Chapter 5: Syntax directed translation

1

• Chapter 5 Syntax directed translation
• use the grammar to direct the translation
• the grammar defines the syntax of the input
  language. Attributes are attached to grammar
  symbols. Semantic rules are associated with
  grammar productions.
• Attributes -- type of values associated with
  grammar symbols representing programming language
  constructs. These values are computed by the
  semantic rules.
• 2 notations for associating semantic rules with
  productions.
• Syntax directed definitions
• Syntax directed translation schemes

2

• Syntax directed definitions
• Each grammar production A-gta is associated with a
  set of semantic rules of the form
  bf(c1, c2, , ck)
• If b is an attributed associated with A, it is
  called a synthesized attribute.
• If b is an attributed associated with a grammar
  symbol on the right side of the production, b is
  called an inherited attribute.
• The definitions does not specify explicitly the
  order in which the attributes can be evaluated.
• The semantic rules implicitly indicate the order
  (b depends on c1, c2, , ck).
• Syntax directed definitions hide many
  implementation details.

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• Example an S-attributed definition
• A syntax directed definition that uses
  synthesized attributes exclusively is said to be
  an S-attributed definition.
• Production semantic rules
• L -gtE n print(E.val)
• E-gtE1 T E.val E1.val
  T.val
• E-gtT E.val
  T.val
• T-gtT1 F T.val T1.val
  F.val
• T-gtF T.val
  F.val
• F-gt(E) F.val
  E.val
• F-gtdigits F.val
  digits.lexval
• 354n

4

• Example inherited attributes
• production semantic
  rules
• D -gtT L L.in
  T.type
• T-gtint T.type
  integer
• T-gtreal T.type
  real
• L-gtL1, id L1.in
  L.in, addtype(id.entry, L.in)
• L-gtid
  addtype(id.entry, L.in)
• real id1, id2, id3

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• Syntax-directed construction of syntax trees
• Tree mkleaf(type, value)
• Tree mknode(op, left, right)
• Production
• E-gtE1T
• E-gtE1-T
• E-gtT
• T-gt(E)
• T-gtnum
• how is the tree for 1-23 constructed?

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• L-attributed definitions
• A syntax directed definition is L-attributed if
  each inherited attribute of Xj, 1ltjltn, on the
  right side of A-gtX1X2Xn depends only on
• attributes of the symbols X1, X2, , Xj-1.
• the inherited attributes of A.
• L stands for Left since information appears to
  flow from left to right in the compilation
  process.
• Example
• A-gtLM L.iA.i M.iL.s A.s
  M.s
• A-gtQR R.i A.i Q.i R.s A.s
  Q.s
• Relation between S-attributed definitions and
  L-attributed definitions?
• Why L-attributed definitions are important?

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• Given a syntax directed definition, how to build
  a translator?
• For general definitions, to evaluate the semantic
  rules correctly, we need to follow the dependence
  of the attributes (defined by the semantic
  rules).
• Build a dependency graph. Topologically sort the
  graph, then evaluate the rules accordingly.
• Example real id, id, id
• For some special definitions, we can perform
  translation while parsing
• e.g. bottom-up evaluation of S-attributed
  definitions.
• Most L-attributed definitions also works.

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• Syntax directed translation scheme
• a context-free grammar in which attributes are
  associated with grammar symbols and the semantic
  actions are enclosed between and are inserted
  within the right side of productions to indicate
  the order in which translation takes place --
  must be careful with the order.
• Example
• E-gtT R
• R-gt T print() R - T print(-) R e
• T-gtnum print(num.val)

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• Syntax directed translation scheme
• another example
• E-gtT R.I T.val
• R E.val R.s
• R-gt
• T R1.I R.I R.val
• R R.s R1.s
• R-gt R.s R.I
• T-gtnum T.val num.val
• evaluation of 9-52

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• S-attributed definitions can directly translated
  into a translation scheme by placing the semantic
  actions at the end of each productions.
• Actions in the middle of productions can be
  removed to be put at the end of productions by
  changing the grammar.
• Example previous page.

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• Syntax directed translation with YACC.
• Only has synthesized attributes
• parser stack with field for synthesized
  attributes
• default attribute is of integer type.
• How to handle inherited attributes?
• Semantic actions only happen at the end of a
  production?