Compilers

J. Bense Tony
Anna University
Chennai

Acompiler is a program that reads a
program written in one language and
translates into equivalent target
language
COMPILER
Source- - Target
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Error Message

 The Front end checks whether the
program is correctly written in terms of the
programming language syntax and
semantics

 The back end is responsible for translating
the source into assembly code.

Front End :

LexicalAnalysis
Preprocessing
Syntax Analysis
Semantic Analysis

Back End

 Analysis
 Optimization
 Code generation

 Lexical Analyzer
 Syntax Analyzer
 Semantic Analyzer
 Intermediate code generator
 Code optimizer
 Code generator

Lexical

Syntax

Semantic
Symbol Error
Table Handler
manager
Intermediate Code Generator

Code Generator

Code Optimizer

 Also called Linear Analysis
 Characters read from left to right and
grouped into tokens that are a sequence of
characters with a collective meaning
Scans Input
Removes White spaces and comments
Manufacture Tokens
Generate Error if Any

◦ Example

◦ A=B+C

◦ Variable tokens - A ,B, C
◦ Symbolic token -- = +

 Also called as Hierarchial Analysis
 A syntax tree[also called as parse tree] is generated
where
◦ Operators  Interior nodes
◦ Operands  Children of node for operators.

=

A + Interior

B C Children

 Characters grouped as tokens in Lexical Analysis are
recorded as Tables. Checks for semantic errors

 Collect TYPE information for the subsequent code
generation phase

 Sophisticated compilers typically perform multiple
passes over various intermediate forms.
 Many algorithms for code optimization are easier to
apply one at a time
 The input to one optimization relies on the
processing performed by another optimization

 Input : Concrete Syntax Tree[Parse Tree] or
Abstract Syntax Tree
 The tree is converted into a linear sequence
of instructions, usually in an intermediate
language such as three address code.
 This is an Early stage of Code generation

Concrete Parse tree
Abstract syntax tree

Converted into a linear sequence of instructions

Results in 3AC [ 3 Address Code]

 This phase attempts to improve the intermediate
code inorder to increase the running time

 Reduce the complexity of the code generated

 Leading to a faster execution of the program

 Increased Performance

 Platform Dependant/ Platform Independent
 Optimization can be automated by compilers or
performed by programmers
 Usually, the most powerful optimization is to find
a superior algorithm.
 Include activities like
◦ Optimization of LOOPS
◦ Optimization of Bottlenecks

 Succeeding step of Intermediate code
optimizer
 Consists of re-locatable machine
code/assembly code
 Intermediate instructions are converted
into a a sequence of machine instructions

 One pass compilers

 Multi pass compilers

 Load and go compilers

 Optimizing compilers

One pass Multi pass

Passes through the source code of Processes the source code of a
each compilation unit only once program several times

Compilation time is faster Compilation time is slower

Has limited scope of passes Has wide scope of passes.

wide compilers Narrow compilers

Pascal Java

 Generates machine code and immediately
executes it.

 Compilers usually produce either absolute
code that is executed immediately upon
conclusion of the compilation or object code that
is transformed by a linking loader into absolute
code.

 Loop optimization
 Data flow
 Code generation
 Functional language
 Interprocedural optimizations
 SSA [Static Single Assignment] based
optimizations

Threaded code compiler
 database lookup program.
 replaces given strings in the source with
given binary code.

 Incremental compiler:
◦ Individual functions can be compiled in a run-time
environment that also includes interpreted
functions.

 Stage compiler
That compiles to assembly language of a
theoretical machine, like some Prolog
implementations

 Just-in-time compiler
◦ Applications are delivered in byte code, which is
compiled to native machine code just prior to
execution

 A Retargetable compiler
◦ object code is frequently of lesser quality than that
produced by a compiler developed specifically for a
processor.
◦ Retargetable compilers are often also cross
compilers
 A parallelizing compiler
◦ converts a serial input program into a form suitable
for efficient execution on a parallel computer
architecture.

 The compiler highlights all the possible
errors which are obstacle in the
program to provide a good meaning.
 Logical errors could be found only at
Run time of the program.

 Array bound missing
 Array size too large
 Bad file name
 Call of on function
 Declaration missing
 Compound statement missing
 Illegal Accessions

 Not abstract
 Not accessible
 Not initialized
 Not found in “import”
 Scope related errors
 Cannot be inherited
 Overloading
 Overriding errors

 Program is free from lexical errors
 Program is free from syntax errors
 Program is free from semantic errors
 Even a complex program could be compiled in a
small interval of time
 In programs related to database accessing,
many risks are reduced.

Compilers

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Compilers