On Thu, 28 May 2026 21:56:58 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: > > normalize_for_read/normalize_for_write => normalize test/hotspot/jtreg/compiler/unsafe/UnsafeBooleanTest.java line 170: > 168: } > 169: } > 170: // Model what we expect the interpreter and/or JIT to do // when accessing a boolean in memory. The `x!=0` // behavior is historical, while `x&1` (truncation) is current. // Note that the interpreter and/or JIT sometimes omit // the normalization step, if the boolean in question is // a being READ from a Java heap variable that is strongly // typed as a boolean. (Not an unsafely generated address, // not off-heap.) When WRITING booleans to the Java heap, // the interpreter and JIT both make sure to normalize as `x&1`, // so the Java heap is never polluted. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/31249#discussion_r3321054829
