RFR: 8365229: ARM32: c2i_no_clinit_check_entry assert failed after JDK-8364269

[Paul Hohensee]

On Tue, 12 Aug 2025 14:06:35 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> When recording adapter entries, we record _offsets_, not the actual addresses:
> 
> 
>   entry_offset[3] = handler->get_c2i_no_clinit_check_entry() - i2c_entry;
> 
> 
> Every platform except ARM32 and Zero have all these entries set up, so offset are always sane. But those two platforms set up `nullptr` as `c2i_no_clinit_check_entry()`, because clinit barriers are unimplemented. So the new assert added in [JDK-8364269](https://bugs.openjdk.org/browse/JDK-8364269) fails encountering effectively `nullptr - i2c_entry` "garbage".
> 
> This PR is the second least horrible (IMO) fix for this: relaxing assert by checking that "out of range" values are actually wrapping around back to `0`/`nullptr`. Had to do it in unsigned ints to avoid UB. For the affected platforms, we do not actually access this problematic/garbage entry offset, since we are always checking if clinit barriers are enabled. So the assert is the only place where it matters.
> 
> The least horrible solution would be storing the actual `address`-es instead of `int` offsets. But that likely has footprint implications.
> 
> Additional testing:
>  - [x] Linux ARM32 server fastdebug, `java -version` now works

You might turn line 454 into a function right after the pointer_delta section of globalDefinitions.hpp. Vis

inline uintptr_t raw_pointer(const volatile void* p, uintptr_t offset) {
    return p2u(p) + offset;
}

so line 454 becomes

raw_pointer(cb->insts_begin(), entry_offset[i]) == 0

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

PR Review: https://git.openjdk.org/jdk/pull/26746#pullrequestreview-3111706868