That explains the results the I am getting. Things make sense to me now. Thank you very much! -Ghassan On Sun, 1 Feb 2004, Jan Hubicka wrote: > > > > 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 > > > >
