Author: Uri Blass
Date: 16:28:01 01/01/03
Go up one level in this thread
On January 01, 2003 at 19:02:03, Bruce Moreland wrote:
>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 () {}
>>
>>-------
>>I know for instance that:
>>
>>ptr=&R[i];
>>if((*ptr==3)||(*ptr==7)) {;}
>>
>>is faster then:
>>
>>if((R[i]==3)||(R[i]==7)) {;}
>
>How do you know this?
>
>It is true or not true, depending upon the local context. If you are using "R"
>and "i" together, the compiler can figure out what you are doing sometimes. If
>this is one of those times, then doing weird pointer stuff is just confusing to
>everyone -- possibly including the compiler.
>
>Also, why do you care if your code goes fast? This seems like a stupid
>question, but often, it is the *best* question, and the answer is, "I don't".
>
>And even if you find something that needs to go fast, the fact that you ask the
>question, and need to know the real answer, means that you need to go to the
>root, which is in this case the generated assembly and profiler output.
>
>bruce
I agree.
I think that in cases that you care about speed then it is more important to
care first about speed of the logic and not about speed of C.
It means that if you have a condition A&&B and it is usually wrong because of B
then it may be better to change it to B&&A.
It is something that the compiler cannot do by itself because the compiler is
not going to take the risk that B can cause a crash when A is false and the
compiler has no way to know that B cannot cause a crash.
Uri
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