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

[David Holmes]

On Tue, 14 Mar 2023 18:17:09 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.

So basically this change is statically deciding that `needs_explicit_null_check` will always be false at these call-sites.

I find it hard to determine where the actual null check does occur for something like `arraylength()`?

Shouldn't the same change be made for all the other platforms that use the same code here?

Thanks.

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