# 3.5 Classical optimizations  (Page 2/4)

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A programmer can sometimes improve performance by making the compiler aware of the constant values in your application. For example, in the following code segment:

`X = X * Y`

the compiler may generate quite different runtime code if it knew that `Y` was 0, 1, 2, or 175.32. If it does not know the value for `Y` , it must generate the most conservative (not necessarily the fastest) code sequence. A programmer can communicate these values through the use of the `PARAMETER` statement in FORTRAN. By the use of a parameter statement, the compiler knows the values for these constants at runtime. Another example we have seen is:

``` DO I = 1,10000 DO J=1,IDIM..... ENDDOENDDO```

After looking at the code, it’s clear that `IDIM` was either 1, 2, or 3, depending on the data set in use. Clearly if the compiler knew that `IDIM` was 1, it could generate much simpler and faster code.

Programs often contain sections of dead code that have no effect on the answers and can be removed. Occasionally, dead code is written into the program by the author, but a more common source is the compiler itself; many optimizations produce dead code that needs to be swept up afterwards.

Dead code comes in two types:

• Instructions that are unreachable
• Instructions that produce results that are never used

You can easily write some unreachable code into a program by directing the flow of control around it — permanently. If the compiler can tell it’s unreachable, it will eliminate it. For example, it’s impossible to reach the statement `I = 4` in this program:

``` PROGRAM MAIN I = 2WRITE (*,*) I STOPI = 4 WRITE (*,*) IEND```

The compiler throws out everything after the `STOP` statement and probably gives you a warning. Unreachable code produced by the compiler during optimization will be quietly whisked away.

Computations with local variables can produce results that are never used. By analyzing a variable’s definitions and uses, the compiler can see whether any other part of the routine references it. Of course the compiler can’t tell the ultimate fate of variables that are passed between routines, external or common, so those computations are always kept (as long as they are reachable). If a compiler does sufficient interprocedural analysis, it can even optimize variables across routine boundaries. Interprocedural analysis can be the bane of benchmark codes trying to time a computation without using the results of the computation. In the following program, computations involving `k` contribute nothing to the final answer and are good candidates for dead code elimination:

``` main () {int i,k; i = k = 1;i += 1; k += 2;printf ("%d\n",i); }```

Dead code elimination has often produced some amazing benchmark results from poorly written benchmarks. See [link] for an example of this type of code.

## Strength reduction

Operations or expressions have time costs associated with them. Sometimes it’s possible to replace a more expensive calculation with a cheaper one. We call this strength reduction . The following code fragment contains two expensive operations:

do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
it is a goid question and i want to know the answer as well
Maciej
Abigail
Do somebody tell me a best nano engineering book for beginners?
what is fullerene does it is used to make bukky balls
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
for screen printed electrodes ?
SUYASH
What is lattice structure?
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
what's the easiest and fastest way to the synthesize AgNP?
China
Cied
types of nano material
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
what is nano technology
what is system testing?
preparation of nanomaterial
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
what is system testing
what is the application of nanotechnology?
Stotaw
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
Azam
anybody can imagine what will be happen after 100 years from now in nano tech world
Prasenjit
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
Azam
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
Prasenjit
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
Damian
silver nanoparticles could handle the job?
Damian
not now but maybe in future only AgNP maybe any other nanomaterials
Azam
Hello
Uday
I'm interested in Nanotube
Uday
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
Prasenjit
how did you get the value of 2000N.What calculations are needed to arrive at it
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