST *rhs)
: Op(op), LHS(lhs), RHS(rhs) {}
virtual Value *Codegen();
};
/// CallExprAST - Expression class for function calls.
class CallExprAST : public ExprAST {
std::string Callee;
std::vector<ExprAST*> Args;
public:
CallExprAST(const std::string &callee, std::vector<ExprAST*> &args)
: Callee(callee), Args(args) {}
virtual Value *Codegen();
};
/// 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).
class PrototypeAST {
std::string Name;
std::vector<std::string> Args;
public:
PrototypeAST(const std::string &name, const std::vector<std::string> &args)
: Name(name), Args(args) {}
Function *Codegen();
};
/// FunctionAST - This class represents a function definition itself.
class FunctionAST {
PrototypeAST *Proto;
ExprAST *Body;
public:
FunctionAST(PrototypeAST *proto, ExprAST *body)
: Proto(proto), Body(body) {}
Function *Codegen();
};
//===----------------------------------------------------------------------===//
// Parser
//===----------------------------------------------------------------------===//
/// CurTok/getNextToken - Provide a simple token buffer. CurTok is the current
/// token the parser is looking at. getNextToken reads another token from the
/// lexer and updates CurTok with its results.
static int CurTok;
static int getNextToken() {
return CurTok = gettok();
}
/// BinopPrecedence - This holds the precedence for each binary operator that is
/// defined.
static std::map<char, int> BinopPrecedence;
/// GetTokPrecedence - Get the precedence of the pending binary operator token.
static int GetTokPrecedence() {
if (!isascii(CurTok))
return -1;
// Make sure it's a declared binop.
int TokPrec = BinopPrecedence[CurTok];
if (TokPrec <= 0) return -1;
return TokPrec;
}
/// Error* - These are little helper functions for error handling.
ExprAST *Error(const char *Str) { fprintf(stderr, "Error: %s\n", Str);return 0;}
PrototypeAST *ErrorP(const char *Str) { Error(Str); return 0; }
FunctionAST *ErrorF(const char *Str) { Error(Str); return 0; }
static ExprAST *ParseExpression();
/// identifierexpr
/// ::= identifier
/// ::= identifier '(' expression* ')'
static ExprAST *ParseIdentifierExpr() {
std::string IdName = IdentifierStr;
getNextToken(); // eat identifier.
if (CurTok != '(') // Simple variable ref.
return new VariableExprAST(IdName);
// Call.
getNextToken(); // eat (
std::vector<ExprAST*> Args;
if (CurTok != ')') {
while (1) {
ExprAST *Arg = ParseExpression();
if (!Arg) return 0;
Args.push_back(Arg);
if (CurTok == ')') break;
if (CurTok != ',')
return Error("Expected ')' or ',' in argument list");
getNextToken();
}
}
// Eat the ')'.
getNextToken();
return new CallExprAST(IdName, Args);
}
/// numberexpr ::= number
static ExprAST *ParseNumberExpr() {
ExprAST *Result = new NumberExprAST(NumVal);
getNextToken(); // consume the number
return Result;
}
/// parenexpr ::= '(' expression ')'
static ExprAST *ParseParenExpr() {
getNextToken(); // eat (.
ExprAST *V = ParseExpression();
if (!V) return 0;
if (CurTok != ')')
return Error("expected ')'");
getNextToken(); // eat ).
return V;
}
/// primary
/// ::= identifierexpr
/// ::= numberexpr
/// ::= parenexpr
static ExprAST *ParsePrimary() {
switch (CurTok) {
default: return Error("unknown token when expecting an expression");
case tok_identifier: return ParseIdentifierExpr();
case tok_number: return ParseNumberExpr();
case '(': return ParseParenExpr();
}
}
/// binoprhs
/// ::= ('+' primary)*
static ExprAST *ParseBinOpRHS(int ExprPrec, ExprAST *LHS) {
// If this is a binop, find its precedence.
while (1) {
int TokPrec = GetTokPrecedence();
// If this is a binop that binds at least as tightly as the current binop,
// consume it, otherwise we are done.
if (TokPrec < ExprPrec)
return LHS;
// Okay, we know this is a binop.
int BinOp = CurTok;
getNextToken(); // eat binop
// Parse the primary expression after the binary operator.
ExprAST *RHS = ParsePrimary();
if (!RHS) return 0;
// If BinOp binds less tightly with RHS than the operator after RHS, let
// the pending operator take RHS as its LHS.
int NextPrec = GetTokPrecedence();
if (TokPrec < NextPrec) {
RHS = ParseBinOpRHS(TokPrec+1, RHS);
if (RHS == 0) return 0;
}
// Merge LHS/RHS.
LHS = new BinaryExprAST(BinOp, LHS, RHS);
}
}
/// expression
/// ::= primary binoprhs
///
static ExprAST *ParseExpression() {
ExprAST *LHS = ParsePrimary();
if (!LHS) return 0;
return ParseBinOpRHS(0, LHS);
}
/// prototype
/// ::= id '(' id* ')'
static PrototypeAST *ParsePrototype() {
if (CurTok != tok_identifier)
return ErrorP("Expected function name in prototype");
std::string FnName = IdentifierStr;
getNextToken();
if (CurTok != '(')
return ErrorP("Expected '(' in prototype");
std::vector<std::string> ArgNames;
while (getNextToken() == tok_identifier)
ArgNames.push_back(IdentifierStr);
if (CurTok != ')')
return ErrorP("Expected ')' in prototype");
// success.
getNextToken(); // eat ')'.
return new PrototypeAST(FnName, ArgNames);
}
/// definition ::= 'def' prototype expression
static FunctionAST *ParseDefinition() {
getNextToken(); // eat def.
