Author: Robert Hyatt
Date: 06:55:02 09/21/01
Go up one level in this thread
On September 21, 2001 at 00:01:42, Antonio Dieguez wrote:
>
>hi Dann
>
>if (turno==0)
>{
> if (eval0id1[indice]==p->idHashseg && eval0id2[indice]==p->idHashseg2)
> { return cualeval0[indice]; }
>}
>else
>{
> if (eval1id1[indice]==p->idHashseg && eval1id2[indice]==p->idHashseg2)
> { return cualeval1[indice]; }
>}
>
>you said to me the other day that putting &s there was faster in the profiling
>you did.
>That operations seemed expensive, I supose because are big arrays. And usually
>if one is false(wich I guess happen at least 80% of the time) then both are
>false. So I was puzzled why the & is faster there.
It depends on the cost of accessing both halves of the &/&& operation. &
needs both, && might bail out on the first. But then && will have two
jumps, while & will have one.
In your case, the branch misprediction seems to overwhelm the cost of
fetching both operands for the "&" operation.
>
>be well.
>
>
>>>can you tell me?
>>
>>Depends on a zillion things. Sometimes, you cannot make that translation.
>>
>>For instance, if you want to substitute & for &&, then you must have both
>>operands be boolean (IOW: _ONLY_ take on the values 0 and/or 1).
>>
>>This is not a valid translation:
>>int a = 1;
>>int b = 2;
>>
>>if ((a && b) == (a & b) puts("My compiler is broken);
>>
>>
>>If the cost of evaluating the operands is very high, then it may be better to
>>use &&.
>>
>>Example:
>>
>>if (foo() && bar()) then foobar();
>>
>>Suppose that foo() is fast, and bar() is really slow. Further, foo() is 0 most
>>of the time. Then you would rather have the short circuit evaluation and
>>branch.
>>
>>Missed branch predictions are expensive on newer chips, but it is not always an
>>easy thing to see when one method is faster than the other.
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