On Thu, 28 May 2026 22:47:10 GMT, Vladimir Ivanov <[email protected]> wrote:
>> On bytecode level booleans are represented as ints and HotSpot JVM >> normalizes boolean values on memory accesses. It unconditionally applies >> normalization on boolean stores, but trusts on-heap boolean locations to >> hold normalized values. Normalization is applied on loads for off-heap and >> mismatched unsafe accesses . >> >> There are 2 normalization procedures used: (1) cast int to byte and test it >> against zero; and (2) truncation to least-significant bit. Truncation is >> preferred (due to performance considerations), but JNI mandates testing >> against zero and, historically, `#1` was used for off-heap unsafe accesses >> as well. It complicated the implementation (leading to subtle bugs) and >> introduced divergence in behavior at runtime (depending on execution mode >> and JIT-compilation peculiarities). >> >> The fix uses truncation uniformly across all execution modes. It simplifies >> implementation and eliminates possible divergence in behavior between >> execution modes. Also, it drastically simplifies future Unsafe API >> refactorings. >> >> There's one scenario left when it's possible to observe non-normalized >> values: when mismatched access pollutes the Java heap with a bogus boolean >> value, but then the value is read with a well-typed boolean access. >> >> Testing: hs-tier1 - hs-tier6 >> >> - [x] I confirm that I make this contribution in accordance with the >> [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai). > > Vladimir Ivanov has updated the pull request incrementally with one > additional commit since the last revision: > > Fix Compile::narrow_value This is very good work. Please see my pre-review comments for small residual issues. src/hotspot/share/opto/compile.cpp line 5383: > 5381: result = phase->transform(new LShiftINode(value, > phase->intcon(24))); > 5382: result = new RShiftINode(result, phase->intcon(24)); > 5383: } else if (bt == T_BOOLEAN) { Great catch. Maybe scan for other T_BOOLEANs near other "FF" numbers or "and" operations. ------------- Changes requested by jrose (Reviewer). PR Review: https://git.openjdk.org/jdk/pull/31249#pullrequestreview-4385368939 PR Review Comment: https://git.openjdk.org/jdk/pull/31249#discussion_r3321122613
