Download presentation

Presentation is loading. Please wait.

Published byJase Hild Modified over 2 years ago

1
Attribute Grammars Prabhaker Mateti ACK: Assembled from many sources

2
About Attribute Grammars Attribute grammars (AGs) add semantic info on parse tree nodes Used for semantic checking and other compile-time analyses, e.g., type checking in a compiler Used for translation, e.g., parse tree to assembly code A traversal of the parse tree and the computation of information. CS7842

3
Attribute Grammars: Definition An attribute grammar is a context-free grammar G = (S, N, T, P) with the following additions. For each terminal and non-terminal X, disjoint sets S(X) synthesized attributes and I(X) inherited attributes: – A(X) = S(X) ∪ I(X) X0 ::= X1... Xn S(X0) = f(I(X0), A(X1),..., A(Xn)) I(Xj) = g(A(X0),..., A(Xn)) – depends on the attribute value at parent and those of siblings. Each rule has a set of predicates/ conditions to check for attribute consistency: P( A(X0), A(X1), A(X2), …, A(Xn) ) CS7843

4
Example-1: Binary Numbers 1.N ::= 0 2.N ::= 1 3.N ::= N 0 4.N ::= N 1 CFGs do not provide semantics. CGGs provide only syntax, that too without context-sensitive details 1.N.val:= 0 2.N.val:= 1 3.N.val:= 2*N.rhs.val 4.N.val:= 2*N.rhs.val+1 N.val is an attribute associated with node N of the parse tree N.rhs Node corresponding to the rhs Synthesized Attributes CS7844

5
Example-2: Type Checking 1.E ::= n 2.E ::= x 3.E ::= E1 + E2 4.E ::= E1 * E2 Semantics we wish to add – n is an int, x is a real. – op + returns an int if both the operands are int, otherwise a real. 1.E.type := int 2.E.type := real 3.if E1.type = E2.type then E.type := E1.type else E.type := real fi Item 3 derived version of the semantics Static semantics CS7845

6
Example-3: Assignment Arithmetic stm ::= var := exp | stm; stm exp ::= var + var | var var ::= A | B | C synthesized actual-type for var and exp inherited expected-type for exp lookup (var.string) a helper function; gives the actual type of A, B, C exp ::= var1 + var2 – subscripts added exp.actual-type := var1.actual-type exp.expected-type – from parent in the parse tree Predicates: – var1.actual-type == var2.actual-type – exp.expected-type == exp.actual-type – var ::= A | B | C var.actual-type := lookup (var.string) CS7846

7
Inherited Attributes Example Declaration and Use { int i, j, k; i := i + j + j; } assign ::= var := exp – env: environment – var.env := assign.env – exp.env := assign.env CS7847

8
8 Information Flow inherited synthesized... computed available available

9
Attribute Value Computation If all attributes were inherited, the tree could be decorated in top-down order. Inherited Attributes pass information – down the parse tree, or – from left siblings to the right siblings If all attributes were synthesized, the tree could be decorated in bottom-up order. Synthesized Attributes pass information up the parse tree In many cases, both kinds of attributes are used, and it is some combination of top-down and bottom-up that must be used. Initially, there are intrinsic attributes on the leaves If a condition in a tree evaluates to false, an error occurs. CS7849

10
Prolog for Example-2 1.type(n, int). 2.type(x, real). 3.type(+(E, F), T) :- type(E, T), type(F, T). 4.type(+(E, F), real) :- type(E, T1), type(F, T2), T1 \= T2. 5. Type Checking ?- type(+(n, x), real). 6. Type Inference ?- type(+(n, x), T). (Definite Clause Grammars) CS78410

11
Example-4: Fractions in Binary 1.F ::=. N 2.N ::= 0 3.N ::= 1 4.N ::= 0 N 5.N ::= 1 N Synthesized: val, value Inherited: pow, the number of bits between left of a non-terminal and the binary point Nr: N-right, Nl: N-left 1: F.val:= N.val; N.pow:= 1 2: N.val := 0 3: N.val := (1/2^N.pow) 4: Nl.val := Nr.val 4: Nr.pow := 1 + Nl.pow 5: Nl.Val := Nr.val+(1/2^N.pow) 5: Nr.pow := 1 + Nl.pow CS78411

12
Ex4: Synthesized Attributes Only Binary Fractions, same grammar as before: 1.F ::=. N 2.N ::= 0 3.N ::= 1 4.N ::= 0 N 5.N ::= 1 N Alternate computation based on synthesized attribute val only 1: F.val := N.val / 2 2: N.val := 0 3: N.val := 1 4: N.val := N.val / 2 5: N.val := N.val / 2 + 1 CS78412

13
Example-5: Distinct Identifiers Compute the number of distinct identifiers in a straight-line program. Semantics specified in terms of sets of identifiers. Attributes – var id – exp ids – stm ids num CS78413

