[fpc-devel] WriteBarrier

Martin fpc at mfriebe.de
Mon Aug 24 02:11:43 CEST 2020

Ok, I got a bit further.

Assuming the following
  (and not counting on Interlocked... doing any Barriers itself)

Thread 1
   Foo := val; // normal assign
   WriteBarrier; // make sure above write to Foo is executed, before the 
next write to flag
   InterLockedIncrement(Flag);  // needed for the lock, if others do 
write access too.

Thread 2
   If InterLockedExchangeAdd(Flag, 0) > 0 then begin
      ReadBarrier; // make sure below read is executed only after the 
read of the flag
      myval := Foo;

That should get the correct value of Foo.
Even if run a million times, and in all of this the read of Flag 
eventually happens exactly one cpu cycle after it was set.

Now the Examples I've seen all are showing cases where the re-orderable 
assignment are in close proximity to each other.
On the other hand compiler and CPU are getting better and better. So 
there is nothing that would in theory stop them to swap read/write order 
over any amount of statements....

So even if (in Thread 1) I returned from a subroutine (which could be 
inlined for all I (don't) know), done some other work, and only then set 
the flag.
If I want to be sure, any value that was written in the subroutine is 
written before the flag gets set, then I should do a WriteBarrier?
Or is that paranoid?

Thanks again

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