COMPILERS

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COMPILER STATEMENT OF PROBLEM A compiler accepts a program written in a higher

level language as input and produces its machine language equivalent as output.

Example:WCM: PROCEDURE (RATE, START, FINISH);DECLARE ( COST , RATE , START , FINISH) FIXED BINARY (31)

STATIC;COST = RATE * (START – FINISH) + 2 * RATE * (START – FINISH –

100);RETURN (COST) ;END;

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COMPILER Steps followed by compiler to produce

machine code Recognize certain string as basic elements.Ex: COST := variable , WCM := label, PROCEDURE : =

keyword and “=“ := operator Recognize combinations of elements as syntactic units and

interpret their meaning.Ex: 1st statement is procedure name with three arguments Allocate storage and assign locations for all variables in this

program. Generate the appropriate object code.

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COMPILER Problem No: 1 – Recognizing Basic Elements (lexical analysis) Uses conceptually simple string processing techniques. Source program is scanned sequentially Tokens (identifiers, literals or terminal symbols)are identified Basic elements(identifiers & literals) are placed into tables with

basic information Example:

WCM : PROCEDURE ( RATE , START , FINISH ) ;

(DECLARE COST , RATE , START , FINISH ) FIXED BINARY (31 ) STATIC ;

COST = RATE * ( START - FINISH ) + 2 *

RATE * ( START - FINISH - 100 ) ;

RETURN ( COST ) ;END ;

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COMPILER Ways of lexical process Single continuous pass used to prepare a chain or

table of tokens Reduces the size of token table by only parsing

tokens as necessary. Discover and note lexical errors.

CLASSES OF UNIFORM SYMBOLS:

IDENTIFIER(IDN)TERMINAL SYMBOL(TRM)

LITERAL(LIT)

CLASS PTR

IDN

TRM

TRM

TRM

IDN

WCM

:

PROCEDURE(

RATE

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COMPILER Problem No: 2 – Recognizing syntactic

units and Interpreting Meaning (syntactic construction)

Compiler must recognize the phrases & interpret the meaning of the constructions.

Note syntactic errors Some compiler guess what the

programmer did wrong & correct it.

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COMPILERExample:

WCM : PROCEDURE ( RATE , START , FINISH ) ;

DECLARE ( COST , RATE , START , FINISH ) FIXED BINARY ( 31 ) STATIC ;

COST = RATE * ( START – FINISH ) + 2 * RATE * ( START - FINISH - 100 ) ;

RETRUN ( COST ) ; END ;

Valid procedure statement

Valid declare statement

Valid end statement

Valid assignment statementValid assignment statement

Basic syntactic constructs

Tokens

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COMPILER INTERMEDIATE FORM Compiler creates an intermediate form of

source program. It facilitates optimization of object code It allows a logical separation between the

machine-independent phases & machine-dependent phases

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COMPILER ARITHMETIC STATEMENT Parse tree - form of an arithmetic

statement Rules for converting an arithmetic

statement into a parse tree are: Any variable is a terminal node of the

tree For every operator, construct a binary

tree whose left branch is the tree for operand 1 & whose right branch is the tree for operand 2.

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COMPILERExample: parse tree

COST = RATE * ( START – FINISH ) + 2 * RATE * ( START - FINISH - 100 ) ;

=

+

* *

-

-

*

COST

RATE-

START FINISH 2 RATE

START FINISH

100

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COMPILER MATRIX : Linear representation of the parse tree Operations are listed sequentially Each matrix entry has one operator and two operands. Operands are uniform symbols

Matrix line no.

O8perator

Operand1

Operand2

1 - START FINISH2 * RATE M13 * 2 RATE4 - START FINISH5 - M4 1006 * M3 M57 + M2 M68 = COST M7

matrix

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COMPILER NONARITHMETIC STATEMENTS DO, IF GO TO ,etc replaced by sequential

ordering of individual matrix entries. Operators are defined in later phases of

compilerOperator Operand1 Operand2 Return COST End

RETURN ( COST) ;END ;

Matrix for RETURN & END statement

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COMPILER NONEXECUTABLE STATEMENTS No intermediate form for these

statements Information of these statements is

entered into tablesName Base Scale Precisi

on(bits)

Storage class

location

COST BINARY FIXED 31 STATIC 0RATE BINARY FIXED 31 STATIC 4START BINARY FIXED 31 STATIC 8FINISH BINARY FIXED 31 STATIC 12

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COMPILER Problem no. 3 – Storage Allocation Storage allocation routine scans the identifier

table and assigns a location to each scalar. Absolute address is unknown at load time. Relative address format

Temporary locations for intermediate results of the matrix(M!,M2,etc)

Operand Sign bit(1)

31 binary digit

COST SRATE ISTART GFINISH N

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COMPILER Problem no. 4 – Code Generation

Based on matrix and table compiler generates object code

A table with matrix operation and associated matrix code is used for creating object deck.

