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(newFile(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 + "\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 = "\n" + embedded + "branch_out" + branch_out + ":\n";this.emitString(s);branch_out++;
}

loopCount则是对嵌套循环的处理

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