Data structure and algorithm.lect-03.ppt

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Front End vs Back End of a Compilers
 The phases of a compiler are collected into
front end and back end.
 The front end consists of those phases that
depend primarily on the source program.These
normally include Lexical and Syntactic
analysis,Semantic analysis ,and the generation
of intermediate code

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Front End vs Back End of a
Compilers(Cont’d)
 A certain amount of code optimization can be
done by front end as well.

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Compilers(Cont’d)
 The BACK END includes the code optimization
phase and final code generation phase,along
with the necessary error handling and symbol
table operations.

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Compilers(Cont’d)
 The front end analyzes the source program
and produces intermediate code while the
back end synthesizes the target program
from the intermediate code.
 A naive approach (front force) to that front end
might run the phases serially

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Compilers(Cont’d)
 It is also tempting to compile several different
languages into the same intermediate
language and use a common back end for the
different front ends, thereby obtaining several
compilers for one machine.
 However, because of subtle difference in the
view points of different language, there has
been only limited success in this direction.

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Passes
 In an implementation of a complier, portion of
one or more phases are combined into a
module called a pass

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Passes(cont’d)
 Several phases of complier are usually
implemented in a single pass consisting of
reading an input file and writing as output file.
 It is common for several phases to be grouped
into one pass and for the activity of these
phases to be interleaved during the pass

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Passes(cont’d)
 For example lexical analysis ,syntax analysis,
semantic analysis and intermediate code
generation might be grouped into one pass.
 If so, the token stream after lexical analysis
may be translated directly into intermediate
code.

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Passes(cont’d)
 A pass reads the source program or output of
the previous pass make the transformation
specified by its phases and writes output into
an intermediate file , which may then be read
by a subsequent pass.

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Multi Pass Compiler
 A multi pass compiler can be made a useless
space than a single pass compiler.
 Since the space occupied by the complier
program for one pass can be reused by the
following pass.

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Multi Pass Compiler(cont’d)
 A multi pass complier is of the course slower
than a single pass compiler, because each
pass reads and writes an intermediate file .

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Multi Pass Compiler(cont’d)
 Thus compiler running in computers with small
memory would normally use several passes
while on a computer with a large random
memory , a compiler with fewer passes would
be possible.

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Reducing The No of Passes
It is desirable to have relatively few passes, since
it takes time to read and write intermediate files.
on the other hand ,if we group several phases
into one pass,we may be forced to keep the
entire program in memory.

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Reducing The No of Passes(cont’d)
Because one phase may need
information in a different order than a previous
phase produce it.

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The internal form of the program may be
considerably larger than either the source
program or the target program ,so this space
may not be a trivial matter.

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For some phases,grouping into one pass
presents few problems.For example,the
interface b/w lexical and syntactic analyzers
can often be limited to a single token.

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On the other hand, it is often very hard to perform
Code generation until the intermediate representation
has been completely generated.

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For example, languages like PL/I and Algol 68
permit variables to be used before they are
declared. we can not generate the target code
for a construct if we do not know the types of
variables involved in that construct.

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 Similarly most languages allow goto`s that
jump forward in the code.
 We can not determine the target addresses of
such a jump until we have seen the intervening
source code and generated target code for it.

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 In some cases ,it is possible to leave a blank
slot for missing information ,and fill in the slot
when the information becomes available.
 In particular ,intermediate and target code
generation can often be merged into one pass
using a technique called “back patching”

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we can combine the action of the passes as
follows.On encountering an assembly statement
that is forward reference ,say
GOTO target.

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We generate a skeletal instruction ,with the
machine operation code for GOTO and blanks
for the address. All instructions with blanks
for the address of target are kept in a list
associated with the symbol table entry for
target.

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The blanks are filled in when we
finally encounter and instruction such as
target : MOV R1, bar

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And determine the value of target ;it is the
address of the current instruction.We then
back patch by going down the list for target
of all the instructions that need its address,
substituting the address of target for the
blanks in the address fields of those
instructions

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This approach is easy to implement
if the instructions can be kept in memory until
all target addresses can be determined.

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This approach is a reasonable one for an
Assembler that can keep all its output in
memory.Since the intermediate and final
representations of code for an assembler
are roughly the same.

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 And surely of approximately the same
enough,back patching over the length of the
entire assembly program is not infeasible.
 However, in a compiler ,with a space
consuming intermediate code ,we may need
to be careful about the distance over which
back patching occurs.

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Compiler Construction Tools
 A number of tools have been developed
specifically to held construct compilers.These
tools variously called compiler-
compilers,compiler-generators, or translator-
writing systems,which produce a compiler from
some form of specification of a source
language and target m/c language.

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Compiler Construction Tools(cont’d)
 Largely ,they are oriented around a
particular model of languages and they are
most suitable for generating compilers of
languages similar to the model.

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 For example , it is tempting to assume that
lexical analyzers for all languages are
essentially the same,except for the particular
key words and signs recognized.

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 Many compiler-compilers do in fact produce
fixed lexical analysis routines for use in the
generated compiler.

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 These routines differ only in the list of key
words recognized ,and this list is all that needs
to be supplied by the user. The approach is
valid, but may be unworkable if it is required to
recognize nonstandard tokens,such as
identifiers that may include certain character
other than letters and digits.

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 Some general tools have been created for the
automatic design of specific compiler
components.
 These tools use specialized languages for
specifying and implementing the
component ,and many use algorithms that are
quite sophisticated.

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 The most successful tools are those that hide
the details of the generation algorithm and
produce components that can be easily
integrated into the remainder of a compiler

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The following is a list of some useful compiler
construction tools.
1) Parser Generators
2) Scanner Generators
3) Syntax-directed translation Engines
4) Automatic Code Generators
5) Data Flow Engines

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1) Parser Generators
 These produce syntax analyzers ,normally
from input that is based on a context free
grammar. In early compilers, syntax analysis
consumed not only a large fraction of the
running time of a compiler but a large fraction
of the intellectual effort of writing a compiler.

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Parser Generators(cont’d)
 This phase is now considered one of the
easiest to implement.
 Many parser generators utilize powerful
parsing algorithms that are too complex to be
carried out by hand.

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2) Scanner Generators
 These automatically generate lexical analyzer
normally from a specification based on regular
expressions.

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3) Syntax Directed Translation Engine
 These produce collections of routines that walk
the parse tree ,generating intermediate cods.

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4) Automatic Code Generators
 Such a tool takes a collection of rules that
defines the translation of each operation of the
intermediate language into the m/c for the
target machine.

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Automatic Code Generators(cont’d)
 The rules must include sufficient detail that we
can handle the different possible access
methods for data e.g. variables may be in
registers or a fixed (static)location in memory
or may be allocated a position on a stack.

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5) Data Flow Engines.
 Much of the information needed to perform
good code optimization involves “data flow
analysis “ the gathering of information about
how values are transmitted from one part of
the programme to each part.

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Source Program
Symbol Table Error Handler
Manager
Target Program
Lexical Analyzer
Syntax Analyzer
Semantic Analyzer
Intermediate Code
Generator
Code Optimizer
Code Generator

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 A Compiler operates in phases ,each of which
transforms the source programme from one
representation to another.
 In practice,some of the phases may be
grouped together.
 The first three phases ,forming the bulk of the
analysis portion of a compiler

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 Two other activities , symbol table
management and error handling, are shown
interacting with the six phases of lexical
analysis, syntax analysis,semantic
analysis,intermediate code generation,code
optimization,and code generation.
 Informally,we shall also call the symbol table
manager and error handler Phases.

Data structure and algorithm.lect-03.ppt

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