RFR: 8254125: Assertion in cppVtables.cpp during builds on 32bit Windows

[Aleksey Shipilev]

On Sun, 11 Oct 2020 04:04:56 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> **Problem:** when iterating over the cloned vtables, the original code assumes that they are laid out consecutively in
>   memory. However, since [JDK-8224509](https://bugs.openjdk.java.net/browse/JDK-8224509), the memory allocated for each
>   of the the cloned vtables is now 8-byte aligned. This introduces gaps between the cloned vtables, and causes the assert
>   to fail.
> 
> **Fix:** the fix is to no longer assume the consecutive memory layout. Instead, use the CppVtables::_index array to
>   access each individual cloned vtable.
> 
> **Note:** I also cleaned up the code significantly. I feel the original code is pretty hard to understand, so if I just
>   do the bare minimum to fix the bug, it will be pretty hard to review.
> 
> I would suggest that the reviewers look at just the new version of the code and see if it's working as described
> (instead of looking at the diff to understand what the bug was and how it has been fixed).
> This version still uses the x-macro CPP_VTABLE_TYPES_DO to enumerate over the classes whose vtables need to be cloned.
> I plan to change that into templates in a future RFE.

This looks reasonable to me, but after testing it on x86_32, I wonder if
https://bugs.openjdk.java.net/browse/JDK-8254606 is the problem with this patch, or a different issue.

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PR: https://git.openjdk.java.net/jdk/pull/591