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

Florian Klämpfl florian at freepascal.org
Tue May 29 22:27:05 CEST 2018

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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