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

src/hotspot/share/opto/library_call.cpp line 4082:

> 4080: 
> 4081:   bool is_array = _gvn.type(addr)->isa_aryptr();
> 4082:   return is_mixed || (in_heap && !is_array);

Why is the is_array check needed?

-------------

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