从零写一个编译器（十二）：代码生成之生成逻辑

项目的完整代码在 C2j-Compiler

前言

在上一篇解释完了一些基础的Java字节码指令后，就可以正式进入真正的代码生成部分了。但是这部分先说的是代码生成依靠的几个类，也就是用来生成指令的操作。

这一篇用到的文件都在codegen下：

• Directive.java
• Instruction.java
• CodeGenerator.java
• ProgramGenerator.java

Directive.java

这个是枚举类，用来生成一些比较特殊的指令

都生成像声明一个类或者一个方法的范围的指令，比较简单。

public enum Directive {
    CLASS_PUBLIC(".class public"),
    END_CLASS(".end class"),
    SUPER(".super"),
    FIELD_PRIVATE_STATIC(".field private static"),
    METHOD_STATIC(".method static"),
    METHOD_PUBLIC(".method public"),
    FIELD_PUBLIC(".field public"),
    METHOD_PUBBLIC_STATIC(".method public static"),
    END_METHOD(".end method"),
    LIMIT_LOCALS(".limit locals"),
    LIMIT_STACK(".limit stack"),
    VAR(".var"),
    LINE(".line");
    
    private String text;
    
    Directive(String text) {
    	this.text = text;
    }
    
    public String toString() {
    	return text;
    }
}

Instruction.java

这也是一个枚举类，用来生成一些基本的指令

public enum Instruction {
    LDC("ldc"),
    
    GETSTATIC("getstatic"),
    SIPUSH("sipush"),
    IADD("iadd"),
    IMUL("imul"),
    ISUB("isub"),
    IDIV("idiv"),
    INVOKEVIRTUAL("invokevirtual"),
    INVOKESTATIC("invokestatic"),
    INVOKESPECIAL("invokespecial"),
    RETURN("return"),
    IRETURN("ireturn"),
    ILOAD("iload"),
    ISTORE("istore"),
    NEWARRAY("newarray"),
    NEW("new"),
    DUP("dup"),
    ASTORE("astore"),
    IASTORE("iastore"),
    ALOAD("aload"),
    PUTFIELD("putfield"),
    GETFIELD("getfield"),
    ANEWARRAY("anewarray"),
    AASTORE("aastore"),
    AALOAD("aaload"),
    IF_ICMPEG("if_icmpeq"),  
    IF_ICMPNE("if_icmpne"),
    IF_ICMPLT("if_icmplt"),
    IF_ICMPGE("if_icmpge"),
    IF_ICMPGT("if_icmpgt"),
    IF_ICMPLE("if_icmple"),
    GOTO("goto"),
    IALOAD("iaload");
    
    private String text;
    Instruction(String s) {
    	this.text = s;
    }
    
    public String toString() {
    	return text;
    }
}

CodeGenerator.java

重点来了，生成的逻辑主要都在CodeGenerator和ProgramGenerator里，CodeGenerator是ProgramGenerator的父类

CodeGenerator的构造函数new了一个输出流，用来输出字节码到xxx.j里

public CodeGenerator() {
      String assemblyFileName = programName + ".j";

      try {
          bytecodeFile = new PrintWriter(new PrintStream(new
                  File(assemblyFileName)));
      } catch (FileNotFoundException e) {
          e.printStackTrace();
      }
  }

emit、emitString、emitDirective、emitBlankLine都属于输出基本指令的方法，都有多个重载方法来应对不一样操作和操作数。需要注意的是，有的指令可能需要先缓存起来，在最后的时候一起提交，比如buffered、classDefine就是用来判断是不是应该先缓存的布尔值

public void emitString(String s) {
    if (buffered) {
        bufferedContent += s + "
";
        return;
    }

    if (classDefine) {
        classDefinition += s + "
";
        return;
    }

    bytecodeFile.print(s);
    bytecodeFile.flush();

}

public void emit(Instruction opcode) {
    if (buffered) {
        bufferedContent += "	" + opcode.toString() + "
";
        return;
    }

    if (classDefine) {
        classDefinition += "	" + opcode.toString() + "
";
        return;
    }

    bytecodeFile.println("	" + opcode.toString());
    bytecodeFile.flush();
    ++instructionCount;
}

public void emitDirective(Directive directive, String operand1, String operand2, String operand3) {
    if (buffered) {
        bufferedContent += directive.toString() + " " + operand1 + " " + operand2 + " " + operand3 + "
";
        return;
    }

    if (classDefine) {
        classDefinition += directive.toString() + " " + operand1 + " " + operand2 + " " + operand3 + "
";
        return;
    }

    bytecodeFile.println(directive.toString() + " " + operand1 + " " + operand2 + " " + operand3);
    ++instructionCount;
}

public void emitBlankLine() {
    if (buffered) {
        bufferedContent += "
";
        return;
    }

    if (classDefine) {
        classDefinition += "
";
        return;
    }

    bytecodeFile.println();
    bytecodeFile.flush();
}

ProgramGenerator.java

ProgramGenerator继承了CodeGenerator，也就是继承了一些基本的操作，在上一篇像结构体、数组的指令输出都在这个类里

处理嵌套

先看四个属性，这四个属性主要是就来处理嵌套的分支和循环。

private int branch_count = 0;
private int branch_out = 0;
private String embedded = "";
private int loopCount = 0;

