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

[J. Gareth Moreton]

That seems fair. Thank you for your 
answer. I'm still learning about the 
nuances of branch prediction and the like.

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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