Chap01-Intro.ppt

Chapter 1. Introduction
Compiler
Dr. Mohammed Khader
ASU
1

Outline
• 1.1 History of Compilation
• 1.2 What Compilers Do
• 1.3 Interpreters
• 1.4 Syntax and Semantics
• 1.5 Organization of a Compiler
3

Language Processors
• Act as Translators
– Transforming human-oriented programming
languages into computer-oriented machine
languages
4

Overview and History
• Compilers are fundamental to modern
computing.
• They act as translators, transforming
human-oriented programming languages
into computer-oriented machine languages.
Programming
Language
(Source)
Compiler
Machine
Language
(Target)
5

Overview and History (Cont’d.)
• The first real compiler
– FORTRAN compilers of the late 1950s
• Today, we can build a simple compiler in
a few month.
• Creating an efficient and reliable compiler
is still challenging.
6

Overview and History (Cont’d.)
• Compiler technology is largely applicable
and has been employed in rather
unexpected areas.
– Text-formatting languages, like LaTeX
– Silicon compiler for the creation of VLSI circuits
– Command languages of OS
– Query languages of Database systems
7

8
Front-end and Back-end
• Front-end performs the analysis of the
source language code to build the internal
representation, called
intermediate representation (IR).
• Back-end performs the target language
synthesis to generate the target code
(assembly code).

1.2 What Compilers Do
• Compilers may be distinguished in two
ways:
1. By the kind of machine code they generate
2. By the format of the target code they generate
9

10
Machine Code Generated by
Compilers(kind of Machine code)
 Pure machine code
– Compiler may generate code for a particular
machine’s instruction set without assuming the
existence of any operating system or library
routines.
– Pure machine code is used in compilers for
system implementation languages, which are
for implementing operating systems or
embedded applications.

11
Compilers (Cont.)
 Augmented machine code
– Compilers generate code for a machine
architecture that is augmented with:
1) operating system routines and
2) runtime language support routines.

12
Compilers (Cont.)
 Virtual machine code
– Compilers generate virtual machine code that is
composed entirely of virtual machine instructions.
– That code can run on any architecture for which a VM
interpreter is available.
• For example, the VM for Java, Java virtual machine (JVM), has a
JVM interpreter.
– Portability is achieved by writing just one virtual
machine (VM) interpreter for all the target architectures.

1.2.2 Target Code Formats
Another way that compilers differ from one another is in the
format of the target code they generate as follows:
1. Assembly Language (Source) Format
• Advantages:
– Simplifies translation:
• A number of code generation decisions (e.g. how to compute
addresses) can be left for the assembler.
– Cross-compilation:
• The compiler executes on one computer but generates code that
executes on another.
• Symbolic form is produced that is easily transferred between different
computers.
– Support experimental programming language designs
• Makes it easier to check the correctness of a compiler, since its output
can be observed.
• Disadvantage:
– Need an additional pass (Assembler)
13

2. Relocatable binary
– External references, local instruction addresses, and data addresses
are not yet bound.
– Addresses are assigned relative either to the beginning of the
module or to symbolically named locations.
• Advantages:
– More efficient and more control over the translation process: because
it eliminates one step of calculating absolute addresses.
– Allow modular compilation: breaking up of a large program into
separately compiled pieces.
– Allow cross-language references: calls of assembler routines or
subprograms in other high-level languages
– Support subroutine libraries for example, I/O, storage allocation,
and math routines
• Disadvantage:
– A linkage step is required to support libraries and to produce
absolute binary format that is executable. 14

3. Absolute binary
• Advantages:
– Program can be directly executed when the compiler is finished
– No need for intermediate step of linking.
– Faster: allows a program to be prepared and executed in a single
step.
– Guarantee the use of only the most current library routines
– Good for debugging (frequent changes)
• Disadvantages:
– Limited ability to interface with other code
– The program must be recompiled for each execution
15

16
1.3 Interpreters
Compiler vs. Interpreter
Compiler has compilation and execution phases:
1) The compilation phase generates target
program from source program.
2) The execution phase executes the target
program.
(Fig. 1.1).
Interpreter directly interprets (executes) the source
program that reads inputs and writes outputs
(Fig. 1.3).

