[Custom Thumbnail] All the Code of the series can be found at the GitHub repository: https://github.com/drifter1/compiler

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Introduction

    Hello it's a me again @drifter1! Today we continue with my Compiler Series, a series where we implement a complete compiler for a simple C-like language by using the C-tools Flex and Bison. This article will be about covering what Semantic Analysis is all about "in theory". Practical stuff will be covered from next time on..

The topics that we will cover today are:

1. What Semantic Analysis is all about
2. Context-Sensitive Grammars (CSG) and Languages (CSL)
3. Attribute Grammars (synthesized and inherited attributes)
4. S- and L-attributed Syntax Directed Translations (SDT)

    Where and how exactly we do all this for our language and more specifically in the C-tool Bison (our parser) will be covered from next time on!

Requirements:

    Actually you need to read and understand all the topics that I covered in the series as a whole, as these articles will give you access to knowledge about:

• What Compiler Design is (mainly the steps)
• For which exact Language the Compiler is build for (Tokens and Grammar)
• How to use Flex and Bison
• How to implement a lexer and parser for the language using those tools
• What the Symbol Table is and how we implement it
• How we combine Flex and Bison together
• How we can pass information from the Lexer to the Parser
• How we define operator priorities, precedencies and associativity

Difficulty:

Talking about the series in general this series can be rated:

• Intermediate to Advanced

Today's topic(s) can be rated:

• Easy

So, without further ado, let's now finally start with the actual Tutorial...

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Actual Tutorial Content

What Semantic Analysis is all about

    Until now we covered how we turn the character stream that we get from the source program into lexical units or tokens, which is the so called Lexical Analysis. Afterwards, we combined these tokens to form the syntax of the language, which is the Syntax Analysis part of Compiler Design. Doing all that we are not quite finished, as the parse- or syntax-tree constructed in the current phase, doesn't really evaluate the tokens used in the parsing process.
You might remember that:

• Lexical analysis uses a regular grammar/language, which can be expressed using regular expressions and in the end turns into a deterministic finite automaton (DFA)
• Syntax analysis uses a context-free grammar/language, which can be expressed using the so called production rules, which in the end become push-down or shift-reduce automata (Bison uses LALR)

    You can clearly see that syntax analysis was needed, because the lexical analysis couldn't determine the syntax of a given sentence, as regular expressions have certain limitations. So, until now the lexical analyzer (lexer) is responsible for the validity of the tokens, that it supplies to the syntax analyzer (parser). The syntax analyzer then combines those "valid" tokens to check if the syntax of the program is correct. But, we still have some problems, cause the parser can't determine if:

• a token is valid (something that the lexer does for it - sortoff)
• a token was declared before being used
• a token was initialized before being used
• a operation performed on a specific token is allowed or not

    These things, and even more, are taks that are accomplished by the semantic analyzer. Constructing a parse-tree in syntax analysis was not bad, but it surely is not enough, cause the parse-tree that we have now doesn't contain any information that helps us evaluate the tree, having it in an actually useful form that can be turned into machine code. This all means that the context-free grammar rules of the syntax analysis don't include how we are supposed to interpret them. The Semantics will provide meaning to the tokens and syntax structure, telling us how we interpret the symbols, the types and relations between them. So, in the end, semantic analysis is all about judging if the syntax-tree constructed from the source program has a meaning or not. Other tasks that are performed during semantic analysis are:

• Scope resolution, which helps us recognize if an variable is being accessed out of scope, if a variable has been declared multiple times in the same scope and if an variable is undeclared
• Type checking, which let's us recognize type mismatch, identifier misuse, actual and formal parameter mismatch etc.
• Array-bound checking

and more...depending on the language!

    One last thing that I would like to mention is that the semantic analysis can be of two forms:

• Static: Semantics are checked at compile-time
• Dynamic: Semantics are checked at run-time, using generated semantic error-checking code

    For now I'm only planning to do static semantic analysis, which is mostly about checking variable declarations and usage in the correct scope, type checking and parameter type checking.

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Context-Sensitive Grammars (CSG) and Languages (CSL)

    From the Chomsky hierarchy, we know that context-sensitive languages (CSL) are more powerful than the context-free languages (CFL) used for the syntax analysis. Such languages can describe more grammars, but are much more difficult to work and implemented in code, cause they need a linear bounded automaton, which is a restricted form of a Turing machine.
Either way, a CSG has the following 4-tuple:
G = {N, Σ, P, S}, where:

• N is the set of non-terminal symbols
• Σ is the set of terminal symbols
• S is the start symbol of the production
• P is a finite set of productions

The rules/productions of P are of the form:
α -> β
or
α1Aα2 -> α1βα2
where α and β can be terminal or non-terminal symbols.

In context-free grammars the left-part was always a non-terminal!

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Attribute Grammars

    Getting into an CSG would make the compiler design procedure even more difficult and so we needed to find another way. To provide context-sensitive information to non-terminal symbols, we extend the context-free grammar and turn it into a so called Attribute Grammar. Such a grammar is a special form of CFG, where additional information, in form of attributes, is appended to one or more of the non-terminals. These attributes are well-defined domains of values, which can be integers, floats, characters, strings or even expressions. I guess that you see now that an CSG is not actually needed, cause adding attributes to non-terminals, we provide semantics to an CFG, helping us specify not only the syntax, but also the semantics of a programming language. This final form of parse-tree, which also contains attributes, is mostly called a decorated parse-tree.

