code optimization final

8/12/2019 Code Optimization FINAL

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Code Optimizations

Introduction Optimized code

Executes faster

efficient memory usage yielding better performance.

Compilers can be designed to provide codeoptimization.

Users should only focus on optimizations notprovided by the compiler such as choosing afaster andor less memory intensive algorithm.

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Introduction

Concerns !ith machine"independent codeoptimization #$"%$ rule& execution spends #$' time in %$'

of the code. It is moderately easy to achieve #$'

optimization. The rest %$' is very difficult.

Identification of the %$' of the code is not

possible for a compiler ( it is the )ob of a profiler. In general* loops are the hot"spots

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Compiler Code

Optimizations + Code optimizer sits bet!een the front

end and the code generator.

,or-s !ith intermediate code. Can do control flo! analysis.

Can do data flo! analysis.

oes transformations to improve the

intermediate code.

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Compiler Code

Optimizations Optimizations provided by a compiler

includes& Inlining small functions

Code hoisting

ead store elimination

Eliminating common sub"expressions

/oop unrolling /oop optimizations& Code motion* Induction

variable elimination* and 0eduction in strength.

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Introduction

Optimization can be done in almost all

phases of compilation.

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Front

end

Code

generator

Source

code

Inter.

code

target

code

Profile and

optimize

(user)

Loop, proc

calls, addr

calculation

improvement

(compiler)

Reg usage,

instruction

choice,

peephole opt

(compiler)


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Themes behind

Optimization Techniques +void redundancy& something already computed

need not be computed again

1maller code& less !or- for C2U* cache* and

memory3 /ess )umps&)umps interfere !ith code pre"fetch

Code locality& codes executed close together intime is generated close together in memory (

increase locality of reference Extract more information about code& 4ore

info ( better code generation

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Redundancy elimination

0edundancy elimination5 etermining that t!ocomputations are equivalent and eliminating one.

There are several types of redundancy elimination& 6alue numbering

+ssociates symbolic values to computations and identifies expressions thathave the same value

Common subexpression elimination Identifies expressions that have operands !ith the same name

ConstantCopy propagation Identifies variables that have constantcopy values and uses the

constantscopies in place of the variables.

2artial redundancy elimination Inserts computations in paths to convert partial redundancy to full

redundancy.

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Levels of optimization

Design level Implementation of this design !ill benefit

from a good choice of efficient algorithms

and the implementation of these algorithms!ill benefit from !riting good quality code.The architectural design of a systemover!helmingly affects its performance. Thechoice of algorithmaffects efficiency more

than any other item of the design and* sincethe choice of algorithm usually is the firstthing that must be decided* argumentsagainst early or 7premature optimization7may be hard to )ustify.

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http://en.wikipedia.org/wiki/Algorithmic_efficiencyhttp://en.wikipedia.org/wiki/Algorithmhttp://en.wikipedia.org/wiki/Algorithmhttp://en.wikipedia.org/wiki/Algorithmic_efficiency


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Sou ce code level +voiding poor quality coding can also

improve performance* by avoiding obvious

7slo!do!ns7. +fter that* ho!ever* some

optimizations are possible that actually

decrease maintainability. 1ome* but not

all* optimizations can no!adays be

performed by optimizing compilers.

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http://en.wikipedia.org/wiki/Optimizing_compilerhttp://en.wikipedia.org/wiki/Optimizing_compiler


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Build level 8et!een the source and compile level*

directivesand build flagscan be used to

tune performance options in the source

code and compiler respectively* such as

using preprocessordefines to disable

unneeded soft!are features* or optimizing

for specific processor models or hard!arecapabilities.

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http://en.wikipedia.org/wiki/Directive_(programming)http://en.wikipedia.org/wiki/Build_automationhttp://en.wikipedia.org/wiki/Build_automationhttp://en.wikipedia.org/wiki/Directive_(programming)


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Compile level Use of an optimizing compiler tends to

ensure that the executable program is

optimized at least as much as the

compiler can predict.