Operators are treat as macro definition, operands as arguments and production table as macro definition.

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COMPILERStandard code definition for -,*,+,=

- L1,&OPERAND1 S1,&OPERAND2 ST 1,M&N-------------------------------* L1,&OPERAND1

S1,&OPERAND2 ST 1,M&N------------------------------------+ L1,&OPERAND1

S1,&OPERAND2

ST 1,M&N------------------------------------= L1,&OPERAND1 S1,&OPERAND2 ST 1,M&N----------------------------------

MATRIX GENERATED CODE

1 - START FINISH L1,START

S1,FINISH

ST 1,M1

2 * RATE M1 L 1,RATEM 0,M1ST 1,M2

3 * 2 RATE L 1,=F’2’M 0,RATEST 1,M3

4 - START FINISH L1,START

S1,FINISH

ST 1,M4

5 - M4 100 L 1,M4S

1,=F’100’ST 1,M5

6 * M3 M5 L 1,M3M 0,M5ST 1,M6

7 + M2 M6 L 1,M2A 1,M6ST 1,M7

8 = COST M7 L 1,M7ST 1,COST

Code definitionCode generation

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COMPILER Questions

Was it a good idea to generate code directly from the matrix?

MACHINE INDEPENDENT OPTIMIZATION Have we made the best use of machine we

have at our disposal? MACHINE DEPENDENT OPTIMIZATION Can we generate machine language directly?

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COMPILER OPTIMIZATION(MACHINE-INDEPENDENT)

Delete all duplicate matrix entries of sub expression occurs in the same statement more than once(a common sub expression)

Modify all references to the deleted entries Done before code generation Example

Matrix with common sub expression

Matrix after elimination of common sub expression

1 - START FINISH 1 - START FINISH

2 * RATE M1 2 * RATE M13 * 2 RATE 3 * 2 RATE4 - START FINISH 4

5 - M4 100 5 - M1 1006 * M3 M5 6 * M3 M57 + M2 M6 7 * M2 M68 = COST M7 8 = COST M7

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COMPILER Other machine independent optimization

steps are: Compile time computation of operations, both

of whose operands are constants Movement of computations involving

nonvarying operands out of loops Use of the properties of Boolean expression to

minimize their computation.

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COMPILER machine dependent optimization steps

are: For temporary storage use registers, this reduce

the number of loads and stores from 14 to 5 in our example

Use shorter and faster instructions whenever possible

Advantages Reduces memory space needed for the program Execution time of the object program by factor

of 2

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COMPILER OPTIMIZATION(MACHINE-DEPENDENT)

Done while generating code

Optimized matrix First try Improved code

1 - START FINISH LSST

1,START1,FINISH1,M1

L 1,START1,FINISH M1-> R1

2 * RATE M1 LMST

1,RATE0,M11,M2

LMR

3,RATE2,1 M2->R3

3 * 2 RATE LMST

1,=F’2’0,RATE1,M3

LM

5,=F’2’4,RATE M3->R5

4

5 - M1 100 LSST

1,M11,=F’100’1,M5

S 1,=F’100’ M5->R1

6 * M3 M5 LMST

1,M30,M51,M6

LRMR

7,56,1

M6->M7

7 + M2 M6 LAST

1,M21,M61,M7

AR 3,7 M7->R3

8 = M7 COST LST

1,M71,COST

ST 3,COST

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COMPILER ASSEMBLY PHASE

Assembly phase is similar to pass 2 of assembler

Defines labels and resolves all references

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GENERAL MODEL OF COMPILER

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PHASES OF COMPILERLEXICAL PHASE

LEXICAL PHASE Task

To parse the source program into basic elements or tokens of the language

Build literal table and an identifier table Build a uniform symbol table

Database Source program(string of characters) Terminal tableSymbol Indicator precedence; yesProcedure no

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PHASES OF COMPILERLEXICAL PHASE

Database Literal table

Identifier table

Uniform symbol table

Literal

Base Scale Precision Other information

Address

31 Decimal Fixed 2

Name Data attribute Address WCM Filled by later phases

Table Index IDN 1(WCM)

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PHASES OF COMPILERLEXICAL PHASE

Algorithm Input string is separated into tokens by break

characters Tokens are checked with entries of terminal table If match then token is classified as terminal symbol

and an uniform symbol is created for type TRM If not matched then checked as identifier or literal If token start with alphabet and contain up to 30

more characters or underscores. then classified as identifier and an uniform symbol is created for type IDN

If not then checked as literal If not fit into one of these categories, it is an error.

