[lworld] RFR: 8367338: [lworld] compiler/gcbarriers/TestImplicitNullChecks.java fails [v2]

[Roberto Castañeda Lozano]

On Mon, 29 Sep 2025 08:54:32 GMT, Damon Fenacci <dfenacci at openjdk.org> wrote:

>> # Issue
>> IR checks of `testCompareAndExchange`, `testCompareAndSwap` and `testGetAndSet` of  `compiler/gcbarriers/TestImplicitNullChecks.java` fail because they expect no  `NULL_CHECK` in the final code graph but it encounters a few in each test instead.
>> 
>> # Cause
>> The tests in question simply call 3 different `VarHandle` functions (one each) but these don't get inlined as intrinsics directly. They instead rely on inline methods (with some conditional statements) that eventually call a method that gets intrinsified. Unfortunately the compilation of some of the inlined code can result in null-checks after being compiled.
>> For instance `testCompareAndExchange` inlines `Unsafe::compareAndExchangeReference`
>> https://github.com/openjdk/valhalla/blob/1ecd2e95a158c80186d4334ca3f0673f660d5042/src/java.base/share/classes/jdk/internal/misc/Unsafe.java#L1723-L1742
>> which calls `compareAndSetReference` or `compareAndExchangeReference` but `if (valueType.isValue() || isValueObject(expected))` adds a null-check for `valueType`.
>> This happens only in Valhalla because of a Valhalla-specific value-type checks (see [JDK-8351569](https://bugs.openjdk.org/browse/JDK-8351569)).
>> 
>> # Fix
>> The goal of the tests is to make sure that there are no null-checks with compare-and-exchange operations with ZGC and G1. Instead of relying on `VarHandle` compare-and-exchange-related methods we can use the equivalent `Unsafe` methods that are intrinsified directly.
>> 
>> # Testing
>> Tier 1-3+
>
> Damon Fenacci has updated the pull request incrementally with one additional commit since the last revision:
> 
>   JDK-8367338: remove unused VarHandle

Marked as reviewed by rcastanedalo (no project role).

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PR Review: https://git.openjdk.org/valhalla/pull/1626#pullrequestreview-3279153831