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Programming Languages
Tevfik Koşar
Lecture - IX February 14th, 2006
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Roadmap
- Semantic Analysis
- Role of Semantic Analysis
- Static vs Dynamic Analysis
- Attribute Grammars
- Evaluating Attributes
- Decoration of Parse Trees
- Synthesized and Inherited Atributes
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Role of Semantic Analysis
- Following parsing, the next two phases of the
"typical" compiler are
- semantic analysis
- (intermediate) code generation
- The principal job of the semantic analyzer is to
enforce static semantic rules
- constructs a syntax tree (usually first)
- information gathered is needed by the code
generator
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Static vs Dynamic Semantics
- Static Semantics
- Dynamic Semantics
- At runtime
- Division by zero
- Out-of-bound indexing an array
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Attribute Grammars
- We'll start with decoration of parse trees, then
consider syntax trees
- Consider the following LR (bottom-up) grammar
for arithmetic expressions made of constants,
with precedence and associativity:
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Attribute Grammars
E → E + T
E → E – T
E → T
T → T * F
T → T / F
T → F
F → - F
F → (E)
F → const
- This says nothing about what the program
MEANS
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Attribute Grammars
- We can turn this into an attribute grammar as
follows (similar to Figure 4.1):
E → E + T E 1 .val = E 2 .val + T.val E → E – T E 1 .val = E 2 .val - T.val E → T E.val = T.val T → T * F T 1 .val = T 2 .val * F.val T → T / F T 1 .val = T 2 .val / F.val T → F T.val = F.val F → - F F 1 .val = - F 2 .val F → (E) F.val = E.val F → const F.val = C.val
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Attribute Grammars
- The attribute grammar serves to define the
semantics of the input program
- Attribute rules are best thought of as
definitions, not assignments
- They are not necessarily meant to be evaluated
at any particular time, or in any particular
order, though they do define their left-hand
side in terms of the right-hand side
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Evaluating Attributes
- This is a very simple attribute grammar:
- Each symbol has at most one
attribute
- the punctuation marks have no attributes
- These attributes are all so-called SYNTHESIZED
attributes:
- They are calculated only from the attributes of
things below them in the parse tree
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Evaluating Attributes
- In general, we are allowed both synthesized and
INHERITED attributes:
- Inherited attributes may depend on things above or to
the side of them in the parse tree
- Tokens have only synthesized attributes, initialized
by the scanner (name of an identifier, value of a
constant, etc.).
- Inherited attributes of the start symbol constitute
run-time parameters of the compiler
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Evaluating Attributes
- The grammar above is called S-ATTRIBUTED
because it uses only synthesized attributes
- Its ATTRIBUTE FLOW (attribute dependence
graph) is purely bottom-up
- It is SLR(1), but not LL(1)
- An equivalent LL(1) grammar requires inherited
attributes:
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LL(1) Grammar
expr const expr_tail
expr_tail - const expr_tail | ε
Create the parse tree for 9 – 4 – 3 and evaluate it.
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Evaluating Attributes– Example
FT 1 → * F FT 2 FT 1 .val = FT 2 .val
FT 2 .st = FT 1 .st * F.val
FT 1 → / F FT 2 FT 1 .val = FT 2 .val
FT 2 .st = FT 1 .st / F.val
FT → ε FT.val = FT.st
F 1 → - F 2 F1.val = - F2.val
F → ( E ) F.val = E.val
F → const F.val = C.val
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Evaluating Attributes– Example
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Evaluating Attributes– Example
- Attribute grammar in Figure 4.3:
- This attribute grammar is a good bit messier than
the first one, but it is still L-ATTRIBUTED, which
means that the attributes can be evaluated in a
single left-to-right pass over the input
- In fact, they can be evaluated during an LL parse
- Each synthetic attribute of a LHS symbol (by
definition of synthetic) depends only on attributes of
its RHS symbols
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Evaluating Attributes – Example
- Attribute grammar in Figure 4.3:
- Each inherited attribute of a RHS symbol (by
definition of L-attributed) depends only on
- inherited attributes of the LHS symbol, or
- synthetic or inherited attributes of symbols to its left in the RHS
- L-attributed grammars are the most general class of
attribute grammars that can be evaluated during an
LL parse
