手拉手教你实现一门编程语言 Enkel, 系列 13

本文系 Creating JVM language 翻译的第 13 篇。
原文中的代码和原文有不一致的地方均在新的代码仓库中更正过，建议参考新的代码仓库。

源码

Github

范围 for 循环

本节中我们来实现范围循环，在范围内迭代值，在 Java 中大概张这个样子：
for (int i=0;i<=5;i++)

Enkel 的等价形式：
for i from 0 to 5

我实现另外一个特性，循环会自动检测是递增还是递减：

for i from 0 to 5 //increment i from 0 to 5  - for (int i=0;i<=5;i++)

for i from 5 to 0 //decremenet i from 5 to 0 - for (int i=5;i>=0;i--)

递增或者递减必须在运行时推断，因为范围的值可能是方法调用的返回值。

for while 循环或者容器迭代器都很相似，本节不做描述。

语法规则更改

statement : block
           //other statement alternatives
           | forStatement ;

forStatement : 'for' ('(')? forConditions (')')? statement ;
forConditions : iterator=varReference  'from' startExpr=expression range='to' endExpr=expression ;

• forConditions 是迭代的条件表达式
• = 提高可读性
• 迭代器必须是变量的名字
• startExpression 用来初始化迭代器
• endExpressions 是迭代器的终止值

for (i from 0 to 5) print i 图形化的解析树如下所示：

image

匹配 Antlr 上下文对象

Antlr 根据语法规则会生成 ForStatementContext 对象，我们用它生成对编译器更加友好的类。可以解决迭代器变量未生明的问题。

public class ForStatementVisitor extends EnkelBaseVisitor<RangedForStatement> {

    //other stuff
    
    @Override
    public RangedForStatement visitForStatement(@NotNull ForStatementContext ctx) {
        EnkelParser.ForConditionsContext forExpressionContext = ctx.forConditions();
        Expression startExpression = forExpressionContext.startExpr.accept(expressionVisitor);
        Expression endExpression = forExpressionContext.endExpr.accept(expressionVisitor);
        VarReferenceContext iterator = forExpressionContext.iterator;
        String varName = iterator.getText();
        //If variable referenced by iterator already exists in the scope
        if(scope.localVariableExists(varName)) { 
            //register new variable value
            Statement iteratorVariable = new AssignmentStatement(varName, startExpression); 
            //get the statement (usually block))
            Statement statement = ctx.statement().accept(statementVisitor); 
            return new RangedForStatement(iteratorVariable, startExpression, endExpression,statement, varName, scope); 
        //Variable has not been declared in the scope
        } else { 
            //create new local variable and add to the scope
            scope.addLocalVariable(new LocalVariable(varName,startExpression.getType())); 
            //register variable declaration statement
            Statement iteratorVariable = new VariableDeclarationStatement(varName,startExpression); 
            Statement statement = ctx.statement().accept(statementVisitor);
            return new RangedForStatement(iteratorVariable, startExpression, endExpression,statement, varName,scope);
        }
    }
}

迭代器变量可能在作用中存在或者未生明，这两种情况都需要被妥善处理:

var iterator = 0
for (iterator from 0 to 5) print iterator

迭代器已经声明过，赋值给 startExpression。
new AssignmentStatement(varName,startExpression);

    for (iterator from 0 to 5) print iterator

迭代器没有声明，首先声明，然后赋值给 startExpression。
new VariableDeclarationStatement(varName,startExpression);

