C optimisations

Using array indices

 

If you wished to set a variable to a particular character, depending upon the value of something, you might do this : 

 

switch ( queue ) {

case 0 :   letter = 'W';

   break;

case 1 :   letter = 'S';

   break;

case 2 :   letter = 'U';

   break;

}

or maybe 

if ( queue == 0 )

  letter = 'W';

else if ( queue == 1 )

  letter = 'S';

else

  letter = 'U';

A neater ( and quicker ) method is to simply use the value as an index into a character array, eg. 

static char *classes="WSU";

 

letter = classes[queue];

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Aliases

 

Consider the following: 

void func1( int *data )

{

    int i;

 

    for(i=0; i<10;>

   {

           somefunc2( *data, i);

    }

}

Even though "*data" may never change, the compiler does not know that somefunc2() did not alter it, and so the program must read it from memory each time it is used - it may be an alias for some other variable that is altered elsewhere. If you know it won't be altered, you could code it like this instead: 

void func1( int *data )

{

    int i;

    int localdata;

 

    localdata = *data;

    for(i=0; i<10;>

    {

           somefunc2( localdata, i);

    }

}

This gives the compiler better opportunity for optimisation. 

 

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Integers

 

Use unsigned ints instead of ints if you know the value will never be negative. Some processors can handle unsigned integer arithmetic considerably faster than signed ( this is also good practise, and helps make for self-documenting code). 

So the best declaration for an int variable in a tight loop would be 

 

register unsigned int   var_name;

 

(although it is not guaranteed that the compiler will take any notice of "register", and "unsigned" may make no difference to the processor.) 

 

Remember, integer arithmetic is much faster than floating-point arithmetic, as it can be usually be done directly by the processor, rather than relying on external FPUs or floating point maths libraries. If you only need to be accurate to two decimal places (e.g. in a simple accounting package), scale everything up by 100, and convert it back to floating point as late as possible.

Loop jamming

Never use two loops where one will suffice:
for(i=0; i<100; i++)
{
stuff();
}

for(i=0; i<100; i++)
{
morestuff();
}
It would be better to do:
for(i=0; i<100; i++)
{
stuff();
morestuff();
}
Note, however, that if you do a lot of work in the loop, it might not fit into your processor's instruction cache. In this case, two separate loops may actually be faster as each one can run completely in the cache.

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Faster for() loops

Simple but effective.
Ordinarily, you would code a simple for() loop like this:
for( i=0; i<10; i++){ ... }
i loops through the values 0,1,2,3,4,5,6,7,8,9
If you don't care about the order of the loop counter, you can do this instead:

for( i=10; i--; ) { ... }
Using this code, i loops through the values 9,8,7,6,5,4,3,2,1,0, and the loop should be faster.
This works because it is quicker to process "i--" as the test condition, which says "is i non-zero? If so, decrement it and continue.".
For the original code, the processor has to calculate "subtract i from 10. Is the result non-zero? if so, increment i and continue.". In tight loops, this make a considerable difference.
The syntax is a little strange, put is perfectly legal. The third statement in the loop is optional (an infinite loop would be written as "for( ; ; )" ). The same effect could also be gained by coding:

for(i=10; i; i--){}
or (to expand it further)
for(i=10; i!=0; i--){}
The only things you have to be careful of are remembering that the loop stops at 0 (so if you wanted to loop from 50-80, this wouldn't work), and the loop counter goes backwards.It's easy to get caught out if your code relies on an ascending loop counter.

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switch() instead of if...else...

For large decisions involving if...else...else..., like this:
if( val == 1)
dostuff1();
else if (val == 2)
dostuff2();
else if (val == 3)
dostuff3();
it may be faster to use a switch:
switch( val )
{
case 1: dostuff1(); break;

case 2: dostuff2(); break;

case 3: dostuff3(); break;
}
In the if() statement, if the last case is required, all the previous ones will be tested first. The switch lets us cut out this extra work. If you have to use a big if..else.. statement, test the most likely cases first.