项目的完整代码在 C2j-Compiler

前言

在上一篇完成了符号表的构建，下一步就是输出抽象语法树(Abstract Syntax Tree，AST)

抽象语法树（abstract syntax tree 或者缩写为 AST），是源代码的抽象语法结构的树状表现形式，这里特指编程语言的源代码。树上的每个节点都表示源代码中的一种结构。

AST对于编译器是至关重要的，现在的编译型语言一般通过AST来生成IR，解释型语言也可以不用虚拟机而直接遍历AST来解释执行，之后要写解释器和编译器都依赖这个AST

这一篇主要文件有：

AstBuilder.java
AstNode.java
AstNodeImpl.java
NodeKey.java
NodeFactory.java

主要数据结构

AST节点的表示

public interface AstNode {

    AstNode addChild(AstNode node);

    AstNode getParent();

    ArrayList<AstNode> getChildren();

    void setAttribute(NodeKey key, Object value);

    Object getAttribute(NodeKey key);

    boolean isChildrenReverse();

    void reverseChildren();

    AstNode copy();

}

这是对AstNode接口的实现，并且继承HashMap，这里的NodeKey是

TokenType, VALUE, SYMBOL, PRODUCTION, TEXT

对应的value，

TokenType就是非终结符的类型
Text用来存储解析对象的文本信息
Symbol对应的就是变量的符号对象
Value是对应对象解析的值，比如int a = 1，那么value的值就为1

public class AstNodeImpl extends HashMap<NodeKey, Object> implements AstNode {

    private Token type;

    private AstNodeImpl parent;

    private ArrayList<AstNode> children;

    String name;

    private boolean isChildrenReverse = false;

    public AstNodeImpl(Token type) {

        this.type = type;

        this.parent = null;

        this.children = new ArrayList<>();

        setAttribute(NodeKey.TokenType, type);

    }

    @Override

    public AstNode addChild(AstNode node) {

        if (node != null) {

            children.add(node);

            ((AstNodeImpl) node).parent = this;

        }

        return node;

    }

    @Override

    public AstNode getParent() {

        return parent;

    }

    @Override

    public void reverseChildren() {

        if (isChildrenReverse) {

            return;

        }

        Collections.reverse(children);

        isChildrenReverse = true;

    }

    @Override

    public boolean isChildrenReverse() {

        return isChildrenReverse;

    }

    @Override

    public ArrayList<AstNode> getChildren() {

        reverseChildren();

        return children;

    }

    @Override

    public void setAttribute(NodeKey key, Object value) {

        if (key == NodeKey.TEXT) {

            name = (String) value;

        }

        put(key, value);

    }

    @Override

    public Object getAttribute(NodeKey key) {

        return get(key);

    }

    @Override

    public String toString() {

        String info = "";

        if (get(NodeKey.VALUE) != null) {

            info += "Node Value is " + get(NodeKey.VALUE).toString();

        }

        if (get(NodeKey.TEXT) != null) {

            info += "\nNode Text is " + get(NodeKey.TEXT).toString();

        }

        if (get(NodeKey.SYMBOL) != null) {

            info += "\nNode Symbol is " + get(NodeKey.SYMBOL).toString();

        }

        return info + "\n Node Type is " + type.toString();

    }

    @Override

    public AstNode copy() {

        AstNodeImpl copy = (AstNodeImpl) NodeFactory.createICodeNode(type);

        Set<Entry<NodeKey, Object>> attributes = entrySet();

        Iterator<Map.Entry<NodeKey, Object>> it = attributes.iterator();

        while (it.hasNext()) {

            Map.Entry<NodeKey, Object> attribute = it.next();

            copy.put(attribute.getKey(), attribute.getValue());

        }

        return copy;

    }

}

NodeFactory

NodeFactory就是简单的返回一个节点的实现

public class NodeFactory {

    public static AstNode createICodeNode(Token type) {

        return new AstNodeImpl(type);

