An Attribute Grammar for Tiny

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An Attribute Grammar for Tiny
Programming Language Concepts

• Prepared by
• Manuel E. Bermúdez, Ph.D.
• Associate Professor
• University of Florida

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An Attribute Grammar for Tiny

• 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"

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An Attribute Grammar for Tiny (contd)

• equality operator ""
• binary operators "" and "-"
• unary "-", "not".
• Assignment statements.
• Variables must be assigned a value before they
  are used.
• No declarations.

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An Attribute Grammar for Tiny (contd)

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

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Tinys Syntax

• Tiny ? 'program' Name '' S 'end' Name '.
  gt 'program'
• S ? 'assign' Name '' Exp gt 'assign'
• ? 'output' Exp gt 'output'
• ? 'if' Exp 'then' S 'else' S 'fi' gt
  'if'
• ? 'while' Exp 'do' S 'od' gt 'while'
• ? S list '' gt ''
• Exp ? Term '' Term gt ''
• ? Term
• Term ? Term '' Factor gt ''
• ? Term '-' Factor gt '-'
• ? Factor

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Tinys Syntax (contd)

• Factor ? '-' Factor gt '-'
• ? 'not' Factor gt 'not'
• ? Name
• ? 'read' gt 'read'
• ? 'ltintegergt'
• ? '(' Expression ')'
• Name ? 'ltidentifiergt'

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Sample Tiny Program

• Copies 10 integers from input to output
• program copy
• assign i 1
• while not (i11) do
• output read
• assign i i 1
• od
• end copy.

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AST Grammar for Tiny

• P ? lt'program' 'ltidentifiergt' E 'ltidentifiergt'gt
• E ? lt'assign' 'ltidentifiergt' Egt
• ? lt'output' Egt
• ? lt'if' E E Egt
• ? lt'while' E Egt
• ? lt'' Egt
• ? lt'not' Egt
• ? lt'' E Egt
• ? lt'' E Egt
• ? lt'-' E Egt
• ? lt'-' Egt
• ? 'ltidentifiergt'
• ? 'ltintegergt'
• ? 'read'

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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.

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Target Machine Instruction Set

• PC 1
• Next
• case CodePC of
• save n stackn stacktop--
• load n stacktop stackn
• negate stacktop -stacktop
• not stacktop not stacktop
• add t stacktop-- stacktop
  stacktop t
• subtract t stacktop-- stacktop
  stacktop - t
• equal t stacktop-- stacktop
  stacktop t

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Target Machine Instruction Set (contd)

• read stacktop getinteger(input)
• print putinteger(stacktop--)
• lit n stacktop n
• goto n PC n goto Next
• iffalse n if stacktop-- 0 then PCn
  goto Next fi
• iftrue n if stacktop-- 1 then PCn goto
  Next fi
• stop halt
• end
• PC
• goto Next

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Target Code for our Sample Tiny Program
program copy assign i 1 while not
(i11) do output read assign i i
1 od end copy.

• 1 lit 1 i 1
• 2 load 1 i 11
• 3 lit 11
• 4 equal
• 5 not
• 6 iffalse 14 while
• 7 read
• 8 print
• 9 load 1 i i 1
• 10 lit 1
• 11 add
• 12 save 1
• 13 goto 2 od
• 14 stop end

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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.

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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.

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Data Structures Required (contd)

• 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.

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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.

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Attributes (contd)

• ALL attributes are both synthesized and
  inherited.
• Convention
• a ? is the synthesized attribute a.
• a ? is the inherited attribute a.

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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".

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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 ?.

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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 lt i lt n
• a ?(?) a ?(n)

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Tiny's Attribute Grammar

• E ? 'ltintegergtn'
• code ?(?) gen (code ?(?), "lit", "n")
• next ?(?) next ?(?) 1
• top ?(?) top ?(?) 1
• type ?(?) "integer"

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Tiny's Attribute Grammar (contd)

• E ? 'ltidentifierxgt'
• code ?(?) gen (code ?(?), "load",
  lookup("x"))
• next ?(?) next ?(?) 1
• top ?(?) top ?(?) 1
• type ?(?) "integer"
• error ?(?) if lookup("x") 0
• then gen (error ?(?),
• "identifier un-initialized")
• else error ?(?)

