Author: Matt Taylor
Date: 19:18:31 01/01/03
Go up one level in this thread
On January 01, 2003 at 20:50:32, Bruce Moreland wrote:
>On January 01, 2003 at 19:28:01, Uri Blass wrote:
>
>>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.
>
>C is defined such that if you have (A && B), A will be evaluated first always.
>
>The case where you get undefined execution order is this one:
>
> func(A(), B());
>
>The order in which A() and B() are called is undefined.
>
>bruce
In a boolean condition like that, yes, C defines A to be evaluated first.
However, consider the following conditional:
x > 10 && !done
In this case, the compiler CAN swap the order of evaluation because there are no
side-effects. The compiler won't swap unless you profile, and even then it might
not swap the order. Order of evaluation is a trivial optimization, though.
-Matt
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