    }

}

构造AST

AST的创建也是需要在语法分析过程中根据reduce操作进行操作的。也就是在takeActionForReduce方法中调用AstBuilder的buildSyntaxTree方法

在AstBuilder里面还是需要两个堆栈来辅助操作

private Stack<AstNode> nodeStack = new Stack<>();

private LRStateTableParser parser = null;

private TypeSystem typeSystem = null;

private Stack<Object> valueStack = null;

private String functionName;

private HashMap<String, AstNode> funcMap = new HashMap<>();

private static AstBuilder instance;

构造AST的主要逻辑在buildSyntaxTree方法里，需要注意的是有一些节点在解释执行和代码生成的时候是不一样的，有时代码生成需要的节点解释执行的话并不需要

在这里提一下UNARY这个非终结符，这个非终结符和NAME很像，但是它一般是代表进行运算和一些操作的时候，比如数组，++，--或者函数调用的时候

其实构建AST的过程和符号表的构建过程有点儿类似，都是根据reduce操作来创建信息和组合信息，符号表是组合修饰符说明符等，而AST则是组合节点间的关系变成一棵树

我们只看几个操作

Specifiers_DeclList_Semi_TO_Def

这个节点需要注意的是，从堆栈的什么地方拿到Symbol，这个需要从reduce次数和推导式中得出

* DEF -> SPECIFIERS  DECL_LIST SEMI

* DECL -> VAR_DECL

* VAR_DECL -> NEW_NAME

*             | VAR_DECL LP RP

*             | VAR_DECL LP VAR_LIST RP

*             | LP VAR_DECL RP

*             | START VAR_DECL

从推导式可以看出，DEF节点的符号应该在valueStack.size() - 3，但是DECL和VAR_DECL没有做reduce操作，所以符号应该在valueStack.size() - 2。这其实和前面的符号表构建算出之前符号的位置是一样的。

TO_UNARY

这里则是变量、数字或者字符串的节点，如果是个变量的号，这个节点就需要一个Symbol的value了

case SyntaxProductionInit.Number_TO_Unary:

case SyntaxProductionInit.Name_TO_Unary:

case SyntaxProductionInit.String_TO_Unary:

    node = NodeFactory.createICodeNode(Token.UNARY);

    if (production == SyntaxProductionInit.Name_TO_Unary) {

        assignSymbolToNode(node, text);

    }

    node.setAttribute(NodeKey.TEXT, text);

    break;

其余的节点无非是把一些语句拆分它的逻辑然后组成节点，真正的求值部分像Name_TO_Unary比较少，更多是比如把一个if else块分成if节点、判断节点、else节点，之后再按照这棵树进行解释执行或者代码生成

public AstNode buildSyntaxTree(int production, String text) {

    AstNode node = null;

    Symbol symbol = null;

    AstNode child = null;

    if (Start.STARTTYPE == Start.INTERPRETER) {

        int p1 = SyntaxProductionInit.Specifiers_DeclList_Semi_TO_Def;

        int p2 = SyntaxProductionInit.Def_To_DefList;

        int p3 = SyntaxProductionInit.DefList_Def_TO_DefList;

        boolean isReturn = production == p1 || production == p2 || production == p3;

        if (isReturn) {

            return null;

        }

    }

    switch (production) {

        case SyntaxProductionInit.Specifiers_DeclList_Semi_TO_Def:

            node = NodeFactory.createICodeNode(Token.DEF);

            symbol = (Symbol) valueStack.get(valueStack.size() - 2);

            node.setAttribute(NodeKey.SYMBOL, symbol);

            break;

        case SyntaxProductionInit.Def_To_DefList:

            node = NodeFactory.createICodeNode(Token.DEF_LIST);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.DefList_Def_TO_DefList:

            node = NodeFactory.createICodeNode(Token.DEF_LIST);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Number_TO_Unary:

        case SyntaxProductionInit.Name_TO_Unary:

        case SyntaxProductionInit.String_TO_Unary:

            node = NodeFactory.createICodeNode(Token.UNARY);

            if (production == SyntaxProductionInit.Name_TO_Unary) {

                assignSymbolToNode(node, text);

