[master] RFR: OMWorld: Cleanups [v2]

[Coleen Phillimore]

On Fri, 24 May 2024 15:06:20 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> src/hotspot/cpu/aarch64/c2_MacroAssembler_aarch64.cpp line 502:
>> 
>>> 500:       ldr(t1_monitor, Address(box, BasicLock::object_monitor_cache_offset_in_bytes()));
>>> 501:       // null check with Flags == NE, no valid pointer below alignof(ObjectMonitor*)
>>> 502:       cmp(t1_monitor, checked_cast<uint8_t>(alignof(ObjectMonitor*)));
>> 
>> I don't know what this means.
>
> Maybe I should say `valid ObjectMonitor* check` instead of `null check`. 
> 
> The reason that the check used to be against `2` is because some iterations of the code have signalled information in the lowest bit. 
> 
> Currently it could just be a check agains `1` which would check for `nullptr` while also setting Flags to NE. 
> 
> The check is simply that based on the fact that no valid ObjectMonitor* except nullptr can exist below `alignof(ObjectMonitor*)`.

But in the current code now, the field should be either nullptr or a valid ObjectMonitor, right?  If you simply checked against nullptr, would it set the NE flags?  All the extra casting etc makes this a puzzler where testing against nullptr does what it says.

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PR Review Comment: https://git.openjdk.org/lilliput/pull/178#discussion_r1613831179