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Re: Superblock Instruction Scheduling in GCC 3.4


> 
> 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


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