Lecture 2: Compilation and Interpretation

Transcription

1 Lecture 2: Compilation and Interpretation COMP 514 Programming Language Concepts Aaron Block January 16, 2007 Based on notes by N. Fisher, F. Hernandez-Campos, and D. Stotts

2 Goals In this lecture, we will discuss how to translate source code into machine executable code.

3 Compilation and Interpretation There are two primary methods for translating high-level code: Compilation Interpretation

4 Compilation Source Program Compiler Input Target Program Output

5 The target program is called the object code. Compilation Source Program Compiler Input Target Program Output

6 Compilation Source Program Compiler You translate once and run many times. Input Target Program Output

7 Interpretation Source Program Input Interpreter Output

8 Interpretation Source Program Input Interpreter You translate for each run. Output

9 Comparison Source Source In Compiler Interpreter In Target Out Out

10 Compilers are usually faster than interpreters. Comparison Source Source In Compiler Interpreter In Target Out Out

11 Comparison Interpreters are usually more flexible and easier to debug than compilers. Source Source In Compiler Interpreter In Target Out Out

12 Compilation and Interpretation Source Program Translator Intermediate Program Virtual Machine Output Input

13 Compilation and Interpretation Source Program The Virtual Machine acts as an interpreter. Translator Intermediate Program Virtual Machine Output Input

14 Compilation and Interpretation The translator can be a compiler or an interpreter. It is considered to be a compiler if: 1. There is a thorough analysis of the program 2. The transformation is non-trivial. Source Program Translator Intermediate Program Virtual Machine Output Input

15 Compilation and Interpretation Source Program Translator This is exactly the process that Java uses. Intermediate Program Virtual Machine Output Input

16 Linking Source Program Compiler Library Routines Incomplete machine language Linker Machine language program

17 Linking Source Program The Linker is used to connect subroutines that Compiler are not contained in the original code. Incomplete machine language Library Routines Linker Machine language program

18 Linking Source Program Library Routines Compiler This is used by Fortran Incomplete machine language Linker Machine language program

19 Preprocessing Source Program Preprocessor Modified source program

20 Preprocessing Source Program For example, this may be used to remove comments or use replacement macros. Preprocessor Modified source program

21 Preprocessing Source Program On the note about macros, consider the macro #define FALSE 0. Preprocessor This code will replace all instances of the word FALSE with the value 0. Modified source program

22 Bootstrapping Pascal came shipped with three things: A Pascal to P-Code Compiler, written in Pascal. Pascal to P-Code compiler, in Pascal A Pascal to P-Code Compiler, written in P-Code Pascal to P-Code compiler, in P-Code A P-code interpreter, in Pascal. P-Code interpreter, in Pascal.

23 Bootstrapping Pascal came shipped with three things: A Pascal to P-Code Compiler, written in Pascal. Pascal to P-Code compiler, in Pascal A Pascal to P-Code Compiler, written in P-Code Pascal to P-Code compiler, in P-Code A P-code interpreter, in Pascal. P-Code is a very simple language that can easily be translating into any machine language. P-Code interpreter, in Pascal.

24 Bootstrapping Pascal came shipped with three things: A Pascal to P-Code Compiler, written in Pascal. Pascal to P-Code compiler, in Pascal A Pascal to P-Code Compiler, written in P-Code Pascal to P-Code compiler, in P-Code A P-code interpreter, in Pascal. P-Code interpreter, in Pascal.

25 Bootstrapping Pascal came shipped with three things: A Pascal to P-Code Compiler, written in Pascal. Pascal to P-Code compiler, in Pascal A Pascal to P-Code Compiler, written in P-Code Pascal to P-Code compiler, in P-Code A P-code interpreter, in Pascal. The interpreter is hand translated into machine lang. P-Code interpreter, in Pascal. P-Code interpreter, in Machine Lang.

