从零写一个编译器（十三）：代码生成之遍历AST

项目的完整代码在 C2j-Compiler

前言

在上一篇完成对JVM指令的生成，下面就可以真正进入代码生成部分了。通常现代编译器都是先把生成IR，再经过代码优化等等，最后才编译成目标平台代码。但是时间水平有限，我们没有IR也没有代码优化，就直接利用AST生成Java字节码

入口

进行代码生成的入口在CodeGen，和之前解释器一样：先获取main函数的头节点，从这个节点开始，先进入函数定义，再进入代码块

函数定义节点

在进入函数定义节点的时候，就要生成一个函数定义对应的Java字节码，即一个静态方法（因为我们对整个C语言文件生成为一个类，main方法为public static main，其它的则是对应的静态方法，结构体则是另外的类）

• 对于函数定义先从节点中拿到对应的函数命和参数
• emitArgs是用来处理参数的，根据参数生成相应的Java字节码
• 如果这个函数是main的话已经是交由前面处理了，逻辑差不多（具体在start.Start中）

case SyntaxProductionInit.NewName_LP_RP_TO_FunctDecl:
    root.reverseChildren();
    AstNode n = root.getChildren().get(0);
    String name = (String) n.getAttribute(NodeKey.TEXT);
    symbol = (Symbol) root.getAttribute(NodeKey.SYMBOL);
    generator.setCurrentFuncName(name);
    if (name != null && !name.equals("main")) {
        String declaration = name + emitArgs(symbol);
        generator.emitDirective(Directive.METHOD_PUBBLIC_STATIC, declaration);
        generator.setNameAndDeclaration(name, declaration);
    }
    copyChild(root, root.getChildren().get(0));
    break;

case SyntaxProductionInit.NewName_LP_VarList_RP_TO_FunctDecl:
    n = root.getChildren().get(0);
    name = (String) n.getAttribute(NodeKey.TEXT);
    symbol = (Symbol) root.getAttribute(NodeKey.SYMBOL);
    generator.setCurrentFuncName(name);
    if (name != null && !name.equals("main")) {
        String declaration = name + emitArgs(symbol);
        generator.emitDirective(Directive.METHOD_PUBBLIC_STATIC, declaration);
        generator.setNameAndDeclaration(name, declaration);
    }

    Symbol args = symbol.getArgList();

    if (args == null || argsList == null || argsList.isEmpty()) {
        System.err.println("generate function with arg list but arg list is null");
        System.exit(1);
    }
    break;

创建结构体和数组

数组

创建结构体和数组的节点在DefGenerate里，可以看到在这里只处理了数组和普通变量，有关结构体的处理是在对结构体第一次使用的时候。顺便提一下代码生成对于赋初值操作是没有进行处理的。

• 如果是个数组，酒直接调用ProgramGenerator直接生成创建数组的指令
• 如果是个普通变量，就直接找到它并且赋值为0（这里变量在队列里的位置是根据符号表来计算的，具体可以看上一篇的getLocalVariableIndex方法）

public class DefGenerate extends BaseGenerate {
    @Override
    public Object generate(AstNode root) {
        int production = (int) root.getAttribute(NodeKey.PRODUCTION);
        ProgramGenerator generator = ProgramGenerator.getInstance();
        Symbol symbol = (Symbol) root.getAttribute(NodeKey.SYMBOL);

        switch (production) {
            case SyntaxProductionInit.Specifiers_DeclList_Semi_TO_Def:
                Declarator declarator = symbol.getDeclarator(Declarator.ARRAY);
                if (declarator != null) {
                    if (symbol.getSpecifierByType(Specifier.STRUCTURE) == null) {
                        generator.createArray(symbol);
                    }
                } else {
                    int i = generator.getLocalVariableIndex(symbol);
                    generator.emit(Instruction.SIPUSH, "" + 0);
                    generator.emit(Instruction.ISTORE, "" + i);
                }

                break;

            default:
                break;
        }

        return root;
    }
}

结构体

处理结构体定义的代码在UnaryNodeGenerate，也就是只有在使用到结构体定义时才会进行定义

• 先拿到当前UNARY的符号，如果instanceof ArrayValueSetter就说明是一个结构体数组，就进入getStructSymbolFromStructArray方法创建一个结构体数组，并返回当前下标的结构体对象
• 设置当前结构体的作用域范围
• 对结构体作为类进行定义
• 然后对读取结构体的域
• 其实可以忽略指针部分，因为代码生成并没有对指针进行模拟

