8288754: GCC 12 fails to build zReferenceProcessor.cpp

[shipilev]

When compiling with GCC 12.1.1 (current in Fedora rawhide), the following warning-as-error is produced and breaks the build by default:

/home/test/shipilev-jdk/src/hotspot/share/gc/z/zReferenceProcessor.cpp: In member function 'oopDesc* ZReferenceProcessor::drop(oop, ReferenceType)':
/home/test/shipilev-jdk/src/hotspot/share/gc/z/zReferenceProcessor.cpp:270:22: error: '%s' directive argument is null [-Werror=format-overflow=]
  270 | log_trace(gc, ref)("Dropped Reference: " PTR_FORMAT " (%s)", p2i(reference), reference_type_name(type));
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The problem is over-zealous compiler seeing NULL from ShouldNotReachHere() block, and complaining about it. In this particular case, we can dodge this by returning a more reasonable constant on failure path.

(This thing should be more reasonably handled when ShouldNotReachHere() is somehow nonreturn-ed, but Hotspot style doc is still undecided on this).

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Issue

• JDK-8288754: GCC 12 fails to build zReferenceProcessor.cpp

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[mlbridge]

Webrevs

• 00: Full (71c600ab)

[shipilev]

Closed in favor of JDK 19 PR: openjdk/jdk19#47