Re: optimization/10625: -freduce-all-givs has negative effect

[Zdenek Dvorak <rakdver@atrey.karlin.mff.cuni.cz>]

The following reply was made to PR optimization/10625; it has been noted by GNATS.

From: Zdenek Dvorak <rakdver@atrey.karlin.mff.cuni.cz>
To: thome@lix.polytechnique.fr
Cc: gcc-gnats@gcc.gnu.org
Subject: Re: optimization/10625: -freduce-all-givs has negative effect
Date: Mon, 5 May 2003 21:12:39 +0200

 Hello,
 
 > -freduce-all-givs impacts negatively on the performance by around 15%.
 > 
 > The code also suffers from the inability of unroll-loops to expand the
 > two nested loops (the inner loop becomes constant when the outer is
 > expanded).
 > 
 > For the record, a hand-made unrolling of these loops can yield a 25%
 > speedup (hence 150ms). Further hand-tweaking of the assembly code can
 > improve the performace by an additional 20% or so.
 > 
 > This code could probably be better optimized by gcc than what happens
 > now.
 
 -freduce-all-givs is not enabled by default exactly for this reason --
 you order loop optimizer to reduce givs even if it does not know that
 there will be any profit, and the increased register presure causes
 the performance loss.
 
 Zdenek