From: Jeffrey A Law <law@redhat.com>
To: jh@suse.com
Cc: gcc@gcc.gnu.org
Subject: Slow profile updating (pr 15524)
Date: Sat, 20 Nov 2004 00:41:00 -0000 [thread overview]
Message-ID: <1100904370.27318.54.camel@localhost.localdomain> (raw)
Believe it or not we're at a point where updating of the profile in
response to a jump thread is the most expensive routine in the
compiler for PR 15524.
If we look up update_bb_profile_for_threading we see one loop of
significance:
else
FOR_EACH_EDGE (c, ei, bb->succs)
c->probability = ((c->probability * REG_BR_PROB_BASE) / (double)
prob);
So anytime we thread through some block BB, we have to walk through
all its successors to rescale their probabilities. Needless to say
that gets rather expensive, especially if BB has a large switch
statement and several of its incoming edges are threadable.
Is it the case that the computation of c->probability actually
has to happen in the order you specified via the parenthesis? If
not, then we could precompute REG_BR_PROB_BASE / (double) prob
outside the loop which would result in a loop like
tmp = REG_BR_PROB_BASE / (double) prob;
FOR_EACH_EDGE (c, ei, bb->succs)
c->probability *= tmp;
Which would probably provide a reasonable improvement.
And if that's safe, then we'd probably want to rewrite it like
else if (prob != REG_BR_PROB_BASE)
{
double tmp = REG_BR_PROB_BASE / (double) prob;
FOR_EACH_EDGE 9c, ei, bb->succs)
c->probability *= tmp;
}
Which avoids the loop completely if nothing is going to change (as is
the case for pr15524).
Doing something like this would give us a net improvement of 15-20% on
PR 15524.
Alternately we might want to look at whether or not we can rescale the
successor blocks en-masse after all the redirections for a particular
block are complete.
Thoughts?
jeff
next reply other threads:[~2004-11-19 22:46 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-11-20 0:41 Jeffrey A Law [this message]
2004-11-20 14:14 ` Jan Hubicka
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