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Compilers and Language Translation Gordon College.

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Presentation on theme: "Compilers and Language Translation Gordon College."— Presentation transcript:

1 Compilers and Language Translation Gordon College

2 2 What’s a compiler? All computers only understand machine language Therefore, high-level language instructions must be translated into machine language prior to execution …… This is a program

3 3 What’s a compiler? Compiler A piece of system software that translates high-level languages into machine language …… Congrats! while (c!='x') { if (c == 'a' || c == 'e' || c == 'i') printf("Congrats!"); else if (c!='x') printf("You Loser!"); } Compiler gcc -o prog program.c program.c prog

4 4 Assembler (a kind of compiler) LOADXAssembly Machine Language (symbol table)(opcode table) One-to-one translation

5 5 Compiler (high-level language translator) a = b + c - d; One-to-many translation LOAD B ADD C SUBTRACT D STORE A …….

6 6 Goals of a compiler Code produced must be correct A = (B+C)-(D+E); Possible translation: LOAD B ADD C STORE B LOAD D ADD E STORE D LOAD B SUBTRACT D STORE A No - STORE B and STORE D changes the values of variables B and D which is the high-level language does not intend Is this correct?

7 7 Goals of a compiler Code produced should be reasonably efficient and concise Compute the sum - 2x 1+ 2x 2+ 2x 3+ 2x 4+…. 2x sum = 0.0 for(i=0;i<50000;i++) { sum = sum + (2.0 * x[i]); Optimizing compiler: sum = 0.0 for(i=0;i<50000;i++) { sum = sum + x[i]; sum = sum * 2.0;49,999 less instructions

8 8 General Structure of a Compiler

9 9 The Compilation Process Phase I: Lexical analysis Compiler examines the individual characters in the source program and groups them into syntactical units called tokens Phase II: Parsing The sequence of tokens formed by the scanner is checked to see whether it is syntactically correct Scanner Source code Groups of tokens Parser Groups of tokens correct not correct

10 10 The Compilation Process Phase III: Semantic analysis and code generation The compiler analyzes the meaning of the high-level language statement and generates the machine language instructions to carry out these actions Code Generator Groups of tokens Machine language

11 11 The Compilation Process Phase IV: Code optimization The compiler takes the generated code and sees whether it can be made more efficient Code Optimizer Machine language Machine language

12 12 Overall Execution Sequence on a High-Level Language Program

13 13 The Compilation Process Source program Original high-level language program Object program Machine language translation of the source program

14 14 Phase I: Lexical Analysis Lexical analyzer The program that performs lexical analysis More commonly called a scanner Job of lexical analyzer Group input characters into tokens Tokens: Syntactical units that are treated as single, indivisible entities for the purposes of translation Classify tokens according to their type

15 15 Phase I: Lexical Analysis Program statement sum = sum + a[i]; Digital perspective: tab,s,u,m,blank,=,blank,s,u,m,blank,+,blank,a,[,i,],; Tokenized: sum,=,sum,+,a[i],;

16 16 Phase I: Lexical Analysis TOKEN TYPECLASSIFICATION NUMBER Symbol1 Number2 = ;6 ==7 If8 Else9 (10 )11 [ 12 ] 13 … Typical Token Classifications

17 17 Phase I: Lexical Analysis Lexical Analysis Process 1. Discard blanks, tabs, etc. - look for beginning of token. 2. Put characters together 3. Repeat step 2 until end of token 4. Classify and save token 5. Repeat steps 1-4 until end of statement 6. Repeat steps 1-5 until end of source code Scanner sum=sum+a[i]; sum 1 =3 +4 a1 [12 i1 ]13 ;6

18 18 Phase I: Lexical Analysis Input to a scanner - A high-level language statement from the source program Scanner’s output - A list of all the tokens in that statement - The classification number of each token found Scanner sum=sum+a[i]; sum 1 =3 +4 a1 [12 i1 ]13 ;6

19 19 Phase II: Parsing Parsing phase A compiler determines whether the tokens recognized by the scanner are a syntactically legal statement Performed by a parser

20 20 Phase II: Parsing Output of a parser A parse tree, if such a tree exists An error message, if a parse tree cannot be constructed Successful construction of a parse tree is proof that the statement is correctly formed

