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Syntax-Directed Translation
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Introduction
Translation of languages guided by context-free grammars Attach attributes to the grammar symbols
- Values of the attributes are computed by semantic rules associated with the grammar productions
- Attribute may represent: Number, type, string, memory location, label, etc. Syntax–directed definitions:
- Each production has a set of semantic rules associated with it Conceptually we parse the input token stream, build the parse tree and then traverse the tree as needed to evaluate the semantic rules.
- The rules may generate code, save information in the symbol table, issue error messages, etc.
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Introduction
In some cases we don’t have to follow this outline literally
- We may evaluate the rules during parsing Synthesized attribute:
- The value of the attribute at a parent node can be found from the attributes of its children It can be evaluated by traversing the tree bottom-up Inherited attribute:
- The value of the attribute at a node can be found from the attributes at its parent node and/or its sibling nodes Can be evaluated by traversing the tree top-down S-attributed definition:
- A syntax-directed definition where all attributes are synthesized (the “S” stands for synthesized).
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Example: Simple Desk Calculator
Production L ÆE n E Æ E 1 + T E Æ T T Æ T 1 * F T Æ F F Æ (E) F Æ digit
Semantic Rules print(E.val) E.val := E 1 .val + T.val E.val := T.val T.val := T 1 .val * F.val T.val := F.val F.val := E.val F.val := digit .lexval
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Inherited Attributes
Inherited attributes are convenient for
expressing the dependence of a
programming language construct on the
context in which it appears.
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Example
Using inherited attributes to parse a declaration and add type information to the symbol table. L Æ id addtype (id.entry, L.in)
L 1 .in := L.in addtype (id.entry, L.in)
L Æ L 1 , id
T Æ real T.Type := real
T Æ int T.Type := integer
D Æ TL L.in := T.type
Production Semantic Rules
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Annotated Parse Tree for real, id1, id2, id
D T.Type = real
real
L.in = real
L.in = real
L.In = real
id
, id
.
. , (^) id
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Bottom-up Parsing
If all attributes are synthesized then a bottom-up parser can evaluate the attributes while it parses Add attribute fields to the nonterminals on the stack
Before each production apply the semantic rule associated with the production E.g. Assume the semantic rule associated with the A Æ XYZ production is : A.a := f (X.x, Y.y, Z.z)
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Bison Generated Parsers
Bison maintains a semantic stack which, in tandem with the parser stack, helps Bison manipulate and determine the meaning of the program it compiles
The values in the semantic stack represent the values or meanings of all the grammar symbols
Exactly what constitutes values for the entries in the semantic stack depends heavily on the semantics of the language being compiled
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Semantic Stack
Manipulation of the semantic stack is
triggered by the parser during reduce moves
Values of symbols on the rhs of the
production are popped off the semantic
stack
A value for the symbol on the lhs is
determined from these values and the
semantic rules of the language, and a new
value is pushed back on the stack
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Semantic Stack
All symbols in the grammar have a value
associated with them in the semantic stack
even though some of these values may be
NULL (indicating no information is needed
about this symbol)
Not all reductions require semantic actions
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Semantic Rules
Bison allows you to specify the semantic
action associated with the symbol on the lhs
of the production
- These actions may return a value and may utilize the values returned by previous actions
The lexical analyzer can return values for
tokens, if desired, through the use of the
global variable yylval
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Union
The values on the semantic stack do not have to be of a single type
- You can define the type of the value using the %union statement in Bison
Bison must know which type to associate with each symbol in the grammar
- This is done using the %type statement Beware, once you starting using %type, you are forced to type all symbols even though you may not be using them
- You are required to use %type statements in your project
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%union and %type
%union { int int_val; char *name_ptr; CONS *cons_ptr; ID *id_ptr; TYPE *ty_ptr; EXPR *exp_ptr; }
%type <int_val> INTCONSTSY IfHead %type <id_ptr> IdList FieldList
