[lworld] RFR: 8377714: [lworld] Re-enable virtual thread tests

[Coleen Phillimore]

On Thu, 12 Feb 2026 00:49:09 GMT, Patricio Chilano Mateo <pchilanomate at openjdk.org> wrote:

> Please review the following patch which re-enables virtual thread tests `TestVirtualThreads.java`‎ and `Fuzz.java`.
> 
> First, this patch fixes the AArch64 virtual thread code to adapt to the changes in [JDK-8367151](https://bugs.openjdk.org/browse/JDK-8367151) where the valid FP is now stored in copy `#1` instead of copy `#2`:
> 
> https://github.com/openjdk/valhalla/blob/c0b679480a10bc9c71aa75ed26b3cfd0e69d294c/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp#L5960-L6007
> 
> With that change we can now simplify the `stackChunk` walking code and the thawing patching logic, since we don't need to keep track of two separate stack pointers, one to access the saved pc (`_unextended_sp[-1]`) and another to access the saved fp (`_sp[-2]`). Also, method `compiled_frame_details()` enables simplification of `FreezeBase::sender`.
> Follow-up changes from [JDK-8371993](https://bugs.openjdk.org/browse/JDK-8371993) uncovered that in `ContinuationHelper::Frame::return_pc_address` we were reading the saved pc from the wrong location. This is also fixed in this PR. Going forward though, it seems the issue is with `f.real_fp()` which should include the extra added size for extended frames.
> 
> Second, the x64 code has been updated so that the layout of the extended frames is the same as with AArch64, and that we also only use the `#1` copies. This not only aligns behavior across both platforms but also allows simplification of the virtual thread code as mentioned above with AArch64. Changes in [JDK-8372806](https://bugs.openjdk.org/browse/JDK-8372806) uncovered the same issue with `ContinuationHelper::Frame::return_pc_address` and that is fixed as well.
> Note: While working on this, I noticed that for extended frames, with `-XX:+PreserveFramePointer`, rbp is set to the location of copy `#2` rather than copy `#1`. Storing bad values in these locations will break the chain of pointers though, so we probably want to set rbp to location of copy `#1`. Same with AArch64.
> 
> Finally, for both AArch64 and x64 I updated `frame::describe_pd` to show correct locations of saved return pc and FP for extended frames. I still kept the locations where `#2` copies are stored. This has proven useful for debugging purposes.
> 
> Changes were tested by running both `TestVirtualThreads.java‎` and `Fuzz.java`, job `valhalla-comp-stress` in mach5, as well as tiers1-3. I also run `TestVirtualThreads.java‎` with extra flags, including `-Xcomp -XX:+SafepointALot -XX:+DeoptimizeALot` as reported in [JDK-8370177](https...

I can't follow the details of the address calculations in this, but I like some things about this change: 1. the consistency between x86 and aarch64, 2. the conditional argument callee_augmented is removed, 3. Using compiled_frame_details is nice, and 4. poisoning the unused pc and rbp addesses in the stack so you can find misuses quickly.
Maybe compiled_frame_details should be merged up to mainline?

src/hotspot/cpu/x86/c1_MacroAssembler_x86.cpp line 243:

> 241:     movl(Address(rsp, VMRegImpl::stack_slot_size), badRegWordVal);
> 242:   }
> 243: #endif

I like this.

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Marked as reviewed by coleenp (Committer).

PR Review: https://git.openjdk.org/valhalla/pull/2085#pullrequestreview-3822125091
PR Review Comment: https://git.openjdk.org/valhalla/pull/2085#discussion_r2824359142