從零寫一個編譯器（十二）：代碼生成之生成邏輯

項目的完整代碼在 C2j-Compiler

前言

在上一篇解釋完了一些基礎的Java字節碼指令后，就可以正式進入真正的代碼生成部分了。但是這部分先說的是代碼生成依靠的幾個類，也就是用來生成指令的操作。

這一篇用到的文件都在codegen下：

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

Directive.java

這個是枚舉類，用來生成一些比較特殊的指令

都生成像聲明一個類或者一個方法的范圍的指令，比較簡單。

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

這也是一個枚舉類，用來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

重點來了，生成的邏輯主要都在CodeGenerator和ProgramGenerator里，CodeGenerator是ProgramGenerator的父類

CodeGenerator的構造函數new了一個輸出流，用來輸出字節碼到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都屬於輸出基本指令的方法，都有多個重載方法來應對不一樣操作和操作數。需要注意的是，有的指令可能需要先緩存起來，在最后的時候一起提交，比如buffered、classDefine就是用來判斷是不是應該先緩存的布爾值

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

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

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

}

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

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

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

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

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

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

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

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

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

ProgramGenerator.java

ProgramGenerator繼承了CodeGenerator，也就是繼承了一些基本的操作，在上一篇像結構體、數組的指令輸出都在這個類里

處理嵌套

先看四個屬性，這四個屬性主要是就來處理嵌套的分支和循環。

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

• 當沒嵌套一個ifelse語句時候 embedded屬性就會加上一個字符‘i’，而當退出一個分支的時候，就把這個‘i’切割掉

• branch_count和branch_out都用來標志相同作用域的分支跳轉

• 也就是說如果有嵌套就用embedded來處理，如果是用一個作用域的分支就用branch_count和branch_out來做標志

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

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

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

loopCount則是對嵌套循環的處理

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

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

public void increaseLoopCount() {
    loopCount++;
}

處理結構體

putStructToClassDeclaration是定義結構體的，也就是new一個類。declareStructAsClass則是處理結構體里的變量，也就是相當於處理類的屬性

• 結構體如果已經類的定義的話，就會加入structNameList，不要進行重復的定義
• symbol.getValueSetter()如果不是空的話就表明是一個結構體數組，這樣就直接從數組加載這個實例，不用在堆棧上創建
• declareStructAsClass則是依照上一篇說的Java字節碼有關類的指令來創建一個類

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);
}

獲取堆棧信息

其它有關Java字節碼其實都是根據上一篇來完成的，邏輯不復雜，現在來看一個方法：getLocalVariableIndex，這個方法是獲取變量當前在隊列里的位置的

• 先拿到當前執行的函數，然后拿到函數的對應參數，再反轉（這和參數壓棧的順序有關）
• 然后把當前符號對應作用域的符號都添加到列表里
• 之后遍歷這個列表就可以算出這個符號對應在隊列里的位置

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;
}

小結

這一篇主要是根據上一篇的JVM字節碼來對不同的操作提供不同的方法來去輸出這些指令

歡迎Star!