前言

本章对应官方教程第5章,本章介绍如何扩展Kaleidoscope以使用if / then / else表达式和一个简单的for循环。

教程如下:

教你使用swift写编译器玩具(0)

教你使用swift写编译器玩具(1)

教你使用swift写编译器玩具(2)

教你使用swift写编译器玩具(3)

教你使用swift写编译器玩具(4)

教你使用swift写编译器玩具(5)

教你使用swift写编译器玩具(6)

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教你使用swift写编译器玩具(8)

仓库在这

开始

if / then / else

if / then / else也是一种表达式,我们需要把它计算为int1类型,0是假,1是真。如果if表达式计算为真返回then表达式,否则返回else表达式。

首先我们需要做的第一件事情是扩展我们的Token枚举

...
case `if`
case then
case `else`
...

接着我们在LexernextToken()方法中补充token的解析

} else if identifierStr == "if" {
		currentToken = CurrentToken(token: .if, val: "if")
} else if identifierStr == "then" {
		currentToken = CurrentToken(token: .then, val: "then")
} else if identifierStr == "else" {
		currentToken = CurrentToken(token: .else, val: "else")
}

if / then / else的AST扩展

为了解析新的表达式我们需要添加新的AST Node。

class IfExprAST: ExprAST {
    
    let cond: ExprAST
    
    let then: ExprAST
    
    let `else`: ExprAST
    
    init(_ cond: ExprAST, _ then: ExprAST, _ `else`: ExprAST) {
        self.cond = cond
        self.then = then
        self.else = `else`
    }
    
}

if / then / else的Parser扩展

有了AST之后我们要做的事情那就是扩展Parser了。

/// 解析条件语句
///
/// - Returns: AST
private func parseIfExpr() -> ExprAST? {
    lexer.nextToken()
  	//解析if表达式
    let cond = parseExpression()
    guard cond != nil else {
        return nil
    }
  	//if表达式后面不是then就报错
    guard lexer.currentToken!.token == .then else {
        fatalError("expected then.")
    }
    lexer.nextToken()
  	//解析then表达式
    let then = parseExpression()
    guard then != nil else {
        return nil
    }
  	//then表达式后面不是else表达式就报错
    guard lexer.currentToken!.token == .else else {
        fatalError("expected else.")
    }
    lexer.nextToken()
    let `else` = parseExpression()
    guard `else` != nil else {
        return nil
    }
    return IfExprAST(cond!, then!, `else`!)
}

接下来我们把它放在parsePrimary中。

/// 解析基本表达式的入口
///
/// - Returns: AST
private func parsePrimary() -> ExprAST? {
    guard lexer.currentToken != nil else {
        return nil
    }
    if lexer.currentToken!.val == "(" {
        return parseParenExpr()
    }
    switch lexer.currentToken!.token {
    case .identifier:
        return parseIdentifierExpr()
    case .number:
        return parseNumberExpr()
    case .if:
        return parseIfExpr()
    default:
        fatalError("unknow token when expecting an expression")
    }
}

if / then / else的代码生成

我们需要在IfExprAST中实现方法codeGen()。这里我们需要使用的是一个SSA操作:Phi操作

func codeGen() -> IRValue? {
    var condV = cond.codeGen()
    guard condV != nil else {
        return nil
    }
  	//这里有个神坑就是build条件时候要使用int1类型
    condV = builder.buildICmp(condV!, IntType.int1.zero(), .equal, name: "ifCond")
    
    let theFunction = builder.insertBlock?.parent
    guard theFunction != nil else {
        return nil
    }
    
    //为then else merge创建basic block并放在函数里
    let thenBB = theFunction!.appendBasicBlock(named: "then")
    let elseBB = theFunction!.appendBasicBlock(named: "else")
    let mergeBB = theFunction!.appendBasicBlock(named: "merge")
    
  	//构建控制流表达式
    builder.buildCondBr(condition: condV!, then: thenBB, else: elseBB)
    
  	//让builder移动到then的基本块里
    builder.positionAtEnd(of: thenBB)
  	//插入then
    let thenVal = then.codeGen()
    guard thenVal != nil else {
        return nil
    }
    builder.buildBr(mergeBB)
    //让builer移动到else的基本块里
    builder.positionAtEnd(of: elseBB)
    let elseVal = `else`.codeGen()
    guard elseVal != nil else {
        return nil
    }
    builder.buildBr(mergeBB)
    //让builder移动到merge的基本块里
    builder.positionAtEnd(of: mergeBB)
    let phi = builder.buildPhi(FloatType.double, name: "phi")
    phi.addIncoming([(thenVal!, thenBB), (elseVal!, elseBB)])
    
    return phi
}

for循环表达式

Kaleidoscope的for循环长下面这样,1.0是可选的步长,默认即为1.0。

for i = 1, i < n, 1.0 in

for循环表达式的处理会复杂一些,但还是同样运用了Phi操作来处理。

同控制流语句的扩展,我们还是先要扩展TokenLexer

case `for`

else if identifierStr == "for" {
		currentToken = CurrentToken(token: .for, val: "for")
}

接着我们扩展for循环的AST NodeForExprAST

class ForExprAST: ExprAST {
    
    let name: String
    
    let start: ExprAST
    
    let end: ExprAST
    
    let step: ExprAST?
    
