Author: Robert Hyatt
Date: 11:58:27 09/21/01
Go up one level in this thread
On September 21, 2001 at 13:56:45, Eugene Nalimov wrote:
>I believe explanation is even simpler. Of course it all depend on the compiler,
>but probably for the code
> (x == y) & (u == w)
>it generated something like
> t1 = 1
> if (x == y) goto L1
> t1 = 0
>L1:
> t2 = 1
> if (u == w) goto L2
> t2 = 0
>L2:
> t3 = t1 & t2
> ...
>So you have *both* mispredicted branches and memory accesses.
>
>Eugene
>
no CMOV's???
>
>On September 21, 2001 at 09:55:02, Robert Hyatt wrote:
>
>>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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