RFR: 8273459: Update code segment alignment to 64 bytes

[Jatin Bhateja]

On Thu, 16 Sep 2021 13:52:24 GMT, Scott Gibbons <github.com+6704669+asgibbons at openjdk.org> wrote:

> Change the default code entry alignment to 64 bytes from 32 bytes.  This allows for maintaining proper 64-byte alignment of data within a code segment, which is required by several AVX-512 instructions.
> 
> I ran into this while implementing Base64 encoding and decoding.  Code segments which were allocated with the address mod 32 == 0 but with the address mod 64 != 0 would cause the align() macro to misalign.  This is because the align macro aligns to the size of the code segment and not the offset of the PC.  So align(64) would align the PC to a multiple of 64 bytes from the start of the segment, and not to a pure 64-byte boundary as requested.  Changing the alignment of the segment to 64 bytes fixes the issue.
> 
> I have not seen any measurable difference in either performance or memory usage with the tests I have run.
> 
> See [this ](https://mail.openjdk.java.net/pipermail/hotspot-dev/2021-August/054180.html) article for the discussion thread.

src/hotspot/cpu/x86/globals_x86.hpp line 47:

> 45: // only assured that the entry instruction meets the 5 byte size requirement.
> 46: #if COMPILER2_OR_JVMCI
> 47: define_pd_global(intx, CodeEntryAlignment,       64);

How about setting dynamic alignment based on MaxVectorSize ? i.e. match the alignment to MaxVectorSize. This way we can save internal fragmentations over AVX2.
It could be done during VM Initialization similar to following code.
https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/x86/vm_version_x86.cpp#L1439

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PR: https://git.openjdk.java.net/jdk/pull/5547