RFR: 8282469: Allow considered use of C++ thread_local in Hotspot

[David Holmes]

On Mon, 7 Mar 2022 19:14:52 GMT, John R Rose <jrose at openjdk.org> wrote:

>> This patch provides a means for using C++ `thread_local` when it is essential - see JBS for more details.
>> 
>> There are three parts:
>> 
>> 1. Add the new #define for `thread_local`
>> 2. Remove `operator_new.cpp` as use of C++ `thread_local` with a non-trival cleanup actions requires use of global operators new/delete. These are still excluded for hotspot use via a link-time check.
>> 3. Remove the prohibition on using `thread_local` from the hotspot style guide
>> 
>> Due to the way hotspot style guide changes must be done, part 3 is being done under a sub-task in PR https://github.com/openjdk/jdk/pull/7720 and the two PR's will integrate at the same time.
>> 
>> Testing:
>>   - manual testing of the Panama usecase as referenced in the JBS issue
>>   - Tiers 1-3
>> 
>> Thanks,
>> David
>
> src/hotspot/share/memory/operator_new.cpp line 37:
> 
>> 35: // a memory leak.  Use CHeapObj as the base class of such objects to make it explicit
>> 36: // that they're allocated on the C heap.
>> 37: // Commented out in product version to avoid conflicts with third-party C++ native code.
> 
> There's a little bit of policy information here that is being deleted.  It overlaps with the section `### Memory Allocation` in `hotspot-style.md`, but includes this information which might not be stated elsewhere:
> 
>> Typically, uses of the C++ global operator new are inadvertent and therefore often associated with memory leaks.
> 
> (This is my rephrasing, perhaps appropriate to the style guide.)
> 
> Or is a point like this made in the config file which prevents direct linkage?  (I don't know where that file is.)

The link-time check is expressed in open/make/hotspot/lib/CompileJvm.gmk but simply states:


# Hotspot disallows the use of global operators 'new' and 'delete'. This build
# time check helps enforce this requirement. ...


The prohibition on using global operator new should definitely be explicitly documented somewhere, so the style guide seems a suitable place.

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PR: https://git.openjdk.java.net/jdk/pull/7719