Optimization beyond the Module Border

[Don Stewart]

bbr:
> > I suspect that if all modules are compiled -O0, then you recompile one
> > module with -O2, high up in the dependency graph (i.e. it depends on
> > many lower-level modules), plus all things that in turn depend on it
> > (--make), you will not get the good performance you expect.  None of the
> > lower-level functions will have exported inlinings or fusion rules into
> > the interface file.  _All_ modules must be recompiled with -O2,
> > especially the bottom of the dependency chain, to get the best benefit
> > from optimisation.
> > 
> > Regards,
> >     Malcolm
> 
> I am very sorry, I think what Malcolm describes might be exactly what
> had happened. Now that I tried to blow up the example from 0.122 msec to
> get a more significant result, I can't reproduce the effect. Funny thing
> though, as I was pretty keen on doing a thorough job as it was all about
> measuring the quality of the work of the previous fortnight. Now I find
> that - after all - I did a much better job than it seemed yesterday :o)
> 
> So there may be two (minor) issues left if you would be interested.
> Firstly, about profiling in connection with optimization. When I
> compiled things with -O2 AND -prof -auto-all no profile would be
> written. Now you might think that having both at once is a silly idea,
> the side effects of profiling might be the first to be "optimized" away.

You almost always want to profile with full optimisations on.
Otherwise its not even close to measuring the kind of code you're
actually running.

> But I think it was not so silly after all as I had introduced a lot of
> overhead into my programs which I was pretty sure could be optimized
> away. Hence, I was not at all interested in the unoptimized profile. And
> I think it is not so unusual to want to improve only those things that
> the compiler cannot improve by itself. Couldn't the profiling things be
> added AFTER all optimization was done?
> 
> And then, secondly, about the connection of optimization with side
> effects. I had programs behave differently when compiling them all in
> one go or module-wise. (And if I am not able to reproduce that effect as
> well I will do a little merry dance!) Is this also interesting? Might it
> be connected with what Don mentioned about the stream fusion package?
> (Although I cannot remember mentioning any side effects in Duncan's talk
> in Freiburg.)

No, I can't think of any issue there.
  
-- Don