[aarch64-port-dev ] RFR: Backport Shenandoah since last merge

[Andrew Dinn]

On 25/07/17 19:28, Roman Kennke wrote:
>> Can you explain the need to use the 48 bit mask?
> It is explained here:
> http://mail.openjdk.java.net/pipermail/shenandoah-dev/2017-June/002756.html

Ah, I see. When needs_explicit_null_check is called from the signal
handler this value is coming straight out of info->si_addr (where
siginfo_t* info). Hmm, it turns out that this is complicated. I think
it's ok for now to push the code as is with one minor change to the comment:

- // AArch64 uses 48-bit addresses
+ // AArch64 addresses passed from the signal handler may have
+ // their top 8 bits zeroed. That affects the case where
+ // Shenandoah tries to load a Brooks pointer via a null oop.

I have identified why this truncation is happening and what it means. I
will start a follow-up thread to discuss why this needs

  i) a better explanation in the code

  ii) possible rework

"thread has been cleaned/unloaded"? Should that not be nmethod?
> Probably, but this is only a comment, and not even ours. We only
> re-instanted the code to where it was before (it's G1 code), so this is
> one minimize-diff-to-upstream change :-)

Ok, I guess we can leave this for later generations of devs to go "Tchh,
ha!" over.

>> Why is the return type a size_t rather than a jint?
> 
> Good catch! This looks like a genuine mistake! I'll push a fix to
> shenandoah-jdk10 and let it trickle down to jdk9, jdk8 and from there
> into this.

Hurrah, so it /was/ worth reviewing after all!

> Do you think it's ok to push with the size_t changed to jint?
Sure. It was arguably 'ok' to push even without my suggested changes but
I had to justify my time spent checking it somehow :-)

It was very impressive to see how many improvements you and Aleksey
managed to add. Nice job!

regards,


Andrew Dinn
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