[fpc-devel] x86-64 compilation of while loops

[Florian Klämpfl]

Am 29.05.2018 um 21:48 schrieb J. Gareth Moreton:
> That seems fair. Thank you for your 
> answer. I'm still learning about the 
> nuances of branch prediction and the like.

Do be honest, I never investigated this particular case in detail, but years I ago we just decided based on the reasons
I mentioned.

> 
> On Tue 29/05/18 21:27 , Florian Klämpfl 
> florian at freepascal.org sent:
>> Am 28.05.2018 um 01:55 schrieb J. Gareth 
> Moreton:
>>
>>>
>>
>>> In this case, the ".balign" hint adds
>> 2 bytes to pad the loop.  However, my 
> question is this... why
>>> does it immediately jump to the end of 
> the loop
>> to check the condition (which is very 
> likely to be
>>> true on the first iteration), only to 
> jump to
>> the beginning again? 
>>
>>
>> Several reasons:
>>
>> - it is not that easy to find a 
> heuristics when the condition should be
>> checked twice
>> - two conditional jumps waste an extra 
> branch prediction table entry
>>
>> - bigger code, higher cache usage
>>
>>
>>
>> Weigthing these reasons against the 
> little overhead of an extra
>> unconditional jump, we decided to
>> use the currently used code generation 
> strategy.
>>
>>
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