从零写一个编译器（九）：语义分析之构造抽象语法树(AST)

项目的完整代码在 C2j-Compiler <https://github.com/dejavudwh/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/

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