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Re: IPA (was: Re: [tree-profiling-branch PATCH] Function cloning + IPCP extension (RESUBMISSION))
> On Sunday 10 October 2004 22:59, Jan Hubicka wrote:
> > While I personally agree with the scheme you outlined (and also
> > described it in the GCC Summit paper), the requirement of not loading
> > whole program into memory at once (so relying only on global date while
> > doing IPA and performing the actual transformations later) actually
> > brings some stress on the implementation of IPA optimizers (ie
> > optimizers done after inlining needs to deal with the clones and it is
> > dificult to iterate optimizations like we commonly do for local
> > optimizations).
> In my mental model, inlining and cloning would be one of the
> last optimizations to do on the call graph. Wouldn't it be a
> simple matter of adding the clones to the call graph and moving
> on as if it is a normal compilation? At least you would not
> actually do any IPA on clones and functions after inlining.
> That would require re-analyzing the function to get the global
> data right, wouldn't it?
The idea is that inlining (as any code specialization transformation) makes analysis
more precise (ie once you constant propagate operand into function
argument it might become very trivial), so one approach how to cope with
this is to simply re-throw the expanded CFG after inlining to the
analysers again to gain extra precision.
One posibility is to update the local information once you decide to do
the cloning (ie to decide in informed way that function X is good to
clone when it's argument is constant, you need to know anyway that it
will simplify after you do so)
> > In fact Kenneth already suggested in private mail to
> > not use this approach and instead load everything in the memory at once
> > and do kind of iteration on the IPA.
> Can you show some pseudo-algorithm of how this would work?
I believe Kenny's idea is to make early optimization, IPA,
clonning/inlining, IPA again on already inlined and compiled functions
and finally compile the function.
> > I would like to hear some opinions
> > here.
> Hmm, obviously it depends on how much we can improve on memory
> efficiency, there is obvioulsy a lot of room for improvements
> there if we could move to saner data structures. Everyone knows
> we're a *little* memory wasteful right now ;-)
My major concern is that even if we are not little wastefull, we will
have scalability problems for very large compilation units that are
going to be more common..
> But even then, I wonder if you could take the source of a very
> large code base (say, GCC itself, or the linux kernel), and do
> link time whole program optimizations on it if you have to keep
> everything in memory.
GCC or Linux is still little. Think of Open Office in 10 years ;)
> And iterating (or perhaps worklist based?) IPA doesn't sound very
> attractive either. I suppose you could offer it as an option at
> some very high optimization level, but does it really pay off in
> general to do IPA so aggressively? Is the benefit large enough to
> justify a complicated and probably compile time expensive framework
> like that?
worklist is probably not good idea. I had in mind simply re-running the
passes, iteration was probably not best way to call it ;)
> In any case, we'll be stuck on doing everything in-memory at first
> anyway, so we'll have the opportunity to experiment a bit, and that
> will be interesting. Of course, making pre-inlining optimizations
> possible is the most important thing for now...