RFR: 8326936: RISC-V: Shenandoah GC crashes due to incorrect atomic memory operations [v6]

[Fei Yang]

On Tue, 5 Mar 2024 07:24:02 GMT, MaxXing <duke at openjdk.org> wrote:

>> With compressed oops enabled, Shenandoah GC crashes in the concurrent marking phase due to some incorrect atomic memory operations. This resulted in the failure of multiple related tests, including `gc/shenandoah/TestSmallHeap.java`, `gc/metaspace/TestMetaspacePerfCounters.java#Shenandoah-64`, and so on, tested on XuanTie C910 and QEMU.
>> 
>> This failure is related to a GCC bug we recently discovered: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114130.
>> 
>> In detail, there's a pattern in method `ShenandoahHeap::conc_update_with_forwarded`:
>> 
>> 
>> if (in_collection_set(obj)) {
>>   // Dereferences `obj` without explicit null check.
>>   oop fwd = ShenandoahBarrierSet::resolve_forwarded_not_null(obj);
>>   // Then calls atomic built-in.
>>   atomic_update_oop(fwd, p, obj);
>> }
>> 
>> 
>> `atomic_update_oop` then compresses `obj` into a 32-bit word and calls the built-in `__atomic_compare_exchange`. The latter produces incorrect assembly that comparing this unsigned 32-bit word with the sign-extended result of `lr.w` instructions.
>> 
>> This bug can be bypassed by adding an explicit null check (like `if (obj && in_collection_set(obj))`), or adding compiler flag `-fno-delete-null-pointer-checks`.
>> 
>> In previous commits, JDK-8316186 removed RISC-V's `PlatformCmpxchg<4>` but left the flag `-fno-delete-null-pointer-checks` enabled. Then JDK-8316893 removed the flag and made the bug visible. This patch adds `PlatformCmpxchg<4>` back.
>
> MaxXing has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add explicit type cast for `old_value`.

LGTM. Thanks. The condition for defining macro `CORRECT_COMPILER_ATOMIC_SUPPORT` needs to be improved when the gcc bugfix is backported to gcc branches 11, 12 and 13 [1].

[1] https://gcc.gnu.org/pipermail/gcc-patches/2024-February/646822.html

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Marked as reviewed by fyang (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/18039#pullrequestreview-1916638598