RFR: 8081833: Clean up JVMFlag getter/setter code

[Ioi Lam]

On Wed, 16 Sep 2020 15:01:52 GMT, Gerard Ziemski <gziemski at openjdk.org> wrote:

>> This is a follow to [JDK-8243208 Clean up JVMFlag implementation](https://bugs.openjdk.java.net/browse/JDK-8243208).
>> 
>> I try to use templates and inheritance (and a bit of macros) to get rid of the large amount of duplicated code in the
>> JVMFlag getters/setters.
>> Stefan Karlsson and I have tried this several times already, but I think the current attempt probably has the least
>> amount of hacks and most functionality:
>> - Type safety is enforced up to the point of loading/storing `JVMFlag::_addr`, without any unchecked typecasts.
>> - Runtime type checking is implemented by the `JVM_FLAG_TYPE(t)` macro (see jvmFlagAccess.hpp), by passing along an
>>   integer (`JVMFlag::TYPE_int`, etc). This is more efficient than previous attempts that passed along a function pointer
>>   (see http://cr.openjdk.java.net/~stefank/8081833/webrev.04/)
>> - The old `JVMFlag::xxxAtPut` functions were undocumented. There are actually two groups that have different usage. I
>>   added documentation in jvmFlagAccess.hpp to explain this.
>> - I got rid of the `JVMFlagEx` class and moved the code into `JVMFlag`. This is possible with C++11 which allows forward
>>   declaration of `enum JVMFlagsEnum : int;`
>> - I changed `JVMFlag::_type` from a string into an enum. I am surprised this had lasted for over 20 years.
>> - I also changed `JVMFlag` from a struct into a class and made some fields private. I probably will do more cleanup in a
>>   separate RFE.
>> 
>> Please start with jvmFlagAccess.hpp, jvmFlagAccess.hpp and then globals_extension.hpp.
>> 
>> Probably not everyone is in love with the `JVM_FLAG_TYPE(t)` macro. I'll be happy to try out alternatives.
>
> src/hotspot/share/runtime/flags/jvmFlagAccess.cpp line 91:
> 
>> 89: public:
>> 90:   JVMFlag::Error set_impl(JVMFlag* flag, void* value_addr, JVMFlag::Flags origin) const {
>> 91:     bool verbose = !JVMFlagLimit::validated_after_ergo();
> 
> Why only verbose when not AfterErgo? Maybe add comment explaining that.

If I understand the old code correctly, we print out error messages only when parsing the command-line given by the
user. When we later override values that are set programmatically by the FLAG_SET_XXX() macros, we don't print out
error messages (or else the user will be confused). I will rewrite the code to make it clear, and add comments.

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PR: https://git.openjdk.java.net/jdk/pull/163