Translation of expression in compiler design
- 1. Unit 3
- 2. Translation of Expressions 1. Operations within Expressions 2. Incremental Translation 3. Addressing Array elements 4. Translation of Array References
- 3. 1. Operations within expressions ● SDD attributes: ○ code - denote three address code ○ addr - address that will hold value of non-terminal ● Top - points to current symbol table ● top.get() - retrieves entry of parameter passed ● gen() - generate 3 address instruction , pass operator in quotes ● new Temp() - Create distinct temp name
- 5. 2. Incremental Translation ● Code attributes can be long strings, so they are usually generated incrementally ● In the incremental approach, gen not only constructs a three-address instruction, it appends the instruction to the sequence of instructions generated so far. ● The sequence may either be retained in memory for further processing, or it may be output incrementally.
- 6. 3. Addressing Array Elements 12 18 24 0 1 2 Address Element Formula for calculation:
- 9. 4. Translation of array references
- 12. Type Checking 1. Rules of type checking 2. Type conversions 3. Overloading of functions and operators 4. Type Inference and Polymorphic Functions 5. An algorithm for unification
- 13. What is type checking? ● assign a type expression to each component of the source program ● The compiler must then determine that these type expressions conform to a collection of logical rules that is called the type system for the source language. ● It can be used to detect errors in the program. ● A sound type system eliminates the need for dynamic checking for type errors, because it allows us to determine statically that these errors cannot occur when the target program runs. ● type checking have been used to improve the security of systems that allow software modules to be imported and executed Example:
- 14. 1. Rules for Type Checking (Synthesis/Inference) Type Synthesis ● builds up the type of an expression from the types of its subexpressions. ● It requires names to be declared before they are used. ● Eg. Type of E1+E2 is determined from types of E1 and E2. Type Inference ● Determines the type of a language construct from the way it is used. ● Let null(x) be a function to check if list is empty. ● Since null function applied on list, we can say x is a list
- 15. 2. Type Conversions/ Type Casting ● Type casting is converting one datatype to some other data type. It is executed using the cast operator. ● Explicit type conversion in C is when the datatype conversion is user-defined according to the program's needs. Explicit type conversion can easily change a particular data type to a different one. ● Implicit type conversion is automatically done by the program. ● Need: a = 5 b = 10.12 print(a+b)
- 19. 3. Overloading of functions and operators a+b
- 20. 4. Type Inference and Polymorphic Functions ● Type inference is useful for a language like ML which is strongly typed, but does not require names to be declared before they are used. Type inference ensures that names are used consistently. ● The term polymorphic refers to any code fragment that can be executed with arguments of different types. ● polymorphism is characterized by parameters or type variables
- 24. 5. Unification Algorithm 1. Unification is the problem of determining whether two expressions s and t can be made identical by substituting expressions for the variables in s and t. 2. Testing equality of expressions is a special case of unification; if s and t have constants but no variables, then s and t unify if and only if they are identical. 3. The unification algorithm in this section extends to graphs with cycles, so it can be used to test structural equivalence of circular types 4. Type variables are represented by leaves and type constructors are represented by interior nodes. 5. Nodes are grouped into equivalence classes; if two nodes are in the same equivalence class, then the type expressions they represent must unify. 6. Thus, all interior nodes in the same class must be for the same type constructor, and their corresponding children must be equivalent.
- 25. Finds representative node of equivalence class Union merges classes with nodes s and t