    let body: ExprAST
    
    init(_ name: String, _ start: ExprAST, _ end: ExprAST, _ step: ExprAST?, _ body: ExprAST) {
        self.name = name
        self.start = start
        self.end = end
        self.step = step
        self.body = body
    }
  
}

step用来表示for循环的步长,即每次变量的增长值。编译器通过检查第二个逗号是否存在来判断,如果不存在我们把它设为nil

for循环的Parser扩展

/// 解析For表达式
///
/// - Returns: AST
private func parseForExpr() -> ExprAST? {
    lexer.nextToken()
  	//第一个得是变量,比如说`i`
    guard lexer.currentToken!.token == .identifier else {
        fatalError("expected identifier after for.")
    }
    let idName = lexer.currentToken!.val
    lexer.nextToken()
    guard lexer.currentToken!.val == "=" else {
        fatalError("expected '=' after for.")
    }
    
    lexer.nextToken()
  	//循环开始值
    let start = parseExpression()
    guard start != nil else {
        return nil
    }
    guard lexer.currentToken!.val == "," else {
        fatalError("expected ',' after start value.")
    }
    
    lexer.nextToken()
  	//循环结束值
    let end = parseExpression()
    guard end != nil else {
        return nil
    }
    
  	//步长
    var step: ExprAST!
    if lexer.currentToken!.val == "," {
        lexer.nextToken()
        step = parseExpression()
        guard step != nil else {
            return nil
        }
    }
    //in作为for循环的关键字不可缺少
    guard lexer.currentToken!.token == .in else {
        fatalError("expected 'in' after for.")
    }
    lexer.nextToken()
    //for循环的循环体解析
    let body = parseExpression()
    guard body != nil else {
        return nil
    }
    
    return ForExprAST(idName, start!, end!, step, body!)
}

我们在parsePrimary()方法中补充调用。

case .for:
		return parseForExpr()

for循环的代码生成

话不多说直接看代码,过程都会体现在注释中。

func codeGen() -> IRValue? {
    let startVal = start.codeGen()
    guard startVal != nil else {
        return nil
    }
    
    //for循环,插在当前的block之后
    let theFunction = builder.insertBlock?.parent
    guard theFunction != nil else {
        return nil
    }
    let preHeaderBB = builder.insertBlock
  	//循环体的基本块
    let loopBB = theFunction!.appendBasicBlock(named: "loop")
    builder.buildBr(loopBB)
  	//让builder移动到
    builder.positionAtEnd(of: loopBB)
    
    //这里控制循环或退出
    let phi = builder.buildPhi(FloatType.double, name: name)
    phi.addIncoming([(startVal!, preHeaderBB!)])
    
  	//防止for循环作用域与外部产生变量命名冲突,所以先记录一下,是nil也无所谓
    let oldVal = namedValues[name]
    namedValues[name] = phi
    
    guard body.codeGen() != nil else {
        return nil
    }
    
    let stepVal: IRValue?
    if step != nil {
      	//有步长就要解析
        stepVal = step!.codeGen()
        guard stepVal != nil else {
            return nil
        }
    } else {
      	//默认步长为1.0
        stepVal = FloatType.double.constant(1)
    }
    //步长的增长指令
    let nextVar = builder.buildAdd(phi, stepVal!, name: "nextVar")
    
    //循环终止条件
    var endCond = end.codeGen()
    guard endCond != nil else {
        return nil
    }
    endCond = builder.buildICmp(endCond!, IntType.int1.zero(), .equal, name: "loopCond")
    
    //循环后的代码basic block
    let loopEndBB = builder.insertBlock
    let afterBB = theFunction?.appendBasicBlock(named: "afterLoop")
    builder.buildCondBr(condition: endCond!, then: loopBB, else: afterBB!)
    builder.positionAtEnd(of: afterBB!)
    
    phi.addIncoming([(nextVar, loopEndBB!)])
    
    if oldVal != nil {
        namedValues[name] = oldVal!
    } else {
        namedValues[name] = nil
    }
    
    //for循环解析总是返回0
    return FloatType.double.constant(0)
}

测试

控制流语句

extern foo();
Read extern:

declare i64 @foo()
extern bar();
Read extern:

declare i64 @bar()
def baz(x) if x then foo() else bar();
Read function definition:

define i64 @baz(i64 %x) {
entry:
  %ifCond = icmp eq i64 %x, 0
  br i1 %ifCond, label %then, label %else

then:                                             ; preds = %entry
  %call = call i64 @foo()
  br label %merge

else:                                             ; preds = %entry
  %call1 = call i64 @bar()
  br label %merge

merge:                                            ; preds = %else, %then
  %phi = phi i64 [ %call, %then ], [ %call1, %else ]
  ret i64 %phi
}

for循环语句

extern putchard(char);
Read extern:

declare i64 @putchard(i64 %char)
def printstar(n) for i = 1, i < n, 1 in putchard(42);
Read function definition:

define i64 @printstar(i64 %n) {
entry:
  br label %loop

loop:                                             ; preds = %loop, %entry
  %i = phi i64 [ 1, %entry ], [ %nextVar, %loop ]
  %call = call i64 @putchard(i64 42)
  %nextVar = add i64 %i, 1
  %boolCmp = icmp slt i64 %i, %n
  %0 = sext i1 %boolCmp to i64
  %loopCond = icmp eq i64 %0, 0
  br i1 %loopCond, label %loop, label %afterLoop

afterLoop:                                        ; preds = %loop
  ret i64 0
}