RFR(S): 8247307: C2: Loop array fill stub routines are not called

[Vladimir Kozlov]

No further comments from me.

Yes, we can work on stubs later.

Thanks,
Vladimir

On 6/16/20 8:30 PM, Pengfei Li wrote:
> Hi Vladimir,
> 
>> Looking on generated code I see that vectorized loop may unroll 16 times (16
>> vector instructions by 256 bytes) where
>> generate_fill() stub on x86 has 2 (256 bytes wide) instructions per iteration
>> and 1 instruction for avx512 [2].
>> Also stub has alot of pre- and post-loop instructions and checks.
> 
> Right, I also take a look at x86 generated stub code and think the performance is potentially to be improved if the loop is unrolled more times. The AArch64 stub code is manually unrolled 8 times and it has almost no performance difference with the auto-vectorized version in general cases.
> 
>> I thought may be we can improve stub. But it seems vectorized loop with
>> predicates is more compact and efficient. And it is auto generated!
>>
>> Base on results I agree with you switching off fill optimization on x86.
>>
>> There could be side effects due to loops code will be larger (vs stub call) but
>> we have it already right now before your changes so I don't think we will see
>> regression for GCs which use strip mining.
> 
> Trying to improve the stub is my next plan. I believe both x86 and AArch64 stubs have room for improvement. So I prefer the keep the stub code for now and check if it can win the auto-vectorized version after been improved in the near future. But I hope some Intel guy could help with the x86 backend part since I'm not quite familiar with new x86 instructions.
> 
> I'm also studying the experimental feature of vectorized loop with predicates optimization in recent days (the PostLoopMultiversioning). But I found it's more complex and not working well now. This could be another long term goal.
> 
> Please let me know if you have further comments.
> 
> --
> Thanks,
> Pengfei
>