[PATCH][ARM] Switch to default sched pressure algorithm

[Ramana Radhakrishnan]

On 30/07/2019 10:08, Christophe Lyon wrote:
> On Mon, 29 Jul 2019 at 18:49, Wilco Dijkstra <Wilco.Dijkstra@arm.com> wrote:
>>
>> Currently the Arm backend selects the alternative sched pressure algorithm.
>> The issue is that this doesn't take register pressure into account, and so
>> it causes significant additional spilling on Arm where there are only 14
>> allocatable registers.  SPEC2006 shows significant codesize reduction
>> with the default pressure algorithm, so switch back to that.  PR77308 shows
>> ~800 fewer instructions.
>>
>> SPECINT2006 is ~0.6% faster on Cortex-A57 together with the other DImode
>> patches. Overall SPEC codesize is 1.1% smaller.
>>
> 
> Hi Wilco,
> 
> Do you know which benchmarks were used when this was checked-in?
> It isn't clear from https://gcc.gnu.org/ml/gcc-patches/2012-07/msg00706.html

It was from my time in Linaro and thus would have been a famous embedded 
benchmark, coremark , spec2000 - all tested probably on cortex-a9 and 
Cortex-A15. In addition to this I would like to see what the impact of 
this is on something like Cortex-A53 as the issue rates are likely to be 
different on the schedulers causing different behaviour.


I don't have all the notes today for that - maybe you can look into the 
linaro wiki.

I am concerned about taking this patch in without some more data across 
a variety of cores.

Thanks,
Ramana


> 
> Thanks,
> 
> Christophe
> 
>> Bootstrap & regress OK on arm-none-linux-gnueabihf --with-cpu=cortex-a57
>>
>> ChangeLog:
>> 2019-07-29  Wilco Dijkstra  <wdijkstr@arm.com>
>>
>>          * config/arm/arm.c (arm_option_override): Don't override sched
>>          pressure algorithm.
>>
>> --
>>
>> diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
>> index 81286cadf32f908e045d704128c5e06842e0cc92..628cf02f23fb29392a63d87f561c3ee2fb73a515 100644
>> --- a/gcc/config/arm/arm.c
>> +++ b/gcc/config/arm/arm.c
>> @@ -3575,11 +3575,6 @@ arm_option_override (void)
>>     if (use_neon_for_64bits == 1)
>>        prefer_neon_for_64bits = true;
>>
>> -  /* Use the alternative scheduling-pressure algorithm by default.  */
>> -  maybe_set_param_value (PARAM_SCHED_PRESSURE_ALGORITHM, SCHED_PRESSURE_MODEL,
>> -                        global_options.x_param_values,
>> -                        global_options_set.x_param_values);
>> -
>>     /* Look through ready list and all of queue for instructions
>>        relevant for L2 auto-prefetcher.  */
>>     int param_sched_autopref_queue_depth;
>>