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

[Kim Barrett]

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 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."

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.

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PR Review Comment: https://git.openjdk.org/jdk/pull/27288#discussion_r2488596063
PR Review Comment: https://git.openjdk.org/jdk/pull/27288#discussion_r2488602736