IRA performance regressions on PPC

[Vladimir Makarov]

Jeff Law wrote:
> H.J. Lu wrote:
>> My understanding is PowerPC is quite sensitive to choice of register
>> as shown in PR 28690. IRA merge may make fixes for PR 28690
>> ineffective. There are a few small testcases in PR 28690. You can
>> check if those problems in PR 28690 come back due to IRA merge.
>> Also, IRA disables regmove:
>>
>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37364
>>
>> I don't know its impact on PowerPC. Can you try
>>
>> --- ./regmove.c.regmove    2008-09-06 10:09:43.000000000 -0700
>> +++ ./regmove.c    2008-09-06 11:34:24.000000000 -0700
>> @@ -1117,8 +1117,7 @@ regmove_optimize (rtx f, int nregs)
>>
>>    for (pass = 0; pass <= 2; pass++)
>>      {
>> -      /* We need fewer optimizations for IRA.  */
>> -      if ((! flag_regmove || flag_ira) && pass >= 
>> flag_expensive_optimizations)
>> +      if ((! flag_regmove) && pass >= flag_expensive_optimizations)
>>      goto done;
>>
>>        if (dump_file)
>> @@ -1167,8 +1166,7 @@ regmove_optimize (rtx f, int nregs)
>>          }
>>          }
>>
>> -      /* All optimizations important for IRA have been done.  */
>> -      if (! flag_regmove || flag_ira)
>> +      if (! flag_regmove)
>>          continue;
>>
>>        if (! find_matches (insn, &match))
>>
>> on the current ira-merge branch.
>>
>>   
> I can't express how badly I feel this is the wrong direction to be 
> taking.    Remove needs to go away and we need to be looking at the 
> root failures rather than re-enabling this crap code in regmove.c
>
> I've got a performance regression as well that ties into the disabling 
> of regmove, but doing a root cause analysis has made it plainly clear 
> that the problem is not regmove, nor IRA, nor the backend port in 
> question.  For my specific problem the root cause is actually subreg 
> lowering.    While I could fix my regression by twiddling regmove 
> and/or the port itself, neither change is actually solving the 
> problem.      I would *STRONGLY* suggest you take the time to do a 
> root cause analysis or at least avoid installing this bandaid patch.
>
I am agree with Jeff.  IRA was designed to replace most of regmove.  It 
has ability to do what regmove does.  Switching on regmove besides 
making RA slower only hides what is wrong with IRA.

Although, it would be interesting to see what regmove can give to IRA.