Questions about selective scheduler and PowerPC

[David Edelsohn]

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.

- David