RFR: 8241613: Suspicious calls to MacroAssembler::null_check(Register, offset) [v2]

[Matias Saavedra Silva]

On Thu, 16 Mar 2023 20:02:56 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:
> 
>   load_klass now emits null check and other platforms supported

I have now added support for the other platforms. In addition, I noticed that in `load_klass_check_null` the check was never generated when by all means it should be. Instead of removing these checks to maintain the current behavior, I think it would be best to implement the intended functionality. Otherwise `load_klass_check_null` does nothing else but call `load_klass` with a potentially null klass. 

Given that this has passed under the radar until recently, there must be existing checks down the line that check for nulls.

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PR: https://git.openjdk.org/jdk/pull/13026