RFR: 8241613: Suspicious calls to MacroAssembler::null_check(Register, offset) [v3]

[Coleen Phillimore]

On Wed, 22 Mar 2023 21:29:13 GMT, Matias Saavedra Silva <matsaave at openjdk.org> wrote:

>> In several places in HotSpot, the method MacroAssembler::null_check(Register, offset) is called in a way that never produces any null check in the assembly code. The method null_check(Register, offset) calls needs_explicit_null_check(offset) to determine if it must emit a null check in the assembly code or not.
>> 
>> needs_explicit_null_check(offset) returns true only if the offset is negative or bigger than the os page size. 
>> the offset being passed is the offset of a field in the header of Java object or a Java array. In both cases, the offset is always positive and smaller than an os page size. A null_check() call with a single parameter will always produce a null check in assembly.
>> 
>> The cases suggested in the issue have been addressed by either removing or preserving the null_check. Verified with tier 1-3 tests.
>
> Matias Saavedra Silva has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Removed load_klass_null_check

This seems to clear up the confusion around this null_check call.  Maybe add two asserts in shared oops code that klass_offset_in_bytes() and array_lengh_offset_in_bytes() are < pagesize would be sufficient to make this bug proof, but honestly seems 100% unlikely enough to not to bother.

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Marked as reviewed by coleenp (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13026#pullrequestreview-1354741670