This is the mail archive of the gcc@gcc.gnu.org mailing list for the GCC project.

Index Nav:	[Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav:	[Date Prev] [Date Next]	[Thread Prev] [Thread Next]
Other format:	[Raw text]

Re: Understanding IRA

From: Jeff Law <law at redhat dot com>
To: Vladimir Makarov <vmakarov at redhat dot com>
Cc: Ian Bolton <bolton at IceraSemi dot com>, gcc at gcc dot gnu dot org
Date: Fri, 16 Oct 2009 09:45:26 -0600
Subject: Re: Understanding IRA
References: <4D60B0700D1DB54A8C0C6E9BE69163700BD6DB18@EXCHANGEVS.IceraSemi.local> <4AD888ED.1020102@redhat.com>

On 10/16/09 08:53, Vladimir Makarov wrote:

The biggest problem of GCC RA is still reload pass. It frequently changes decisions of IRA not in a good way.

Agreed. Not only may reload make a bad choice, it's horribly unpredictable. Trivial changes often lead to drastically different reloading decisions which in turn drastically change the final output.

One of my favorites right now is the round-robin selection of spill registers to encourage reload inheritance. While I certainly understand the reasoning behind the code, it's amazingly frustrating to watch reload choose the worst possible spill register simply because of the round-robin selection.

I've got a little hack in the reload-v2 branch which queries IRA to mitigate this problem, but it's merely a short-term hack until I can make reload inheritance go away.

jeff

Follow-Ups:
- RE: Understanding IRA
  - From: Ian Bolton

References:
- Understanding IRA
  - From: Ian Bolton
- Re: Understanding IRA
  - From: Vladimir Makarov

Index Nav:	[Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav:	[Date Prev] [Date Next]	[Thread Prev] [Thread Next]