phases of a compiler
- 2. 2 Phases of a Compiler Compiler operates in phases Each phase transforms source program from one representation to naother
- 3. 3 Phases of the Compiler Source Program Lexical Analyzer 1 Syntax Analyzer 2 Semantic Analyzer 3 Intermediate Code Generator 4 Code Optimizer 5 Code Generator 6 Target Program Symbol-table Manager Error Handler
- 4. 4 Lexical Analysis Linear Analysis / Scanning Stream of characters are read and grouped into tokens Tokens – Sequence of characters having a collective meaning Input: position := initial + rate * 60 Tokens Position := initial + rate * 60 ;
- 5. 5 Lexical Analysis During Scanning Blanks separating tokens are eliminated Char sequences forming a token – lexeme Entered into the symbol table id1 = id2 + id3 * 60
- 6. 6 Syntax Analysis Hierarchical Analysis/Parsing Tokens are grouped hierarchically into nested collections with collective meaning – grammatical phrases that are used by the compiler to synthesize the output Grammatical phrases are represented by Parse Trees
- 7. 7 Syntax Analysis Hierarchical structure is expressed using recursive rules Any identifier is an expr Any number is an expr If expr1 and expr2 are expressions expr1 + expr2, expr1 * expr2; (expr1) are all expressions If id1 is and identifier and exp2 is an expression id1 = exp2 is a statement If exp1 is an expression and stmt2 is a statement while (exp1) do stmt2 if (exp1) then stmt2 are all statements
- 8. 8 Syntax Analysis Division between lexical and syntax analysis is arbitrary Constructs of the source language can be used to decide Non-recursive – lexical analysis – Recursion is not required to recognize identifiers. Identifiers are strings of letters and digits beginning with a letter. Identifiers are recognized by a simple scan of the input stream. Recursive – Syntactic analysis , begin-end statements; parenthesis matching in expressions etc.
- 10. 10 Syntax Analysis Compressed representation – Syntax tree In Parse tree interior node: record with an operator field and 2 fields for children Leaf: record with 2/more fields; one for token and other information about token position initial rate := + * 60
- 11. 11 Semantic Analysis Ensures that the components of a program fit together meaningfully Performs Type checking Gathers type information and checks for type compatibility Checks operands are permitted by the source language specification position initial rate := + * inttoreal 60
- 12. 12 Intermediate Code Generation Intermediate representation is a Program for an abstract machine Two Properties Should be easy to produce Easy to translate into target program 3-address code is a common form It is a sequence of instructions Each instruction has at most 3 operands
- 13. 13 Intermediate Code Generation Properties of Three address code in Intermediate form 1. Each three address instruction has at most one operator in addition to the assignment Order of operations must be decided Multiplication precedes the addition 2. Compiler must generate temporary names to hold the value computed by each instruction 3. Some instructions may have less than 3 operands
- 14. 14 Intermediate Code Generation Example temp1 := inttoreal(60) temp2 := id3 * temp1 temp3 := id2 + temp2 id1 := temp3
- 15. 15 Code Optimization Aims to produce faster running machine code Perform the same calculation using the two instructions Optimizing Compilers The fraction of time the compiler is spent in this phase Amount of optimization varies Optimized code temp1 := id3 * 60.0 id1 := id2 + temp1 -int to real conversion at compile time -temp3 can be eliminated
- 16. 16 Code Generation Generates target code Relocatable machine code / Assembly code Memory locations are selected for variables Assignment of variables to registers is done MOVF id3, R2 MULF #60.0, R2 MOVF id2, R1 ADDF R2, R1 MOVF R1, id F -> floating point numbers, # -> Constant Source Destination
- 17. 17 Symbol Table Management Essential function of a compiler is to record for each identifier and fields for the various attribute of each identifier Attributes Stored Identifiers - Storage allocated, type, Scope Procedures – Number and type of arguments, Passing by reference/ value, return type Lexical Analysis stage makes entries, other phases fill in details and refer
- 18. 18 Error detection and Reporting All phases may encounter errors Lexical Syntax Semantic Must not stop on first error
- 19. 19 Example E r r o r s position := initial + rate * 60 lexical analyzer syntax analyzer semantic analyzer intermediate code generator id1 := id2 + id3 * 60 := id1 id2 id3 + * 60 := id1 id2l id3 + * inttoreal 60 Symbol Table position .... initial …. rate….
- 20. 20 Example E r r o r s intermediate code generator code optimizer final code generator temp1 := inttoreal(60) temp2 := id3 * temp1 temp3 := id2 + temp2 id1 := temp3 temp1 := id3 * 60.0 id1 := id2 + temp1 MOVF id3, R2 MULF #60.0, R2 MOVF id2, R1 ADDF R2, R1 MOVF R1, id1 position .... initial …. rate…. Symbol Table 3 address code