RFR: 8297534: Specify the size of MEMFLAGS

[Thomas Schatzl]

On Wed, 23 Nov 2022 22:19:52 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

> 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.

Imo the `_flags` member of `MallocHeader` should then also be of type `MEMFLAGS` as it is now defined what size it is. That saves some casting around in related code too.
I tried the change and it at least compiled, and do not see a reason not to.

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Changes requested by tschatzl (Reviewer).

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