C Language, optimisation

yomgu
#1 / 3

optimisation

Hello,

As an exercise, I was wondering if
I can make something faster then :

char lTest ;
while ( lTest = Fonction ( ) )
{
.....

Quote:

}

to be faster, I am trying to avoid
the comparison in the while.
could this work ?

for ( ; ; )
{
char lTest = Fonction ( ) ;

switch ( lTest )
{
case 0 : continue;
}

break ;

Quote:

}

if it doesn't, could you tell me
how to make the switch in a such way
that it won't be just a comparison,
but something faster, as an indexed array ?

thanks

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Sun, 27 Jun 2004 20:15:23 GMT

Joona I Palast
#2 / 3

optimisation

Quote:

> Hello,
> As an exercise, I was wondering if
> I can make something faster then :
> char lTest ;
> while ( lTest = Fonction ( ) )
> {
> .....
> }
> to be faster, I am trying to avoid
> the comparison in the while.
> could this work ?
> for ( ; ; )
> {
> char lTest = Fonction ( ) ;

> switch ( lTest )
> {
> case 0 : continue;
> }

> break ;
> }
> if it doesn't, could you tell me
> how to make the switch in a such way
> that it won't be just a comparison,
> but something faster, as an indexed array ?

The C standard doesn't say this or that about which language construct
is faster or slower than which.
As far as ISO C cares, the compiler could insert 100 ms delays between
all function calls UNLESS there's a statement 1; between them. As long
as the flow of logic and overall effect is the same, execution time
does not matter.

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Sun, 27 Jun 2004 20:43:58 GMT

- Kees van der Bent
#3 / 3

optimisation

Quote:

> Hello,

> As an exercise, I was wondering if
> I can make something faster then :

> char lTest ;
> while ( lTest = Fonction ( ) )
> {
> .....
> }

> to be faster, I am trying to avoid
> the comparison in the while.
> could this work ?

> for ( ; ; )
> {
> char lTest = Fonction ( ) ;

> switch ( lTest )
> {
> case 0 : continue;
> }

> break ;
> }

> if it doesn't, could you tell me
> how to make the switch in a such way
> that it won't be just a comparison,
> but something faster, as an indexed array ?

Don't bother, most modern compilers do optimizations like this
for you and often better than you can express yourself in C.

The matter of performance falls outside the scope of this group.
Still it't fun to play with things like this. Keep in mind that:
"Premature optimization is the root of all evil." Donald Knuth.

Good luck,

Kees

Sun, 27 Jun 2004 22:55:05 GMT