RFR: JDK-8294902: Undefined Behavior in C2 regalloc with null references [v9]

[Andrew Haley]

> This patch fixes the remaining null pointer dereference bugs that I know of.
> 
> For the main bug, C2 was using a null reference to indicate an uninitialized `Node_List`. I replaced the null reference with a static sentinel.
> 
> I also turned on `-fsanitize=null` and found and fixed a bunch of other null pointer dereferences. With this,I have run a full bootstrap and tier1 tests with `-fsanitize=null` enabled.
> 
> I have checked that the code generated by GCC is not worse in any significant way, so I don't expect to see any performance regressions.
> 
> I'd like to enable `-fsanitize=null` in debug builds to prevent regressions in this area. What do you think?

Andrew Haley has updated the pull request incrementally with one additional commit since the last revision:

  Reviewer feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10920/files
  - new: https://git.openjdk.org/jdk/pull/10920/files/812ce8fc..35e93fe8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10920&range=08
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10920&range=07-08

  Stats: 12 lines in 1 file changed: 7 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/10920.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10920/head:pull/10920

PR: https://git.openjdk.org/jdk/pull/10920