PLLab, NTHU Cs2403 Programming Languages Implementation Issues Cs2403 Programming Language Spring...

PLLab, NTHUCs2403 Programming Languages

Implementation Issues

Cs2403 Programming Language

Spring 2005

Kun-Yuan Hsieh


Why studying?

• Increased capacity to express ideas– Language feature limits the form of algorithm you

construct– Simulating other language’s feature to break this limit

• Improved ability of choosing right PL– To give you the ability to make informed language

choices• Increase ability to learn new PL

– Programming language is in a state of continuous evolution

– Vocabulary and fundamental concepts helps you learn


Why studying? (cont.)

• Better understanding of implementation– To use a language in the way it was designed to

be used

• Overall advancement of computing– Most popular languages are the best available?– Better educated programmer leads to better

language population?


Evaluation criteria

• Readability– Important because maintenance is cost– Overall simplicity since we tend to learn subset of it

• Feature multiplicity: more than one way to accomplish one operation

– Count = count +1– Count += 1– Count ++

• operation overloading– Reducing # of operations

• Less readable if carried too far– Assembly language is simple but less readable

• Orthogonality


Evaluation criteria

• Readability (cont.)– Control statement

• Read from top to bottom is easier to understand than jump to follow the execution order

– Data type and structure makes your meaning clear• timeOut = 1 v.s. timeOut = true!!

– Syntax– Identifier forms: using connotative names

– Special words: program appearance

– Form and meaning: statement indicates the purpose


Evaluation criteria

• Writability– What affects readability affect writability

– Simplicity and orthogonality• Simplicity avoid misusing but may leads to undetected error

– Support for abstraction to define then use complicated structures in a simple way

• Subprogram

• Tree (using point or array!!)

– Expressivity


Evaluation criteria

• Reliability– Performs to its specification under all

conditions– Type checking– Exception handling– Aliasing– Readability and writability


Evaluation criteria

• Cost

– Training programmers to use language

– Writing programs

– Compiling programs, optimization

– Executing programs

– Language implementation system• Java interpreter is free

– Reliability

– Maintaining programs


Influences on language design

• Computer architecture– Software executes on hardware

• Programming methodologies– 1950s and early 1960s: machine efficiency– Late 1960s: programming efficiency,

readability, better control structures– Late 1970s: data abstraction– Middle 1980s: object-oriented programming


The von Neumann Arch (cont.)

Control unitArithmetic and

logic unit

Instructions and dataResults of operations

Memory (stores both instructions and data)

Input and output devices


A grid-computer arch(non-Von Neumann)

CPU CPU CPU

CPU CPU CPU

CPU CPU CPU


Categories & trade-off

• PL categories:– Imperative (C, Pascal, FORTRAN)– Functional (LISP, Scheme)– Logic (Prolog)– Object-Oriented (C++, C Sharp, Java)

• Design trade-off– Reliability v.s. cost of execution– Writability v.s. readability– Flexibility v.s. safety


Implementation methods• Compilation (Pascal, C, C++, FORTRAN)

– Translate high-level program to machine code– Slow translation – Fast execution

• Interpretation (LISP, Scheme, scripts)– No translation, interpreted by interpreter– Slow execution (translated on execution)– Easy to debug, run-time error refers to source level units

• Hybrid method (Java, CSharp, Perl)– Translated to intermediate language (Java byte code, MSIL)– Medium execution speed


Compiler examples


Interpreter v.s. Compiler

Input data

ResultsInterpreterSource Program

CompilerExecutable(sample.exe)Source Program


Hybrid method

Java Byte Code

Java Source Code Java Compiler

Java Interpreter on Windows

Java Interpreter on Unix

(Stored in the disk & Publicly available)

(private property)


C# VB .NET Visual J# .NET

MSIL

Linux native code

Windows native code

Mac OS native code

Compile into

MSIL

CLR do this

Support now

Will Support soon

Will Support soon

Hybrid method example- .Net


Helloworld.il


Virtual computer conceptC++

compiler

FORTRAN compiler

C compiler

Ada compiler …

Assembler

OperatingSystem

Commandinterpreter

LISP interpreter

Operating system

Macroinstruction interpreter

Bare machine

Virtual CComputer

Virtual C++Computer


Levels of Programming Languages

High-level program class Triangle { ... float surface() return b*h/2; }

class Triangle { ... float surface() return b*h/2; }

Low-level program

ldloc.i4 bldloc.i4 hmulldc.i4 2divret

ldloc.i4 bldloc.i4 hmulldc.i4 2divret

Executable Machine code

0001001001000101001001001110110010101101001...

0001001001000101001001001110110010101101001...


Compiler phases

Lexical analyzer

Syntax analyzer

Intermediate codegenerator

Semantic analyser

Code optimizer

Code generator

Symbol-table manager

Error handler

Source program

Target program


Lexical analysis

• Source program is read from left-to-right and group into tokens

• Tokens: sequences of characters have a meaning• Ex:

– a= b + c * 10• identifier a• assignment symbol =• identifier b• plus sign +• identifier c• multiplication sign *• number 10


Syntax analysis

• Grouping the tokens into grammatical phrases (hierarchical)

• Usually presented as a parse tree

• Expressed by recursive rules– Any identifier is an

expression– Any number is an

expression– If exp1 and exp2 are

expressions, then so are• exp1 + exp2• exp1 * exp3

Assignment statement

expression

identifier expression

expression

expressionexpression

a

identifier

b

c

identifier

10

number

=

+

*


Semantics• Specification of semantics is concerned with

specifying the “meaning” of well-formed programs.

• Terminology:– Expressions are evaluated and yield values (and may or

may not perform side effects)– Commands are executed and perform side effects.– Declarations are elaborated to produce bindings

• Side effects:– change the values of variables– perform input/output


Syntax Analysis

Scanner

Source Program

Abstract Syntax Tree

Error Reports

Parser

Stream of “Tokens”

Stream of Characters

Error Reports

Dataflow chart


Type Checking• For most statically typed programming languages,

a bottom up algorithm over the AST:• Types of expression AST leaves are known

immediately:– literals => obvious– variables => from the ID table– named constants => from the ID table

• Types of internal nodes are inferred from the type of the children and the type rule for that kind of expression

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