8374446: Fix -Wzero-as-null-pointer-constant warnings in test_compressedKlass.cpp

[kimbarrett]

Please review this simple change to a recently added gtest to avoid triggering
-Wzero-as-null-pointer-constant warnings when that warning is enabled.

There are two parts to the change. One is just an obvious replacement of 0
with nullptr.

The other is the removal of a gtest assertion. That assertion's predicate is
attempting to verify the CompressedKlassPointers state has been initialized.
But the test being used (null pointer <= base pointer) is technically invalid;
ordered comparisons between a pointer and null are UB. And even ignoring that,
the compiler may decide the comparison is tautologially true, or it may always
be true at runtime, because pointer comparison is treated as unsigned by at
least some compilers. It would be better done via an is_initialized() function
in CompressedKlassPointers, or use == with the initial "uninitialized" value
of -1. But we're already asserting initialization in the implementation of the
base() function, so at least in debug builds we've already verified
initialization. It doesn't seem worthwhile adding is_initialized() or breaking
through the abstraction and using -1.

Testing: mach5 tier1

---

Progress

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Issue

• JDK-8374446: Fix -Wzero-as-null-pointer-constant warnings in test_compressedKlass.cpp (Enhancement - P4)

Reviewers

• David Holmes (@dholmes-ora - Reviewer)

Reviewing

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[openjdk]

@kimbarrett This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8374446: Fix -Wzero-as-null-pointer-constant warnings in test_compressedKlass.cpp

Reviewed-by: dholmes

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At the time when this comment was updated there had been 17 new commits pushed to the master branch:

• de81d38: 8374456: JVM crashes with "assert(resolved_method->method_holder()->is_linked()) failed: must be linked" when run with large value for PreallocatedOutOfMemoryErrorCount
• fa8ea6b: 8374168: Resolve disabled warnings in JDWP agent
• 5fd095f: 8372591: assert(!current->cont_fastpath() || freeze.check_valid_fast_path()) failed
• ... and 14 more: https://git.openjdk.org/jdk/compare/d9bd300c6eddfd30a83e53e7ae03c47ea43a9e08...master

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[mlbridge]

Webrevs

• 00: Full (33066341)

[dholmes-ora]

Fix seems trivially fine.

Removal of the assertion sounds reasonable based on your explanation

Thanks

[kimbarrett]

Thanks for reviewing @dholmes-ora

[kimbarrett]

/integrate

[openjdk]

Going to push as commit 3214428.
Since your change was applied there have been 26 commits pushed to the master branch:

• e27309f: 8374350: Convert hotspot gtests to use Atomic
• df5b49e: 8374525: RISC-V: Several masked float16 vector operations are not supported
• 532a0a6: 8372950: Pem.pemEncoded should cache the Pattern
• ... and 23 more: https://git.openjdk.org/jdk/compare/d9bd300c6eddfd30a83e53e7ae03c47ea43a9e08...master

Your commit was automatically rebased without conflicts.

[openjdk]

@kimbarrett Pushed as commit 3214428.

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