RFR: 8253402: Convert vmSymbols::SID to enum class

[Ioi Lam]

On Wed, 23 Sep 2020 16:38:39 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> Convert `vmSymbols::SID` to an `enum class` to provide better type safety.
>> 
>> - The original enum type `vmSymbols::SID` cannot be forward-declared. I moved it out of the `vmSymbols` class and
>>   renamed, so now it can be forward-declared as `enum class vmSymbolID : int;`, without including the large vmSymbols.hpp
>>   file.
>>   - This also breaks  the mutual dependency between the `vmSymbols` and `vmIntrinsics` classes. Now  the declaration of
>>     `vmIntrinsics` can be moved from vmSymbols.hpp to vmIntrinsics.hpp, where it naturally belongs.
>> - Type-safe enumeration (contributed by Kim Barrett)
>> for (vmSymbolID index : vmSymbolsIterator()) {
>>    vm_symbol_index[as_int(index)] = index;
>> }
>> - I moved `vmSymbols::_symbols[]` to `Symbol::_vm_symbols[]`, and made it accessible via `Symbol::vm_symbol_at()`. This
>>   way, header files (e.g. fieldInfo.hpp) that need to convert from `vmSymbolID` to `Symbol*` don't need to include the
>>   large vmSymbols.hpp file.
>> - I changed the `VM_SYMBOL_ENUM_NAME` macro so that the users don't need to explicitly add the `vmSymbolID::` scope.
>> - I removed many unnecessary casts between `int` and `vmSymbolID`.
>> - The remaining casts are done via `vmSymbol::as_int()` and `vmSymbols::as_SID()` with range checks.
>> 
>> -----
>> If this is successful, I will do the same for `vmIntrinsics::ID`.
>
> src/hotspot/share/ci/ciObjectFactory.hpp line 32:
> 
>> 30: #include "utilities/growableArray.hpp"
>> 31:
>> 32: enum class vmSymbolID : int;
> 
> Do you need this declaration to avoid #include "classfile/vmSymbols.hpp" ?

Yes, that's the intention, because vmSymbols.hpp is big.

I am thinking whether I should move such forward declarations of enums into globalDefinitions.hpp, so header files that
use `vmSymbolID` don't need to know that the base type is `int`.

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PR: https://git.openjdk.java.net/jdk/pull/276