Lecture 03: Languages Implementation Methods & Compilation

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describe Compilation
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[1] Translate high-level program (source language) into machine code (machine language) [3] Slow translation, fast execution [2] Use: Large commercial applications.

Describe Pure Interpretation
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[1] Programs are interpreted by another program known as an interpreter. [2] Use: Small programs or when efficiency is not an issue.

Describe Hybrid Implementation Systems
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[1] A compromise between compilers and pure interpreters. [2] Use: Small and medium systems when efficiency is not the first priority.

what are the phases of Compilation?
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[1] lexical analysis [2] syntax analysis [3] Semantics analysis [4] code generation

what is lexical analysis
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converts characters in the source program into lexical units

What is syntax analysis
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transforms lexical units into parse trees which represent the syntactic structure of program

what is Semantics analysis
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generate intermediate code

what is code generation
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machine code is generated

What is a compiler
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A compiler is a program that translates a program in a source language into an equivalent program in a target language.

What is the structure of a compiler?
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(Source program->[Lexical analyser]->[Synax analyser]->[Semantic analyser]->[Code optomiser]->[Code generator]->Target program) everything in [] is also conected to a symbol table

what does the lexical Analysis entail?
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[1] The lexical analyser (or scanner): reads the source program’s text (one character at a time); and returns a sequence of tokens to send to the next phase. [2] Tokens are symbolic names for the lexical elements of the source language

What is the Symbols Table
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The symbol table is a data structure containing all the identifiers (together with their attributes) of a source program.

What are typical Attributes of variables?
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type, size, scope.

What are typical attributes for methods procedures or functions
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number of arguments and their types and passing mechanisms; the return type (if any).

What is Syntax Analisis (parsing)
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[1] analyses the syntactic structure of the source program for valid syntax[2] input to a parser is the sequence of output tokens from the lexical analyser [3] It attempts to apply the rules that define the syntax of the language on the sequence of tokens

what is Semantic Analysis
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determines if the source program (based on the AST and symbol table) is semantically valid.

What does AST stand for? and what is it?
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Abstract syntax trees are data structures widely used in compilers to represent the structure of program code

What is Error Handling
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[1] The three analysis phases of a compiler must be able to deal with errors in the source program. [2] On detecting an error a compiler should ideally: report the error in a helpful way, and continue processing to look for further errors.

What is Code Optimisation
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[1] Attempts to improve the time and space requirements of a program, i.e., makes a smaller or faster or both. [2] Example optimisations include: -constant folding (e.g., replacing 3+7 with 10); -eliminating unreachable code

What is Code Generation
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[1] The final task of the compiler is to generate code for a specific machine. In this phase we need to consider such [2] The output from this phase is usually programs in machine language or assembly language or code for a virtual machine.

What things may be considered during code generation
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[1] Instruction selection: The machine language instr' to use [2] Instr' scheduling: The order to put those instr'[3] Register allocation: The allocation of variables to processor registers[4] Debug data generation if required so the code can be debugged

What is Pure Interpretation
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An interpreter parse the source code and execute it directly, e.g., early versions of the Lisp and BASIC.

describe some dissadvantages of pure implimentation compared with compilation.
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[1] Slower execution than compiled programs [2] Often requires more space

Why is pure interpritation slower than compilation?
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[1] decoding high-level language statement is slower than decoding machine language instructions [2] Regardless how many times a statement is executed, it must be decoded every time.

Why does pure interpritation need more space than compilation?
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the source code and the symbol table must be present during interpretation

give examples of Pure interpritation languages
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Now rare for traditional high-level languages but has significant comeback with some Web scripting languages (e.g., JavaScript, PHP)

What is Hybrid Implementation Systems
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A compromise between compilers and pure interpreters (not as fast as compiled languages, but faster than interpreted languages)

Describe the process used byHybrid Implementation Systems
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A high-level language program is translated/compiled to an intermediate code that will be interpreted during execution.

with Hybrid Implementation Systems is the intermediate code ever translated further?
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no never, Instead, a virtual machine is supplied that takes the intermediate language as its machine language, and interpret the intermediate code (much faster than it would be able to interpret a high-level language)

Give examples of some Hybrid Implementation Systems languages
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E.g., Perl, Python, and MATLAB, JDK1.0

describe Just-in-Time Implementation Systems
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[1] compile programs to an intermediate language[2] intermediate language is loaded into memory [3] segments/subprograms are translated into machine code just before its execution (or when they are called)[4] The machine code is kept for subsequent calls


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