Re: Poor performance with GNAT-GPL-2017

[Robert Eachus <rieachus@comcast.net>]

On Saturday, June 10, 2017 at 7:23:34 AM UTC-4, Charly wrote:
 
> Any ideas??
 
Three ideas really.  First there may be some change in the tasking and rendezvous parts of GNAT-GPL that results in twice as many scheduling actions (or more) and thus the slowdown.  Most probable.

Second there is no guarantee that the tasking actions will be identical between the two instances.  Worst case, you have one thread which is the high pole in the tent starting later.  The "easy" way to check this is to take out the tasking and make the program single-threaded with identical program behavior.

You could run some tasking tests which I hope have been around forever (ACES), to see what changed in tasking timings.

Finally, and highly unlikely given the manitude of the timing change, the change in versions will change the order (to some extent) in which alternatives are tested.  These changes can result in the program run time being different because more instances are checked before determining that there is no valid way to continue from that point.  You could check this by putting in an instance counter that is incremented each time a tile is placed.  (Keeping separate counters for each task is an obvious optimization.)

It would be very interesting if the number of instances checked varied, either from run-to-run or compiler version to compiler version.