an attribute grammar for tiny

An Attribute Grammar for Tiny

Prepared by

Manuel E. Bermúdez, Ph.D.Associate ProfessorUniversity of Florida

Programming Language PrinciplesLecture 18


• Tiny: a very small Pascal-like language with the following: • Every program is given a name.• Variables of type integer.• Expressions composed of:

• variables• integers• intrinsic function "read"• boolean operator "not"

An Attribute Grammar for Tiny (cont’d)

• equality operator "="• binary operators "+" and "-"• unary "-", "not".

• Assignment statements.• Variables must be assigned a value

before they are used.• No declarations.

An Attribute Grammar for Tiny (cont’d)

• Statements:• if-then-else• while• statement sequencing (";")• output statement.

Tiny’s Syntax

Tiny → 'program' Name ':' S 'end' Name '.’ => 'program';S → 'assign' Name ':=' Exp => 'assign' → 'output' Exp => 'output' → 'if' Exp 'then' S 'else' S 'fi' => 'if' → 'while' Exp 'do' S 'od' => 'while' → S list ';' => ';' ;Exp → Term '=' Term => '=' → Term;Term → Term '+' Factor => '+' → Term '-' Factor => '-' → Factor;

Tiny’s Syntax (cont’d)

Factor → '-' Factor => '-' → 'not' Factor => 'not'

→ Name

→ 'read' => 'read'

→ '<integer>'

→ '(' Expression ')';

Name → '<identifier>';

Sample Tiny Program

• Copies 10 integers from input to output:

program copy: assign i := 1; while not (i=11) do output read; assign i := i + 1 odend copy.

AST Grammar for Tiny

P → <'program' '<identifier>' E '<identifier>'> E → <'assign' '<identifier>' E> → <'output' E> → <'if' E E E> → <'while' E E> → <';' E+> → <'not' E> → <'=' E E> → <'+' E E> → <'-' E E> → <'-' E> → '<identifier>' → '<integer>' → 'read'

Note

• AST grammar simplified.• No distinction between expressions

and statements.

• Our attribute grammar will specify the translation (compilation) of Tiny programs to assembly for an abstract machine.

Target Machine Instruction Set

PC := 1;

Next:

case Code[PC] of

save n:stack[n] := stack[top--]

load n:stack[++top] := stack[n]

negate:stack[top] := -stack[top]

not: stack[top] := not stack[top]

add: t := stack[top--]; stack[top] := stack[top] + t

subtract: t := stack[top--]; stack[top] := stack[top] - t

equal: t := stack[top--]; stack[top] := stack[top] = t

Target Machine Instruction Set (cont’d)

read: stack[++top] := getinteger(input)

print: putinteger(stack[top--])

lit n: stack[++top] := n

goto n: PC := n; goto Next

iffalse n: if stack[top--] = 0 then PC:=n; goto Next fi

iftrue n: if stack[top--] = 1 then PC:=n; goto Next fi

stop: halt

end;

++PC;

goto Next;

Target Code for our Sample Tiny Program

1: lit 1 # i := 12: load 1 # i = 113: lit 114: equal5: not6: iffalse 14 # while7: read8: print9: load 1 # i := i + 110: lit 111: add12: save 113: goto 2 # od14: stop # end

program copy:

assign i := 1;

while not (i=11) do

output read;

assign i := i + 1

od

end copy.

Semantic Errors

• Variables used before being initialized.• Non-boolean expression in "while", "not",

or "if".• Non-integer expression in "=", "-", or "+".• Program names do not match.

Data Structures Required

• Declaration table.• Function enter(name,l).

• Binds "name" with stack location "l".• Returns "l".

• Function lookup(name).• Returns the location of "name" .• Returns 0 if "name" is not found.

Data Structures Required (cont’d)

• Files.

• Function gen(file, arg1 , ..., argn). Writes a new line to "file". The line contains arg1 , ..., argn. Returns the new, modified file.

• function Open. Creates a new file.• function Close. Closes a file.

Attributes

• code: File of code generated.• next: Label of the next

instruction on the code file.• error: File of semantic errors.• top: Current (predicted) size

of run-time stack.• type: Type of subtree.

Used for type-checking.

Attributes (cont’d)

• ALL attributes are both synthesized and inherited.

• Convention:• a is the synthesized attribute a.• a is the inherited attribute a.

