On Fri, Aug 7, 2009 at 9:24 PM, bala chandar<[email protected]> wrote:
> I am Balachandar.Final year B.Tech - IT student.I want to develop
> a c language code optimizer.
Good choice! Code optimization is a fascinating topic. I did this
during my college days too :-)
I must warn you that compiler design is a difficult topic which needs
lots of time and guidance. You could spend many years on this subject.
> I know the compiler's can optimize the code but its not effective always.
There's a vast body of theory behind compiler optimization -- trees,
control graphs, basic blocks, code invariants, dead code elimination,
loop optimizations, array traversal optimizations, intermediate code
generation, peep-hole optimization, machine specific optimization,
etc. which need to be carefully understood before attempting to build
a code optimizer.
See the internal implementation docs for GCC for more:
http://gcc.gnu.org/onlinedocs/gccint/
I also suggest that you read the Dragon Book (new cover looks nice):
http://en.wikipedia.org/wiki/Dragon_Book_(computer_science)
> I did a small experiment with the
> below code and got a positive result for me.I want to highlight the
> codes which can be optimized to get quicker execution.
...
> If i execute this each time ( j/100 ) is calculated and takes
> more amount of time.
...
> Here j /100 is calculated only once and the execution time
> reduces automatically.So i want to highlight these kind of places in a
> code.
In optimization parlance, this technique is called "constant folding".
http://en.wikipedia.org/wiki/Constant_folding
If you use
const double pi = 22/7
...
double area = 2 * pi * r
in your code, the compiler will calculate 22/7 at compile time itself
and just substitute the values for all the constants. The second line
will be optimized to:
double area = XX * r
XX is whatever the output of 2 * 22 / 7 for that specific 16/32/64 bit ISA
Compiler optimization must be used with care, as it may introduce
subtle bugs. Here is an example:
You may be tempted to optimize the below code:
for (i = 0; i < 1000; i++)
c = i;
to just
i = 1000
c = 999
But if "c" refers to the memory mapped address of a serial port, then
you've obviously left out a lot of data.
Compiler optimization also has problems when you are debugging. See
the man page for gcc's "-g" option. Best to disable optimization
entirely while debugging "gcc -O0 -g".
While code optimization is a fairly advanced topic the moment, the
constant advances in programming languages and paradigms offers
an ever increasing opportunities and areas of study for the keen student.
Enjoy the ride!
- Raja
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