On Thu, 21 May 2026 21:42:49 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). src/hotspot/share/gc/shared/c1/barrierSetC1.cpp line 208: > 206: } > 207: > 208: // Truncate boolean values returned by unsafe operations. The new code is faster, because it is now consistent with the rule of `x&1`. Note also the word-size issue here: the `x!=0` comparison is done on a 32-bit JVM value. The verifier allows any of the 32 bits to be set or clear independently. That’s a nightmare to test. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/31249#discussion_r3293553282