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ssembly level +t the lo!est level* !riting code using

an assembly language* designed for a

particular hard!are platform can produce

the most efficient and compact code if the

programmer ta-es advantage of the full

repertoire of machine instructions.

4any operating systems usedon embedded systems have been

traditionally !ritten in assembler code for

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Optimizing

Transformations Compile time evaluation Constant folding

Constant propagation

Common sub"expression elimination Code motion

1trength 0eduction

ead code elimination

Copy propagation

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Compile!Time "valuation

Expressions !hose values can be pre"

computed at the compilation time

T!o !ays& Constant folding

Constant propagation

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Constant #olding

Evaluation of an expression !ith constant

operands to replace the expression !ith

single value

Example&

area := (22.0/7.0) * r ** 2

area := 3.14286 * r ** 2

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Constant $ropagation

,hat does it mean9 :iven an assignment x 5 c* !here c is a constant

value. 1imilar to copy propagation Extra feature& It can analyze constant"value conditionals to

determine !hether a branch should be executed or not.

,hen is it performed9 Early in the optimization process.

,hat is the result9 1maller code ;e!er registers

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Constant $ropagation

0eplace a variable !ith constant !hich

has been assigned to it earlier.

Example&pi := 3.14286

area = pi * r ** 2

area = 3.14286 * r ** 2

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Common Sub!e%pression

"valuation Identify common sub"expression present in differentexpression* compute once* and use the result in allthe places. The definitionof the variables involved should not change

Example&

a := b * c temp := b * c

a := temp

x := b * c + 5 x := temp + 5

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Code &otion

4oving code from one part of the program

to other !ithout modifying the algorithm 0educe size of the program

0educe execution frequency of the code

sub)ected to movement

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Code &otion

1. Code Space reduction& 1imilar to commonsub"expression elimination but !ith theob)ective to reduce code size.

Example& Code hoisting

temp & 5 x a? b@ then if >a? b@ then

z &5 x


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Code &otion

2 Execution frequency reduction& reduce executionfrequency of partially available expressions>expressions available atleast in one path@

Example&if >a?b@ then if >a?b@ then

z 5 x < = temp 5 x < =

z 5 temp

else else

y 5 %$ y 5 %$

temp 5 x < =g 5 x < = g 5 tempB

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Code &otion

4ove expression out of a loop if the

evaluation does not change inside the

loop.

Example&

whie ( i ! (max"2) )

Equivalent to&

t := max " 2

whie ( i ! t )

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Code &otion

1afety of Code movement4ovement of an expression efrom a basic bloc- bito another bloc- bj* is safe if it does not introduceany ne! occurrence of ealong any path.

Example& Unsafe code movementtemp 5 x < =

if >a?b@ then if >a?b@ then

z 5 x < = z 5 temp

else else

y 5 %$ y 5 %$

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Strength Reduction

0eplacement of an operator !ith a less costly one.

Example&temp 5 DB

for i5% to %$ do for i5% to %$ do

x 5 i < D x 5 temp

temp 5 temp A D

end end

Typical cases of strength reduction occurs in addresscalculation of array references.

+pplies to integer expressions involving inductionvariables >loop optimization@

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'ead Code "limination

ead Code are portion of the program !hich!ill not be executed in any path of theprogram.

Can be removed Examples&

Fo control flo!s into a basic bloc-

+ variable is dead at a point "G its value is not

used any!here in the program +n assignment is dead "G assignment assigns a

value to a dead variable

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'ead Code "limination

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!"amples#

#$%&':=0

i (#$%&') prit Can $e

eliminated


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Copy $ropagation

,hat does it mean9 :iven an assignment x 5 y* replace later uses of x

!ith uses of y* provided there are no intervening

assignments to x or y.

,hen is it performed9 +t any level* but usually early in the optimization

process.

,hat is the result9 1maller code

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Copy $ropagation

:= are called copy statements or copies Use of for * !henever possible after copy

statement

Example&

xHi 5 aB xHi 5 aB

sum 5 xHi A aB sum 5 a A aB

4ay not appear to be code improvement* butopens up scope for other optimizations.

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code optimization final

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