this document is design document

1. Chapter 4
Semantic Analysis

2.  Introduction
 A Short Program
 Semantic Analysis
 Annotated Abstract Syntax tree
(AST)
 Syntax-Directed Translation
 Syntax-Directed Definitions (SDD)
 Translation Schemes

3. Where are we?
Semantic
Analyzer

4. Introduction
 Program is lexically well-formed:
 Identifiers have valid names.
 Strings are properly terminated.
 Program is syntactically well-formed:
 Class declarations have the correct structure.
 Expressions are syntactically valid.
 Does this mean that the program is legal?

5. A Short Program
class MyClass implements MyInterface {
string myInteger;
void doSomething() {
int[] x = new string;
x[5] = myInteger * y;
}
void doSomething() {
}
int fibonacci(int n) {
return doSomething() + fibonacci(n – 1);
}
}

6. A Short Program…
class MyClass implements MyInterface {
string myInteger;
void doSomething() {
int[] x = new string;
x[5] = myInteger * y;
}
void doSomething() {
}
int fibonacci(int n) {
return doSomething() + fibonacci(n – 1);
}
}

7. Semantic Analysis
 Ensure that the program has a well-defined
meaning.
 Verify properties of the program that aren't caught
during the earlier phases:
 Variables are declared before they're used.
 Expressions have the right types.
 Classes don't inherit from nonexistent base classes
 Type consistence;
 Inheritance relationship is correct;
 A class is defined only once;
 A method in a class is defined only once;
 Reserved identifiers are not misused;
 …
 Once we finish semantic analysis, we know that the
user's input program is legal.

8. A Simple Semantic Check
 “Matching identifier declarations with uses”
 Important analysis in most languages
 If there are multiple declarations, which one to match?
The scope of an identifier
is the portion of a program
in which that identifier is
accessible

9. Semantic Analysis…
 Semantic Analysis is the third phase of Compiler.
 Semantic Analysis makes sure that declarations and
statements of program are semantically correct.
 It is a collection of procedures which is called by parser as and
when required by grammar.
 Both syntax tree of previous phase and symbol table are used
to check the consistency of the given code.
 Last compiler phase that can report errors to the user.
 Type checking is an important part of semantic analysis
where compiler makes sure that each operator has matching
operands

10. Semantic Analysis…
Semantic Analyzer:
 It uses syntax tree and symbol table to check whether the
given program is semantically consistent with language
definition.
 It gathers type information and stores it in either syntax tree
or symbol table.
 This type information is subsequently used by compiler
during intermediate-code generation.
Semantic Errors:
 Errors recognized by semantic analyzer are as follows:
 Type mismatch
 Undeclared variables
 Reserved identifier misuse
 Access of an out of scope variable.
 Actual and formal parameter mismatch.

11. Semantic Analysis…
 Semantic analysis also gathers useful information
about program for later phases:
 E.g. count how many variables are in scope at each point.
 Why can't we just do this during parsing?
 Read about Limitations of CFGs

12. Semantic Analysis…
Functions of Semantic Analysis:
 Type Checking –
Ensures that data types are used in a way consistent with their
definition.
 Label Checking –
A program should contain labels references.
 Flow Control Check –
Ensures that control structures are used in the right way example:
no break statement outside a loop)
 Scope resolution.
This involves determining the scope of a name
 Array-bound checking.
This is determining whether all array references in a program are within the
declared range
Example:
float x = 10.1;
float y = x*30;
In the above example integer 30 will be typecasted to float 30.0
before multiplication, by semantic analyzer.

13. Semantic Analysis…
 Semantic Analysis can be implemented using
Annotated Abstract Syntax tree (AST)
 The input for Semantic Analysis (syntax analyzer) is
Abstract Syntax tree and the output is Annotated
Abstract Syntax tree.
 Annotated Abstract Syntax tree is parse-tree that also
shows the values of the attributes at each node.
Y=3*x+z

14. Syntax-Directed Translation(SDT)
 SDT is used to drive semantic analysis tasks based on
the language’s syntax structures
 What semantic tasks?
 Generate AST (abstract syntax tree)
 Check type errors
 Generate intermediate representation (IR)
 What is synatx structures?
 Context free grammar (CFG)
 Parse tree generated from parser
 How?
 Attach attributes to grammar symbols/parse tree
 Attach either rules or program fragments to productions in
a grammar
 Evaluate attribute values using semantic actions/
semantic rules associated with the production rules.

