Author: Eugene Nalimov
Date: 17:42:34 09/21/01
Go up one level in this thread
GCC uses CMOVs. MSVC does not. So you can see yourself that CMOVs only hurts :-)
Eugene
On September 21, 2001 at 14:58:27, Robert Hyatt wrote:
>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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