14
Example-5: Distinct Identifiers exp::= var – exp.ids = { var.id } exp::= exp1 + exp2 – exp.ids = exp1.ids U exp2.ids stm::= var:= exp – stm.ids = { var.id } U exp.ids – stm.num = | stm.ids | stm::= stm1;stm2 – stm.ids = stm1.ids U stm2.ids – stm.num = | stm.ids | CS78414

15
Example-5: Using Lists Attributes – envi: list of vars in preceding context – envo: list of vars for following context – dnum: number of new variables exp ::= var exp.envo = if member(var.id, exp.envi) then exp.envi else cons(var.id, exp.envi) fi CS78415

16
Example-5: Using Lists exp ::= exp1 + exp2 envi envi envi envo envo envo dnum dnum dnum exp1.envi := exp.envi exp2.envi := exp1.envo exp.envo := exp2.envo exp.dnum := length(exp.envo) exp.envo = append-sans-duplicates(exp1.envo, exp2.envo ) CS78416

17
Complete Evaluation Rules Synthesized attribute associated with N: – Each alternative in “N ::= …” should contain a rule for evaluating the Synthesized attribute. Inherited attribute associated with N: – For every occurrence of N in “… ::= … N …” there must be a rule for evaluating the Inherited attribute. Whenever you create an attribute grammar (in home work/ exams), make sure it satisfies these requirements. CS78417

18
One Pass Attribute Computation To enable one-pass top-down left-to-right computation of the attributes: – each inherited attribute of the right-hand side symbol can depend on all the attributes associated with preceding right-hand side symbols and the inherited attribute of the left-hand side non-terminal. – Similarly, the synthesized attribute of the left-hand side non-terminal can depend on all the attributes associated with all the right-hand side symbols and the inherited attribute of the left-hand side non- terminal. CS78418

19
More than Context-Free Power LABC = { a^nb^nc^n | n > 0 } – Unlike LAB = { a^nb^n | n > 0 }, here we need explicit counting of a’s, b’s and c’s LWCW = { wcw | w ∈ {a, b}* } – The “flavor” of checking whether identifiers are declared before their uses LABC, LWCW cannot be defined with a context- free grammar Syntax analysis (i.e., parser based on CFGs) cannot handle semantic properties CS78419

20
LABC = { a^n b^n c^n | n > 0 } ls ::= as bs cs – ExpNb(bs) := Na(as); ExpNc(cs) := Na(as) as ::= a | a as1 – Na(as) := 1; Na(as) := Na(as1) + 1 bs ::= b | b bs1 – cond(ExpNb(bs) = 1); ExpNb(bs1) := ExpNb(bs) - 1 cs ::= c | c cs1 – Cond(ExpNc(cs) = 1); ExpNc(cs1) := ExpNc(cs) – 1 Na:synthesized by as ExpNb:inherited from bs ExpNc: inherited from cs CS78420

21
Uses of Attribute Grammars Compiler Generation – Top-down Parsers (LL(1)) FIRST sets, FOLLOW sets, etc – Code Generation Computations Type, Storage determination, etc. Databases – Optimizing Bottom-up Query Evaluation (Magic Sets) Programming and Definitions CS78421

22
Uses of Inherited Attributes ex: 5.0 + 2 – need to generate code to coerce int 2 to real 2.0 Determination of un-initialized variables Determination of reachable non-terminals Evaluation of an expression containing variables CS78422

23
Use of Attribute Grammars Useful for expressing arbitrary cycle-free computational walks over CFG derivation trees – Synthesized and inherited attributes – Conditions to reject invalid parse trees – Evaluation order depends on attribute dependencies Realistic applications: – type checking – code generation “Global” data structures must be passed around as attributes Any container data structure (sets, etc.) can be used The evaluation rules can call auxiliary/helper functions but the functions cannot have side effects CS78423

24
References T. K. Prasad, Attribute Grammars and their Applications, In: Encyclopedia of Information Science and Technology, pp. 268-273, 2008. Attribute-Grammars.pdf Attribute-Grammars.pdf PL Text Book Sections – Pagan: 2.1, 2.2, 2.3, 3.2 – Stansifer: 2.2, 2.3 – Slonneger and Kurtz: 3.1, 3.2 CS78424

Similar presentations

OK

1 Compiler Construction (CS-636) Muhammad Bilal Bashir UIIT, Rawalpindi.

1 Compiler Construction (CS-636) Muhammad Bilal Bashir UIIT, Rawalpindi.

© 2017 SlidePlayer.com Inc.

All rights reserved.

Ads by Google

Ppt on fibonacci numbers in flowers Ppt on different model of atom Seven segment display ppt online Ppt on 500 mw turbo generators Ppt on earth damage Ppt on aerobics workout Ppt on edge detection tutorial Numeric display ppt online Ppt on indian supreme court Ppt on 4g wireless technology free download