1 October 2, 2015 1 October 2, 2015October 2, 2015October 2, 2015 Azusa, CA Sheldon X. Liang Ph. D....

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April 21, 20231

April 21, 2023April 21, 2023 Azusa, CAAzusa, CA

Sheldon X. Liang Ph. D.

Computer Science at Computer Science at Azusa Pacific UniversityAzusa Pacific University

Azusa Pacific University, Azusa, CA 91702, Tel: (800) 825-5278 Department of Computer Science, http://www.apu.edu/clas/computerscience/

CS400 Compiler ConstructionCS400 Compiler Construction

http://www.apu.edu/clas/computerscience/

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Syntax AnalysisPart I

Chapter 4

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CS@APU: CS400 Compiler ConstructionCS@APU: CS400 Compiler Construction


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Position of a Parser in the Compiler Model

LexicalAnalyzer

Parser and rest offront-end

SourceProgram

Token,tokenval

Symbol Table

Get nexttoken

Lexical error Syntax errorSemantic error

Intermediaterepresentation

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Keep in mind following questionsKeep in mind following questions

• Parser– Twofold:– Checking syntax– Invoking semantic action

• Error Handling – Two abilities:– Identifying errors – Locating errors

• Chomsky Hierarchy – From Turing machine– to Finite Automaton– From non to more restriction

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The Parser

• A parser implements a C-F grammar• The role of the parser is twofold:1. To check syntax (= string recognizer)

– And to report syntax errors accurately2. To invoke semantic actions

– For static semantics checking, e.g. type checking of expressions, functions, etc.

– For syntax-directed translation of the source code to an intermediate representation

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Syntax-Directed Translation

• One of the major roles of the parser is to produce an intermediate representation (IR) of the source program using syntax-directed translation methods

• Possible IR output:– Abstract syntax trees (ASTs)– Control-flow graphs (CFGs) with triples, three-address

code, or register transfer list notation– WHIRL (SGI Pro64 compiler) has 5 IR levels!

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WHIRL

• Winning Hierarchical Intermediate Representation

• 5 Levels:– VH, H, M L, VL– Lowering happens when needed – Each optimization performed at the right level

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Error Handling

• A good compiler should assist in identifying and locating errors– Lexical errors: important, compiler can easily

recover and continue– Syntax errors: most important for compiler,

can almost always recover– Static semantic errors: important, can

sometimes recover– Dynamic semantic errors: hard or impossible

to detect at compile time, runtime checks are required

– Logical errors: hard or impossible to detect

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Viable-Prefix Property

• The viable-prefix property of LL/LR parsers allows early detection of syntax errors– Goal: detection of an error as soon as possible without

further consuming unnecessary input

– How: detect an error as soon as the prefix of the input does not match a prefix of any string in the language

…for (;)…

…DO 10 I = 1;0…

Error isdetected here

Error isdetected here

Prefix Prefix

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Error Recovery Strategies

• Panic mode– Discard input until a token in a set of designated synchronizing

tokens is found

• Phrase-level recovery– Perform local correction on the input to repair the error

• Error productions– Augment grammar with productions for erroneous constructs

• Global correction– Choose a minimal sequence of changes to obtain a global least-

cost correction

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Grammars (Recap)

• Context-free grammar is a 4-tupleG = (N, T, P, S) where– T is a finite set of tokens (terminal symbols)

– N is a finite set of nonterminals

– P is a finite set of productions of the form

where (NT)* N (NT)* and (NT)*

– S N is a designated start symbol

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Notational Conventions Used

• Terminalsa,b,c,… Tspecific terminals: 0, 1, id, +

• NonterminalsA,B,C,… Nspecific nonterminals: expr, term, stmt

• Grammar symbolsX,Y,Z (NT)

• Strings of terminalsu,v,w,x,y,z T*

• Strings of grammar symbols,, (NT)*

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Derivations (Recap)

• The one-step derivation is defined by A

where A is a production in the grammar• In addition, we define

is leftmost lm if does not contain a nonterminal is rightmost rm if does not contain a nonterminal– Transitive closure * (zero or more steps)– Positive closure + (one or more steps)

• The language generated by G is defined byL(G) = {w T* | S + w}

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Derivation (Example)Grammar G = ({E}, {+,*,(,),-,id}, P, E) withproductions P = E E + E

E E * EE ( E )E - EE id

E - E - id

E * E

E + id * id + id

E rm E + E rm E + id rm id + id

Example derivations:

E * id + id

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15

Chomsky Hierarchy: Language Classification

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Avram Noam Chomsky is an American linguist, philosopher,, and lecturer. He is a professor emeritus of linguistics at the MIT. Chomsky is well known in the academic and scientific community as the father of modern linguistics. In the 1950s, Chomsky began developing his theory of generative grammar, which has had a profound influence on linguistics. He established the Chomsky hierarchy, a classification of formal languages in terms of their generative power. His naturalistic approach to the study of language has affected the philosophy of language and mind.

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• A grammar G is said to be– Regular if it is right linear where each production is of the form

A w B or A wor left linear where each production is of the form

A B w or A w– Context free if each production is of the form

A where A N and (NT)*

– Context sensitive if each production is of the form A

where A N, ,, (NT)*, || > 0– Unrestricted

Chomsky Hierarchy: Language Classification

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Chomsky Hierarchy

L(regular) L(context free) L(context sensitive) L(unrestricted)

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Chomsky Hierarchy

L(regular) L(context free) L(context sensitive) L(unrestricted)

Where L(T) = { L(G) | G is of type T }That is: the set of all languages

generated by grammars G of type T

L1 = { anbn | n 1 } is context free

L2 = { anbncn | n 1 } is context sensitive

Every finite language is regular! (construct a FSA for strings in L(G))

Examples:

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unrestricted

context sensitive

context free

regular

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Parsing

• Universal (any C-F grammar)– Cocke-Younger-Kasimi– Earley

• Top-down (C-F grammar with restrictions)– Recursive descent (predictive parsing)– LL (Left-to-right, Leftmost derivation) methods

• Bottom-up (C-F grammar with restrictions)– Operator precedence parsing– LR (Left-to-right, Rightmost derivation) methods

• SLR, canonical LR, LALR

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Got it with following questionsGot it with following questions

• Parser– Twofold:– Checking syntax– Invoking semantic action

• Error Handling – Two abilities:– Identifying errors – Locating errors

• Chomsky Hierarchy – From Turing machine– to Finite Automaton– From non to more restriction

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Thank you very much!

Questions?

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1 October 2, 2015 1 October 2, 2015October 2, 2015October 2, 2015 Azusa, CA Sheldon X. Liang Ph. D....

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