RFR: 8364588: Export the NPE backtracking functionality to general null-checking APIs [v3]

[Chen Liang]

On Tue, 5 Aug 2025 14:24:37 GMT, Chen Liang <liach at openjdk.org> wrote:

>> I'd like to see the description for `slot` pushed into an enum, and make any negative number except `-1` explicitly forbidden.
>> 
>> ```c++
>> enum class NPESlot : int {
>>   // docs here
>>   None = -1,
>>   // docs here
>>   Explicit // Anything >= 0
>> };
>> bool get_NPE_message_at(outputStream* ss, Method* method, int bci, NPESlot slot) {
>>   if (slot != NPESlot::None) {
>>     // Explicit search
>>   }
>> }
>
> I don't think I know how to use C++ enums. I tried to define this enum in bytecodeUtils.hpp and use it from jvm.cpp, and don't know how in any way I can ever express `BytecodeUtils::NullSlot slot = search_slot >= 0 ? search_slot : ???;` because `NullSlot::Search` or `BytecodeUtils::NullSlot::Search` or `BytecodeUtils::NullSlot.Search` are all erroneous. I don't think this may worth the hassle since the attempt to use C++ enum only creates more pain, unless you tell me how to use it properly.

I think I figured out how to use static_cast with C++ enum classes.

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PR Review Comment: https://git.openjdk.org/jdk/pull/26600#discussion_r2254630773