From: Jan Hubicka <hubicka@ucw.cz>
To: Ghassan Shobaki <gshobaki@ece.ucdavis.edu>
Cc: Jan Hubicka <jh@suse.cz>,
gcc-help@gcc.gnu.org, "Vladimir N. Makarov" <vmakarov@redhat.com>,
gcc@gnu.org, hubicka@ucw.cz
Subject: Re: Superblock Instruction Scheduling in GCC 3.4
Date: Thu, 29 Jan 2004 11:15:00 -0000 [thread overview]
Message-ID: <20040129101203.GB30883@atrey.karlin.mff.cuni.cz> (raw)
In-Reply-To: <Pine.GHP.4.58.0401282211460.5040@dante.ece.ucdavis.edu>
>
> 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
next prev parent reply other threads:[~2004-01-29 10:14 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-12-03 7:07 Superblock Instruction Scheduling in GCC Ghassan Shobaki
2003-12-03 14:05 ` Vladimir N. Makarov
2003-12-04 0:13 ` Jan Hubicka
2003-12-04 1:30 ` Ghassan Shobaki
2003-12-04 10:49 ` Jan Hubicka
2003-12-04 17:40 ` Vladimir Makarov
2003-12-09 21:45 ` Ghassan Shobaki
2004-01-28 3:26 ` Superblock Instruction Scheduling in GCC 3.4 Ghassan Shobaki
2004-01-28 12:51 ` Jan Hubicka
2004-01-29 10:14 ` Ghassan Shobaki
2004-01-29 11:15 ` Jan Hubicka [this message]
2004-02-01 18:19 ` Ghassan Shobaki
2004-02-01 21:56 ` Jan Hubicka
2004-02-02 5:54 ` Ghassan Shobaki
2004-02-10 18:58 ` Superblock Scheduling Alg in GCC Ghassan Shobaki
2004-02-10 20:10 ` Vladimir Makarov
2004-02-10 20:18 ` Vladimir Makarov
2004-02-10 20:28 ` Jan Hubicka
2004-04-14 17:40 ` Ghassan Shobaki
2004-04-15 8:44 ` Vladimir Makarov
2004-05-27 14:12 ` Ghassan Shobaki
2004-01-28 13:57 Superblock Instruction Scheduling in GCC 3.4 S. Bosscher
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