• 当没嵌套一个ifelse语句时候 embedded属性就会加上一个字符‘i’，而当退出一个分支的时候，就把这个‘i’切割掉

• branch_count和branch_out都用来标志相同作用域的分支跳转

• 也就是说如果有嵌套就用embedded来处理，如果是用一个作用域的分支就用branch_count和branch_out来做标志

public void incraseIfElseEmbed() {
    embedded += "i";
}

public void decraseIfElseEmbed() {
    embedded = embedded.substring(1);
}

public void emitBranchOut() {
    String s = "
" + embedded + "branch_out" + branch_out + ":
";
    this.emitString(s);
    branch_out++;
}

loopCount则是对嵌套循环的处理

public void emitLoopBranch() {
    String s = "
" + "loop" + loopCount + ":" + "
";
    emitString(s);
}

public String getLoopBranch() {
    return "loop" + loopCount;
}

public void increaseLoopCount() {
    loopCount++;
}

处理结构体

putStructToClassDeclaration是定义结构体的，也就是new一个类。declareStructAsClass则是处理结构体里的变量，也就是相当于处理类的属性

• 结构体如果已经类的定义的话，就会加入structNameList，不要进行重复的定义
• symbol.getValueSetter()如果不是空的话就表明是一个结构体数组，这样就直接从数组加载这个实例，不用在堆栈上创建
• declareStructAsClass则是依照上一篇说的Java字节码有关类的指令来创建一个类

public void putStructToClassDeclaration(Symbol symbol) {
    Specifier sp = symbol.getSpecifierByType(Specifier.STRUCTURE);
    if (sp == null) {
        return;
    }

    StructDefine struct = sp.getStruct();
    if (structNameList.contains(struct.getTag())) {
        return;
    } else {
        structNameList.add(struct.getTag());
    }

    if (symbol.getValueSetter() == null) {
        this.emit(Instruction.NEW, struct.getTag());
        this.emit(Instruction.DUP);
        this.emit(Instruction.INVOKESPECIAL, struct.getTag() + "/" + "<init>()V");
        int idx = this.getLocalVariableIndex(symbol);
        this.emit(Instruction.ASTORE, "" + idx);
    }

    declareStructAsClass(struct);
}

private void declareStructAsClass(StructDefine struct) {
    this.setClassDefinition(true);

    this.emitDirective(Directive.CLASS_PUBLIC, struct.getTag());
    this.emitDirective(Directive.SUPER, "java/lang/Object");

    Symbol fields = struct.getFields();
    do {
        String fieldName = fields.getName() + " ";
        if (fields.getDeclarator(Declarator.ARRAY) != null) {
            fieldName += "[";
        }

        if (fields.hasType(Specifier.INT)) {
            fieldName += "I";
        } else if (fields.hasType(Specifier.CHAR)) {
            fieldName += "C";
        } else if (fields.hasType(Specifier.CHAR) && fields.getDeclarator(Declarator.POINTER) != null) {
            fieldName += "Ljava/lang/String;";
        }

        this.emitDirective(Directive.FIELD_PUBLIC, fieldName);
        fields = fields.getNextSymbol();
    } while (fields != null);

    this.emitDirective(Directive.METHOD_PUBLIC, "<init>()V");
    this.emit(Instruction.ALOAD, "0");
    String superInit = "java/lang/Object/<init>()V";
    this.emit(Instruction.INVOKESPECIAL, superInit);

    fields = struct.getFields();
    do {
        this.emit(Instruction.ALOAD, "0");
        String fieldName = struct.getTag() + "/" + fields.getName();
        String fieldType = "";
        if (fields.hasType(Specifier.INT)) {
            fieldType = "I";
            this.emit(Instruction.SIPUSH, "0");
        } else if (fields.hasType(Specifier.CHAR)) {
            fieldType = "C";
            this.emit(Instruction.SIPUSH, "0");
        } else if (fields.hasType(Specifier.CHAR) && fields.getDeclarator(Declarator.POINTER) != null) {
            fieldType = "Ljava/lang/String;";
            this.emit(Instruction.LDC, " ");
        }

        String classField = fieldName + " " + fieldType;
        this.emit(Instruction.PUTFIELD, classField);

        fields = fields.getNextSymbol();
    } while (fields != null);

    this.emit(Instruction.RETURN);
    this.emitDirective(Directive.END_METHOD);
    this.emitDirective(Directive.END_CLASS);

    this.setClassDefinition(false);
}

获取堆栈信息

其它有关Java字节码其实都是根据上一篇来完成的，逻辑不复杂，现在来看一个方法：getLocalVariableIndex，这个方法是获取变量当前在队列里的位置的

• 先拿到当前执行的函数，然后拿到函数的对应参数，再反转（这和参数压栈的顺序有关）
• 然后把当前符号对应作用域的符号都添加到列表里
• 之后遍历这个列表就可以算出这个符号对应在队列里的位置

public int getLocalVariableIndex(Symbol symbol) {
    TypeSystem typeSys = TypeSystem.getInstance();
    String funcName = nameStack.peek();
    Symbol funcSym = typeSys.getSymbolByText(funcName, 0, "main");
    ArrayList<Symbol> localVariables = new ArrayList<>();
    Symbol s = funcSym.getArgList();
    while (s != null) {
        localVariables.add(s);
        s = s.getNextSymbol();
    }
    Collections.reverse(localVariables);

    ArrayList<Symbol> list = typeSys.getSymbolsByScope(symbol.getScope());
    for (int i = 0; i < list.size(); i++) {
        if (!localVariables.contains(list.get(i))) {
            localVariables.add(list.get(i));
        }
    }

    for (int i = 0; i < localVariables.size(); i++) {
        if (localVariables.get(i) == symbol) {
            return i;
        }
    }

    return -1;
}

小结

这一篇主要是根据上一篇的JVM字节码来对不同的操作提供不同的方法来去输出这些指令

欢迎Star!