26 Bootstrapping To get a simple (but slow) compiler, one could use the Pascal to P-Code compiler, in P-Code and the P-Code to machine lang. interpreter to compile Pascal code. Source Source, in P- Code. Pascal to P-Code compiler, in P-Code P-Code interpreter, in Machine Lang. P-Code interpreter, in Machine Lang. Target, in machine

27 Bootstrapping To get a simple (but slow) compiler, one could use the Pascal to P-Code compiler, in P-Code and the P-Code to This interpreter is needed to run the machine lang. interpreter to compile Pascal code. Pascal to P-Code compiler. Source Source, in P- Code. Pascal to P-Code compiler, in P-Code P-Code interpreter, in Machine Lang. P-Code interpreter, in Machine Lang. Target, in machine

28 Bootstrapping (Faster) To create a faster compiler, we modify the Pascal to P- Code compiler, in Pascal so that it is a Pascal to machine language compiler, in Pascal. Pascal to P-Code compiler, in Pascal Pascal to machine lang. compiler, in Pascal

29 Bootstrapping (Faster) To create a faster compiler, we modify the Pascal to P- This is MUCH HARDER than creating a P-Code to Code compiler, in Pascal so that it is a Pascal to Machine language interpreter. machine language compiler, in Pascal. Pascal to P-Code compiler, in Pascal Pascal to machine lang. compiler, in Pascal

30 Bootstrapping (Faster) Pascal to machine lang. compiler, in Pascal Pascal to P-Code compiler, in P-Code Pascal to machine lang. compiler, in P-Code P-Code interpreter, in Machine Lang. Input Output P-Code interpreter, in Machine Lang. Pascal to machine lang. compiler, in machine Pascal to machine lang. compiler, in P-Code

31 Bootstrapping (Faster) Input Output Pascal to machine lang. compiler, in Pascal Pascal to P-Code compiler, in P-Code The first step is to construct the Pascal to machine lang. compiler, in P-Code. Pascal to machine lang. compiler, in P-Code P-Code interpreter, in Machine Lang. P-Code interpreter, in Machine Lang. Pascal to machine lang. compiler, in machine Pascal to machine lang. compiler, in P-Code

32 Bootstrapping (Faster) Pascal to machine lang. compiler, in Pascal Pascal to P-Code compiler, in P-Code The P-Code second interpreter, step in is to run the Pascal Machine to Lang. machine lang. compiler, in Pascal through the P-Code version to produce the Machine lang. version. Pascal to machine lang. compiler, in P-Code Pascal to machine lang. compiler, in P-Code P-Code interpreter, in Machine Lang. Input Output Pascal to machine lang. compiler, in machine

33 Bootstrapping (Faster) Pascal to machine lang. compiler, in Pascal Pascal to P-Code compiler, in P-Code Pascal to machine lang. compiler, in P-Code P-Code interpreter, in Machine Lang. Input Output P-Code interpreter, in Machine Lang. Pascal to machine lang. compiler, in machine Pascal to machine lang. compiler, in P-Code

34 Compiling Character Stream Token Stream Parse Tree Abstract syntax tree Modified intermediate form Scanner (lexical analysis) Parser (syntax analysis) Semantic analysis & intermediate code gen. Machine-independent optimization (optional) Symbol Table Machine language Modified target language Target code generation. Machine-specific optimization (optional)

35 Example Program GCD program gcd(input, output); var i, j: integer; begin read(i,j); // get i & j from read while i<>j do if i>j then i := i-j else j := j-1; writeln(i) end.

36 Lexical Analysis Scanner (lexical analysis) Recognize structures without regard to meaning and groups them into tokens. program gcd(input, ouput); program gcd ( input, output ) ; The purpose of the scanner is to simplify the parser by reducing the size of the input.

37 Syntax Analysis Parser (syntax analysis) Parsing organizes the tokens into a context-free grammar (i.e., syntax). program gcd ( input, output ) ; program id(gcd) ( id(input) more_ids ) ; block, id(output) more_ids Rest of code The University empty of North Carolina at Chapel Hill

38 Syntax Analysis Parser (syntax analysis) Parsing organizes the tokens into a context-free grammar (i.e., syntax). program gcd ( input, output ) ; The Syntax analysis catches all malformed statements program id(gcd) ( id(input) more_ids ) ; block, id(output) more_ids Rest of code The University empty of North Carolina at Chapel Hill

39 Syntax Analysis Parser (syntax analysis) Parsing organizes the tokens into a context-free grammar (i.e., syntax). The parse tree is sometimes called a program gcd ( input, output ) ; concrete syntax tree because it contains how all tokens are derived... program id(gcd) ( id(input) more_ids ) ; block, id(output) more_ids Rest of code The University empty of North Carolina at Chapel Hill

