Questions about selective scheduler and PowerPC

[Jeff Law]

  On 10/18/10 09:22, David Edelsohn wrote:
> On Mon, Oct 18, 2010 at 8:27 AM, Nathan Froyd<froydnj@codesourcery.com>  wrote:
>> 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.
> I am not sure when it last was re-checked, but it was checked after
> sched_pressure was added.  When that option is not enabled, the
> issue_rate change still helped.
Did anyone check this after Bernd's work to better handle allocation of 
double-word pseudos in IRA?  That code should be handling the false 
conflicts created by movement of clobbers.

jeff