Error Processing, Intermediate Representation, Semantic Analysis

(エラー処理、中間表現、意味解析)

11th lecture, June 16, 2017

Language Theory and Compilers

http://www.sw.it.aoyama.ac.jp/2014/Compiler/lecture11.html

Martin J. Dürst

© 2005-17 Martin J. Dürst 青山学院大学

Today's Schedule

• Summary and leftovers of last lecture
• Additional information and hints for homework
• Repetition: Grammar patterns
• Error processing
• Intermediate Representations
• Semantic analysis

Leftovers from Last Week

Summary of Last Week

• There are many different orders of derivation, in particular leftmost derivation and rightmost derivation
  
  • Recursive descent parsing corresponds to leftmost derivation
  • Bottom-up parsing (LALR parsing) uses the reverse order of rightmost derivation
• LALR parsing operations: shift/reduce/goto/accept
  • shift pushes a token and a state onto the stack
  • reduce converts a number of tokens and nonterminals at the top of the stack to a single nonterminal
• These operations can be checked in the .output file produced by bison -v
  and when debugging by setting #define YYDEBUG 1
• For ambiguous grammars, bison reports shift/reduce and reduce/reduce conflicts

Hints for Homework

Deadline: June 29, 2017 (next Thursday), 19:00

Expand the simple calculator of calc.y to a small programming language (see last week for details).

Now is your chance to ask questions!

Additional hints:

For the variables, use an arrary of appropriate length

Example of inputs and outputs: test.in; test.check

Processing Syntax Errors

• Why is error processing difficult
• Requirements for error processing
• Techniques for error processing

Why is Error Processing Difficult

• To make programs shorter, 'unnecessary' symbols should be avoided
  
• For each correct program, there are many different erroneous programs
• It is difficult for programs to distinguish between errors easily made by humans and errors rarely made by humans
• Parsing is based on language theory, but there is not much theory for error processing

Requirements for Error Processing

• Output error messages that are easy to understand
• Find as many actual errors as possible
• Avoid secondary errors
• Do not slow down processing of correct programs
• Do not make the compiler much more complicated

Techniques for Error Processing

• Throw away tokens until finding a token that matches the grammar (panic mode)
• Try to add or exchange a small number of tokens
• Add productions to the grammar that catch errors (error productions)
• Search for the correct program closest to the input

Error Processing in bison

• Rules may contain a special error token
  Example:

  statement: ... SEMICOLON { ... }
           | error SEMICOLON { yyerror("Statement error.\n"); yyerrok; }

  (yyerror is called automatically, therefore this call may be unnecessary)
• If there is an error, bison ignores all the tokens and nonterminals before the closest error token
• The tokens in the rule after the error token are also ignored

Compilation Stages

1. Lexical analysis
2. Parsing (syntax analysis)
3. Semantic analysis
4. Optimization (or 5)
5. Code generation (or 4)

Intermediate Representation: Symbol Table

• Functionality provided:
  • Search of symbols
  • Registration and removal (for local variables) of symbols
  • Management of data for each symbol
• Main points:
  • Frequent use, large number of symbols → efficiency is important
  • The same symbol may be used for different things in different contexts → distinction by scope and type is important

Data Stored by Symbol Table

• Kind of symbol (variable, argument, function, type,...)
• Locations of declarations/definition/use
• Type for variables, functions,...
• Scope where a symbol is visible (e.g. global, file, function, compound statement,...)
• For variables,...: Size (amount of memory needed)
• For functions, variables,...: (relative) address

Example of Scope

extern int a; // declaration only, global scope
static int b; // file scope
int f (int a) // function scope
{
    int a;    // function scope
    static int b; // function scope, but persists across function calls
    while (...) {
        int a;    // block scope
    }
}

Intermediate Representation: Abstract Syntax Tree

How to construct an abstract syntax tree:
Create nodes of the syntax tree as attributes of the attributed grammar.

For example, rewrite this

exp: exp '+' term { $$ = $1 + $3; }

to this:

exp: exp '+' term
        { $$ = newnode(PLUS, $1, $3); }

(YYSTYPE has to be changed)

Most parts of an abstract syntax tree are binary (two branches), but for some constructs (e.g. arguments of a function), special treatment is necessary.

(For very simple programming languages (e.g. Pascal) and simple architectures (e.g. stack machine), it is possible to create code during parsing and to avoid the creation of an abstract syntax tree.)

Semantic Analysis

• Mainly analysis and processing of type information:
  • Check whether types match
  • If necessary, add automatic type conversion (to syntax tree)
  • For C and similar languages, relatively easy
  • For object-oriented languages, has to consider inheritance,...
  • Some languages (e.g. Haskell) use type inference
• Timing: When abstract syntax tree is constructed or just before or during code generation

Type Equivalence

There are different ways to define type equivalence:

• Same name, same type (simple, but inconvenient for user)
• Same components, same type (complicated)
• For object-oriented languages, many choices exist

Example for C: type-equivalence.c (does this program compile?)

Type Equivalence in Haskell

• Haskell: Functional programming language with strong theoretical background
• Characteristics:
  • No assignment (only initialization), no loops (only recursion)
  • Lazy evaluation
  • Type inference
• Example of type inference:
  • Type of function f: (a, Char) -> (a, [Char])
    (function from a pair of a and Char to a pair of a and a list of Chars
  • Type of function g: (Int, [b]) -> Int
    (function from a pair of Int and a list ofbs to an Int)
  • What is the type of the function h(x) := g(f(x))?

Topic Next Week: Turing Machines

Glossary

syntax error

構文エラー

secondary error

二次エラー

symbol table

名前表

scope

有効範囲、スコープ

compound statement

複文

inheritance

継承

type inference

型推論

type equivalence

型の等価

functional (programming) language

関数型 (プログラミング) 言語

lazy evaluation

遅延評価