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

[Dmitry Samersoff]

On Wed, 9 Nov 2022 17:36:05 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 532:
> 
>> 530: 
>> 531: #else
>> 532:   unsigned char code_buffer[5];
> 
> Should this be aligned?

I would prefer to keep original 32bit code, that is here for ages, as it is. Verified entry point is always aligned, so alignment shouldn't be a problem.

> src/hotspot/cpu/x86/nativeInst_x86.cpp line 562:
> 
>> 560: 
>> 561:   // Patch bytes 0-3 (from jump instruction)
>> 562:   *(int32_t*)verified_entry = *(int32_t *)code_buffer;
> 
> Is this store and at line 552 atomic?

This code is also inherited. 
On x86 pointer sized stores is atomic, I used Atomic::store in 64bit code above just to improve readability.

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