Re: scheduler queue flush (was Re: [gcc-15 0/3] RISC-V improve stack/array access by constant mat tweak)

[Jeff Law <jeffreyalaw@gmail.com>]

On 3/22/24 2:47 AM, Richard Biener wrote:
> On Thu, Mar 21, 2024 at 8:56 PM Jeff Law <jeffreyalaw@gmail.com> wrote:
>>
>>
>>
>> On 3/21/24 11:19 AM, Vineet Gupta wrote:
>>
>>>>
>>>> So if we go back to Robin's observation that scheduling dramatically
>>>> increases the instruction count, perhaps we try a run with
>>>> -fno-schedule-insns -fno-schedule-insns2 and see how the instruction
>>>> counts compare.
>>>
>>> Oh yeah ! Robin hinted to this in Tues patchworks meeting too
>>>
>>> default           : 2,565,319,368,591
>>> 128       : 2,509,741,035,068
>>> 256       : 2,527,817,813,612
>>> no-sched{,2}: 1,295,520,567,376
>> Now we're getting somewhere.  That's in line with expectations.
>>
>> I would strongly suspect it's -fno-schedule-insns rather than
>> -fno-schedule-insns2.  The former turns off scheduling before register
>> allocation, the second turns it off after register allocation.  So if
>> our theory about spilling is correct, then it must be the first since
>> the second won't affect register allocation.   While I can speculate
>> about other potential scheduler impacts, spilling due to sched1's
>> actions is by far the most likely.
> 
> Another option is to enable -fsched-pressure which should help with
> this issue.
In theory we're already using that by default -- it's part of what makes 
me so curious to understand what's going on.

jeff