15. Two Types of Attributes
 In semantic rule attributes are value that holds or
represent anything we need:
 a string,
 a type,
 a number,
 a memory location , … represented as (VAL) and they are
two types
Reading assignment Synthesized attributes vs
Inherited attributes

16. Two Types of Syntax Directed
Translation
 Syntax-directed definition
 A general way to associate actions (i.e., programs) to
production rules of a context-free grammar.
 Used for carrying out most semantic analyses as well as
code translation
 A syntax-directed definition associates:
 With each grammar symbol, a set of attributes, and
 With each production, a set of semantic rules for
computing the values of the attributes associated with the
symbols appearing in the production
 A grammar with attributes and semantic rules is called
an attributed grammar
 A parse tree augmented with the attribute values at
each node is called an annotated parse tree.

17. Two Types of Syntax Directed
Translation
 When we associate semantic rules with productions, we
use two notations:-
 Syntax-Directed Definitions
 associates a production rule with a set of semantic
actions, and we do not say when they will be evaluated.
 don’t specify order of evaluation/ Translation - hides
implementation details
 Grammar +semantic rule
 Translation Schemes
 indicate the order of evaluation of semantic actions
associated with a production rule.
 shows more implementation details

18. Attribute Grammar
 An attribute grammar is an extension of context-
free grammar that enables definition of context-sensitive
aspects of a language and its translation.
 Attribute grammar is a special form of context-free grammar
 where some additional information (attributes) are appended to
one or more of its non-terminals in order to provide context-
sensitive information.
 Each attribute has well-defined domain of values, such as
integer, float, character, string, and expressions.

19. Attribute Grammar
 Attribute grammar is a medium to provide semantics to the
context-free grammar and
 It can help specify the syntax and semantics of a
programming language.
 Attribute grammar (when viewed as a parse-tree) can pass
values or information among the nodes of a tree.
Example:
E → E + T { E.value = E.value + T.value }
 The right part of the CFG contains the semantic rules that
specify how the grammar should be interpreted.

20. Attribute Grammar
 Semantic attributes may be assigned to their values from their domain
at the time of parsing and
 Evaluated at the time of assignment or conditions.
 Based on the way the attributes get their values, they can be broadly
divided into two categories : synthesized attributes and inherited
attributes.
 Expansion : When a non-terminal is expanded to terminals as per a
grammatical rule
 Reduction : When a terminal is reduced to its corresponding non-
terminal according to grammar rules.
 Syntax trees are parsed top-down and left to right.
 Whenever reduction occurs, we apply its corresponding semantic

21. Attribute Grammar
Semantic analysis uses Syntax Directed Translations to perform the above tasks.
Semantic analyzer attaches attribute information with AST, which are called
Attributed AST.
Attributes are two tuple value, <attribute name, attribute value>
Example
int value = 5; <type, “integer”> <presentvalue, “5”>
For every production, we attach a semantic rule.

22. Two Types of Syntax Directed
Translation
E  E+T | T
T  T*F | F
F  id
 This is a grammar to syntactically validate an expression having
additions and multiplications in it.
 Now, to carry out semantic analysis we will augment SDT rules to
this grammar, in order to pass some information up the parse tree
and check for semantic errors, if any.
 In this example, we will focus on the evaluation of the given
expression, as we don’t have any semantic assertions to check in
this very basic example.
E  E+T { E.val = E.val + T.val }
E  T { E.val = T.val }
T  T*F { T.val = T.val * F.val }
T  F { T.val = F.val }
F  id { F.val = id.lexval }

23. Two Types of Syntax Directed
Translation
To construct the SDT
Step1: Generate the syntax tree from the given
Grammar
Step2:Associate production rule with semantic ruel
Step3:Parse the tree from left to right and top to
down
Step4: When ever there is a reduction from Right
hand side to left side go to the production and