    Talking about the parse-tree of the Attribute grammar, we understand that we can easily use and pass values or information between the different nodes of this tree. The only restriction we have is that information can only be passed directly between nodes of the same production rule, while parsing. After finishing the syntax Analysis, we can off course access all the rules as we like! These attributes will be accessed and modified in the actions of each grammar production of non-terminals. Depending on how exactly attributes get their values, we divide attributes into two categories:

1. Synthesized attributes
2. Inherited attributes

Let's get into each category on it's own!

Synthesized attributes

    Such attributes get values only from attribute values of their child nodes. So, when having a rule A -> BC, A is only taking values from it's childs B and C. The values of B and C are synthesized into A. So, such attributes cannot take values from their parent nodes or any sibling nodes. This all shows us that synthesized attributes can only flow upwards in the syntax tree.

Inherited attributes

    Such attributes can get values from parent and/or siblings. So, in the previous example rule, B could take a value from A and/or C. In the same way, C could take a value from A and/or B.

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Syntax Directed Translations (SDT)

    For the Syntax Analysis we define the following two terms:

• Expansion: when a non-terminal is expanded to terminals according to a grammatical rule
• Reduction: When a terminal is reduced to it's corresponding non-terminal.

    The semantic rules are applied whenever an reduction occurs. This shows us that semantic analysis uses an Syntax Directed Translation (SDT) to perform the semantic tasks. The SDT is of two types:

1. S-attributed SDT
2. L-attributed SDT

Let's get into each type on it's own!

S-attributed SDT

    Such an SDT uses only synthesized attributes, which is why we call it S-attributed. The semantic rules in a S-attributed SDT are written right after the production (right hand side). This shows us that the attributes can only be evaluated in bottom-up parsing, cause only then the parent nodes can get the values of the child nodes they depend on.

L-attributed SDT

    Such an SDT uses both types of attributes, with one very important restriction of not taking values from right siblings. So, a node can get values from it's parent, child and "left" sibling nodes. Considering the example of before (A -> BC), A can of course take values from B and C (synthesized), B can only take a value from A (C is to the right!) and C can take a value from both A and B! This shows us that L-attributed SDT's are evaluated depth-first and left-to-right. This is the most practical and essential grammar property for an one-pass compiler, cause semantic rules can therefore be applied directly during parsing. There is no need for keeping the entire parse-tree in memory!

    I guess it's also worth noting that S-attributed SDT's are also L-attributed SDT's, BUT an L-attributed is not always an S-attributed SDT! So, the L-attributed definition encloses the S-attributed definitions. The way Bison works (LALR parser) allows the use of both types, which means that the grammar that we created in Bison can be extended into an L-attributed grammar!

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RESOURCES

References:

The following helped me refresh my knowledge around Semantics:

1. https://www.tutorialspoint.com/compiler_design/compiler_design_syntax_analysis.htm
2. https://www.tutorialspoint.com/compiler_design/compiler_design_semantic_analysis.htm
3. https://www.geeksforgeeks.org/toc-context-sensitive-grammar-csg-language-csl/
4. https://www.cs.fsu.edu/~engelen/courses/COP402003/notes4.html
5. https://web.cs.wpi.edu/~kal/courses/cs4533/module4/mysa.html

Images:

All of the images are custom-made!

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Previous parts of the series

• Introduction -> What is a compiler, what you have to know and what you will learn
• A simple C Language -> Simplified C, comparison with C, tokens, basic structure
• Lexical Analysis using Flex -> Theory, Regular Expressions, Flex, Lexer
• Symbol Table (basic structure) ->Why Symbol Tables, Basic Implementation
• Using Symbol Table in the Lexer -> Flex and Symbol Table combination
• Syntax Analysis Theory -> Syntax Analysis, grammar types and parsing
• Bison basics -> Bison tutorial actually
• Creating a grammar for our Language -> Grammar and first Parser
• Combine Flex and Bison -> lexer and parser combined
• Passing information from Lexer to Parser -> Bug Fix, yylval variable, YYSTYPE union, Setting up the Parser, Passing information "directly" and through the symbol table (special case) with examples.
• Finishing Off The Grammar/Parser (part 1) -> Adding the missing grammar rules, small fixes
• Finishing Off The Grammar/Parser (part 2) -> Operator priorities, precedencies and associativity, complete example file (for testing), grammar rule visualization, finish off grammar/parser

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Final words | Next up on the project

     And this is actually it for today's post! I hope that I explained everything as much as I needed to, meaning that you learned something out of it.
Next up on this series are:

• Semantic analysis (visualization of semantics for our grammar rules, "extending" the symbol table for scope resolution and creating/using semantic rules in Bison)
• Intermediate Code generation (Abstract Syntax Tree)
• Machine Code generation (MIPS Assembly)

     Which are all topics that will need more than one article to complete. Also, note that we might also get into Optimizations later on, or could even extend the Language by adding complex datatypes (structs and unions), more rules etc.
So, see ya next time!

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GitHub Account:

https://github.com/drifter1

Keep on drifting! ;)