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RE: How to figure out the gcc -dP output?


Thanks David.

I thought -mmininal-toc might have been a better workaround as well :-) .

Is there a Bugzilla number for this issue?

-----Original Message-----
From: David Edelsohn [mailto:dje.gcc@gmail.com] 
Sent: Tuesday, July 28, 2009 9:46 AM
To: Tim Crook
Subject: Re: How to figure out the gcc -dP output?

Tim,

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.

David

> 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<tcrook@adobe.com> 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.
>


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