RFR: 8253640: Make MEMFLAGS an enum class [v2]

[Thomas Stuefe]

On Mon, 28 Sep 2020 13:00:10 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> 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.
>
> Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Review 1

lgtm

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Marked as reviewed by stuefe (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/378