RFR: 8339113: AccessFlags can be u2 in metadata [v5]

[Martin Doerr]

On Fri, 20 Dec 2024 13:17:17 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:
> 
>   Restore ACC in comment.

src/hotspot/cpu/ppc/templateInterpreterGenerator_ppc.cpp line 149:

> 147:     // _access_flags must be a 16 bit value.
> 148:     assert(sizeof(AccessFlags) == 2, "wrong size");
> 149:     __ lha(R11_scratch1/*access_flags*/, method_(access_flags));

Using `lhz` would be more consistent. `lha` uses sign extend instead of zero extend. Feel free to clean this up if you want. Both instructions should work.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22246#discussion_r1897422214