Re: c/7344: performance regression on huge case statements

[Daniel Berlin <dberlin at dberlin dot org>]

The following reply was made to PR c/7344; it has been noted by GNATS.

From: Daniel Berlin <dberlin@dberlin.org>
To: Nathanael Nerode <neroden@twcny.rr.com>
Cc: gcc-gnats@gcc.gnu.org, <gcc-prs@gcc.gnu.org>,
	<rschiele@uni-mannheim.de>, <gcc-bugs@gcc.gnu.org>,
	<nobody@gcc.gnu.org>
Subject: Re: c/7344: performance regression on huge case statements
Date: Thu, 10 Oct 2002 22:06:09 -0400

 On Thu, 10 Oct 2002, Nathanael Nerode wrote:
 
 > Partial analysis:
 >
 > http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit- 
 > trail&database=gcc&pr=7344
 >
 > It's stuck in a loop in et-forest.c at line 420,
 > in caclulate_value(node) -- this loop is taking a very, very, long  
 > time,
 > which indicates that that the tree being looked at is very deep.  Oh,
 > and node->parent->right is equal to node->parent every time.
 
 >
 > This is called from et_forest_common_ancestor as the condition (!!) on  
 > a
 > while
 > statement, making it doubly nested.  (Horrible... horrible...)
 >
 > This is all, eventunally, called from note_prediction_to_br_prob in
 > predict.c.
 
 Through nearest_common_dominator, which is called by
 dominated_by_p, perchance?
 If so, this is a known problem.
 It doesn't cache the info (I think Jan said he could make it constant  
 time
 in the alternative, which would also solve the problem), so it is
 constantly recomputing a value that hasn't changed.
 
 Try replacing dominance.c with the one from the tree-ssa-branch, and see
 if that helps.
 
 If so, that's yer problem
 --Dan