Overview of Compilation
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Transcript of Overview of Compilation
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