Working of the Compilers

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1 Working of the Compilers Manisha Yadav Nisha Thakran IT DEPARTMENT IT DEPARTMENT DCE,GURGAON DCE,GURGAON Abstract- The objective of the paper is to depict the working of the compilers that were designed to convert the higher level languages to the low level languages.the entire working of the complier is divided into different operations that are lexical analysis, preprocessing, parsing, semantic analysis (Syntax-directed translation), code generation, and code optimization due to which the high-level source programs are translated into target programs in machine languages for the specific hardware which is used by the users.this paper also depicts the different types of the compilers used along with the difference between the wellknown compilers C++.and the JAVA compilers. Both the compilers work for the different languages although both the compilers have many differences yet have similarities too that has been impended in the paper. It also demonstrates their working operating systems, their work with different operating systems and their licence type. 1. INTRODUCTION A compiler is a computer program that translates a program in a source language into an equivalent program in a target language.a source program/code is a program/code written in the source language, which is usually a high-level language. A target program/code is a program/code written in the target language, which often is a machine language or an intermediate codea compiler acts as a translator, transforming human-oriented programming languages into computer-oriented machine languages. Ignore machine-dependent details for programmer. A program that reads a program written in one language (source language) and translates it into an equivalent program in another language (target language). Two components (i)understand the program (make sure it is correct) (ii)rewrite the program in the target language. Traditionally, the source language is a high level language and the target language is a low level language (machine code). Fig.1 Structure of the compiler Scanner the scanner begins the analysis of the source program by reading the input, character by character, and grouping characters into individual words and symbols (tokens) o RE (Regular Expression) IJIRT INTERNATONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 511

2 o NFA (Non -deterministic finite automata) o DFA (Deterministic finite automata) o LXA Parser Given a format syntax specification (typically as a context-free grammar [CFG]), the parser reads tokens and groups them into units as specified by the productions of the CFG being used. As syntactic structure is recognized, the parser either calls corresponding semantic routines directly or builds a syntax tree. o CFG (Context Free Grammar) o BNF (Backus-Naur Form) o GAA (Grammar Analysis Algorithms) o LL, LR, SLR, LALR Parsers o YACC Semantic Routines perform two functions o check the static semantics of each construct o Do the actual translation o heart of the compiler syntax Directed Translation semantic Processing Techniques IR (intermediate Representation) Optimizer The IR generated by the semantic routines is analyzed and transformed into functionally equivalent but improved IR code This phase can be very complex and slow peephole optimization loop optimization, register allocation, code scheduling o register and temporary management o peephole optimization Code Generator interpretive code generation generating code from Tree/Dag Grammar-Based code generator Different phases of compiler/working: The different phases can be seen as different transformation steps to transform source code into object code. The different phases correspond roughly to the different parts of the language specification: Syntax analysis <-> Syntax A program that reads a program written in one language (source language) and translates it into an equivalent program in another language (target language). Two components (i)understand the program (make sure it is correct) (ii)rewrite the program in the target language. Traditionally, the source language is a high level language and the target language is a low level language (machine code). It gets the parse tree from the parser together with information about some syntactic elements. It determines if the semantics or meaning of the program is correct. This part deals with static semantic:(i)semantic of programs that can be checked by reading off from the program only.(ii)syntax of the language which cannot be described in contextfree grammar. Mostly, a semantic analyzer does type checking. It modifies the parse tree in order to get that (static) semantically correct code. (fig. 1.1) Fig.(1.1) IJIRT INTERNATONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 512

