RFR: 8358957: [ubsan]: The assert in layout_helper_boolean_diffbit() in klass.hpp needs UB to fail [v4]

[Dean Long]

On Mon, 3 Nov 2025 09:29:28 GMT, Afshin Zafari <azafari at openjdk.org> wrote:

>> Avoid using loop and UB in left-shift operation as suggested by Kim's comment in the JBS-issue.
>> 
>> Tests:
>>  mach5 tiers 1-5 {macosx-aarch64, linux-x64, windows-x64} x {debug, product}
>
> Afshin Zafari has updated the pull request incrementally with one additional commit since the last revision:
> 
>   comments and post-cond

src/hotspot/share/oops/klass.hpp line 525:

> 523:     // So use alternate form of negation to avoid warning.
> 524:     uint result = candidates & (~candidates + 1);
> 525:     assert(((result - 1) & result) == 0, "post-condition");

Maybe use "must be power of 2" instead of  "post-condition".  Also, this value is never going to change.  Can we make the function `constexpr`?

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PR Review Comment: https://git.openjdk.org/jdk/pull/27288#discussion_r2492344606