RFR: 8247691: [aarch64] Incorrect handling of VM exceptions in C1 deopt stub/traps

[Patric Hedlin]

On Sat, 26 Sep 2020 09:34:03 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> The immediate issue that causes the error symptom is due to the procedure link register (LR) not being preserved across
>> the actual call to the "patching" function (in code generated from '_generate_patching()_'), invoked as part of the C1
>> deoptimization traps.  However, this update addresses a larger issue with the current implementation.
>> 
>> Since the Aarch64 implementation does not adopt patching but deoptimization to support load-mirror et.al., the
>> "patching" stub-code does not need to support the exception protocol otherwise necessary when a VM transition might be
>> needed in the actual patching function. The deoptimization is compulsory and the transition to the interpreter will
>> cater for proper interaction with the VM.  The proposed solution is thus: 1. The version of '_patch_code()_' used
>> with **DEOPTIMIZE_WHEN_PATCHING** should not be modelled as a **JRT_ENTRY**. The "patching" functions will not transfer
>> control to VM (for up-call or otherwise). 2. The patch-stub code does not need to handle the VM exception protocol
>> (code can be removed). 3. The patch-stub code need only to support deoptimization and re-execute (it's a fatal error
>> otherwise). 4. The patch-stub code does not need to utilise any instruction stream synchronisation barrier as no
>> patching/modifying of the instruction stream is done (including indirectly through a VM transition).
>
> Marked as reviewed by aph (Reviewer).

Thanks for reviewing Erik and Andrew.
Additional thanks to Erik for discussing the issue and the proposed solution.

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PR: https://git.openjdk.java.net/jdk/pull/344