字节码生成

RangedForStatement 生成后，下面我们开始生成字节码。

JVM 中没有为 for 循环设计特殊的指令。一种实现方式就是使用控制流指令。

public void generate(RangedForStatement rangedForStatement) {
    Scope newScope = rangedForStatement.getScope();
    StatementGenerator scopeGeneratorWithNewScope = new StatementGenerator(methodVisitor, newScope);
    ExpressionGenrator exprGeneratorWithNewScope = new ExpressionGenrator(methodVisitor, newScope);
    Statement iterator = rangedForStatement.getIteratorVariableStatement();
    Label incrementationSection = new Label();
    Label decrementationSection = new Label();
    Label endLoopSection = new Label();
    String iteratorVarName = rangedForStatement.getIteratorVarName();
    Expression endExpression = rangedForStatement.getEndExpression();
    Expression iteratorVariable = new VarReference(iteratorVarName, rangedForStatement.getType());
    ConditionalExpression iteratorGreaterThanEndConditional = new ConditionalExpression(iteratorVariable, endExpression, CompareSign.GREATER);
    ConditionalExpression iteratorLessThanEndConditional = new ConditionalExpression(iteratorVariable, endExpression, CompareSign.LESS);

    //generates varaible declaration or variable reference (istore)
    iterator.accept(scopeGeneratorWithNewScope);

    //Section below checks whether the loop should be iterating or decrementing
    //If the range start is smaller than range end (i from 0 to 5)  then iterate (++)
    //If the range start is greater than range end (i from 5 to 0) then decrement (--)

    //Pushes 0 or 1 onto the stack 
    iteratorLessThanEndConditional.accept(exprGeneratorWithNewScope);
    //IFNE - is value on the stack (result of conditional) different than 0 (success)?
    methodVisitor.visitJumpInsn(Opcodes.IFNE,incrementationSection);

    iteratorGreaterThanEndConditional.accept(exprGeneratorWithNewScope);
    methodVisitor.visitJumpInsn(Opcodes.IFNE,decrementationSection);

    //Incrementation section
    methodVisitor.visitLabel(incrementationSection);
    rangedForStatement.getStatement().accept(scopeGeneratorWithNewScope); //execute the body
    methodVisitor.visitIincInsn(newScope.getLocalVariableIndex(iteratorVarName),1); //increment iterator
    iteratorGreaterThanEndConditional.accept(exprGeneratorWithNewScope); //is iterator greater than range end?
    methodVisitor.visitJumpInsn(Opcodes.IFEQ,incrementationSection); //if it is not go back loop again 
    //the iterator is greater than end range. Break out of the loop, skipping decrementation section
    methodVisitor.visitJumpInsn(Opcodes.GOTO,endLoopSection); 

    //Decrementation section
    methodVisitor.visitLabel(decrementationSection);
    rangedForStatement.getStatement().accept(scopeGeneratorWithNewScope);
    methodVisitor.visitIincInsn(newScope.getLocalVariableIndex(iteratorVarName),-1); //decrement iterator
    iteratorLessThanEndConditional.accept(exprGeneratorWithNewScope);
    methodVisitor.visitJumpInsn(Opcodes.IFEQ,decrementationSection);

    methodVisitor.visitLabel(endLoopSection);
}

这看起来有点复杂，因为递增递减的推测逻辑是在运行时决定的。

以 for (i from 0 to 5) 为例，我们来看一下整个流程:

1. 声明迭代器变量 i 并且赋予初始值 0
2. 检测迭代器的值 0 是否大于结束值 5
3. 因为 0 < 5, 因此递增，跳到递增部分
4. 执行 for 循环体内的语句
5. 递增 1
6. 检查迭代器的值是否大于 5
7. 如果条件不成立，跳到 4
8. 循环体执行 5 次后，跳到结束部分

示例

如下 Enkel 代码：

Loops {
    main(string[] args) {
        for i from 1 to 5 {
            print i
        }
    }
}

生成后的字节码反编译后的 Java 代码：

public class Loops {
    public static void main(String[] var0) {
        int var1 = 1;
        if(var1 >= 5 ) { //should it be decremented?
            do {
                System.out.println(var1);
                --var1;
            } while(var1 >= 5);
        } else { //should it be incremented?
            do {
                System.out.println(var1);
                ++var1;
            } while(var1 <= 5);
        }

    }
}

运行结果:

$ java Loops 
1
2
3
4
5