Hi,
I was curious if there are any gcc compiler optimizations that can
improve this code:
void foo10( )
{
for ( int i = 0; i < 10; ++i )
{
[...]
if( i == 15 ) { [BLOCK1] }
}
}
void foo100( )
{
for ( int i = 0; i < 100; ++i )
{
[...]
if( i == 15 ) { [BLOCK2] }
}
}
int main( void )
{
foo10( );
foo100( );
return 0;
}
1) For the function foo10:
The if-block following "if( i == 15 )" will be never executed since
'i' will never become 15 here. So, this entire block could be removed
without changing the semantics. This would improve the program execution
since the if-condition does not need to be evaluated in each loop iteration.
Can this code transformation be automatically performed by a compiler? If so,
which techniques/analyses and optimizations must be applied? Would gcc simplify
this loop?
2) For the function foo100:
This code is not optimal as well. The if-condition is just once met but
has to be evaluated for all of the 100 loop iterations. An idea
I had in mind was to split the loop in two parts:
for ( int i = 0; i <= 15; ++i ) {...}
BLOCK2
for ( int 16 = 0; i < 100; ++i ) {...}
So, here the evaluation of "i==15" is not required any more and we save
100 comparisions. Is this a good idea and always applicable? Or are there
better compiler optimzations?
Thank you very much for your help.
Regards,
Chris
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