RFR: 8339113: AccessFlags can be u2 in metadata [v4]

[David Holmes]

On Thu, 19 Dec 2024 22:22:13 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Please review this change that makes AccessFlags and modifier_flags u2 types and removes the last remnants of Hotspot adding internal access flags.  This change moves AccessFlags and modifier_flags in Klass to alignment gaps saving 16 bytes.  From pahole: so it's a bit better.
>> 
>> before:
>> 
>>         /* size: 216, cachelines: 4, members: 25, static members: 17 */
>>         /* sum members: 194, holes: 3, sum holes: 18 */
>> 
>> 
>> after:
>> 
>>         /* size: 200, cachelines: 4, members: 25, static members: 17 */
>>         /* sum members: 188, holes: 4, sum holes: 12 */
>> 
>> 
>> We may eventually move the modifiers to java.lang.Class but that's WIP.
>> 
>> Tested with tier1-7 on oracle platforms.  Did test builds on other platforms (please try these changes ppc/arm32 and s390).  Also requires minor Graal changes.
>
> Coleen Phillimore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use ldrh rather than load_unsigned_short for aarch64, use testbitdi for ppc.

Things become somewhat clearer once one realizes/recalls that `AccessFlags` do not specifically pertain to access, but to all class/method modifiers as per the various JVMS ACC_XXX constants. We need to restore the leading comment in accessFlags.hpp to read

// AccessFlags is an abstraction over Java ACC flags.

as it originally did.

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PR Comment: https://git.openjdk.org/jdk/pull/22246#issuecomment-2556293078