RFR: 8255598: [PPC64] assert(Universe::heap()->is_in(result)) failed: object not in heap

[Albert Mingkun Yang]

On Thu, 5 Nov 2020 16:46:43 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

> JDK-8237363 introduced "assert(Universe::heap()->is_in..." check in CompressedOops::decode functions.
> This assertion restricts the usability of the decode functions. There are periods of time (during GC) at which we can't use " Universe::heap()->is_in" because the pointer gets switched between old and new location, but "Universe::heap()->is_in" is not yet accurate.
> PPC64 code has a usage of CompressedOops::decode which is affected by this problem. (It was observed with SerialGC, see JBS.)
> We could also use a weaker assertion, but seems like other people value the stronger assertion more. So I suggest to use decode_raw as workaround for PPC64.

Given the following naming pair, I would have guess that `decode_raw` can handle null properly, so the explicit check of null in this PR struck me as a surprise. Why so?

  static inline oop decode_raw_not_null(narrowOop v);
  static inline oop decode_raw(narrowOop v);

I think having some concise inline comments to explain why `decode_raw` is used over `decode` could improve readability.

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Changes requested by ayang (Author).

PR: https://git.openjdk.java.net/jdk/pull/1078