RFR: 8297534: Specify the size of MEMFLAGS [v2]

[Johan Sjölen]

> Hi!
> 
> In MallocHeader we assume that MEMFLAGS can be stored in a uint8_t, see:
> 
> ```c++
> class MallocHeader {
> 
>   NOT_LP64(uint32_t _alt_canary);
>   const size_t _size;
>   const uint32_t _mst_marker;
>   const uint8_t _flags;
>   const uint8_t _unused;
>   uint16_t _canary;
> // SNIP!
>   inline MEMFLAGS flags() const { return (MEMFLAGS)_flags; }
> 
> 
> With C++11 we can specify the underlying type of an `enum class`, so let's just do that. If we generate 257 memflags we'll get a compile error. I don't believe that this would lead to any change in the behavior of the code.
> 
> Testing: Compiled the code. Running tier1 testing right now.

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  Make _flags be of type MEMFLAGS

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/11336/files
  - new: https://git.openjdk.org/jdk/pull/11336/files/046f6abc..ce9a1d24

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11336&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11336&range=00-01

  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11336.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11336/head:pull/11336

PR: https://git.openjdk.org/jdk/pull/11336