            }

            node.setAttribute(NodeKey.TEXT, text);

            break;

        case SyntaxProductionInit.Unary_LP_RP_TO_Unary:

            node = NodeFactory.createICodeNode(Token.UNARY);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Unary_LP_ARGS_RP_TO_Unary:

            node = NodeFactory.createICodeNode(Token.UNARY);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Unary_Incop_TO_Unary:

        case SyntaxProductionInit.Unary_DecOp_TO_Unary:

        case SyntaxProductionInit.LP_Expr_RP_TO_Unary:

        case SyntaxProductionInit.Start_Unary_TO_Unary:

            node = NodeFactory.createICodeNode(Token.UNARY);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Unary_LB_Expr_RB_TO_Unary:

            node = NodeFactory.createICodeNode(Token.UNARY);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Uanry_TO_Binary:

            node = NodeFactory.createICodeNode(Token.BINARY);

            child = nodeStack.pop();

            node.setAttribute(NodeKey.TEXT, child.getAttribute(NodeKey.TEXT));

            node.addChild(child);

            break;

        case SyntaxProductionInit.Binary_TO_NoCommaExpr:

        case SyntaxProductionInit.NoCommaExpr_Equal_NoCommaExpr_TO_NoCommaExpr:

            node = NodeFactory.createICodeNode(Token.NO_COMMA_EXPR);

            child = nodeStack.pop();

            String t = (String) child.getAttribute(NodeKey.TEXT);

            node.addChild(child);

            if (production == SyntaxProductionInit.NoCommaExpr_Equal_NoCommaExpr_TO_NoCommaExpr) {

                child = nodeStack.pop();

                t = (String) child.getAttribute(NodeKey.TEXT);

                node.addChild(child);

            }

            break;

        case SyntaxProductionInit.Binary_Plus_Binary_TO_Binary:

        case SyntaxProductionInit.Binary_DivOp_Binary_TO_Binary:

        case SyntaxProductionInit.Binary_Minus_Binary_TO_Binary:

        case SyntaxProductionInit.Binary_Start_Binary_TO_Binary:

            node = NodeFactory.createICodeNode(Token.BINARY);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Binary_RelOP_Binary_TO_Binray:

            node = NodeFactory.createICodeNode(Token.BINARY);

            node.addChild(nodeStack.pop());

            AstNode operator = NodeFactory.createICodeNode(Token.RELOP);

            operator.setAttribute(NodeKey.TEXT, parser.getRelOperatorText());

            node.addChild(operator);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.NoCommaExpr_TO_Expr:

            node = NodeFactory.createICodeNode(Token.EXPR);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Expr_Semi_TO_Statement:

        case SyntaxProductionInit.CompountStmt_TO_Statement:

            node = NodeFactory.createICodeNode(Token.STATEMENT);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.LocalDefs_TO_Statement:

            node = NodeFactory.createICodeNode(Token.STATEMENT);

            if (Start.STARTTYPE == Start.CODEGEN) {

                node.addChild(nodeStack.pop());

            }

            break;

        case SyntaxProductionInit.Statement_TO_StmtList:

            node = NodeFactory.createICodeNode(Token.STMT_LIST);

            if (nodeStack.size() > 0) {

                node.addChild(nodeStack.pop());

