> > 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? With -fbranch-probabilities and no profile, GCC will assume that all the blocks were never executed and conclude that they are cold. This limits several algorithms to more or less optimize for size rather than speed. So if you want traces based on static predictions, you should be using -fguess-branch-probability (default by -O2) and not -fbranch-probabilities. Honza > > 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 > >
