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

[Afshin Zafari]

On Tue, 4 Nov 2025 23:47:40 GMT, Dean Long <dlong at openjdk.org> wrote:

>> 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`?

Assert's message updated.
`constexpr` cannot be used unless all the called functions (alh and its descendents) are also `constexpr`.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/27288#discussion_r2498708371