Structure of a Compiler, Compiler and Interpreter, Lexical Analysis: Role of the lexical analyzer

Structure of a Compiler, Compiler and Interpreter, Lexical Analysis: Role of the lexical analyzer

1. Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon-423 603
(An Autonomous Institute, Affiliated to Savitribai Phule Pune University, Pune)
NACC ‘A’ Grade Accredited, ISO 9001:2015 Certified
Department of Computer Engineering
(NBA Accredited)
Dr. S. N. Gunjal
Assistant Professor
E-mail : [email protected]
Contact No: 91301 91301 Ext :145, 9503916876
Course-System Software
(CO313)
Structure of a Compiler, Compiler and Interpreter, Lexical Analysis: Role of the lexical
analyzer
Dr. S.N Gunjal

2. Compiler
Compiler is a program that can read a program in one language -the source language -and translate
it into an equivalent program in another language -the target language.
An important role of the compiler is to report any errors in the source program that it detects during
the translation process.
Compiler
Source
Program
Target
Program
Errors
Figure 1.1 : A compiler

3. Compiler
If the target program is an executable machine-language program, it can then be called by the user to
process inputs and produce outputs;
Target Program
Input
Output
Figure 1.2: Running the target program

4. Interpreter
Aninterpreterisanothercommonkindoflanguageprocessor.Insteadofproducinga
targetprogramasatranslation,aninterpreterappearstodirectlyexecutetheoperations
specifiedinthesourceprogramoninputssuppliedbytheuser.
Interpreter
Input
Output
Source
Program
Figure 1.3: An interpreter

5. The machine-language target program produced by a compiler is usually much faster than an
interpreter at mapping inputs to outputs .
An interpreter, however, can usually give better error diagnostics than a compiler, because it
executes the source program statement by statement.
Interpreter

6. 5
Structure of
Compiler

7. Structure of Compiler

9. The analysis part breaks up the source program into constituent pieces and imposes a
grammatical structure on them. It then uses this structure to create an intermediate representation of
the source program.
The analysis part also collects information about the source program and stores it in a data
structure called a symbol table, which is passed along with the intermediate representation to the
synthesis part.
Analysis and Synthesis Phase of Compiler
Analysis Phase :

10. Synthesis Phase
The synthesis part constructs the desired target program from the interme-diate representation and the
information in the symbol table.
The analysis part is often called the front end of the compiler; the synthesis part is the back end.

11. The first phase of a compiler is called lexical analysis or scanning.
Thelexicalanalyzerreadsthestreamofcharactersmakingupthesourceprogram.and
groupsthecharactersintomeaningfulsequencescalledlexemes.
Foreachlexeme,thelexicalanalyzerproducesasoutputatokenoftheform
(token-name,attribute-value)
position=initial+rate*60;
sequenceoftokens: <id,1><=><id,2><+><id,3><*><60><;>
Compiler Phases:
Lexical Analysis Phase :

12. Thesecondphaseofthecompilerissyntaxanalysisorparsing.
Theparserusesthefirstcomponentsofthetokensproducedbythelexical
analyzertocreateatree-likeintermediaterepresentationthatdepictsthe
grammaticalstructureofthetokenstream.
Atypicalrepresentationisasyntaxtreeinwhicheachinteriornode
representsanoperationandthechildrenofthenoderepresentthearguments
oftheoperation.
Compiler Phases:
Syntax Analysis :

13. Thesemanticanalyzerusesthesyntaxtreeandtheinformationinthe
symboltabletocheckthesourceprogramforsemanticconsistencywiththe
languagedefinition.
Italsogatherstypeinformationandsavesitineitherthesyntaxtreeorthe
symboltable,forsubsequentuseduringintermediate-codegeneration.
Compiler Phases:
Sematic Analysis :

14. Intheprocessoftranslatingasourceprogramintotargetcode,acompiler
mayconstructoneormoreintermediaterepresentations,whichcanhavea
varietyofforms.
Anintermediateformcalledthree-addresscode,whichconsistsofa
sequenceofassembly-likeinstructionswiththreeoperandsperinstruction.
tl = inttofloat(60)
t2 = id3 * tl
t3 = id2 + t2
id1 = t3
Compiler Phases:
Intermediate Code Generation:

15. Themachine-independentcode-optimizationphaseattemptstoimprovethe
intermediatecodesothatbettertargetcodewillresult.
Compiler Phases:
Code Optimization:

16. Thecodegeneratortakesasinputanintermediaterepresentationofthe
sourceprogramandmapsitintothetargetlanguage.
Theintermediateinstructionsaretranslatedintosequencesofmachine
instructionsthatperformthesametask.
Compiler Phases:
Code Generation:

18. THE ROLE OF THE LEXICAL ANALYZER
Thefirstphaseofacompileriscalledlexicalanalysisorscanning.
Thelexicalanalyzerreadsthestreamofcharactersmakingupthesource
program.andgroupsthecharactersintomeaningfulsequencescalled
lexemes.
Foreachlexeme,thelexicalanalyzerproducesasoutputatokenofthe
form (token-name,attribute-value)
position=initial+rate*60;
sequenceoftokens:<id,1><=><id,2><+><id,3><*><60><;>
Thelexicalanalyzerisresponsibleforremovingthewhitespacesand
commentsfromthesourceprogram.

19. 15/06/20 DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon
THE ROLE OF THE LEXICAL ANALYZER

20. THE ROLE OF THE LEXICAL ANALYZER
Token:Atokenisapair–atokennameandanoptional
tokenvalue.
Apatternisadescriptionoftheformthatthelexemesofatokenmaytake.
Alexemeisasequenceofcharactersinthesourceprogramthatmatchesthe
patternforatoken
Alexicalerroroccurswhenasequenceofcharactersdoesnotmatch
thepatternofanytoken.

21. 15/06/20 DEPARTMENT OF COMPUTER ENGINEERING, Sanjivani COE, Kopargaon
Attributes for tokens
• E = M * C ** 2
–<id, pointer to symbol table entry for E>
–<assign‐op>
–<id, pointer to symbol table entry for M>
–<mult‐op>
–<id, pointer to symbol table entry for C>
–<exp‐op>
–<number, integer value 2>

22. References
1.Alfred V.Aho,MonicaS.Lam,RaviSethi, Jeffrey D. Ullman, “Compilers-
Principles,Techniquesand Tools”, Pearson,ISBN:978-81-317-2101-8