[lilliput-jdk17u:lilliput] RFR: 8303771: [Lilliput/JDK17] Fix interpreter asymmetric fast-locking [v2]

[Thomas Stuefe]

On Thu, 9 Mar 2023 17:40:19 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> Backport of https://github.com/openjdk/lilliput/pull/76.
>> 
>> Currently we get the asymmetric locking check in the interpreters wrong:
>> 
>> ldr(header_reg, Address(rthread, JavaThread::lock_stack_current_offset()));
>> cmpoop(header_reg, obj_reg);
>> br(Assembler::NE, slow_case);
>> 
>> The intention is to load the top of the lock-stack, and compare it to the unlocked object, and, if not equal, branch to the slow-path to handle it. However, what it really does is, it loads the *address* of the top of lock-stack, and compares that to the unlocked object. This can never succeed, and therefore we always call the slow-path. Additionally, the address is not the address of the topmost object, it is the address of the next free slot. What we really want to load is the element at -1 oop from that address. This is not incorrect, but it's unnecessarily slow.
>> 
>> Testing:
>>  - [x] tier1 (x86_64, x86_32, aarch64)
>>  - [ ] tier2 (x86_64, x86_32, aarch64)
>
> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Alias tmp registers

+1

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Marked as reviewed by stuefe (Reviewer).

PR: https://git.openjdk.org/lilliput-jdk17u/pull/9