Author: Heiner Marxen
Date: 07:14:50 02/27/03
Go up one level in this thread
On February 27, 2003 at 05:11:18, Tony Werten wrote:
>On February 26, 2003 at 14:58:52, Heiner Marxen wrote:
>
>>On February 26, 2003 at 14:04:16, Uri Blass wrote:
>>
>>>I had the following code in my program
>>>
>>>do
>>>{
>>>...
>>>if (info[target]==EMPTA)
>>> target+=8;
>>>else
>>> target=64;
>>>}
>>>while (target<64)
>>>
>>>I found that changing it to else break did not do my program faster.
>>
>>Assuming that the variable "target" is not read after this loop (the value
>>is "dead"), I would assume that any decent optimizer will produce code
>>as if there had been a "break" instead of "target=64".
>>
>>Obviously, your compiler does so.
>>You may even inspect the generated assembler for both versions.
>>When they are just equal you need not even understand the assembly.
>>
>>>I think that with break I have branches and without break the compiler can
>>>translate it to
>>>
>>>target=((info[target]==EMPTA)?target+8:64);
>>>
>>>My questions are
>>>1)How can I write the code as branchless code without target=64 that is a waste
>>>of time?
>>
>>I expect it to be optimized away.
>>Hence it is not really a waste of time.
>>
>>I'd suggest you choose the version that best fits your style, i.e. which
>>is most readable for you. I would prefer to terminate a loop with a break
>>instead of manipulating the loop variable to do so in the next iteration.
>>YMMV.
>>
>>
>>>2)I understood from previous discussion that for some reason ? is considered
>>>branchless code.
>>>What is the reason for it.
>>
>>I do not consider ?: to be branchless.
>>In most cases it _does_ generate branches.
>>
>>>It seems to me that using ? is the same as using if.
>>
>>Yes, I agree.
>>
>>>If it is not the case then what is the difference?
>>
>>The ?: is an expression.
>>"if" is a statement.
>>
>>Using ?: within a larger expression can avoid repeating the surrounding
>>part in the then- and else-part of the equivalent if/else code.
>>Sometimes ?: is just more readable, since it is more compact.
>>
>> x = (a<b) ? a : b;
>>
>>vs
>>
>> if (a<b) x=a; else x=b;
>>
>>or even
>>
>> if (a<b) {
>> x=a;
>> }else {
>> x=b;
>> }
>>
>>I clearly prefer the first version. (Using a min macro may be even better).
>
>You should prefer
>
>x=b;
>if a<b x=a;
Actually, I don´t.
If this is really more efficient, I want the optimizer to do this
mini-optimization. The optimizer has to consider what is the best way
to do it on the local hardware.
In my source I prefer the most readable version.
Cheers,
Heiner
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