PrototypeAST *Proto = ParsePrototype();
if (Proto == 0) return 0;
if (ExprAST *E = ParseExpression())
return new FunctionAST(Proto, E);
return 0;
}
/// toplevelexpr ::= expression
static FunctionAST *ParseTopLevelExpr() {
if (ExprAST *E = ParseExpression()) {
// Make an anonymous proto.
PrototypeAST *Proto = new PrototypeAST("", std::vector<std::string>());
return new FunctionAST(Proto, E);
}
return 0;
}
/// external ::= 'extern' prototype
static PrototypeAST *ParseExtern() {
getNextToken(); // eat extern.
return ParsePrototype();
}
//===----------------------------------------------------------------------===//
// Code Generation
//===----------------------------------------------------------------------===//
static Module *TheModule;
static IRBuilder<> Builder(getGlobalContext());
static std::map<std::string, Value*> NamedValues;
Value *ErrorV(const char *Str) { Error(Str); return 0; }
Value *NumberExprAST::Codegen() {
return ConstantFP::get(getGlobalContext(), APFloat(Val));
}
Value *VariableExprAST::Codegen() {
// Look this variable up in the function.
Value *V = NamedValues[Name];
return V ? V : ErrorV("Unknown variable name");
}
Value *BinaryExprAST::Codegen() {
Value *L = LHS->Codegen();
Value *R = RHS->Codegen();
if (L == 0 || R == 0) return 0;
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(getGlobalContext()),
"booltmp");
default: return ErrorV("invalid binary operator");
}
}
Value *CallExprAST::Codegen() {
// Look up the name in the global module table.
Function *CalleeF = TheModule->getFunction(Callee);
if (CalleeF == 0)
return ErrorV("Unknown function referenced");
// If argument mismatch error.
if (CalleeF->arg_size() != Args.size())
return ErrorV("Incorrect # arguments passed");
std::vector<Value*> ArgsV;
for (unsigned i = 0, e = Args.size(); i != e; ++i) {
ArgsV.push_back(Args[i]->Codegen());
if (ArgsV.back() == 0) return 0;
}
return Builder.CreateCall(CalleeF, ArgsV, "calltmp");
}
Function *PrototypeAST::Codegen() {
// Make the function type: double(double,double) etc.
std::vector<Type*> Doubles(Args.size(),
Type::getDoubleTy(getGlobalContext()));
FunctionType *FT = FunctionType::get(Type::getDoubleTy(getGlobalContext()),
Doubles, false);
Function *F = Function::Create(FT, Function::ExternalLinkage, Name, TheModule);
// If F conflicted, there was already something named 'Name'. If it has a
// body, don't allow redefinition or reextern.
if (F->getName() != Name) {
// Delete the one we just made and get the existing one.
F->eraseFromParent();
F = TheModule->getFunction(Name);
// If F already has a body, reject this.
if (!F->empty()) {
ErrorF("redefinition of function");
return 0;
}
// If F took a different number of args, reject.
if (F->arg_size() != Args.size()) {
ErrorF("redefinition of function with different # args");
return 0;
}
}
// Set names for all arguments.
unsigned Idx = 0;
for (Function::arg_iterator AI = F->arg_begin(); Idx != Args.size();
++AI, ++Idx) {
AI->setName(Args[Idx]);
// Add arguments to variable symbol table.
NamedValues[Args[Idx]] = AI;
}
return F;
}
Function *FunctionAST::Codegen() {
NamedValues.clear();
Function *TheFunction = Proto->Codegen();
if (TheFunction == 0)
return 0;
// Create a new basic block to start insertion into.
BasicBlock *BB = BasicBlock::Create(getGlobalContext(), "entry", TheFunction);
Builder.SetInsertPoint(BB);
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();
return 0;
}
//===----------------------------------------------------------------------===//
// Top-Level parsing and JIT Driver
//===----------------------------------------------------------------------===//
static void HandleDefinition() {
if (FunctionAST *F = ParseDefinition()) {
if (Function *LF = F->Codegen()) {
fprintf(stderr, "Read function definition:");
LF->dump();
}
} else {
// Skip token for error recovery.
getNextToken();
}
}
static void HandleExtern() {
if (PrototypeAST *P = ParseExtern()) {
if (Function *F = P->Codegen()) {
fprintf(stderr, "Read extern: ");
F->dump();
}
} else {
// Skip token for error recovery.
getNextToken();
}
}
static void HandleTopLevelExpression() {
// Evaluate a top-level expression into an anonymous function.
if (FunctionAST *F = ParseTopLevelExpr()) {
if (Function *LF = F->Codegen()) {
fprintf(stderr, "Read top-level expression:");
LF->dump();
}
} else {
// Skip token for error recovery.
getNextToken();
}
}
/// top ::= definition | external | expression | ';'
static void MainLoop() {
while (1) {
fprintf(stderr, "ready> ");
switch (CurTok) {
case tok_eof: return;
case ';': getNextToken(); break; // ignore top-level semicolons.
case tok_def: HandleDefinition(); break;
case tok_extern: HandleExtern(); break;
default: HandleTopLevelExpression(); break;
}
}
}
//===----------------------------------------------------------------------===//
// "Library" functions that can be "extern'd" from user code.
//===----------------------------------------------------------------------===//
/// putchard - putchar that takes a double and returns 0.
extern "C"
double putchard(double X) {
putchar((char)X);
return 0;
}
//===----------------------------------------------------------------------===//
// Main driver code.
//===----------------------------------------------------------------------===//
int main() {
LLVMContext &Context = getGlobalContext();
// 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();
// Make the module, which holds all the code.
TheModule = new Module("my cool jit", Context);
// Run the main "interpreter loop" now.
MainLoop();
// Print out all of the generated code.
TheModule->dump();
return 0;
}
Reference: