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Re: Questions about selective scheduler and PowerPC
- From: Nathan Froyd <froydnj at codesourcery dot com>
- To: Jie Zhang <jie at codesourcery dot com>
- Cc: gcc at gcc dot gnu dot org
- Date: Mon, 18 Oct 2010 08:27:46 -0400
- Subject: Re: Questions about selective scheduler and PowerPC
- References: <4CBBEDF1.2010807@codesourcery.com>
On Mon, Oct 18, 2010 at 02:49:21PM +0800, Jie Zhang wrote:
> 3. The aforementioned rs6000 hack rs6000_issue_rate was added by
>
> 2003-03-03 David Edelsohn <edelsohn@gnu.org>
>
> * config/rs6000/rs6000.c (rs6000_multipass_dfa_lookahead): Delete.
> (TARGET_SCHED_FIRST_CYCLE_MULTIPASS_DFA_LOOKAHEAD): Delete.
> (rs6000_variable_issue): Do not return negative value.
> (rs6000_issue_rate): Uniformly set issue rate to 1 for first
> scheduling pass.
>
> , which was more than 7 years ago. Is this still needed now?
I asked David about this on IRC several days ago. He indicated that it
was necessary to prevent the first scheduling pass from unnecessarily
increasing register pressure. I don't know whether anybody has actually
tested it with recent GCC, though presumably it did help when it was
installed.
-Nathan