24. Syntax-Directed Definitions (SDD)
 SDD is a context-free grammar together with, attributes
and rules.
 Attributes are associated with grammar symbols and rules
are associated with productions.
 If X is a symbol and a is one of its attributes, then we write
X.a to denote the value of a at a particular parse-tree node
labeled X.
Production Semantic Rules
L → E return { print(E.val) }
E → E1 + T { E.val = E1.val + T.val }
E → T { E.val = T.val }
T → T1 * F { T.val = T1.val * F.val }
T → F { T.val = F.val }
F → ( E ) { F.val = E.val }
 Symbols E, T,
and F are
associated with
a synthesized
attribute val.
 The token digit
has a
synthesized
attribute lexval
(it is assumed
that it is

25. Annotated Parse Tree -- Example

26. Dependency Graph
Input: 5+3*4 L
E.val=17
E.val=5 T.val=12
T.val=5 T.val=3
F.val=4
F.val=5 F.val=3 digit.lexval=4
digit.lexval=5 digit.lexval=3
A dependency graph suggests possible evaluation
orders for an annotated parse-tree.

27. Example2
exp -> exp + term exp1.val = exp2.val + term.val
exp -> term exp.val = term.val
term -> term * factor term1.val = term2.val *
factor.val
term -> factor term.val = factor.val
factor ->num factor.val = num.lexval
factor -> ( exp ) factor.val = exp.val
Input : 2 * (4 + 5)

28. A Translation Scheme Example (cont.)
Reading assignment
1. S-attributed SDT
2. L-attributed SDT
3. Translation Schemes

29. Type Checking in Compiler Design
Type is an idea that varies from language to language
Limit the programmer what type may be used.
Type checking is the process of verifying and enforcing constraints of types in
values.
 A compiler must check that the source program should follow the syntactic and
semantic conventions of the source language and
 it should also check the type rules of the language.
 It allows the programmer to limit what types may be used in certain
circumstances and assigns types to values.

30. Type Checking in Compiler Design
It checks the type of objects and reports a type error
in the case of a violation, and incorrect types are
corrected.
Whatever the compiler we use, while it is compiling
the program, it has to follow the type rules of the
language.
 Every language has its own set of type rules for the
language.

31. Type Checking in Compiler Design
The information about data types like INTEGER, FLOAT, CHARACTER, and all
the other data types is maintained and computed by the compiler.
The compiler contains modules, where the type checker is a module of a compiler
and its task is type checking.
Conversion from one type to another type is known as implicit if it is to be
done automatically by the compiler.
Implicit type conversions are also called Coercion and coercion is limited in many
languages.
Example:
 An integer may be converted to a real but real is not converted to an integer.
Conversion is said to be Explicit if the programmer writes something to do the
Conversion

32. Type Checking in Compiler Design
Types of Type Checking:
There are two kinds of type checking:
1. Static Type Checking.
2. Dynamic Type Checking.
Static Type Checking:
Static type checking is defined as type checking performed at compile time.
 It checks the type variables at compile-time, which means the type of the
variable is known at the compile time

33. Type Checking in Compiler Design
1. Task of static Type Checking
1. Type checking. Operator applied to incompatible operands
1. If array variable is added with the function variable
2. Flow of control check: statement that result in a branch need to be
determined correctly.
1. Break statements
3. Uniqueness check: object must be defined exactly once for some
scenarios
1. Labels in case statements need to be unique.
4. Name related check: some name appear more than once

34. Type Checking in Compiler Design
Dynamic Type Checking:
Dynamic Type Checking is defined as the type checking being done at run
time.
In Dynamic Type Checking, types are associated with values, not
variables.
Implementations of dynamically type-checked languages runtime objects
are generally
associated with each other through a type tag, which is a reference to a type
containing
its type information
. Dynamic typing is more flexible. A static type system always restricts what
can be conveniently expressed.

35. Next
 reading assignment rules of Type Checking in compiler
Intermediate Languages
Run time- Environments
Code Generation and Optimization