RFR: 8339113: AccessFlags can be u2 in metadata [v13]

[Dean Long]

On Tue, 7 Jan 2025 02:36:36 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:
> 
>   Make more AccessFlags fetches more specific and remove an assert and remove this->s.

src/hotspot/share/classfile/vmIntrinsics.cpp line 39:

> 37: 
> 38: // These are flag-matching functions:
> 39: inline bool match_F_R(u2 flags) {

I wish more code could be size-agnostic.  So instead of using `u2` here, there could be a typedef in accessFlags.hpp that we could use that hides the size.  However, it's not a big deal, because it seems unlikely this type will change much in the future without a JVM spec change.  I'm tempted to suggesting using AccessFlags here, but it's a class.  Since this is an end-point "consumer" of the type that doesn't store it or pass it along, we could even use something like `uint` here.

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PR Review Comment: https://git.openjdk.org/jdk/pull/22246#discussion_r1906033152