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

[Vladimir Kozlov]

On Wed, 9 Nov 2022 12:41:59 GMT, Dmitry Samersoff <dsamersoff 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)?

src/hotspot/cpu/x86/nativeInst_x86.cpp line 532:

> 530: 
> 531: #else
> 532:   unsigned char code_buffer[5];

Should this be aligned?

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?

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