Author: Robert Hyatt
Date: 19:07:07 01/02/03
Go up one level in this thread
On January 01, 2003 at 11:50:58, Lieven Clarisse wrote:
>I was wondering if there is a good book about how to write efficient C code. I
>am not talking about algorithms, but the way *how* to write things, eg
>
>which is faster :
>do {} while() or while () {}
What is required in the program? The two statements are _not_ identical.
The first one has a single jump at the bottom. The second has a jump at the
top to skip the loop if the condition is initially false, and a jump at the
bottom that is always taken, although it could be optimized so that you put
the test outside the loop and make both look _exactly_ the same after the
initial test for the while has been done...
It's simply not important enough to worry about.
>
>-------
>I know for instance that:
>
>ptr=&R[i];
>if((*ptr==3)||(*ptr==7)) {;}
>
>is faster then:
>
>if((R[i]==3)||(R[i]==7)) {;}
I don't see any reason why that would be true. When the C compiler sees
r[i] it simply references *(r+i) anyway. If I is already handy in a register,
there is a clever trick in the CPUs addressing that will scale I, and add it to
R with no penalty in extra instructions, and I is _still_ in a register for
use on other things.
>
>Is there a good book that reviews all those kinds of tricks?
>
>regards,
>
>Lieven
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