8253640: Make MEMFLAGS an enum class

[stefank]

I'd like to propose that we make MEMFLAGS an enum class, to prevent them from being implicitly converted to integers.

I've been refactoring code that uses MEMFLAGS and have found that the functions often take integers and/or use default values. Because of this the compiler won't always tell you if you mess up the order. If we make the MEMFLAGS more type safe, the compiler will complain immediately if you send in the arguments in the wrong order.

Changes in the patch:

1. G1 is the only code that uses MemoryType instead of the MEMFLAGS typedef. I changed those to make the code base uniform w.r.t. this.
2. Added some code generation so that we still can use mtGC, and don't have to write MEMFLAGS::mtGC.
3. Some stricter checks around the values stored in MEMFLAGS variables. There was a masking of 0xFF that I removed in favor of an assert. Need to run some more extensive testing see if there was a need for this masking.
4. Removed unused by_index

Notes:

1. I didn't specify the underlying type of the enum, but it's clear for its usage that code will start to break if the value range is larger than 8 bits. Maybe something to clean up in a separate patch?
2. The code sometimes talks about "flags" and sometimes about "memory type", it's quite clear that the name changed, but the comments and names didn't get cleaned up accordingly.

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Progress

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Issue

• JDK-8253640: Make MEMFLAGS an enum class

Reviewers

• Thomas Stuefe (@tstuefe - Reviewer)
• Thomas Schatzl (@tschatzl - Reviewer)

Download

$ git fetch https://git.openjdk.java.net/jdk pull/378/head:pull/378
$ git checkout pull/378

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[mlbridge]

Webrevs

• 01: Full - Incremental (3f225ac)
• 00: Full (24cb2fb)

[tstuefe]

Hi Stefan,

I was just looking this morning at this enum and wished it were a class. This will make reworking the reservation APIs less error prone.

Found some small nits, but leave it up to you if you change anything.

(For some reason I always believed that using enum class would prevent us from using sentinel values but for the life of me I cannot remember why I thought this...)

Thanks, Thomas

[tstuefe]

Small nit, can you please clarify that this includes "mtNone"?

[stefank]

Good point!

[tstuefe]

Remark (possible for a later change): As you wrote, I think using uint8 would be okay and safer here.

[openjdk]

@stefank This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8253640: Make MEMFLAGS an enum class

Reviewed-by: stuefe, tschatzl

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At the time when this comment was updated there had been 5 new commits pushed to the master branch:

• 16b8c39: 8253053: Javadoc clean up in Authenticator and BasicAuthenicator
• 840aa2b: 8253424: Add support for running pre-submit testing using GitHub Actions
• 8e87d46: 8252857: AArch64: Shenandoah C1 CAS is not sequentially consistent
• c2692f8: 8225329: -XX:+PrintBiasedLockingStatistics causes crash during initia…
• e9c1782: 8252752: Clear card table for old regions during scan in G1

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[tstuefe]

lgtm

[stefank]

Thanks!

[tschatzl]

Lgtm.

[stefank]

Thanks, Thomas!

[stefank]

/integrate

[openjdk]

@stefank Since your change was applied there have been 24 commits pushed to the master branch:

• 86491a5: 8253584: Redunant errors for partial member selects
• ebf443a: 8253590: java/foreign tests are still failing on x86_32 after foreign-memaccess integration
• 431338b: 8212107: VMThread issues and cleanup
• 6bddeb7: 8238761: Asynchronous handshakes
• 6d19fe6: 8253763: ParallelObjectIterator should have virtual destructor
• 55c90a1: 6514600: AbstractAction can throw NullPointerException when clone()d
• b659132: 8252888: Collapse G1MMUTracker class hierarchy
• e63b90c: 8251358: Clean up Access configuration after Shenandoah barrier change
• 9c17a35: 8253748: StressIGV tests fail with release VMs
• 70b0fcc: 8253728: tests fail with "assert(fr.is_compiled_frame()) failed: Wrong frame type"
• ... and 14 more: https://git.openjdk.java.net/jdk/compare/276fcee79572de64134625e0d184fc3c99713595...master

Your commit was automatically rebased without conflicts.

Pushed as commit 3ed960e.

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