RFR: JDK-8294947: Use 64bit atomics in patch_verified_entry on x86_64

[Dean Long]

On Wed, 9 Nov 2022 17:19:55 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> In the void NativeJump::patch_verified_entry() we atomically patch first 4 bytes, then atomically patch 5th byte, then atomically patch first 4 bytes again. But from CMC (cross-modified code) point of view it's better to patch atomically 8 bytes at once.
>> 
>> The patch was tested with hotspot jtreg tests in bare-metal and virtualized environments.
>
> src/hotspot/cpu/x86/nativeInst_x86.cpp line 514:
> 
>> 512:   // complete jump instruction (to be inserted) is in code_buffer;
>> 513: #ifdef AMD64
>> 514:   unsigned char code_buffer[8];
> 
> Should we align this buffer too (to 8/jlong)?

I suggest using a union.

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PR: https://git.openjdk.org/jdk/pull/11059