Syntax Directed Translation

1. Syntax Directed Translation

2. Why do we need Syntax-directed? • To translate a programming language construct, a compiler may need to keep track of many quantities besides the code generated for the construct. • Syntax-directed • We augment a context-free grammar by adding semantic rules or actions to each production. • These rules define the values of attributes and how to compute them. • We call this augmented grammar an attribute grammar. • Two types of attributes: • synthesized attribute which computes the value based on the children • inherited attribute computed based on the parent and the older siblings. • We can use two methods to translate the programming language: • a formalism, syntax- directed definition • a more procedural notation called translation scheme.

3. Syntax-directed definition(1/2) • Syntax-directed definition • A grammar -oriented compiling technique • It is very helpful for organizing the compiler's front end and it translates infix expression into post form • Postfix notation 1) If E is a variable or constant, then the postfix notation for E is E itself. 2) If E is an expression of the form E1 op E2 where op is any binary operator, then the postfix notation foe E is E1' E2' op. 3) If E is an expression of the form (E1), then the postfix notation for E1 is also the postfix notation for E.

4. Syntax-directed definition(2/2) • Essentially a formal treatment of two simple concepts • Each node of a parse tree can have a set of associated values (attributes). • The creation and traversal of parse trees can be automated • Example if we have a production A --> X1 X2 ... Xn we might have a rule A.a = f(X1.a,X1.b,X2.a,X2.b,...,Xn.a,Xn.b) which describes how the attribute a of A is defined in terms of attributes of the children of A.

5. Translation scheme(1/2) • A translation is an iuput-output mapping • The output for each input x is specified in the following way • construct a parse tree for x • compute the value using semantic rule for attribute associated with the X-production used at node n • two types of attributes • synthesized attribute which computes the value based on the children • inherited attribute computed based on the parent and the older siblings • A translation scheme gives a procedural description of an action to be taken when a production is applied

6. Translation scheme(2/2) • The actions may be interspersed with the symbols on the right-hand side of the production • thus indicating an evaluation order • For example, in a translation scheme used to generate assembly code while-stmt --> while { generate label L1,L2; emit("L1:" ) expression { emit("beqz L2"); } statement { emit("b L1); emit("L2:"); } • Attributes may be evaluated during parsing or in a separate pass over the parse tree • Different strategies are used, depending whether a top-down or bottom-up parser is used

7. How attributes are evaluated depends on whether they are synthsized or inherited • A synthesized attribute of a symbol is one which depends only on the attributes of its children (in the parse tree) • An inherited attribute is one which depends on attributes of the parent or siblings (ex. the data type of an identifier) • In general, inherited attributes are more difficult to deal with, particularly for bottom-up parsers

8. How attributes are evaluated depends on whether they are synthesized or inherited. • A synthesized attribute of a symbol is one which depends only on the attributes of its children (in the parse tree); • for example, the value of an expression. • An inherited attribute is one which depends on attributes of the parent or siblings; • for example the data type of an identifier. • In general, inherited attributes are more difficult to deal with, particularly for bottom-up parsers.

13. When the syntax-directed definition has only synthesized attribute, it is a S-attribute definitions. It can be computed by using bottom-up parser. While a syntax-directed definition is L-attribute if each inherited attribute of X_j, 1<= j <= n, on the right side of A-> X1X2¡­Xn, depends only on • 1) the attribute of the symbols X1,X2,¡­,Xj-1 to the left of Xj in the production and • 2) the inherited attributes of A.

14. Every S-attributed definition is L-attributed, because the restriction 1) and 2) apply only to inherited attributes. It can be computed in depth-first order. All syntax-directed definition based on LL(1) grammars are L-attributed. • Following are some examples of S-attributed and L-attributed: 1)synthesized attribute on the parser stack: 2) A non-L-attributed syntax-directed definition: