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Once the intermediate language is broken into basic blocks, there are a number of optimizations that can be performed on the code in these blocks. Some optimizations are very simple and affect a few tuples within a basic block. Other optimizations move code from one basic block to another without altering the program results. For example, it is often valuable to move a computation from the body of a loop to the code immediately preceding the loop.

In this section, we are going to list classical optimizations by name and tell you what they are for. We’re not suggesting that you make the changes; most compilers since the mid-1980s automatically perform these optimizations at all but their lowest optimization level. As we said at the start of the chapter, if you understand what the compiler can (and can’t) do, you will become a better programmer because you will be able to play to the compiler’s strengths.

Copy propagation

To start, let’s look at a technique for untangling calculations. Take a look at the following segment of code: notice the two computations involving X .


X = Y Z = 1.0 + X

As written, the second statement requires the results of the first before it can proceed — you need X to calculate Z . Unnecessary dependencies could translate into a delay at runtime. This code is an example of a flow dependence. I describe dependencies in detail in [link] . With a little bit of rearrangement we can make the second statement independent of the first, by propagating a copy of Y . The new calculation for Z uses the value of Y directly:


X = Y Z = 1.0 + Y

Notice that we left the first statement, X=Y , intact. You may ask, “Why keep it?” The problem is that we can’t tell whether the value of X is needed elsewhere. That is something for another analysis to decide. If it turns out that no other statement needs the new value of X , the assignment is eliminated later by dead code removal.

Constant folding

A clever compiler can find constants throughout your program. Some of these are “obvious” constants like those defined in parameter statements. Others are less obvious, such as local variables that are never redefined. When you combine them in a calculation, you get a constant expression . The little program below has two constants, I and K :


PROGRAM MAIN INTEGER I,KPARAMETER (I = 100) K = 200J = I + K END

Because I and K are constant individually, the combination I+K is constant, which means that J is a constant too. The compiler reduces constant expressions like I+K into constants with a technique called constant folding .

How does constant folding work? You can see that it is possible to examine every path along which a given variable could be defined en route to a particular basic block. If you discover that all paths lead back to the same value, that is a constant; you can replace all references to that variable with that constant. This replacement has a ripple-through effect. If the compiler finds itself looking at an expression that is made up solely of constants, it can evaluate the expression at compile time and replace it with a constant. After several iterations, the compiler will have located most of the expressions that are candidates for constant folding.

Questions & Answers

what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
Damian
yes that's correct
Professor
I think
Professor
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
research.net
kanaga
sciencedirect big data base
Ernesto
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Source:  OpenStax, High performance computing. OpenStax CNX. Aug 25, 2010 Download for free at http://cnx.org/content/col11136/1.5
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