17
Compiler versus Interpreter (Cont.)

1.4 Syntax and Semantics
• Syntax: structure
– E.g. context-free grammars (CFGs)
•a=b+c is legal,
•b+c=a is not legal
• Semantics: meaning
– E.g. a = b + c
•is illegal if any of the variables are
undeclared or if b or c is of type Boolean
– Static semantics
– Runtime semantics
18

Static Semantics
• A set of rules that specify which
syntactically legal programs are actually
valid
– E.g.: Identifier declaration, type-compatibility
of operators and operands, proper number of
parameters in procedure calls
• Can be specified either formally or
informally
– E.g.: attribute grammars
19

An Example of Attribute
Grammars
• Production rule:
– E  E+T
• Augmented production rule:
– Eresult  Ev1 + Tv2
•if v1.type = numeric and v2.type = numeric
then result.type  numeric
else call ERROR( )
20

Runtime Semantics
•Runtime, or execution, semantics are
used to specify what a program
computes.
A. Informal These semantics are often specified
very informally in a language manual or report;
B. Formal approaches to defining the runtime
semantics of programming languages.
21

The Structure of a Compiler
• Modern compilers are syntax-directed
– Compilation is driven by the syntactic
structure of programs;
– i.e., actions are associated with the structures.
22

Tasks performed by compilers
Analysis of the source program such as
scanning and parsing.
•Syntax analysis
•Semantic analysis
Synthesis of a target program such as code
generation, that when executed, will
correctly perform the computations
described by the source program.
23

1.5 Organization of a Compiler
(Used by all Phases
of The Compiler)
24

The Scanner
• The scanner begins the analysis of the source
program by reading the input text (character
by character) and grouping individual
characters into tokens such as :
– Identifiers
– Integers
– Reserved words
– Delimiters
• It eliminate unneeded information like comments
• It process compiler control directives.
• It enters preliminary information into symbol table.
26

The Parser
• Reading tokens and grouping them into phrases
according to the syntax specification such as
context free grammar CFG
• It usually builds an Abstract Syntax Tree (AST)
as a concise representation of program structure
– (Chap. 2 & 7)
27

The Type Checker
(Semantic Analysis)
• Checking the static semantics of each AST
node
– the type checker decorates the AST node by
adding type information to it
– It checks type correctness, reachability and
termination, and exception handling.
– Otherwise, a suitable error message is issued
28

Translator (Program Synthesis)
• Translating AST nodes into Intermediate
Representation code (IR) (such as byte code
in Java) that correctly implements the
meaning of the program
• In simple, nonoptimizing compilers, the
translator may generate target code
directly
29

The Optimizer
• Optimizer improves IR code’s performance.
• Analyzing and transforming the IR code
generated by the translator into functionally
equivalent but improved code
– Complex
• Optimization can also be done after code
generation
30

The Code Generator
• Mapping the IR code generated by the translator
into target machine code (assembly code)
– Machine-dependent, complex
• Register allocation
• Code scheduling
• Automatic construction of code generators has
been actively studied
– Matching a low-level IR to target-instruction
templates
– gcc has machine description files for more than ten
popular computer.
31

328
Organization of a Compiler (Cont.)
Symbol Tables
Scannerr Parser Type Checker
Translator
Optimizer
Code
Generator
Source
Program
Tokens AST
Decorated
AST
Intermediate
Representation
Intermediate
Representation
Target Code
Revised Figure 1.4: A syntax-directed compiler. AST denotes the Abstract
Syntax Tree.

Symbol Tables
• A mechanism that allows information to
be associated with identifiers and shared
among compiler phases
– Identifier declaration
– Identifier use
– Type checking
33

The Structure of a Compiler
34
Scanner
[Lexical Analyzer]
Parser
[Syntax Analyzer]
Semantic Process
[Semantic analyzer]
Code Generator
[Intermediate Code Generator]
Code Optimizer
Tokens
Parse tree
Abstract Syntax Tree w/ Attributes
Non-optimized Intermediate Code
Optimized Intermediate Code
Code Generator
Target machine code

Phases of a Compiler
[Aho, Lam, Sethi, Ullman]
Syntax Analyzer
character stream
target machine code
Lexical Analyzer
Intermediate Code Generator
Code Generator
token stream
syntax tree
intermediate representation
Symbol
Table
Semantic Analyzer
syntax tree
Machine-Independent
Code Optimization
Machine-Dependent
Code Optimization
(optional)
(optional)
35

Compiler Writing Tools
• Compiler generators (compiler compilers)
– Scanner generator
– Parser generator
– Symbol table manager
– Attribute grammar evaluator
– Code-generation tools
• Much of the effort in crafting a compiler lies in
writing and debugging the semantic phases
– Usually hand-coded
36

Compiler-writing tools
Regular expressions
Scanner generators
Lex (The input to lex consists of a definition of
each token as a regular expression )
Parser generators:
Yacc
Code generator generators
http://catalog.compilertools.net/
 37

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