Question about memorySegment with safepoint checks?

[xichen liu]

I learned more by watching the devoxx video:
https://www.youtube.com/watch?v=xFb_LcapbXw&ab_channel=Devoxx. I got it
now, it's not the safepoints that are causing the problem, it's the nested
loop issue. Thanks for your explanation!

Maurizio Cimadamore <maurizio.cimadamore at oracle.com> 于2024年10月10日周四 00:56写道：

> Hello,
> I'm not aware of anything specifically related to safepoints and long
> loops, but I'm cc'ing Roland just in case.
>
> My understanding of the problem is that, when we have a loop with a long
> induction variable, C2 really emits two nested loops, as it wants to go
> back to a place where the inner loop works with an int induction variable.
>
> This double-loop can sometimes cause performance regressions, especially
> for short-loops. I believe Roland is working on a fix to specialize code
> when the limit can be speculated to be "small" enough.
>
> On the general topic of safepoints, my udnerstanding is that we'd like
> long loops to have safepoints every now and then. Even intrinsics such
> as System::arrayCopy have, over time, changed to insert some safepoint,
> not to stall GC completely when running a long bulk operation.
>
> But if you have evidence and/or benchmark where you think long loops are
> generating too many safepoints, and that developers are falling back to
> use ints as a result, please let us know. I'm not aware of such issues
> but somebody else might.
>
> Maurizio
>
>
> On 09/10/2024 17:18, xichen liu wrote:
> > I would like to raise a question regarding the use
> > of `MemorySegment` for pointer traversal in memory. I have noticed
> > that when using `long` as a loop condition variable for traversal, the
> > JVM inserts safepoint checks, which leads to an overall performance
> > decrease. As a result, it seems that developers are advised to
> > manually convert the `long` variable to an `int` type to avoid this
> issue.
> >
> > While I am not an expert in this area, my own testing in GraalVM shows
> > that it is capable of achieving similar performance when
> > using `long` in loops with fewer iterations, comparable to
> > using `int`, whereas C2 does not seem to handle this as efficiently.
> >
> > I recall that there has been some previous discussion on this issue,
> > and I would like to inquire about the current status of this matter.
> > Could you please provide any updates or insights on how this has been
> > addressed?
> >
> > Best regards.
>
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