Implicit null-check in hot-loop?

[Benedikt Wedenik]

Okay :)

Thank you again!

Benedikt.
On 27 May 2015, at 14:25, Vitaly Davidovich <vitalyd at gmail.com> wrote:

> I don't think there's any fundamental reason, it's just that int loops are more common and so were optimized.
> 
> sent from my phone
> 
> On May 27, 2015 8:09 AM, "Benedikt Wedenik" <benedikt.wedenik at theobroma-systems.com> wrote:
> First of all, thanks for your quick response :)
> You were absolutely right - but I do not understand why the long counter was the problem.
> 
> Is there any reason why the loop-unrolling is only available for int loops?
> I mean I guess so - because else it would probably be implemented already.
> But still it would be a nice opportunity for an optimisation :)
> 
> Benedikt.
> On 27 May 2015, at 13:44, Vitaly Davidovich <vitalyd at gmail.com> wrote:
> 
>> Also, the safepoint check on each iteration should go away when using an int loop; it'll be coalesced into once per unroll factor or may go away entirely if the loop turns into a counted loop.
>> 
>> sent from my phone
>> 
>> On May 27, 2015 7:39 AM, "Vitaly Davidovich" <vitalyd at gmail.com> wrote:
>> Cause you're using a long as induction variable; change to int and it should unroll.
>> 
>> sent from my phone
>> 
>> On May 27, 2015 6:46 AM, "Benedikt Wedenik" <benedikt.wedenik at theobroma-systems.com> wrote:
>> Oh!
>> 
>> Thanks :) So then it's not so bad, cause it is (I guess) obligatory.
>> 
>> Do you have any idea why this loop does not get unrolled?
>> 
>> Benedikt.
>> 
>> On 27 May 2015, at 12:36, Andrew Haley <aph at redhat.com> wrote:
>> 
>> > On 05/27/2015 11:27 AM, Benedikt Wedenik wrote:
>> >>
>> >> In the hot-loop there is this “ldr” which looked a little “strange” at the first glance.
>> >> I think that this load is a null-check? Is that the case?
>> >
>> > No.  It's a safepoint check.  HotSpot has to insert one of these because
>> > it can't prove that the loop is reasonably short-lived.
>> >
>> >> I also investigated the generated code on x86 which is quite similar, but instead of a load, they are
>> >> using the “test”-instruction which performs an “and” but only sets the flags discarding the result.
>> >> Is there any similar instruction available on aarch64 or is this already the closest solution?
>> >
>> > Closest to what?  A load to XZR is the best solution: it does not hit
>> > the flags.  x86 cannot do this.
>> >
>> > It looks like great code.
>> >
>> > Andrew.
>> >
>> 
> 

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