• 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
• Syntax directed definitions
• Each grammar production A->a is associated with a
set of semantic rules of the form
b=f(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 implementation
details.
– An S-attributed definition:
• A syntax directed definition that uses synthesized
attributes exclusively is said to be an S-attributed
definition.
Production
L ->E n
E->E1 + T
E->T
T->T1 * F
T->F
F->(E)
F->digits
3*5+4n
semantic rules
print(E.val)
E.val = E1.val + T.val
E.val = T.val
T.val = T1.val * F.val
T.val = F.val
F.val = E.val
F.val = digits.lexval
– Example: inherited attributes
production
D ->T L
T->int
T->real
L->L1, id
L->id
real id1, id2, id3
semantic rules
L.in = T.type
T.type = integer
T.type = real
L1.in = L.in, addtype(id.entry, L.in)
addtype(id.entry, L.in)
– Syntax-directed construction of syntax trees
• Tree mkleaf(type, value);
• Tree mknode(op, left, right);
• Production
E->E1+T
E->E1-T
E->T
T->(E)
T->num
– how is the tree for 1-2+3 constructed?
– L-attributed definitions:
• A syntax directed definition is L-attributed if each inherited
attribute of Xj, 1<=j<=n, on the right side of A->X1X2…Xn
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->LM
A->QR
{L.i=A.i; M.i=L.s; A.s = M.s}
{R.i = A.i; Q.i = R.s; A.s = Q.s}
– Relation between S-attributed definitions and Lattributed definitions?
– Why L-attributed definitions are important?
• 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 for the parsing tree.
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.
• 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->T R
R->+ T {print(‘+’)} R | - T {print(‘-’)} R | e
T->num {print(num.val)}
• Syntax directed translation scheme:
– another example:
E->T {R.I = T.val}
R {E.val = R.s}
R->+
T {R1.I = R.I + T.val}
R1 {R.s = R1.s}
R-> {R.s = R.I}
T->num {T.val := num.val}
evaluation of 9-5+2
– S-attributed definitions can directly translated
into a translation scheme by placing the
semantic actions at the end of each productions.
• Perfect for bottom up parsing (LR parsing)
– Actions in the middle of productions can be
removed to be put at the end of productions by
changing the grammar (adding markers).
• Example: previous page.
– 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?
– The information of a symbol is on the stack, which can be
accessed directly.
• Semantic actions only happen at the end of a
production?
– Automatically done