c/7344: performance regression on huge case statements

[Daniel Berlin]

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-bugs@gcc.gnu.org, nobody@gcc.gnu.org,
	rschiele@uni-mannhiem.de, jh <jh@suse.cz>
Subject: Re: c/7344: performance regression on huge case statements
Date: Thu, 10 Oct 2002 22:21:36 -0400

 On Thursday, October 10, 2002, at 10:13  PM, Nathanael Nerode wrote:
 
 > Daniel Berlin wrote:
 >>>
 >> 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.
 >
 > (...and one which is expensive to compute using the current algorithm)
 >
 
 This is the ironic part: It's not that expensive to compute.
 It's O(log n)
 It just used to be O(1), so it's 1-15 times slower than it used to be.
 
 You wouldn't notice in most cases, because it scales so slowly.
 In fact, I only cached it because on the tree-ssa-branch, because there 
 were cases where it was called 238 million times.
 You start to notice at that point :).
 
 
 --Dan