Synthesized and Inherited Attributes

S(program) = { code , error }I(program) = { }

S(assign) = { code , next , error , top , type }I(assign) = { code , next , error , top }

• All other nodes:• Same synthesized and inherited

attributes as "assign".

Synthesized and Inherited Attributes

• Most nodes have:• Four inherited attributes (all except type

).• All five synthesized attributes.

• One exception:• "program" node:

• no inherited attributes.• two synthesized attributes: code

and error .

Tree Walking Assumptions

• Top-down on the left,• Bottom-up on the right.

• Defaults:• If axiom is missing, assume

• no kids: a () = a ()• n kids:

•a (1) = a ()•a (i) = a (i-1), for 1 < i <= n•a () = a (n)

Tiny's Attribute Grammar

E → '<integer>:n'

code () = gen (code (), "lit", "n")next () = next () + 1top () = top () + 1type () = "integer"

Tiny's Attribute Grammar (cont’d)

E → '<identifier:x>'

code () = gen (code (), "load", lookup("x"))

next () = next () + 1top () = top () + 1type () = "integer"error () = if lookup("x") = 0 then gen (error (), "identifier un-initialized")

else error ()


E → read

code () = gen (code (), "read")next () = next () + 1top () = top () + 1type () = "integer"


E → < - E >

code () = gen (code (1), "negate")next () = next (1) + 1type () = "integer"error () = if type (1) = "integer" then error (1) else gen (error (1), “Illegal type for minus"))


E → <not E >

code () = gen (code (1), “not")next () = next (1) + 1type () = “boolean"error () = if type (1) = “boolean" then error (1) else gen (error (1), "Illegal type for not")


E → < + E E >

code () = gen (code (2), "add")next () = next (2) + 1top () = top (2) - 1type () = "integer"error () = if type (1)=type (2)= "integer" then error (2) else gen (error (2), "Illegal type for plus")


E → < - E E >

code () = gen (code (2), “subtract")next () = next (2) + 1top () = top (2) - 1type () = "integer"error () = if type (1)=type (2)= "integer" then error (2) else gen (error (2), "Illegal type for minus")


E → < = E E >

code () = gen (code (2), “equal")next () = next (2) + 1type () = “boolean"top () = top (2) - 1error () = if type (1) = type (2) then error (2) else gen (error (2), "Type clash in equal comparison")


E → < output E >

code () = gen (code (1), “print")next () = next (1) + 1top () = top (1) - 1type () = “statement"

error () = if type (1) = "integer" then error (1) else gen (error (1), "Illegal type for output")


E → < assign '<identifier:x>' E >

code () = if lookup("x") = 0 then enter("x",top (2)); code (2) else gen (code (2), "save", lookup("x"))next () = if lookup("x") = 0 then next (2) else next (2) + 1

E → < assign '<identifier:x>' E >(cont’d)top () = if lookup ("x") = 0

then top (2) else top (2) - 1error () = if type (2) = "integer" then error (2)

else gen (error (2), "Assignment type clash")

type () = "statement"


E -> < ; E* >

Use Defaults !


E → < if E E E >

code (2) = gen (code (1),"iffalse", next (2) + 1)

next (2) = next (1) + 1top (2) = top (1) - 1code (3) = gen (code (2), "goto", next (3))next (3) = next (2) + 1error (2) = if type (1) = "boolean" then error (1) else gen (error (1), "Illegal expression for if")

E → < if E E E > (cont’d)

error (3) = if type (2) = "statement" then error (2) else gen (error (2), "Statement required for if")

error () = if type (3) = "statement" then error (3) else gen (error (3), "Statement required for if")


E → < while E E >

code (2) = gen (code (1), "iffalse", next (2) + 1)next (2) = next (1) + 1top (2) = top (1) - 1code () = gen (code (2), "goto", next ())next () = next (2) + 1type () = "statement"

E → < while E E > (cont’d)

error (2) = if type (1) = "boolean" then error (1) else gen (error (1),

"Illegal expression in while")

error () = if type (2) = "statement" then error (2)

else gen (error (2), "Statement required in while")


Tiny → < program '<identifier:x>' E '<identifier:y>' >

code (2) = Openerror (2) = Opennext (2) = 1top (2) = 0code () = close (gen (code (2), "stop"))error () = close (if x = y then error (2) else gen (error (2), "program names don't match") )


Prepared by

Manuel E. Bermúdez, Ph.D.Associate ProfessorUniversity of Florida

Programming Language PrinciplesLecture 18

an attribute grammar for tiny

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