case SyntaxProductionInit.Unary_StructOP_Name_TO_Unary:
    child = root.getChildren().get(0);
    String fieldName = (String) root.getAttribute(NodeKey.TEXT);
    Object object = child.getAttribute(NodeKey.SYMBOL);
    boolean isStructArray = false;

    if (object instanceof ArrayValueSetter) {
        symbol = getStructSymbolFromStructArray(object);
        symbol.addValueSetter(object);
        isStructArray = true;
    } else {
        symbol = (Symbol) child.getAttribute(NodeKey.SYMBOL);
    }

    if (isStructArray) {
        ArrayValueSetter vs = (ArrayValueSetter) object;
        Symbol structArray = vs.getSymbol();
        structArray.addScope(ProgramGenerator.getInstance().getCurrentFuncName());
    } else {
        symbol.addScope(ProgramGenerator.getInstance().getCurrentFuncName());
    }

    ProgramGenerator.getInstance().putStructToClassDeclaration(symbol);

    if (isSymbolStructPointer(symbol)) {
        copyBetweenStructAndMem(symbol, false);
    }

    Symbol args = symbol.getArgList();
    while (args != null) {
        if (args.getName().equals(fieldName)) {
            args.setStructParent(symbol);
            break;
        }

        args = args.getNextSymbol();
    }

    if (args == null) {
        System.err.println("access a filed not in struct object!");
        System.exit(1);
    }

    if (args.getValue() != null) {
        ProgramGenerator.getInstance().readValueFromStructMember(symbol, args);
    }

    root.setAttribute(NodeKey.SYMBOL, args);
    root.setAttribute(NodeKey.VALUE, args.getValue());

    if (isSymbolStructPointer(symbol)) {
        checkValidPointer(symbol);
        structObjSymbol = symbol;
        monitorSymbol = args;

        GenerateBrocasterImpl.getInstance().registerReceiverForAfterExe(this);
    } else {
        structObjSymbol = null;
    }
    break;

一元操作节点

这个节点和在解释器的有很多相同，除了有对结构体的操作，其它的也是有非常重要的作用

• 像数字、字符串或者是变量和之前的操作都是把信息传递到父节点，交由父节点处理

case SyntaxProductionInit.Number_TO_Unary:
    text = (String) root.getAttribute(NodeKey.TEXT);
    boolean isFloat = text.indexOf('.') != -1;
    if (isFloat) {
        value = Float.valueOf(text);
        root.setAttribute(NodeKey.VALUE, value);
    } else {
        value = Integer.valueOf(text);
        root.setAttribute(NodeKey.VALUE, value);
    }
    break;

case SyntaxProductionInit.Name_TO_Unary:
    symbol = (Symbol) root.getAttribute(NodeKey.SYMBOL);
    if (symbol != null) {
        root.setAttribute(NodeKey.VALUE, symbol.getValue());
        root.setAttribute(NodeKey.TEXT, symbol.getName());
    }
    break;

case SyntaxProductionInit.String_TO_Unary:
    text = (String) root.getAttribute(NodeKey.TEXT);
    root.setAttribute(NodeKey.VALUE, text);
    break;

case SyntaxProductionInit.Unary_LB_Expr_RB_TO_Unary:
    child = root.getChildren().get(0);
    symbol = (Symbol) child.getAttribute(NodeKey.SYMBOL);

    child = root.getChildren().get(1);
    int index = 0;
    if (child.getAttribute(NodeKey.VALUE) != null) {
        index = (Integer) child.getAttribute(NodeKey.VALUE);
    }
    Object idxObj = child.getAttribute(NodeKey.SYMBOL);

    try {
        Declarator declarator = symbol.getDeclarator(Declarator.ARRAY);
        if (declarator != null) {
            Object val = declarator.getElement((int) index);
            root.setAttribute(NodeKey.VALUE, val);
            ArrayValueSetter setter;
            if (idxObj == null) {
                setter = new ArrayValueSetter(symbol, index);
            } else {
                setter = new ArrayValueSetter(symbol, idxObj);
            }

            root.setAttribute(NodeKey.SYMBOL, setter);
            root.setAttribute(NodeKey.TEXT, symbol.getName());

