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 >
