生成AST是否为创建编译器或实现编程语言的必需步骤？

Great question—this is such a common point of confusion when starting out with compiler or interpreter development, so let’s break it down clearly.

Short Answer: No, it’s not strictly required—but it’s almost always the best choice for anything beyond the simplest languages. #

When you can skip the AST #

There are niche scenarios where you can generate code directly during parsing, without building an explicit AST:

• Tiny, single-purpose languages: For example, a simple arithmetic expression evaluator or a minimal shell command parser. You could use recursive descent parsing and emit assembly instructions the moment you recognize a syntax node (like adding two numbers).
• One-pass compilers: Early compilers (think some old BASIC implementations) worked this way—they parsed code line by line, generated machine code on the fly, and never built a full structured representation of the program.
• Syntax-directed translation: Using tools like Bison/YACC, you can attach code-generation logic directly to grammar production rules. When the parser matches a rule, it runs the associated code to emit assembly, no AST needed.

These approaches are fast (no extra memory overhead for AST storage) and simple for trivial use cases. But they fall apart quickly as your language grows in complexity.

Why everyone recommends ASTs #

ASTs are a structured, abstract representation of your code that decouples parsing from semantic analysis, optimization, and code generation. Here’s why they’re indispensable for non-trivial languages:

• Separation of concerns: Parsing only handles syntax validation, while AST traversal takes care of the hard stuff—type checking, constant folding, dead code elimination, etc. Without an AST, you’d have to cram all this logic into your parser, turning it into an unmaintainable mess.
• Flexibility: You can traverse an AST multiple times for different tasks. For example:
  1. First pass: Check for undefined variables or type mismatches.
  2. Second pass: Optimize (e.g., replace if (2 + 3 > 0) with a direct jump, since the condition is always true).
  3. Third pass: Generate optimized assembly.
• Maintainability: Adding new syntax features (like loops, functions, or classes) is far easier when you can just update how you build the AST, rather than rewriting parser logic that’s tied directly to code generation.

Simpler/High-Performance Alternatives to ASTs #

If you want to avoid ASTs but still need something better than direct one-pass code generation, consider these options:

• Bytecode as an intermediate step: Instead of generating assembly directly, emit a compact bytecode format (like Java’s .class files or Python’s .pyc). Bytecode is often easier to generate than assembly, and you can later translate it to assembly (or execute it via a VM). Many bytecode implementations still use ASTs under the hood, but you could skip the explicit AST if your bytecode is simple enough.
• Direct recursive descent code generation: For a tiny language (e.g., only supports if-else and arithmetic), write a recursive descent parser that emits assembly instructions as it parses each node. For example:

  // Pseudocode for parsing an if statement and emitting assembly
  void parse_if() {
      consume_token("if");
      parse_expression(); // Emits code to evaluate the condition and set flags
      emit("jne else_label"); // Jump if condition is false
      parse_block(); // Emits code for the true branch
      emit("jmp end_if");
      emit("else_label:");
      parse_else_block(); // Emits code for the false branch
      emit("end_if:");
  }
  

  This is fast and simple, but don’t try to scale it beyond a handful of syntax features.

How These Choices Impact Assembly Generation #

• AST-based approach: You’ll generate higher-quality assembly. Since you can optimize the AST before code generation, you can eliminate redundant instructions, fold constants, and rearrange code for better performance. The tradeoff is a small overhead from building and traversing the AST, but this is negligible for most real-world use cases.
• Direct code generation: Assembly will be more "naive"—no optimizations, just a direct translation of the parsed syntax. This is fine for small programs, but as your language gets more complex, you’ll end up with bloated, inefficient assembly.
• Bytecode intermediate step: Assembly generation becomes a separate task (translating bytecode to machine code). This can make assembly generation simpler, since bytecode is more uniform than raw syntax, but you still miss out on AST-based optimizations unless you add them at the bytecode level.

Final Recommendation #

If you’re building a toy language with just a few features (basic expressions, simple conditionals), skip the AST and go direct—you’ll get something working faster. But if you plan to add loops, functions, scoping, or any kind of optimization, bite the bullet and build an AST. It’ll save you hours of pain later when you need to extend or maintain your compiler.

内容的提问来源于stack exchange，提问作者JCN