Overview of Compilation

Prepared by

Manuel E. Bermúdez, Ph.D.Associate ProfessorUniversity of Florida

Programming Language ConceptsLecture 2

Overview of Translation

• Definition: A translator is an algorithm that converts source programs into equivalent target programs.

• Definition: A compiler is a translator whose target language is at a “lower” level than its source language.

TranslatorSource Target

Overview of Translation (cont’d)

• When is one language’s level “lower” than another’s?

• Definition: An interpreter is an algorithm that simulates the execution of programs written in a given source language.

InterpreterSource

input

output

Overview of Translation (cont’d)

• Definition: An implementation of a programming language consists of a translator (or compiler) for that language, and an interpreter for the corresponding target language.

InterpreterTarget

input

output

CompilerSource

Translation

• A source program may be translated an arbitrary number of times before the target program is generated.

Translator1

Source

Translator2

TranslatorN

Target

...

Translation (cont’d)

• Each of these translations is called a phase, not to be confused with a pass, i.e., a disk dump.

Q: How should a compiler be divided into phases?

A: So that each phase can be easily described by some formal model of computation, and so the phase can be carried out efficiently.

Translation (cont’d)

Q: How is a compiler usually divided?A: Two major phases, with many possibilities

for subdivision.• Phase 1: Analysis (determine correctness)• Phase 2: Synthesis (produce target code)

• Another criterion:• Phase 1: Syntax (form).• Phase 2: Semantics (meaning).

Typical Compiler Breakdown

• Scanning (Lexical analysis).• Goal: Group sequences of characters

that occur on the source, into logical atomic units called tokens.

• Examples of tokens: Identifiers, keywords, integers, strings, punctuation marks, “white spaces”, end-of-line characters, comments, etc., …

Scanner (Lexical analysis)

Source

Sequence of Tokens

Lexical Analysis

• Must deal with end-of-line and end-of-file characters.

• A preliminary classification of tokens is made. For example, both ‘program’ and ‘Ex’ are classified as Identifier.

• Someone must give unambiguous rules for forming tokens.

Screening

• Goals:• Remove unwanted tokens.• Classify keywords.• Merge/simplify tokens.

Screener

Sequence of Tokens

Sequence of Tokens

Screening

• Keywords recognized.• White spaces (and comments) discarded.• The screener acts as an interface between

the scanner and the next phase, the parser.

Parsing (Syntax Analysis)

• Goals• To group together the tokens, into

the correct syntactic structures, if possible.

• To determine whether the tokens appear in patterns that syntactically correct.

Parsing (Syntax Analysis)

• Syntactic structures:• Expressions• Statements• Procedures• Functions• Modules

• Methodology:• Use “re-write” rules (a.k.a. BNF).

String-To-Tree Transduction

• Goal: To build a “syntax tree” from the sequence of rewrite rules. The tree will be the functional representation of the source.

• Method: Build tree “bottom-up,” as the rewrite rules are emitted. Use a stack of trees.

Contextual Constraint Analysis

• Goal: To analyze static semantics, e.g.,• Are variables declared before they are used?• Is there assignment compatibility?

e.g., a:=3• Is there operator type compatibility?

e.g., a+3• Do actual and formal parameter types match?

e.g. int f(int n, char c) {…} ...

f('x', 3); • Enforcement of scope rules.

Contextual Constraint Analysis

• Method: Traverse the tree recursively, deducing type information at the bottom, and passing it up.

– Make use of a DECLARATION TABLE, to record information about names.

– “Decorate” tree with reference information.

Example

Chronologically,1. Enter x into the DCLN table, with its type.2. Check type compatibility for x=5.3. X2 not declared!4. Verify type of ’>’ is boolean.5. Check type compatibility for ‘+’.6. Check type compatibility between x and

int, for assignment.

Code Generation

• Goal: Convert syntax tree to target code.

Target code could be:• Machine language.• Assembly language.• Quadruples for a fictional machine:

• label• opcode• operands (1 or 2)

Code Generation

• Example: • “pc” on UNIX generates assembly code• “pi” on UNIX generates code for the “p”

machine, which is interpreted by… an interpreter.

• pc: slow compilation, fast running code.• pi: fast compilation, slow running code.

• Method: Traverse the tree again.

Code (for a stack machine)

LOAD 5STORE X

LOAD XLOAD 10BGTCOND L1 L2

L1 LOAD XLOAD 1BADDSTORE XGOTO L3

L2 . . .L3

Code Optimization

• Goals:• Reduce the size of the target program.• Decrease the running time of the target.

• Note: “Optimization” is a misnomer. Code improvement would be better.

• Two types of optimization:• Peephole optimization (local).• Global optimization (improve loops, etc.).

Code Optimization (cont’d)

• Example (from previous slide):

LOAD 5 can be LOAD 5

STORE X replaced STND X

LOAD X with

Store non-destructively, i.e., store in X, but do not destroy value on top of stack.

Summary

Parser

Source

Constrainer

Code Generator

Code (for an abstract machine)

Interpreter

Screener

Scanner

Input Output

Table Routines

Error Routines

Tokens

Tokens

Tree

Tree

Overview of Compilation

Prepared by

Manuel E. Bermúdez, Ph.D.Associate ProfessorUniversity of Florida

Programming Language ConceptsLecture 2