[lworld] Integrated: 8365996: [lworld] Register corruption in GC barrier slow paths in scalarized calling convention on AArch64

[Marc Chevalier]

On Tue, 2 Sep 2025 15:19:58 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

> That fixes a pair of issues. @TobiHartmann fixed one where some native function call would mess up registers. Pushing and popping them solves it.
> 
> The second part is about using `x29`, aka `fp` (frame pointer), as a regular register. Even when not used as the frame pointer register `x29` is nevertheless saved on the stack when setting up the frame, for when it's used as actually the frame pointer. On exit, `x29` and `x30` are restored from there. But with Valhalla, they can be saved twice on the stack! When the non-scalarized entry point of a C2-compiled method is used, one might need a bit more space to scalarize the arguments. This leads to shape such as drawn here.
> 
> https://github.com/openjdk/valhalla/blob/63314117aa0c30f1a5928b56d21d944de063b8c6/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp#L6107-L6126
> 
> Currently, `LR` and `FP` are restored from the copy `#1`. But the comment is not quite exact: `FP #1` and `FP #2` won't be always the same in case it contained an oop to an object that the GC moved between frame creation and frame destruction: the GC knows only one spilled location for each register, and in the case of `x29`, it's `FP #2.`
> 
> It seems difficult to adapt the GC mechanisms to handle more than one location to update for a register. Likewise, we know that `LR #1` is update, but `LR #2` doesn't seem to be: it's fine to update only one, if we know which one is the trustworthy one. Updating only one `FP` won't mess with the cases where we are actually using `x29` as frame pointer since then, the GC won't update any of them as they won't contain oops.
> 
> The process is rather simple: since we need to load `sp_inc` to know how much to pop from the stack, by changing the `ldr` into a `ldp`, we can load `FP #2` into `x29` at the same time. Then, we reduce the stack by `sp_inc`, we read LR#1 (always 1 word under the new top), and we pop the two last words left. This works whether we need some stack extension or not, and makes sure `sp` stays 16-byte aligned when reading `LR #1` (I've learned the hard way, the SIGBUS way, it's a thing...). This requires only one more instruction than the old way. Let's call that a lesser evil.
> 
> Thanks,
> Marc

This pull request has now been integrated.

Changeset: 3699b1c1
Author:    Marc Chevalier <mchevalier at openjdk.org>
Committer: Tobias Hartmann <thartmann at openjdk.org>
URL:       https://git.openjdk.org/valhalla/commit/3699b1c1423f56211777e14bdcfd9931f7875378
Stats:     271 lines in 14 files changed: 183 ins; 30 del; 58 mod

8365996: [lworld] Register corruption in GC barrier slow paths in scalarized calling convention on AArch64

Reviewed-by: thartmann

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PR: https://git.openjdk.org/valhalla/pull/1540