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PHASES OF COMPILERSYNTAX PHASE

SYNTAX PHASE TASK

Recognize the major construct of the language and to call the appropriate action routines that will generate the intermediate from or matrix for these constructs.

Databases Uniform symbol table

stack

Table Index

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PHASES OF COMPILERSYNTAX PHASE

Algorithm Reduction are tested consecutively for match

between old top of stack field and the actual top of stack until match is found.

When matched is found, the action routines specified in the action field are executed in order from left to right.

When control returns to the syntax analyzer, it modifies the top of stack to agree with the new top of stack field.

Step 1 is then repeated starting with the reduction specified in the next reduction field.

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PHASES OF COMPILERInterpretation phase

Interpretation phase It is a collection of routines that are called

when a construct is recognized in the syntactic phase.

Routines are used to create an intermediate form of the source program and add information to the identifier table.

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PHASES OF COMPILERInterpretation phase

Databases Uniform symbol table Stack Identifier table

Matrix

Name

Base

Scale

Precision

Storage class

Array bound

Structure info

Literal value

Block info

other address

Uniform symbol

Uniform symbol

Uniform symbol

chaining

Operator Operand1 operand2

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PHASES OF COMPILERInterpretation phase

Temporary storage table

Algorithm It contains collection of individual action

routines that accomplish specific tasks when invoked by the syntax analysis phase

Routines are Do any necessary additional parsing Create new entries in the matrix or add data

attributes to the identifier operator and operands and insert them into the matrix.

MTXN Base

Scale Precision

Storage class

Other address

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PHASES OF COMPILERoptimization

Optimization(machine independent optimization)

Databases Matrix

Identifier table Literal table

Operator

Operand1

Operand2

Forward pointer

Backward pointer

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PHASES OF COMPILERoptimization

Algorithm Elimination of common sub expressions Common sub expression must be identical and

must be in same statement1. Place the matrix in a form so that common sub

expressions can be recognized2. Recognize two sub expressions as being equivalent3. Eliminate one of them4. Alter the rest of the matrix to reflect the elimination of

this entry5. Rescan the matrix for possible crated common sub

expressions repeat 1 to 5 until no change occur6. Eliminate from the temporary storage table any MTX

entries that are no longer needed

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PHASES OF COMPILERoptimization

ExampleSource code

B=AA=C*D*(D*C+B)

MATRIX BEFORE OPTIMIZATIONM line no

Operator

Operand1

Operand2

Backward

forward

M1 = B A 0 2M2 * C D 1 3M3 * D C 2 4M4 + M3 B 3 5M5 * M2 M4 4 6M6 = A M5 5 ?

MATRIX BEFORE OPTIMIZATIONM line no

Operator

Operand1

Operand2

Backward

forward

M1 = B A 0 2M2 * C D 1 3M3 * C D 2 4M4 + B M3 3 5M5 * M2 M4 4 6M6 = A M5 5 ?

MATRIX AFTER OPTIMIZATIONM line no

Operator

Operand1

Operand2

Backward

forward

M1 = B A 0 2M2 * C D 1 4M3 * C D 2 4M4 + B M2 2 5M5 * M2 M4 4 6M6 = A M5 5 ?

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PHASES OF COMPILERoptimization

Compile time compute Saves space and execution time for object

program Used for arithmetic computation within loopExampleA=2*276/92*B = (2*276/92)*B = 6*BBefore optimization

M1 * 2 276M2 / M1 92M3 * M2 BM4 = A M3

After optimizationM1M2M3 * 6 BM4 = A M3

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PHASES OF COMPILERoptimization

Boolean expression optimization Use properties of Boolean expression to

shorten their computation Example

If(a or b or c)If a is true the no need to check b and cSo eliminate that part

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PHASES OF COMPILERoptimization

Move invariant computations outside of loops If the computation within a loop depends on a

variable that does not change within that loop, the computation may be moved outside the loop Recognition of invariant computations

Example For(i=0;i<10;i++)

{a=10;i=i*2;}

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PHASES OF COMPILERoptimization

Discovering where to move the invariant computation Move the computation to a position directly

preceding the LOOP from which it comes Moving the invariant computation

Delete the invariant computation from its original position in the matrix and insert it into the appropriate place