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Tiny's Attribute Grammar (contd)

• E ? read
• code ?(?) gen (code ?(?), "read")
• next ?(?) next ?(?) 1
• top ?(?) top ?(?) 1
• type ?(?) "integer"

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Tiny's Attribute Grammar (contd)

• E ? lt - E gt
• code ?(?) gen (code ?(1), "negate")
• next ?(?) next ?(1) 1
• type ?(?) "integer"
• error ?(?) if type ?(1) "integer"
• then error ?(1)
• else gen (error ?(1),
• Illegal type for
  minus"))

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Tiny's Attribute Grammar (contd)

• E ? ltnot E gt
• code ?(?) gen (code ?(1), not")
• next ?(?) next ?(1) 1
• type ?(?) boolean"
• error ?(?) if type ?(1) boolean"
• then error ?(1)
• else gen (error ?(1),
• "Illegal type for
  not")

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Tiny's Attribute Grammar (contd)

• E ? lt E E gt
• code ?(?) gen (code ?(2), "add")
• next ?(?) next ?(2) 1
• top ?(?) top ?(2) - 1
• type ?(?) "integer"
• error ?(?) if type ?(1)type ?(2) "integer"
• then error ?(2)
• else gen (error ?(2),
• "Illegal type for
  plus")

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Tiny's Attribute Grammar (contd)

• E ? lt - E E gt
• code ?(?) gen (code ?(2), subtract")
• next ?(?) next ?(2) 1
• top ?(?) top ?(2) - 1
• type ?(?) "integer"
• error ?(?) if type ?(1)type ?(2) "integer"
• then error ?(2)
• else gen (error ?(2),
• "Illegal type for minus")

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Tiny's Attribute Grammar (contd)

• E ? lt E E gt
• code ?(?) gen (code ?(2), equal")
• next ?(?) next ?(2) 1
• type ?(?) boolean"
• top ?(?) top ?(2) - 1
• error ?(?) if type ?(1) type ?(2)
• then error ?(2)
• else gen (error ?(2),
• "Type clash in equal comparison")

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Tiny's Attribute Grammar (contd)

• E ? lt output E gt
• code ?(?) gen (code ?(1), print")
• next ?(?) next ?(1) 1
• top ?(?) top ?(1) - 1
• type ?(?) statement"
• error ?(?) if type ?(1) "integer"
• then error ?(1)
• else gen (error ?(1),
• "Illegal type for output")

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Tiny's Attribute Grammar (contd)

• E ? lt assign 'ltidentifierxgt' E gt
• 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

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E ? lt assign 'ltidentifierxgt' E gt(contd)

• top ?(?) if lookup ("x") 0
• then top ?(2)
• else top ?(2) - 1
• error ?(?) if type ?(2) "integer"
• then error ?(2)
  else gen (error ?(2),
  "Assignment type clash")
• type ?(?) "statement"

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Tiny's Attribute Grammar (contd)

• E -gt lt E gt
• Use Defaults !

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Tiny's Attribute Grammar (contd)

• E ? lt if E E E gt
• code ?(2) gen (code ?(1),"iffalse", next ?(2)
  1)
• next ?(2) next ?(1) 1
• top ?(2) top ?(1) - 1
• code ?(3) gen (code ?(2), "goto", next ?(3))
• next ?(3) next ?(2) 1
• error ?(2) if type ?(1) "boolean"
• then error ?(1)
• else gen (error ?(1),
• "Illegal expression for if")

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E ? lt if E E E gt (contd)

• 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")

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Tiny's Attribute Grammar (contd)

• E ? lt while E E gt
• code ?(2) gen (code ?(1), "iffalse", next ?(2)
  1)
• next ?(2) next ?(1) 1
• top ?(2) top ?(1) - 1
• code ?(?) gen (code ?(2), "goto", next ?(?))
• next ?(?) next ?(2) 1
• type ?(?) "statement"

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E ? lt while E E gt (contd)

• 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")

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Tiny's Attribute Grammar (contd)

• Tiny ? lt program 'ltidentifierxgt' E
  'ltidentifierygt' gt
• code ?(2) Open
• error ?(2) Open
• next ?(2) 1
• top ?(2) 0
• code ?(?) close (gen (code ?(2), "stop"))
• error ?(?) close (if x y
• then error ?(2)
• else gen (error ?(2),
• "program names don't match")
• )

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An Attribute Grammar for Tiny
Programming Language Concepts

• Prepared by
• Manuel E. Bermúdez, Ph.D.
• Associate Professor
• University of Florida