RFR: 8221554: aarch64 cross-modifying code [v6]

[Alan Hayward]

On Wed, 11 Nov 2020 15:04:32 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> src/hotspot/cpu/aarch64/templateInterpreterGenerator_aarch64.cpp line 1413:
>> 
>>> 1411:     __ blr(rscratch2);
>>> 1412:     // An instruction sync is required here after the call into the VM. However,
>>> 1413:     // that will have been caught in the VM by a cross_modify_fence call.
>> 
>> I think this wording is confusing; I had to read it several times. 
>> 
>> Would not something like
>> '''
>>     // When we return from the VM, the instruction stream may have
>>     // been modified. Previously we emitted an ISB at this point, but
>>     // it's now unnecessary because the VM itself calls cross_modify_fence()
>> '''
>> 
>> be better?
>
> This is still pending.

I wanted to avoid mentioning code that no longer exists. (Maybe it's best to just drop the comment?)

How about:

// When we return from the VM, the instruction stream may have
// been modified. Therefore needs an isb is required. The VM will
// have already done this by calling cross_modify_fence().

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PR: https://git.openjdk.java.net/jdk/pull/428