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

[Dean Long]

On Fri, 11 Nov 2022 12:09:10 GMT, Dmitry Samersoff <dsamersoff at openjdk.org> wrote:

>> 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)?
>
> @vnkozlov 
> 
> CXX optimizes that code to a few of register operations, and optimize out local variable _code_buffer_,  so we need not to care about its alignment.

> ```
> union {
>   jlong cb_long;
>   unsigned char code_buffer[8];
> } u;
> ```
> 

This version is fine.  It removes any question about the alignment of cb_long at the same time.

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