21 21 Example High-level language statement: a = b + c

22 22 Grammars, Languages, and BNF Syntax The grammatical structure of the language The parser must be given the syntax of the language BNF (Backus-Naur Form) Most widely used notation for representing the syntax of a programming language literal_expression ::= integer_literal | float_literal | string | character

23 23 Grammars, Languages, and BNF In BNF The syntax of a language is specified as a set of rules (also called productions) A grammar The entire collection of rules for a language Structure of an individual BNF rule left-hand side ::= “definition”

24 24 Grammars, Languages, and BNF BNF rules use two types of objects on the right- hand side of a production Terminals The actual tokens of the language Never appear on the left-hand side of a BNF rule Nonterminals Intermediate grammatical categories used to help explain and organize the language Must appear on the left-hand side of one or more rules

25 25 Grammars, Languages, and BNF Goal symbol The highest-level nonterminal The nonterminal object that the parser is trying to produce as it builds the parse tree All nonterminals are written inside angle brackets Java BNF

26 26 BNF Example ::= | ::= | "." ::= ::= "," ::= "Sr." | "Jr." | | "" Identify the following: Goal symbol, terminals, nonterminals, a individual rule Is this a legal postal address? Steve Moses Sr. 215 Rose Ave. Everywhere, NC 43563

27 27 Parsing Concepts and Techniques Fundamental rule of parsing: By repeated applications of the rules of the grammar- If the parser can convert the sequence of input tokens into the goal symbol the sequence of tokens is a syntactically valid statement of the language else the sequence of tokens is not a syntactically valid statement of the language

28 28 Parsing Example ::= [ / ] [ ? ] ::= [ : ] ::= | ::= [. ] ::=... ::= ::= | [ / ] ::= [ + ] ::= | | | | ::= [ ] ::= | + ::= [ ] ::= a | b | … | z | A | B | … | Z ::= 0 |1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 ::= $ | - | _ |. | & | ~ ::= ! | * | " | ' | ( | ) | : | ; |, | ::= % ::= | a | b | c | d | e | f | A | B | C | D | E | F ::= [ ] ::= Is the following http address legal:

29 29 Parsing Concepts and Techniques Look-ahead parsing algorithms - intelligent parsers One of the biggest problems in building a compiler is designing a grammar that: Includes every valid statement that we want to be in the language Excludes every invalid statement that we do not want to be in the language

30 30 Parsing Concepts and Techniques Another problem in constructing a compiler: Designing a grammar that is not ambiguous An ambiguous grammar allows the construction of two or more distinct parse trees for the same statement NOT GOOD - multiple interpretations

31 31 Phase III: Semantics and Code Generation Semantic analysis The compiler makes a first pass over the parse tree to determine whether all branches of the tree are semantically valid If they are valid the compiler can generate machine language instructions else there is a semantic error; machine language instructions are not generated

32 32 Phase III: Semantics and Code Generation Semantic analysis Syntactically correct, but semantically incorrect example: sum = a + b; int a; double sum;data type mismatch char b; Semantic records ainteger sumdouble bchar

33 33 Phase III: Semantics and Code Generation Semantic analysis a integer b char temp ? + Parse tree Semantic record

34 34 Phase III: Semantics and Code Generation Semantic analysis a integer b integer temp integer + Parse tree Semantic record

35 35 Phase III: Semantics and Code Generation Code generation Compiler makes a second pass over the parse tree to produce the translated code

36 36 Phase IV: Code Optimization Two types of optimization Local Global Local optimization The compiler looks at a very small block of instructions and tries to determine how it can improve the efficiency of this local code block Relatively easy; included as part of most compilers:

37 37 Phase IV: Code Optimization Examples of possible local optimizations Constant evaluation x = > x = 2 Strength reduction x = x * 2 ---> x = x + x Eliminating unnecessary operations

38 38 Phase IV: Code Optimization Global optimization The compiler looks at large segments of the program to decide how to improve performance Much more difficult; usually omitted from all but the most sophisticated and expensive production-level “optimizing compilers” Optimization cannot make an inefficient algorithm efficient - “only makes an efficient algorithm more efficient”

39 39 Summary A compiler is a piece of system software that translates high-level languages into machine language Goals of a compiler: Correctness and the production of efficient and concise code Source program: High-level language program

40 40 Summary Object program: The machine language translation of the source program Phases of the compilation process Phase I: Lexical analysis Phase II: Parsing Phase III: Semantic analysis and code generation Phase IV: Code optimization


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