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RE: How to figure out the gcc -dP output?
- From: Tim Crook <tcrook at adobe dot com>
- To: David Edelsohn <dje dot gcc at gmail dot com>
- Cc: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Tue, 28 Jul 2009 07:44:29 -0700
- Subject: RE: How to figure out the gcc -dP output?
- References: <6DF2A46812807943AB2102A35BC5121927790F1D93@NAMBX02.corp.adobe.com> <firstname.lastname@example.org>
I thought -mmininal-toc might have been a better workaround as well :-) .
Is there a Bugzilla number for this issue?
From: David Edelsohn [mailto:email@example.com]
Sent: Tuesday, July 28, 2009 9:46 AM
To: Tim Crook
Subject: Re: How to figure out the gcc -dP output?
I do not fully understand the complete explanation of the original problem. You mention extraneous lwz and TOC. I think you are referring to a bug in GCC 4.1 that incorrectly emitted loads after the TOC already had been changed for an indirect call. GCSE probably is producing code that requires a constant and GCC needs to place that constant in the TOC. The late creation of the TOC reference is not scheduled correctly.
GCSE is an optimization. -mminimal-toc is an option to avoid TOC overflow. Both of these are work-arounds to the problem. Disabling GCSE probably will slow down the application. -mminimal-toc probably will have less of a performance impact.
As I mentioned to Chris when I spoke with him last week, I would recomment upgrading to a newer version of GCC because GCC 4.1 no longer is maintained. Many bug fixes, such as one for the problem you are encountering, are incorporated into newer releases.
> I found a possible compiler workaround, compiling with -mminimal-toc. Would
> I get better performance by using this, instead of turning off gcse?
On Fri, Jul 24, 2009 at 4:34 PM, Tim Crook<firstname.lastname@example.org> wrote:
> Hello there.
> I am trying to track down a problem with gcc 4.1 which has to do with inlining and templates on PowerPC. Is there any documentation I can look related to the output generated with -fdump? I am getting extraneous lwz (load word and zero extend) instructions inserted when calling various methods - after $toc (r2) has been switched to the destination method's global data, just before the method call with the bctrl instruction. This lwz instruction causes a crash on IBM AIX when 32-bit shared libraries are loaded non-contiguously in memory. It looks like various code blocks are not being combined correctly when code is inlined - the extra lwz is being left behind.
> I have figured out that turning off gcse optimizations will stop this behavior, but doing this causes a performance hit. I would prefer not to upgrade the compiler at this time. With the compiler dump using -fdump, I am looking for a better way to work around this problem.
> Tim Crook.