Building a VHDL RTL Parser: A Step-by-Step Guide for Engineers

From Tokens to AST: Implementing a VHDL RTL Parser in 10 Minutes

Overview

A concise, focused tutorial showing how to implement a minimal but practical VHDL RTL parser: tokenize VHDL source, build a concrete/abstract syntax tree (AST), and extract RTL constructs (entities, architectures, signals, ports, processes) useful for synthesis or analysis.

Goal

Produce a working prototype that parses small-to-medium VHDL RTL files and yields an AST suitable for traversals, simple semantic checks, and basic RTL extraction — all demonstrated in roughly 10 minutes of focused coding.

Scope (reasonable assumptions)

• Supports VHDL-⁄₂₀₀₈ RTL subset commonly used in synthesizable designs.
• Handles entity/architecture, port/signal declarations, basic concurrent statements (signal assignment, component instantiation, process with sensitivity list, if/elsif/else, case), simple expressions and binary operators.
• Not a full standard-compliant parser: skips packages, complex generics, full type system, attributes, configurations, and advanced VHDL constructs.

Approach (high level)

1. Tokenization: simple lexer producing tokens (identifiers, numbers, symbols, keywords, string literals, comments).
2. Parser: recursive-descent parser that consumes tokens and builds an AST with node types (Entity, Architecture, Port, Signal, Process, Assignment, Instantiation, If, Case, Expression).
3. AST normalization: transform concrete syntax nodes to a simpler AST (flatten nested constructs, resolve anonymous signals).
4. RTL extraction: walk the AST to collect entities, ports, signals, instances, and process behavior for downstream tools.
5. Quick validation: basic semantic checks (undefined signals, port-direction consistency).

Implementation sketch (concise)

• Lexer: regex-driven, skip comments, produce Token(type, value, pos).
• Parser: functions like parse_entity(), parse_architecture(), parse_declarative_items(), parse_statement().
• AST nodes as simple classes or dicts: e.g., Entity{name, ports[]}, Signal{name,type,init}, Process{sensitivity, stmts[]}.
• Example traversal to list ports and signals.

Example usage

• Input: small VHDL file with entity + architecture containing ports, signals, a process with if statement and assignments.
• Output: printed AST or JSON with entities, architectures, ports, signals, and process statements.

Limitations & Next steps

• Not robust for all VHDL; add better expression parsing, type checking, full generics, packages, attributes.
• Improve error reporting, recovery, and performance for large codebases.
• Optionally generate Verilog, FIRRTL, or netlists from the AST.

If you want, I can:

• Provide a compact working implementation in Python (lexer + parser + AST) you can run.
• Expand the supported VHDL subset (give examples of constructs to include).