RFR: JDK-8318119: Invalid narrow Klass base on aarch64 post 8312018

[Thomas Stuefe]

On Fri, 3 Nov 2023 14:32:54 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> Anyone?
>
>> Anyone?
> 
> I find those comments utterly incomprehensible.

@theRealAph I remove the questionable `CompressedKlassPointers::is_valid_base()` which in hindsight confused more than it helped. I removed all asserts using that function, since on the one platform where it matters we will run against other asserts early if encoding does not work.

For the one legitimate use of that function, validating -XX:SharedBaseAddress at CDS dump time, I just added an aarch64-specific line and a clear comment. I feel this is much clearer to the code reader. Sometimes ifdef is the best way to go.

I also added some comments to the reservation function. Please check if this agrees with you.

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PR Comment: https://git.openjdk.org/jdk/pull/16215#issuecomment-1814008436