Author: Eugene Nalimov
Date: 10:56:45 09/21/01
Go up one level in this thread
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
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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