        }
        Declarator pointer = symbol.getDeclarator(Declarator.POINTER);
        if (pointer != null) {
            setPointerValue(root, symbol, index);
            PointerValueSetter pv = new PointerValueSetter(symbol, index);
            root.setAttribute(NodeKey.SYMBOL, pv);
            root.setAttribute(NodeKey.TEXT, symbol.getName());
        }
    } catch (Exception e) {
        e.printStackTrace();
        System.exit(1);
    }
    break;

赋值操作

• 如果当前是一个数组，先拿到它的符号和下标
• 如果不是结构体数组，那么拿到下标直接用readArrayElement生成读取数组元素的指令
• 如果是一个符号则用getLocalVariableIndex读取这个符号的值
• 如果是一个常数，则直接生成IPUSH指令
• 最后进行赋值操作，如果不是对结构体的域进行赋值就直接用getLocalVariableIndex拿到队列位置然后生成ISTORE
• 如果是对结构体数组的元素的域的赋值，就调用assignValueToStructMemberFromArray生成代码，如果只是结构体就直接调用assignValueToStructMember生成代码

ProgramGenerator generator = ProgramGenerator.getInstance();

if (BaseGenerate.resultOnStack) {
    this.value = obj;
    BaseGenerate.resultOnStack = false;
} else if (obj instanceof ArrayValueSetter) {
    ArrayValueSetter setter = (ArrayValueSetter) obj;
    Symbol symbol = setter.getSymbol();
    Object index = setter.getIndex();
    if (symbol.getSpecifierByType(Specifier.STRUCTURE) == null) {
        if (index instanceof Symbol) {
            ProgramGenerator.getInstance().readArrayElement(symbol, index);
            if (((Symbol) index).getValue() != null) {
                int i = (int) ((Symbol) index).getValue();
                try {
                    this.value = symbol.getDeclarator(Declarator.ARRAY).getElement(i);
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        } else {
            int i = (int) index;
            try {
                this.value = symbol.getDeclarator(Declarator.ARRAY).getElement(i);
            } catch (Exception e) {
                e.printStackTrace();
            }

            ProgramGenerator.getInstance().readArrayElement(symbol, index);
        }
    }
} else if (obj instanceof Symbol) {
    Symbol symbol = (Symbol) obj;
    this.value = symbol.value;
    int i = generator.getLocalVariableIndex(symbol);
    generator.emit(Instruction.ILOAD, "" + i);
} else if (obj instanceof Integer) {
    Integer val = (Integer) obj;
    generator.emit(Instruction.SIPUSH, "" + val);
    this.value = obj;
}

if (!this.isStructMember()) {
    int idx = generator.getLocalVariableIndex(this);
    if (!generator.isPassingArguments()) {
        generator.emit(Instruction.ISTORE, "" + idx);
    }
} else {
    if (this.getStructSymbol().getValueSetter() != null) {
        generator.assignValueToStructMemberFromArray(this.getStructSymbol().getValueSetter(), this, this.value);
    } else {
        generator.assignValueToStructMember(this.getStructSymbol(), this, this.value);
    }
}

最后

完成这部分后，对下面的代码

void quicksort(int A[10], int p, int r) {
    int x;
    int i;
    i = p - 1;
    int j;
    int t;
    int v;
    v = r - 1;
    if (p < r) {
        x = A[r];
        for (j = p; j <= v; j++) {
            if (A[j] <= x) {
                i++;
                t = A[i];
                A[i] = A[j];
                A[j] = t;
            }
        }
        v = i + 1;
        t = A[v];
        A[v] = A[r];
        A[r] = t;
        t = v - 1;
        quicksort(A, p, t);
        t = v + 1;
        quicksort(A, t, r);
    }
}

void main () {
    int a[10];
    int i;
    int t;
    printf("before quick sort:");
    for(i = 0; i < 10; i++) {
        t = (10 - i);
        a[i] = t;
        printf("value of a[%d] is %d", i, a[i]);
    }
    quicksort(a, 0, 9);
    printf("after quick sort:");
    for (i = 0; i < 10; i++) {
        printf("value of a[%d] is %d", i, a[i]);
    }
}

