RFR: 8269119: C2: Avoid redundant memory barriers in Unsafe.copyMemory0 intrinsic [v4]

[Tobias Hartmann]

On Fri, 3 Sep 2021 14:24:57 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:
> 
>   Stress instruction scheduling

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

> 4080:   bool is_mixed  = !in_heap && !in_native;
> 4081: 
> 4082:   bool is_prim_array = (addr_t != NULL) && (addr_t->elem() != Type::BOTTOM);

What does `is_prim_array` stand for? I read this as "is primitive array" which does not make sense :)

test/hotspot/jtreg/compiler/unsafe/UnsafeCopyMemory.java line 31:

> 29:  *
> 30:  * @run main/othervm -Xbatch -XX:CompileCommand=dontinline,compiler.unsafe.UnsafeCopyMemory::test*
> 31:  *                   -XX:+UnlockDiagnosticVMOptions -XX:+StressGCM -XX:+StressLCM

You should add `@key stress` to the test.

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