            }

            break;

        case SyntaxProductionInit.FOR_OptExpr_Test_EndOptExpr_Statement_TO_Statement:

            node = NodeFactory.createICodeNode(Token.STATEMENT);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.StmtList_Statement_TO_StmtList:

            node = NodeFactory.createICodeNode(Token.STMT_LIST);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Expr_TO_Test:

            node = NodeFactory.createICodeNode(Token.TEST);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.If_Test_Statement_TO_IFStatement:

            node = NodeFactory.createICodeNode(Token.IF_STATEMENT);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.IfElseStatemnt_Else_Statemenet_TO_IfElseStatement:

            node = NodeFactory.createICodeNode(Token.IF_ELSE_STATEMENT);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.While_LP_Test_Rp_TO_Statement:

        case SyntaxProductionInit.Do_Statement_While_Test_To_Statement:

            node = NodeFactory.createICodeNode(Token.STATEMENT);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Expr_Semi_TO_OptExpr:

        case SyntaxProductionInit.Semi_TO_OptExpr:

            node = NodeFactory.createICodeNode(Token.OPT_EXPR);

            if (production == SyntaxProductionInit.Expr_Semi_TO_OptExpr) {

                node.addChild(nodeStack.pop());

            }

            break;

        case SyntaxProductionInit.Expr_TO_EndOpt:

            node = NodeFactory.createICodeNode(Token.END_OPT_EXPR);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.LocalDefs_StmtList_TO_CompoundStmt:

            node = NodeFactory.createICodeNode(Token.COMPOUND_STMT);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.NewName_LP_RP_TO_FunctDecl:

        case SyntaxProductionInit.NewName_LP_VarList_RP_TO_FunctDecl:

            node = NodeFactory.createICodeNode(Token.FUNCT_DECL);

            node.addChild(nodeStack.pop());

            child = node.getChildren().get(0);

            functionName = (String) child.getAttribute(NodeKey.TEXT);

            symbol = assignSymbolToNode(node, functionName);

            break;

        case SyntaxProductionInit.NewName_TO_VarDecl:

            nodeStack.pop();

            break;

        case SyntaxProductionInit.NAME_TO_NewName:

            node = NodeFactory.createICodeNode(Token.NEW_NAME);

            node.setAttribute(NodeKey.TEXT, text);

            break;

        case SyntaxProductionInit.OptSpecifiers_FunctDecl_CompoundStmt_TO_ExtDef:

            node = NodeFactory.createICodeNode(Token.EXT_DEF);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            funcMap.put(functionName, node);

            break;

        case SyntaxProductionInit.NoCommaExpr_TO_Args:

            node = NodeFactory.createICodeNode(Token.ARGS);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.NoCommaExpr_Comma_Args_TO_Args:

            node = NodeFactory.createICodeNode(Token.ARGS);

            node.addChild(nodeStack.pop());

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Return_Semi_TO_Statement:

            node = NodeFactory.createICodeNode(Token.STATEMENT);

            break;

        case SyntaxProductionInit.Return_Expr_Semi_TO_Statement:

            node = NodeFactory.createICodeNode(Token.STATEMENT);

            node.addChild(nodeStack.pop());

            break;

        case SyntaxProductionInit.Unary_StructOP_Name_TO_Unary:

            node = NodeFactory.createICodeNode(Token.UNARY);

            node.addChild(nodeStack.pop());

            node.setAttribute(NodeKey.TEXT, text);

            break;

        default:

            break;

    }

    if (node != null) {

        node.setAttribute(NodeKey.PRODUCTION, production);

        nodeStack.push(node);

    }

    return node;

}

小结

其实构造AST和创建符号表上非常相似，都是依据reduce操作的信息来完成。在AST的构建中的主要任务就是对源代码语句里的逻辑进行分块，比如对于一个ifelse语句：

上面的图是我依据这个意思话的，和上面构造出来的AST不完全一致

另外我的github博客：https://dejavudwh.cn/

巴特西

从零写一个编译器（九）：语义分析之构造抽象语法树(AST)

前言

主要数据结构

AST节点的表示

NodeFactory

构造AST

Specifiers_DeclList_Semi_TO_Def

TO_UNARY

小结

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