RFR: 8270083: -Wnonnull errors happen with GCC 11.1.1

[Kim Barrett]

On Fri, 9 Jul 2021 00:28:38 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

> It is better if we can treat registers as a class **normally**. Similar change happens for markWord in [JDK-8229258](https://bugs.openjdk.java.net/browse/JDK-8229258). Should we change like now?

The potential difficulty with doing that is the same as for markWord - see [JDK-8235362](https://bugs.openjdk.java.net/browse/JDK-8235362).  A different option might be to make the various categories of Register be enum classes without any defined enumerators.  That wouldn't support the existing XXXRegisterImpl derivation from AbstractRegisterImpl.  I'm not sure that relationship is needed; it may not even be good.  The only use I can find is in the various assert_different_registers, and that could be done differently.  (The numerous overloads could also probably be simplified using variadic templates.)

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PR: https://git.openjdk.java.net/jdk/pull/4719