RFR: 8338383: Implement JEP 491: Synchronize Virtual Threads without Pinning

[Dean Long]

On Fri, 1 Nov 2024 07:14:35 GMT, Dean Long <dlong at openjdk.org> wrote:

>>> OK, so you're saying it's the stack adjustment that's the problem. It sounds like there is code that is using rsp instead of last_Java_sp to compute the frame boundary. Isn't that a bug that should be fixed?
>>>
>> It's not a bug, it's just that the code from the runtime stub only cares about the actual rsp, not last_Java_sp. We are returning to the pc right after the call so we need to adjust rsp to what the runtime stub expects. Both alternatives will work, either changing the runtime stub to set last pc and not push those two extra words, or your suggestion of just setting the last pc to the instruction after the adjustment. Either way it requires to change the c2 code though which I'm not familiar with. But if you can provide a patch I'm happy to apply it and we can remove this `possibly_adjust_frame()` method.
>
> It turns out if we try to set last pc to the instruction after the adjustment, then we need an oopmap there, and that would require more C2 changes.  Then I thought about restoring SP from FP or last_Java_fp, but I don't think we can rely on either of those being valid after resume from preemption, so I'll try the other alternative.

Here's my suggested C2 change:

diff --git a/src/hotspot/cpu/aarch64/aarch64.ad b/src/hotspot/cpu/aarch64/aarch64.ad
index d9c77a2f529..1e99db191ae 100644
--- a/src/hotspot/cpu/aarch64/aarch64.ad
+++ b/src/hotspot/cpu/aarch64/aarch64.ad
@@ -3692,14 +3692,13 @@ encode %{
       __ post_call_nop();
     } else {
       Label retaddr;
+      // Make the anchor frame walkable
       __ adr(rscratch2, retaddr);
+      __ str(rscratch2, Address(rthread, JavaThread::last_Java_pc_offset()));
       __ lea(rscratch1, RuntimeAddress(entry));
-      // Leave a breadcrumb for JavaFrameAnchor::capture_last_Java_pc()
-      __ stp(zr, rscratch2, Address(__ pre(sp, -2 * wordSize)));
       __ blr(rscratch1);
       __ bind(retaddr);
       __ post_call_nop();
-      __ add(sp, sp, 2 * wordSize);
     }
     if (Compile::current()->max_vector_size() > 0) {
       __ reinitialize_ptrue();

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PR Review Comment: https://git.openjdk.org/jdk/pull/21565#discussion_r1826252551