SEMANTIC ANALYSIS PRINCIPLES OF PROGRAMMING LANGUAGES Norbert Zeh - PowerPoint PPT Presentation

SEMANTIC ANALYSIS PRINCIPLES OF PROGRAMMING LANGUAGES Norbert Zeh Winter 2018 Dalhousie University 1/28

PROGRAM TRANSLATION FLOW CHART code improvement Symbol table other intermediate form Abstract syntax tree or Back end target language Modified (e.g., assembly) Target language intermediate form Modified Machine-specific Scanner (lexical analysis) Target code generation code improvement Machine-independent Front end Parse tree Token stream (Character stream) Source program code generation Semantic analysis and Parser (syntactic analysis) 2/28

ROAD MAP • Syntax, semantics, and semantic analysis • Attribute grammars • Action routines • Abstract syntax trees 3/28

ROAD MAP • Syntax, semantics, and semantic analysis • Attribute grammars • Action routines • Abstract syntax trees 3/28

SYNTAX AND SEMANTICS Syntax • Describes form of a valid program • Can be described by a context-free grammar Semantics • Describes meaning of a program • Cannot be described by a context-free grammar Some constraints that may appear syntactic are enforced by semantic analysis! Example: Use of identifier only after its declaration 4/28

SYNTAX AND SEMANTICS Syntax • Describes form of a valid program • Can be described by a context-free grammar Semantics • Describes meaning of a program • Cannot be described by a context-free grammar Some constraints that may appear syntactic are enforced by semantic analysis! Example: Use of identifier only after its declaration 4/28

SYNTAX AND SEMANTICS Syntax • Describes form of a valid program • Can be described by a context-free grammar Semantics • Describes meaning of a program • Cannot be described by a context-free grammar Some constraints that may appear syntactic are enforced by semantic analysis! Example: Use of identifier only after its declaration 4/28

SEMANTIC ANALYSIS Semantic analysis • Enforces semantic rules • Builds intermediate representation (e.g., abstract syntax tree) • Fills symbol table • Passes results to intermediate code generator Two approaches • Interleaved with syntactic analysis • As a separate phase Formal mechanism • Attribute grammars 5/28

SEMANTIC ANALYSIS Semantic analysis • Enforces semantic rules • Builds intermediate representation (e.g., abstract syntax tree) • Fills symbol table • Passes results to intermediate code generator Two approaches • Interleaved with syntactic analysis • As a separate phase Formal mechanism • Attribute grammars 5/28

SEMANTIC ANALYSIS Semantic analysis • Enforces semantic rules • Builds intermediate representation (e.g., abstract syntax tree) • Fills symbol table • Passes results to intermediate code generator Two approaches • Interleaved with syntactic analysis • As a separate phase Formal mechanism • Attribute grammars 5/28

ENFORCING SEMANTIC RULES Static semantic rules • Enforced by compiler at compile time • Example: Do not use undeclared variable. Dynamic semantic rules • Compiler generates code for enforcement at runtime. • Examples: Division by zero, array index out of bounds • Some compilers allow these checks to be disabled. 6/28

ENFORCING SEMANTIC RULES Static semantic rules • Enforced by compiler at compile time • Example: Do not use undeclared variable. Dynamic semantic rules • Compiler generates code for enforcement at runtime. • Examples: Division by zero, array index out of bounds • Some compilers allow these checks to be disabled. 6/28

ROAD MAP • Syntax, semantics, and semantic analysis • Attribute grammars • Action routines • Abstract syntax trees 7/28

ROAD MAP • Syntax, semantics, and semantic analysis • Attribute grammars • Action routines • Abstract syntax trees 7/28

ATTRIBUTE GRAMMARS Attribute grammar An augmented context-free grammar: • Each symbol in a production has a number of attributes. • Each production is augmented with semantic rules that • Copy attribute values between symbols, • Evaluate attribute values using semantic functions, • Enforce constraints on attribute values. 8/28

E 1 val add E 2 val T val E 1 val sub E 2 val T val T 1 val mul T 2 val F val T 1 val div T 2 val F val F 1 val neg F 2 val F const val T 1 T 2 F T F F val F val T val E val F 1 const F 2 F E F val ATTRIBUTE GRAMMAR: EXAMPLE T 2 F T E 1 T E 1 E 2 E 2 E T E val T val T 1 9/28 E → E + T E → E − T E → T T → T ∗ F T → T / F T → F F → − F F → ( E ) F → const

ATTRIBUTE GRAMMAR: EXAMPLE 9/28 E → E + T E 1 → E 2 + T { E 1 . val = add ( E 2 . val , T . val ) } E → E − T E 1 → E 2 − T { E 1 . val = sub ( E 2 . val , T . val ) } E → T E → T { E . val = T . val } T → T ∗ F T 1 → T 2 ∗ F { T 1 . val = mul ( T 2 . val , F . val ) } T → T / F T 1 → T 2 / F { T 1 . val = div ( T 2 . val , F . val ) } T → F T → F { T . val = F . val } F → − F F 1 → − F 2 { F 1 . val = neg ( F 2 . val ) } F → ( E ) F → ( E ) { F . val = E . val } F → const F → const { F . val = const . val }

SYNTHESIZED AND INHERITED ATTRIBUTES Synthesized attributes Attributes of LHS of production are computed from attributes of RHS of production. Inherited attributes Attributes flow from left to right: • From LHS to RHS, • From symbols on RHS to symbols later on the RHS. 10/28

