Re: += in ada

["Robert I. Eachus" <rieachus@comcast.net>]

Hyman Rosen wrote:

> This statement means that we get to talk about the Ariane 5 again.
> If you recall, this involved careful case-by-case analysis of every
> aritmetic statement in a module to determine which ones could have
> their range checks disabled, in order to free up a few more machine
> cycles. No "delegation to the compiler" there.

No, but unless you missed the last discussion of Ariane 5, you would 
know that the problem with Ariane 5 was not spending enough time on 
things other than counting machine cycles.  And the machine cycle 
counting, which are never wrong in a hard-real-time system was done 
right in the Ariane 4 context where it occurred.  Ariene 4 is/was a 
great success and is still being used today.  There have been several 
Ariane 5 failures since the first, and each of them has eventually been 
traced back to the Ariane 5 development team skimping on requirements 
analysis at the highest levels.

> We can also bring up protected types. Apparently the real-time
> community felt that leaving efficiency to the compiler and using
> rendezvous was not acceptable.

If you don't understand protected types, I'm not going to try to explain 
it all here.  But to vastly shorten the discussion, some Ada (83) 
compilers supported special optimizations for what were called things 
like monitor tasks (Verdix).  Basically these were tasks with a special 
structure that allowed the compiler to produce more efficient code.  All 
of the code ran in the stacks of calling tasks, none in the monitor 
task, so it didn't need some resources, including a task control block, 
or any visibility to the scheduler.  Protected types are basically a 
cleaner notation for what many programs did with monitor tasks.

I once said that tasks in Ada were like rabbits, once you had two or 
more, if you didn't kept them segregated, you started finding baby tasks 
everywhere.  Protected objects and protected types make this distinction 
clean.  There are tasks which contain threads of control, and protected 
objects that are used for syncronization and communication between them. 
  If you find the name funny, maybe now you understand why it is so 
perfect. ;-)


-- 
                                                     Robert I. Eachus

"Quality is the Buddha. Quality is scientific reality. Quality is the 
goal of Art. It remains to work these concepts into a practical, 
down-to-earth context, and for this there is nothing more practical or 
down-to-earth than what I have been talking about all along...the repair 
of an old motorcycle."  -- from Zen and the Art of Motorcycle 
Maintenance by Robert Pirsig