RFR: 8269119: C2: Avoid redundant memory barriers in Unsafe.copyMemory0 intrinsic [v2]

[Roland Westrelin]

On Wed, 1 Sep 2021 14:17:16 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> `Unsafe::copyMemory0` intrinsic unconditionally inserts barriers arounds the call to `unsafe_arraycopy` stub.
>> It is a conservative approach and barriers can be avoided in most common cases (similar to what is done for scalar unsafe accesses).
>> 
>> `Unsafe::copyMemory()` performs argument validation which limits inputs either
>> to off-heap location (null + absolute address) or primitive on-heap array.
>> 
>> The only cases when barriers are still needed are:
>>  * mixed accesses (`Object+offset`);
>>  * mismatched access due to lack of type info on base oop (`Object:NotNull+offset`). 
>> 
>> Testing: hs-tier1 - hs-tier6
>
> Vladimir Ivanov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove memory barriers

Looks good to me.

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Marked as reviewed by roland (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/5259