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 04:19:13 GMT, Kim Barrett <kbarrett 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 514:
> 
>> 512:   }
>> 513: 
>> 514:   // Find the right-most non-zero (e.g., ...1000) bit of the diff of array-of-boolean and array-of-byte layout helpers.
> 
> Callers don't care whether it's the rightmost bit, only that it's a single
> bit. (Some callers use log2_exact to get the bit position.) So a more
> pedantically correct description might be something like "Return a value
> containing a single set bit that is in the bitset difference between the
> layout helpers for array-of-boolean and array-of-byte."

Comment of the function is replaced with this one.

> 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");
> 
> Use `power_of_2(result)`.  For completeness, also consider checking other post-conditions - result is set
> in zlh and clear in blh.

Done.

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

PR Review Comment: https://git.openjdk.org/jdk/pull/27288#discussion_r2498695917
PR Review Comment: https://git.openjdk.org/jdk/pull/27288#discussion_r2498696841