[jdk21u-dev] RFR: 8358334: C2/Shenandoah: incorrect execution with Unsafe

[William Kemper]

On Wed, 18 Jun 2025 13:48:14 GMT, Roland Westrelin <roland at openjdk.org> wrote:

> Patch doesn't apply cleanly:
> 
> - jdk21u still has IU barriers: context for some of the changes is
>   different.
>   
> - Because of IU barriers, there's an extra call to `fix_ctrl()` for
>   which the renaming to `nodes_above_barriers` must be applied.
>   
> - The initial patch makes a subtle change that doesn't affect jdk
>   25/26 in the code that was
>   refactored. `ShenandoahBarrierC2Support::push_data_inputs_at_control()`
>   is introduced with logic:
> 
>     if (in != nullptr && phase->has_ctrl(in) && phase->get_ctrl(in) == ctrl) {
> 
> 
>   which replaces the same logic in
>   `ShenandoahBarrierC2Support::fix_ctrl()` but also a slightly
>   different logic in
>   `ShenandoahBarrierC2Support::is_dominator_same_ctrl()`:
>   
> 
>     if (m->in(i) != nullptr && phase->ctrl_or_self(m->in(i)) == c) {
> 
> 
>   that is, it uses `ctrl_or_self()` which works for both data nodes
>   and control nodes but the new method uses `has_ctrl(in) &&
>   get_ctrl(in)` which can only be true for data nodes. That change
>   causes failures in jdk 21 again because of IU barriers that produce
>   a new memory state when expanded and need the logic from
>   `MemoryGraphFixer`. What I propose in this backport to be on the
>   safe side is to leave
>   `ShenandoahBarrierC2Support::is_dominator_same_ctrl()` alone (not
>   apply that part of the refactoring).
>   
> Tested with hotspot_gc_shenandoah + tier1 with -XX:+UseShenandoahGC

My approval doesn't count for anything here, but in case it helps grease the wheels: I ran this test without the patch and observed it fail. I ran the test with the patch and observed it pass. Thank you for the backport!

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Marked as reviewed by wkemper (Author).

PR Review: https://git.openjdk.org/jdk21u-dev/pull/1901#pullrequestreview-2943789176