Re: Superblock Instruction Scheduling in GCC 3.4

[Ghassan Shobaki <gshobaki@ece.ucdavis.edu>]

Eventually, I will be doing profile-based experiments. However, at this
point I am interested in static probabilities because it is an easier option
that will allow me to get some initial results more quickly.
Now, my question is: when I used the f-branch-probablities switch without
doing profiling first, gcc still accepted it and generated some
superblocks. Were these invalid superblocks or what?

Thanks

-Ghassan


On Thu, 29 Jan 2004, Jan Hubicka wrote:

> >
> > Ok, I have just verified that gcc DOES accept the -fsched2-use-tracer and
> > invoke the ebb scheduler as expected. However, it does not set the
> > flag_branch_probabilities automatically. It only sets it when I
> > explicitly use the -fbranch-probabilities command-line switch. Here are
> > the two cases that I have tried:
> >
> > g++ -O3 -fsched2-use-traces
> > Generates ~151K superblocks on my benchmark suite with lots of large
> > superblocks that include 10 basic blocks or more
> >
> > g++ -O3 -fsched2-use-traces -fbranch-probabilities
> > Generates only ~123K superblocks on my benchmark suite with the vast
> > majority of superblocks consisting of less than 10 basic blocks
>
> -fbranch-probabilities can be accpeted only when program has been
> earlier profiled.  GCC does have logic for statically guessing the
> branch outcomes when these are not available
> (-fguess-branch-probability) so the superblocks can be built, just they
> are inferrior to those built with feedback available.
> >
> > So, the question is: Why did the compiler generate more superblocks
> > when branch probabilities were not computed? Do the superblocks generated
> > in that case make any sense?
> > And the bottom line question for me is: which setting should I use in my
> > research on superblocks?
>
> It is always better to use the profile, so I would recommend you
> -fbranch-probabilities unless you are interested in experiments with
> static prediction algorithms.
>
> Honza
>