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Re: [vox-tech] loop efficiency and testing against zero.
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Re: [vox-tech] loop efficiency and testing against zero.

On Fri, Jun 16, 2006 at 12:52:17PM -0700, Tim Riley wrote:
> Peter Jay Salzman wrote:
> > I've read somewhere that a loop that runs from 0 to some number should be
> > written to go in reverse order, e.g. instead of:
> > 
> >    for ( int i = 0;  i < 10;  ++i )
> > 
> > we should write:
> > 
> >    for ( int i = 9;  i >= 0;  --i )
> I would think this:
> 	for( int i = MAXINT; i; --i );
> is faster than this:
> 	for( int i = 0; i < MAXINT; ++i );
> However, on my machine they both took the
> same time -- 3.24 seconds.
> I would think the test of 'i'
> would be a single instruction and the test
> of 'i < MAXINT' would take multiple
> instructions. But surprise!

No, they are both exactly identical in meaning, and thus representable
with exactly the same instructions.

I don't know how GCC's optimizations work, but it's the compiler's job
to handle rote optimizations like this; I imagine that if you enable
-O3, GCC might (should) notice that you're not actually using the value
of i, and optimize it to the decrement operation if appropriate.

Optimization should always happen after implementation.

Micah J. Cowan
Programmer, musician, typesetting enthusiast, gamer...
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