RFR: 8263375: Support stack watermarks in Zero VM

[Aleksey Shipilev]

On Mon, 12 Jul 2021 11:51:16 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> Zero VM supports most of GCs. Since JDK 16, Shenandoah uses stack watermarks, so Zero has to support those if Shenandoah+Zero support is to remain. This PR adds the stack watermark support in Zero VM. This should also be useful as other projects, notably Loom, mature and depend on stack watermarks.
>> 
>> Zero already calls into Hotspot safepoint machinery to do things, and it seems only the hooks for `on_iteration` and `on_unwind` are missing. AFAICS, Zero only has on-return safepoints, renamed it to be more precise.
>> 
>> @fisk, do you see any obvious problems with this patch?
>> 
>> Additional testing:
>>  - [x] Linux x86_64 Zero `hotspot_gc_shenandoah` now passes
>
> src/hotspot/cpu/zero/zeroInterpreter_zero.cpp line 205:
> 
>> 203:     // Notify the stack watermarks machinery that we are unwinding.
>> 204:     // Should do this before resetting the frame anchor.
>> 205:     stack_watermark_unwind_check(thread);
> 
> I wonder if this should maybe move down a bit to where we inspect the reason we left the interpreter loop. There are multiple reasons and only some involve unwinding. I'm thinking BytecodeInterpreter::return_from_method and BytecodeInterpreter::do_osr
> 
> Regarding BytecodeInterpreter::throwing_exception the current contract for exception handing is that an unwind handler is called *after* unwinding instead. We have some exception handler function in the interpreter runtime that gets called after unwinding with an exception into an interpreted frame. Hopefully that still gets called when using zero. Worth double checking.

Thanks! I'll take a look after I am back from extended time off. 

Zero does not actually do OSR anymore (Zero gradually eroded to interpreter-only mode), so only return_from_method might need handling. I'll see what happens in throwing_exception case; worst case I think I can set up a one-off frame anchor and call the unwind handler with it.

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PR: https://git.openjdk.java.net/jdk/pull/4728