Semantics analysis

Semantic Analysis
• Semantics of a language provide meaning to its
constructs like tokens and syntax structure.
• Semantic analysis judge whether the syntax structure
constructed in the source program provide any meaning
or not.
• For example -
Int a=“book”; is syntactically correct but
generates a semantic error.
• Solution –
CFG + Semantic Rules = Syntax directed
definition.

Semantic Analysis
• output of syntax analysis which is a parse tree is the input of semantic
analysis which is also a parse tree with some additional attributes
called annotated parse tree.
• Some semantic rules are associated with the production rules in
semantic analysis, like:
• Production Semantic Rule
E->E1+T E.code =E1.code||T.code||'+'

Semantic Analysis
◦In which certain checks are performed to ensure that the
components of a program fit together meaningfully.
◦The semantic analysis phase checks the source
program for semantic errors and gathers type
information for the subsequent code- generation phase.

Ways to associate semantic rules
◦There are two ways to represent the semantic rules associated
with grammar symbols
◦ Syntax-Directed Definitions (SDD)
◦ Syntax-Directed Translation Schemes (SDT)

Syntax-Directed Definitions
◦ A syntax-directed definition (SDD) is a context-free grammar
together with attributes and rules.
◦ Attributes are associated with grammar symbols and rules are
associated with productions.
◦ PRODUCTION SEMANTIC RULE
E → E1 + T E.code = E1.code || T.code || ‘+’

Attribute Grammar
◦ Attribute grammar is a 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.
◦ 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.
◦ Example
E → E + T { E.value = E.value + T.value }

synthesized attributes
◦ Synthesized attribute get values from the attribute values of their
child nodes.
◦ Example
S → ABC
◦ If S is taking values from its child nodes (A,B,C), then it is said to be
a synthesized attribute, as the values of ABC are synthesized to S.

Inherited attributes
◦ Inherited attributes can take values from parent and/or siblings
Example
S → ABC
◦ A can get values from S, B and C.
◦ B can take values from S, A, and C.
◦ C can take values from S, A, and B.

SDD for expression grammar with synthesized attributes
Annotated Parse Tree for 3*5+4n

SDD for expression grammar with inherited attributes
Annotated Parse Tree for 3*5

S-attributed SDT
◦ If an SDT uses only synthesized attributes it is called S-attributed
SDT.
◦ These attributes are evaluated using S-attributed SDTs that have their
semantic actions written after the production (right hand side).
Example: S → ABC
◦ S can take values from A, B, and C only.

L-attributed SDT
◦ This form of SDT uses both synthesized and inherited attributes with restriction
of not taking values from right siblings.
◦ In L-attributed SDTs, a non-terminal can get values from its parent, child, and
sibling nodes. As in the following production
Example: S → ABC
◦ S can take values from A, B, and C.
◦ A can take values from S only.
◦ B can take values from S and A.
◦ C can get values from S, A, and B.
No non-terminal can get values from the sibling to its right.

SYNTAX -Directed TRANSLATION (SDT)
Background:
◦ Parser uses a CFG (Context-free-Grammer) to validate the
string and produce output for next phase of the compiler.
Output could be either a parse tree or abstract syntax tree
◦ Now to interleave semantic analysis with syntax analysis phase
of the compiler, we use Syntax Directed Translation.

SYNTAX -Directed TRANSLATION (SDT)
Definition:
Syntax Directed Translation are augmented rules to the grammar that
facilitate semantic analysis.
Grammar + semantic rules = SDT

Example:
E -> E+T | T
T -> T*F | F
F -> Id
Grammar RULES
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 }

Application of SDT
◦ Executing arithmetic expression
◦ Conversion from infix to postfix
◦ Conversion from infix to prefix
◦ Conversion from binary to decimal
◦ Counting number of reductions
◦ Creating syntax tree
◦ Generating the intermediate code
◦ Type checking
◦ Storing type info into symbol table

Example:
Grammar :
E -> E+T | T.
T -> T*F | F
F -> digit
input string:
3*5+4

Syntax directed translation schemes
◦ The syntax directed translation scheme is used to evaluate the order
of semantic rules.
◦ In translation scheme, the semantic rules are embedded within the
right side of the productions.
◦ The position at which an action is to be executed is shown by
enclosed between braces.

Errors
Program submitted to a compiler often have errors of
various kinds
So, good compiler should be able to detect as many
errors as possible in various ways and also recover from
them.
Even in the presence of errors ,the compiler should scan
the program and try to compile all of it.

CLASSIFICATION OF COMPILE TIME
ERRORS
•lexical errors
•Syntactic errors
•Semantic errors

Type Mismatch
Int a = 10;
String b=“Hello World”;
Double sum=a+b;
Undeclared or multiple declare variable
Int a , b;
int b;
a = 10;
b = 10;
Sum = a+b;

Actual and formal Parameter mismatch
Fun (int a)
{
STATEMENTS;
}
String value = “ Hello World ” ;
Fun (value);
Reserved identifier mismatch
int void = 10 ;

Manage type casting
When a binary arithmetic operator is applied to an integer and
real. In this case, the compiler may need to be converting the
integer to a real. As shown in figure given below
Float position, initial, Rate;
Initial = 10.5 ;
Rate = 10.5 ;
Position = initial + Rate * 60 ;
int a=10 ;
float b=15.5 ;
float c= a+int(b) ;

References
www.tutorialspoint.com
www.geeksforgeeks.org

Semantics analysis

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