Constructing LALR parsing tables and syntax directed translation schemes

LALR parsing tables and syntax-directed translation schemes are used in compiler design to translate source language into a target language:

LALR parsing tables
LALR parsers are efficient at finding the correct bottom-up parse of an input stream. LALR stands for lookahead left-right, and the "(1)" in LALR(1) indicates one-token lookahead.

1. NADAR SARASWATHI COLLEGE OF ARTS AND SCIENCE Constructing LALR Parsing Tables and Syntax directed translation schemes By G.Keerthika I M.Sc(CS ) COMPILER DESIGN

2. Introduction to LALR Parsing LALR parsing is a popular technique for parsing context-free grammars. It's a bottom-up parsing method that combines features of LR parsing with the simplicity of LL parsing. This presentation will guide you through LALR parsing concepts, from basic grammar definitions to the construction and use of LALR parsing tables.

3. Context-free Grammars A context-free grammar (CFG) defines the syntax of a language. It's a set of production rules that describe how to construct valid strings in the language. CFGs are essential for understanding the structure of programming languages, natural languages, and other formal languages. Terminals These are the basic symbols of the language, like words or punctuation marks. Nonterminals These represent categories of symbols, like "expression" or "statement." They can be replaced by other symbols. Production Rules These define how nonterminals can be replaced by terminals or other nonterminals.

4. Parsing Techniques Parsing techniques are methods for analyzing a string of symbols (usually code) and determining its grammatical structure. They are crucial for compilers, interpreters, and other language processing tools. Top-Down Parsing Starts with the start symbol and tries to derive the input string. Examples include LL parsing and recursive descent. Bottom-Up Parsing Builds the parse tree from the input string, gradually combining symbols to reach the start symbol. Examples include LR parsing and LALR parsing.

5. LALR(1) Grammars An LALR(1) grammar is a restricted form of LR grammar that allows for more efficient parsing. It uses a single lookahead symbol to determine the appropriate production rule to apply during parsing. 1 Lookahead Symbol This is the next symbol in the input string, which is used to make parsing decisions. 2 LR Parsing Table An LALR(1) grammar is defined by a parsing table that maps states and lookahead symbols to actions (shift, reduce, or accept). 3 Limitations Not all context-free grammars can be expressed in LALR(1) form, which might require using other parsing techniques.

6. LALR Parsing Algorithm The LALR parsing algorithm uses the parsing table to process the input string. It builds a parse stack to track the parsing progress and uses the lookahead symbol to guide the parsing process. 1 Initialization Start with an empty stack and the initial state of the parsing table. 2 Shift/Reduce Based on the current state and the lookahead symbol, either shift the symbol onto the stack or reduce the top of the stack using a production rule. 3 Acceptance When the stack contains the start symbol and the input is exhausted, the parse is successful. Otherwise, it's a syntax error.

7. Constructing LALR Parsing Tables The process of constructing an LALR parsing table involves converting the CFG into an equivalent LR(0) grammar, then using the LR(0) grammar to build the parsing table. This process involves several steps, including item sets, closure, and state transitions. LR(0) Item Action Go-to State S → .E Shift (E) 1 E → .T Shift (T) 2 E → T . + T Reduce (E → T + T) - EXAMPLE

8. Handling Conflicts in LALR Tables Conflicts arise when multiple actions are possible for a given state and lookahead symbol. These conflicts can lead to ambiguous parses and syntax errors. Shift/Reduce Conflict When the table suggests both shifting a symbol onto the stack and reducing the stack using a production rule. Reduce/Reduce Conflict When the table suggests reducing the stack using multiple production rules. Resolution Techniques Conflicts can be resolved by modifying the grammar or using alternative parsing techniques.

9. Advantages and Limitations of LALR Parsing LALR parsing offers a balance between efficiency and expressive power. It's a widely used technique in compiler design due to its performance benefits, but it has limitations in handling certain types of grammars. Efficiency LALR parsing is generally faster than LR parsing because it requires smaller parsing tables. Expressiveness LALR parsing can handle a wide range of context-free grammars, making it suitable for many programming languages. Limitations Some grammars are not LALR(1), which means they cannot be parsed using this technique.

10. Introduction to Syntax Directed Translation Schemes Syntax-directed translation schemes (SDTs) are a powerful tool for designing compilers and interpreters. They provide a structured way to translate a program written in one language into an equivalent program in another language. This process involves building a parse tree for the source code and using the parse tree to generate the target code.

11. Purpose and Applications 1 Code Generation SDTs are widely used for generating machine code or intermediate code from source programs. 2 Language Translation They can be used to translate programs from one high-level language to another, such as from Python to C++. 3 Code Optimization SDTs can be used to optimize the generated code, making it faster or more efficient. 4 Data Validation They can be used to enforce semantic constraints and ensure that data is processed correctly.

12. Formal Definition of Translation Schemes Grammar An SDT is based on a context-free grammar that defines the syntax of the source language. Attributes Each nonterminal symbol in the grammar has associated attributes, which represent information about the corresponding subtree. Semantic Rules Semantic rules specify how the attributes are computed and how the target code is generated.

13. Attributes and Advantage and Limitation: Attribute Type Description Synthesized Computed from the values of attributes in the children nodes. Inherited Computed from the values of attributes in the parent node or sibling nodes. Advantages SDTs provide a clear and concise way to specify translation rules and ensure semantic correctness. Limitations Attribute evaluation can be computationally expensive, especially for complex grammars.

14. Evaluation Strategies Bottom-Up Attributes are evaluated in a bottom-up fashion, starting with the leaf nodes and working up to the root. Top-Down Attributes are evaluated in a top-down fashion, starting with the root node and working down to the leaf nodes. Mixed A combination of bottom-up and top-down evaluation strategies.

15. Implementation Techniques Attribute Grammar Tools Tools that provide automated support for generating code based on attribute grammars. Manual Implementation Implementing the attribute evaluation rules and code generation logic by hand.

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