40 Syntax Analysis Parser (syntax analysis) Parsing organizes the tokens into a context-free grammar (i.e., syntax)....however, much of this information is program gcd ( input, output ) ; extraneous for the meaning of the code (e.g., the only purpose of ; is to end a statement). program id(gcd) ( id(input) more_ids ) ; block, id(output) more_ids Rest of code The University empty of North Carolina at Chapel Hill

41 Semantic Analysis Semantic analysis & intermediate code gen. Semantic analysis discovers the meaning of a program. by creating an abstract syntax tree that removes extraneous tokens. To do this, the analyzer builds & maintains a symbol table to map identifiers to information known about it. (i.e., scope, internal structure, etc...) By using the symbol table, the semantic analyzer can catch problems not caught by the parser. For example, Identifiers are declared before used subroutine calls provide correct number and type of arguments.

42 Semantic Analysis Semantic analysis & intermediate code gen. Not all semantic rules can be checked at compile time. Those that can are called static semantics of the language. Those that cannot are called dynamic semantics of the language. For example, Arithmetic operations do not overflow. Array subscripts expressions lie within the bounds of the array.

43 Semantic Analysis Semantic analysis & intermediate code gen. program id(gcd) ( id(input) more_ids ) ; block program (5) read (3) (6) read (3) (7) Index Symbol Type 1 INTEGER type 2 TEXTFILE type 3 INPUT 2 4 OUTPUT 2 5 GCD program Rest of 6 I 1 code The University of North 7 Carolina Jat Chapel Hill 1

44 Example Program GCD program gcd(input, output); var i, j: integer; begin read(i,j); // get i & j from read while i<>j do if i>j then i := i-j else j := j-1; writeln(i) end.

45 Semantic Analysis Semantic analysis & intermediate code gen. program id(gcd) ( id(input) more_ids ) ; block program (5) read (3) (6) read (3) (7) Index Symbol Type 1 INTEGER type 2 TEXTFILE type 3 INPUT 2 4 OUTPUT 2 5 GCD program Rest of 6 I 1 code The University of North 7 Carolina Jat Chapel Hill 1

46 Target code generation Target code generation. Code generation takes the abstract syntax tree and the symbol table to produce machine readable code. Simple code follows directly from the abstract syntax tree and symbol table.

47 Optimization Machine-independent optimization (optional) Machine-specific optimization (optional) The process so far will produce correct code, but it may not be fast. Optimization will adjust the code to improve performance. A possible machine-indp. optimization would be to keep the variables i and j in registers throughout the main loop. A possible machine-spec. optimization would be to assign the variables i and j to specific registers.

48 Now for something completely different... Writing

49 Writing The best resource for writing English is The Elements of Style by Strunk and White. The philosophy of this book is that less is more. The goal of good technical writing is to convey information, not to outsmart the reader. The best way to do this is to be clear and succinct.

50 Some Technical Writing Tips Define your terms! If you use a technical term or an acronym, take the space to formally define it, unless you are certain that the reader knows what it means. (Italics helps a reader pick out formal terms.) Some words are very easy to use without ever defining them (e.g., effective as in This program is more effective than previous generations. How is it more effective?)

51 Some Technical Writing Tips Use your terms You ve taken the time to define your terms, use them where appropriate. This can be hard at times, because we naturally avoid word (or even phrase) repetition. For example, the sentence The man managed his manager sounds odd. Also, if say for example, you ve defined the word effective then use it as opposed to similar phases like better.

52 Some Technical Writing Tips Use examples and pictures The point of good technical writing is convey an idea or an argument. If you add an example, then even esoteric concepts can become clear. Suppose, for example, we tried to explain bootstrapping without pictures!

53 Some Technical Writing Tips i.e. and e.g. can be your best friend, but use them correctly. i.e. means that is to say This can be a great way to thrown in an informal definition or explanation after a more complex formal one, or vice versa. e.g. means for example This can be a great way to thrown in a quick example. I remember the difference by e.g. is short for egg-ample.

54 Some Technical Writing Tips Technical writing is not mystery writing If there is some key concept or result tell the reader in the beginning. Don t keep it a surprise until the end. A good model for a paper is Tell em what your gonna tell em. Tell em Tell em what you told em.