RFR: 8270179: Rename Amalloc_4

[Kim Barrett]

On Mon, 12 Jul 2021 06:10:18 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> Consider this sequence, on 32bit:
> 
>     1. AmallocWords(4) -> _hwm = 4
> 
>     2. Amalloc(8) -> will return a pointer to offset 4, so not 64bit aligned.

I thought that's what ARENA_ALIGN at the beginning of Amalloc was for.  But you are right, there's nothing to align `_hwm`.  This looks like a pre-existing bug, and I wonder how this ever worked on 32bit platforms.

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PR: https://git.openjdk.java.net/jdk/pull/4750