3 Contextual analysis <-> Contextual constraints Contextual analysis: Scope checking: verify that all applied occurrences of identifiers are declared. Type checking: verify that all operations in the program are used according to their type rules. Annotate AST:Applied identifier occurrences => declaration Expressions => Type Code generation <-> Semantics Semantic analysis is applied by a compiler to discover the meaning of a program by analyzing its parse tree or abstract syntax tree. A program without grammatical errors may not always be correct program. pos = init + rate * 60 What if pos is a class while init and rate are integers? This kind of errors cannot be found by the parser Semantic analysis finds this type of error and ensure that the program has a meaning. Static semantic checks (done by the compiler) are performed at compile time Type checking Every variable is declared before used Identifiers are used in appropriate contexts Check subroutine call arguments Check labels Dynamic semantic checks are performed at run time, and the compiler produces code that performs these checks Array subscript values are within bounds Arithmetic errors, e.g. division by zero Pointers are not dereference unless pointing to valid object A variable is used but hasn't been initialized When a check fails at run time, an exception is raised Semantic Analysis and Strong Typing A language is strongly typed "if (type) errors are always detected" Errors are either detected at compile time or at run time Examples of such errors are listed on previous slide Languages that are strongly typed are Ada, Java, ML, Haskell Languages that are not strongly typed are Fortran, Pascal, C/C++, Lisp Strong typing makes language safe and easier to use, but potentially slower because of dynamic semantic checks In some languages, most (type) errors are detected late at run time which is detrimental to reliability e.g. early Basic, Lisp, Prolog, some script languages Code Generation and Intermediate Code Forms A typical intermediate form of code produced by the semantic analyzer is an abstract syntax tree (AST) The AST is annotated with useful information such as pointers to the symbol table entry of identifiers Example AST for the gcd program in Pascal fig.(1.2) IJIRT INTERNATONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 513

4 Fig.(1.2) C++ and java compilers : C++ compiler : A compiler is a program that translates human readable source code into computer executable machine code. To do this successfully the human readable code must comply with the syntax rules of whichever programming language it is written in. The compiler is only a program and cannot fix our programs for us. If we make a mistake, we have to correct the syntax or it won't compile. A compiler's complexity depends on the syntax of the language and how much abstraction that programming language provides. A C compiler is much simpler than C++ Compiler and C# Compiler. Here is what happens when we compile code. Lexical Analysis: This is the first process where the compiler reads a stream of characters (usually from a source code file) and generates a stream of lexical tokens. For example the C++ code Int C= (A*B)+10; might be analyzed as these tokens: type "int" variable "C" Equals Left bracket Variable "A" Times Variable "B" Right bracket Plus literal "10" Next is Syntactical Analysis: This output from Lexical Analyzer goes to the Syntactical Analyzer part of the compiler. This uses the rules of grammar to decide whether the input is valid or not. Unless variables A and B had been previously declared and were in scope, the compiler might say 'A' : undeclared identifier.had they been declared but not initialized. The compiler would issue a warning local variable 'A' used without been initialized.we should never ignore compiler warnings. They can break your code in weird and unexpected ways.always fix compiler warnings One Pass Or Two?:Some languages have been written so that a compiler can get away with reading the source code once and generating the machine code. Pascal is one such language. Many compilers require at least two passes. Why is this Sometimes it is because of Forward Declarations of functions or classes. In C++ a class can be declared but not defined until later. The compiler will be unable to work out how much memory the class needs until it has compiled the body of the class. Then it must reread the code before generating correct code. Generating Machine Code: Assuming that the compiler has successfully completed these stages Lexical Analysis. Syntactical Analysis. The final stage is generating machine code. This can be an extremely complicated process, especially with modern CPUs.The speed of the compiled executable should be as fast as possible and can vary enormously according to The quality of the generated code and How much optimization has been requested. Most compilers let us specify the amount of optimization. Typically none for debugging (quicker compiles!) and full optimization for the released code. Java vs. C++ In Java, a reference is a data type. It can be assigned to, compared, copied, stored, etc. Same reference can refer to different objects at different times during execution In C++, a reference is an alias for an object It cannot be assigned to; assignment is through the reference to the underlying object. Similar to dereferencing a pointer in C.A reference always refers to the same object for the duration of its scope IJIRT INTERNATONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 514

5 REFERENCES [1] [2] Textbooks: [3] Compilers: Principles, Techniques and Tools, Aho, [4] Lam, Sethi & Ullman, 2007 [5] (Required) [6] Lex & yacc, Levine et. Al. [7] Slides [8] Sample code for Lex/YACC (C, C++, Java) [9] ramming/p/compiler.htm IJIRT INTERNATONAL JOURNAL OF INNOVATIVE RESEARCH IN TECHNOLOGY 515

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