则会生成下面的Java字节码

.class public C2Bytecode
.super java/lang/Object

.method public static main([Ljava/lang/String;)V
	sipush	10
	newarray	int
	astore	0
	sipush	0
	istore	1
	sipush	0
	istore	2
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"before quick sort:"
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"
"
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	sipush	0
	istore	1

loop0:
	iload	1
	sipush	10
if_icmpge branch0
	sipush	10
	iload	1
	isub
	istore	2
	aload	0
	iload	1
	iload	2
	iastore
	aload	0
	iload	1
	iaload
	istore	3
	iload	1
	istore	4
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"value of a["
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	iload	4
	invokevirtual	java/io/PrintStream/print(I)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"] is "
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	iload	3
	invokevirtual	java/io/PrintStream/print(I)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"
"
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	iload	1
	sipush	1
	iadd
	istore	1
goto loop0
branch0:
	aload	0
	sipush	0
	sipush	9
	invokestatic	C2Bytecode/quicksort([III)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"after quick sort:"
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"
"
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	sipush	0
	istore	1

loop2:
	iload	1
	sipush	10
if_icmpge branch4
	aload	0
	iload	1
	iaload
	istore	3
	iload	1
	istore	4
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"value of a["
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	iload	4
	invokevirtual	java/io/PrintStream/print(I)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"] is "
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	iload	3
	invokevirtual	java/io/PrintStream/print(I)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"
"
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	iload	1
	sipush	1
	iadd
	istore	1
goto loop2
branch4:
	return
.end method
.method public static quicksort([III)V
	sipush	2
	newarray	int
	astore	6
	sipush	0
	istore	5
	sipush	1
	istore	5
	aload	6
	iload	5
	sipush	1
	iastore
	aload	6
	sipush	1
	iaload
	istore	10
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"before quick sort: "
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	iload	10
	invokevirtual	java/io/PrintStream/print(I)V
	getstatic	java/lang/System/out Ljava/io/PrintStream;
	ldc	"
"
	invokevirtual	java/io/PrintStream/print(Ljava/lang/String;)V
	sipush	0
	istore	9
	sipush	0
	istore	3
	iload	1
	sipush	1
	isub
	istore	3
	sipush	0
	istore	4
	sipush	0
	istore	7
	sipush	0
	istore	8
	iload	2
	sipush	1
	isub
	istore	8
	iload	1
	iload	2
if_icmpge branch1

	aload	0
	iload	2
	iaload
	istore	9
	iload	1
	istore	4

loop1:

	iload	4
	iload	8
if_icmpgt ibranch1

	aload	0
	iload	4
	iaload
	iload	9
if_icmpgt ibranch2

	iload	3
	sipush	1
	iadd
	istore	3
	aload	0
	iload	3
	iaload
	istore	7
	aload	0
	iload	3
	aload	0
	iload	4
	iaload
	iastore
	aload	0
	iload	4
	iload	7
	iastore
ibranch2:

	iload	4
	sipush	1
	iadd
	istore	4
goto loop1

ibranch1:

	iload	3
	sipush	1
	iadd
	istore	8
	aload	0
	iload	8
	iaload
	istore	7
	aload	0
	iload	8
	aload	0
	iload	2
	iaload
	iastore
	aload	0
	iload	2
	iload	7
	iastore
	iload	8
	sipush	1
	isub
	istore	7
	aload	0
	iload	1
	iload	7
	invokestatic	C2Bytecode/quicksort([III)V
	iload	8
	sipush	1
	iadd
	istore	7
	aload	0
	iload	7
	iload	2
	invokestatic	C2Bytecode/quicksort([III)V
branch1:

	return
.end method

.end class

小结

这篇的代码生成和之前解释器的思路很相似，都是根据AST和对应的产生式来执行或者生成代码。

其实主要的思路是很清晰的，只是其中有太多细节容易让人太过纠结。这个系列算作是我自己的学习笔记，到这也有十三篇了，下一篇可能写写总结就正式结束了。

欢迎Star!