SYNTHESIZED AND INHERITED ATTRIBUTES Synthesized attributes Attributes of LHS of production are computed from attributes of RHS of production. Inherited attributes Attributes flow from left to right: • From LHS to RHS, • From symbols on RHS to symbols later on the RHS. 10/28

A 2 a A 1 count A 2 count B 2 b B 1 count B 2 count C 2 c C 1 count C 2 count SYNTHESIZED ATTRIBUTES: EXAMPLE c 1 C 1 C count 1 C 1 1 B 1 B B count C count is not context-free but can be defined using an attribute grammar: S A B C Condition: A count B count A b a A count 1 A 1 1 The language 11/28 L = { a n b n c n | n > 0 } = { abc , aabbcc , aaabbbccc , . . . }

SYNTHESIZED ATTRIBUTES: EXAMPLE The language 11/28 is not context-free but can be defined using an attribute grammar: L = { a n b n c n | n > 0 } = { abc , aabbcc , aaabbbccc , . . . } S → A B C { Condition: A . count = B . count = C . count } A → a { A . count = 1 } A 1 → A 2 a { A 1 . count = A 2 . count + 1 } B → b { B . count = 1 } B 1 → B 2 b { B 1 . count = B 2 . count + 1 } C → c { C . count = 1 } C 1 → C 2 c { C 1 . count = C 2 . count + 1 }

SYNTHESIZED ATTRIBUTES: PARSE TREE DECORATION (1) 2 C S 1 2 3 1 3 C 1 2 3 3 3 3 c c Input: aaabbbccc A Parse tree: A a a A a B c b b B b B C 12/28

SYNTHESIZED ATTRIBUTES: PARSE TREE DECORATION (1) 2 C S 1 2 3 1 3 C 1 2 3 3 3 3 c c Input: aaabbbccc A Parse tree: A a a A a B c b b B b B C 12/28

SYNTHESIZED ATTRIBUTES: PARSE TREE DECORATION (1) 2 C S 1 2 3 1 3 C 1 2 3 3 3 3 c c Input: aaabbbccc A Parse tree: A a a A a B c b b B b B C 12/28

SYNTHESIZED ATTRIBUTES: PARSE TREE DECORATION (1) c 3 2 1 3 2 1 3 2 1 S C c C c Input: aaabbbccc a Parse tree: A a a A A C B b b B b B 12/28 3 = 3 = 3

SYNTHESIZED ATTRIBUTES: PARSE TREE DECORATION (2) c 3 2 3 3 2 1 2 1 3 2 1 S C C Input: aaabbccc c c C B b b B A a A a a A Parse tree: 13/28

SYNTHESIZED ATTRIBUTES: PARSE TREE DECORATION (2) c 3 2 3 3 2 1 2 1 3 2 1 S C C Input: aaabbccc c c C B b b B A a A a a A Parse tree: 13/28

SYNTHESIZED ATTRIBUTES: PARSE TREE DECORATION (2) c 3 2 3 3 2 1 2 1 3 2 1 S C C Input: aaabbccc c c C B b b B A a A a a A Parse tree: 13/28

SYNTHESIZED ATTRIBUTES: PARSE TREE DECORATION (2) Input: aaabbccc 3 2 1 2 1 3 2 1 S C c C c c C B b b B A a A a a A Parse tree: 13/28 3 ̸ = 2 ̸ = 3

A 2 a A 1 count A 2 count B 2 b B 2 inhCount B 1 inhCount C 2 c C 2 inhCount C 1 inhCount INHERITED ATTRIBUTES: EXAMPLE 1 c C 1 Condition C inhCount 1 C 1 1 B 1 Condition B inhCount A S A B C B inhCount A count C inhCount A count a Again, we consider the language A count 1 A 1 1 B b 14/28 L = { a n b n c n | n > 0 } = { abc , aabbcc , aaabbbccc , . . . } .

INHERITED ATTRIBUTES: EXAMPLE Again, we consider the language 14/28 L = { a n b n c n | n > 0 } = { abc , aabbcc , aaabbbccc , . . . } . S → A B C { B . inhCount = A . count ; C . inhCount = A . count } A → a { A . count = 1 } A 1 → A 2 a { A 1 . count = A 2 . count + 1 } B → b { Condition : B . inhCount = 1 } B 1 → B 2 b { B 2 . inhCount = B 1 . inhCount − 1 } C → c { Condition : C . inhCount = 1 } C 1 → C 2 c { C 2 . inhCount = C 1 . inhCount − 1 }

INHERITED ATTRIBUTES: PARSE TREE DECORATION (1) 2 S 1 2 3 3 3 2 c 1 1 1 1 1 1 C C Input: aaabbbccc A Parse tree: A a a A a B c b b B b B C c 15/28

INHERITED ATTRIBUTES: PARSE TREE DECORATION (1) 2 S 1 2 3 3 3 2 c 1 1 1 1 1 1 C C Input: aaabbbccc A Parse tree: A a a A a B c b b B b B C c 15/28

INHERITED ATTRIBUTES: PARSE TREE DECORATION (1) 2 S 1 2 3 3 3 2 c 1 1 1 1 1 1 C C Input: aaabbbccc A Parse tree: A a a A a B c b b B b B C c 15/28