Compiler Error Handling, Intermediate Code, and Optimization

Posted on Jan 30, 2025 in Computers

Error Detection and Recovery in Compilers

In the compilation process, error detection and reporting are crucial. This phase identifies and informs the user about any errors made during coding. This process is known as Error Handling.

Compile-Time Errors

Compile-time errors are categorized into three types:

Lexical Phase Errors

These errors occur during the lexical analysis phase. Examples include:

  • Exceeding the length of identifiers or numeric constants
  • Appearance of illegal characters
  • Unmatched strings

Error Recovery: Panic Mode Recovery

  • Successive characters are removed until a synchronizing token (e.g., ; or }) is found.
  • Advantage: Easy to implement and avoids infinite loops.
  • Disadvantage: Skips a considerable amount of input, potentially missing additional errors.

Syntactic Phase Errors

These errors are detected during syntax analysis. Examples include:

  1. Errors in structure
  2. Missing operators
  3. Misspelled keywords
  4. Unbalanced parentheses

Error Recovery: Panic Mode Recovery

  • Successive characters are removed until a synchronizing token is found.
  • The parser attempts corrections on the remaining input to allow further parsing.
  • Corrections may include deleting extra semicolons, replacing commas with semicolons, or inserting missing semicolons.

Error Production

  • Common errors can be incorporated into the grammar with error productions.
  • This allows for generating appropriate error messages and continuing parsing.

Global Correction

  • The parser examines the entire program to find the closest error-free match.
  • The closest match involves minimal insertions, deletions, and changes of tokens.

Semantic Errors

These errors are detected during semantic analysis. Examples include:

  • Incompatible operand types
  • Undeclared variables
  • Mismatch between actual and formal arguments

Error Recovery for Semantic Errors

  • For “Undeclared Identifier” errors, a symbol table entry is created.
  • For incompatible data types, automatic type conversion is performed.

Advantages of Error Detection and Recovery

  • Improved code quality: Errors are identified early and addressed before they become bigger issues.
  • Increased productivity: The compiler continues processing after an error, saving time and effort.
  • Better user experience: Graceful error handling reduces user frustration.
  • Better debugging: Detailed error messages help developers pinpoint the source of errors.

Disadvantages of Error Detection and Recovery

  • Slower compilation time: Complex recovery mechanisms can slow down compilation.
  • Increased complexity: Error recovery can make the compiler harder to maintain and debug.
  • Risk of silent errors: Errors may be masked, leading to unnoticed issues.
  • Potential for incorrect recovery: Incorrect implementation can introduce new errors.

Intermediate Code Generation in Compiler Design

In the analysis-synthesis model, the front end translates source code into an intermediate code, which the back end uses to generate target code.

Benefits of Intermediate Code

  1. Simplification of Code Generation:
    • Abstract Machine Model: Intermediate code acts as an abstract machine model, simplifying the complexities of direct machine code generation.
    • Easier Translation: Complex high-level constructs are broken down into simpler pieces.
  2. Machine Independence:
    • Portability: Intermediate code is independent of the target machine architecture.
    • Retargeting: The same intermediate representation can be reused for different hardware platforms.
  3. Optimization Opportunities:
    • Platform-Agnostic Optimizations: Many optimizations can be performed at the intermediate code level.
    • Global Optimization: Techniques like common subexpression elimination, constant folding, loop optimization, and dead code elimination are possible.
  4. Improved Error Detection and Debugging:
    • Intermediate Code Analysis: Potential runtime errors can be detected.
    • Debugging Information: Provides a clearer view of the transformed program, aiding in debugging.

Intermediate Languages

Postfix Notation: Also known as reverse Polish notation. Operators follow operands, eliminating the need for parentheses.

DzKRWi+iWkj0AAAAAElFTkSuQmCC

DAG Representation for Basic Blocks

A directed acyclic graph with labels on nodes:

  • Leaves are labeled by unique identifiers (variable names or constants).
  • Interior nodes are labeled by an operator symbol.
  • Nodes also have a sequence of identifiers to store the computed value.

DAGs are used to implement transformations on basic blocks and determine common sub-expressions.

Algorithm for Construction of DAG:

  1. If an operand is undefined, create a node for it.
  2. Create a node for the operator with the operand nodes as children.
  3. Append the result identifier to the node’s identifier list.

Application of DAG in Compiler Design

  • Expression Optimization: Identifying and representing common subexpressions.
  • Code Generation: Representing and optimizing intermediate code.
  • Register Allocation: Identifying variables that can share the same register.
  • Control Flow Analysis: Analyzing and optimizing control flow structures.

Advantages of DAG in Compiler Design

  • Space Efficiency: Minimizing memory usage by eliminating redundancy.
  • Time Efficiency: Reducing time for expression evaluation and code generation.
  • Improved Code Quality: Producing more efficient and optimized code.
  • Simplifies Optimization Passes: Providing a structured representation of intermediate code.
  • Facilitates Register Allocation: Enabling efficient allocation of hardware resources.

Directed Acyclic Graph Characteristics

  • Leaves have unique identifiers (variable names or constants).
  • Interior nodes are labeled with an operator symbol.
  • Nodes have a string of identifiers to store the computed value.
  • DAGs have definitions for transitive closure and transitive reduction.
  • DAGs have topological orderings defined.

Peephole Optimization

An optimization technique performed on a small set of compiler-generated instructions (the peephole or window). It is applied after the target code has been generated.

Characteristics:

  1. Redundant instruction elimination (e.g., redundant loads and stores).
  2. Unreachable code elimination (e.g., removing unlabeled instructions after unconditional jumps).
  3. Machine idioms (replacing target instructions with equivalent machine instructions).

Principle Sources of Optimization

Common Sub-expressions Elimination: Avoiding recomputing expressions that have been previously computed.

Copy Propagation: Using one variable instead of another after a copy statement (e.g., f := g).

Dead-Code Elimination: Removing statements that compute values that are never used.

Constant Folding: Deducing at compile time that the value of an expression is a constant and using the constant instead.

Reduction in Strength: Replacing expensive operations with equivalent cheaper ones (e.g., using shifts instead of multiplication by powers of two).