RFR: 8273459: Update code segment alignment to 64 bytes

[Vladimir Kozlov]

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.

I share Dean's concern from the discussion before. `CodeEntryAlignment` is used in a lot of places and we have to be careful about changes to it.
I found only 7 cases with `align(64)`:

src/hotspot/cpu/x86//stubGenerator_x86_64.cpp:    __ align(64);
src/hotspot/cpu/x86//stubGenerator_x86_64.cpp:    __ align(64);
src/hotspot/cpu/x86//stubGenerator_x86_64.cpp:    __ align(64);
src/hotspot/cpu/x86//stubGenerator_x86_64.cpp:    __ align(64);
src/hotspot/cpu/x86//stubGenerator_x86_32.cpp:    __ align(64);
src/hotspot/cpu/x86//stubGenerator_x86_32.cpp:    __ align(64);
src/hotspot/cpu/x86//stubGenerator_x86_32.cpp:    __ align(64);

It does not justify such general changes.
May suggestion would be to add new `align64()` method to call pc() as you suggested in original proposal.

New function may also use `MaxVectorSize` as Jatin proposed if it helps.

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Changes requested by kvn (Reviewer).

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