[jdk21u-dev] RFR: 8358334: C2/Shenandoah: incorrect execution with Unsafe [v2]

[Aleksey Shipilev]

On Mon, 7 Jul 2025 08:06:45 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
>
> Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

Looks good to me, thank you!

-------------

Marked as reviewed by shade (Reviewer).

PR Review: https://git.openjdk.org/jdk21u-dev/pull/1901#pullrequestreview-2992671111