8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops

[kimbarrett]

Please review this change that fixes -Wzero-as-null-pointer-constant warnings
in CompressedOops code. These all relate to CompressedOops::base().

I also added a couple of asserts to verify our assumptions about null pointer
constants being representationally zero. That isn't a Standard-conforming
assumption, but holds for all platforms we currently support. I considered,
and even explored, a couple of different options.

(1) Continue to have CompressedOops::base() be a pointer, but avoid that
assumption, being more careful about how zero-valued pointers are treated. But
that adds significant complexity that we can't test, since we don't support
any platforms needing that extra work.

(2) Change CompressedOops::base() to an integral adjustment. This is probably
the correct approach, but is much more intrusive and wide ranging in the
changes required. Maybe something for the future.

Testing: mach5 tier1-5
GHA testing, verifying builds on some platforms not supported by Oracle.

There are some simple changes to s390 and ppc code that I haven't tested,
beyond verifying compilation.

---

Progress

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Issue

• JDK-8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops (Enhancement - P4)

Reviewers

• Stefan Karlsson (@stefank - Reviewer) Review applies to ef0dc56e
• Aleksey Shipilev (@shipilev - Reviewer)
• Hamlin Li (@Hamlin-Li - Reviewer) Review applies to ef0dc56e
• Amit Kumar (@offamitkumar - Committer) Review applies to ef0dc56e

Reviewing

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Update a local copy of the PR:
$ git checkout pull/21172
$ git pull https://git.openjdk.org/jdk.git pull/21172/head

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$ git pr checkout 21172

View PR using the GUI difftool:
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https://git.openjdk.org/jdk/pull/21172.diff

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[bridgekeeper]

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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:

8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops

Reviewed-by: shade, stefank, mli, amitkumar

You can use pull request commands such as /summary, /contributor and /issue to adjust it as needed.

At the time when this comment was updated there had been 89 new commits pushed to the master branch:

• 6587909: 8341015: OopStorage location decoder crashes accessing non-initalized OopStorage
• 9003e2c: 8341027: Crash in java/runtime/Unsafe/InternalErrorTest when running with -XX:-UseCompressedClassPointers
• 2a2ecc9: 8339475: Clean up return code handling for pthread calls in library coding
• 85dba47: 8325090: javadoc fails when -subpackages option is used with non-modular -source
• 1bc13a1: 8340552: Harden TzdbZoneRulesCompiler against missing zone names
• e6373b5: 8340679: Misc tests fail assert(!set || SafepointSynchronize::is_at_safepoint()) failed: set once or at safepoint
• 2349bb7: 8340974: Ambiguous name of jtreg property vm.libgraal.enabled
• 5d062e2: 8340576: Some JVMCI flags are inconsistent
• 1447967: 8339261: Logs truncated in test javax/net/ssl/DTLS/DTLSRehandshakeTest.java
• a02d895: 8333403: Write a test to check various components events are triggered properly
• ... and 79 more: https://git.openjdk.org/jdk/compare/dd498794f20df0ac1a73d84e54591905c8a5a5c7...master

As there are no conflicts, your changes will automatically be rebased on top of these commits when integrating. If you prefer to avoid this automatic rebasing, please check the documentation for the /integrate command for further details.

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[openjdk]

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[mlbridge]

Webrevs

• 01: Full - Incremental (a99a6cd9)
• 00: Full (ef0dc56e)

[stefank]

Looks good. One inquiry below.

[stefank]

Could this be a static_assert?

[shipilev]

+1. Although I would expect any sane compiler to fold it, maybe it is still not optimized with something like -O0. Or maybe just move these asserts to CompressedOops::initialize, so whatever happens, happens once.

[merykitty]

This cannot be used in static_assert because reinterpret_cast is not allowed here.

I believe reinterpret_cast<uintptr_t>(nullptr) will always return 0. You may need to do it the other way around.

[kimbarrett]

Correct that static_assert can't be used, because of the reinterpret_cast.

I considered putting the assert in CompressedOops::set_base, with comments to
connect encode/decode with that assertion, but prefer putting the assert near
the code that is actually relying on the property. Even with -O0 gcc seems to
constant fold the expression and (pretty much?) compile away the assertion.

A null pointer value is not guaranteed to have a zero representation. The
conversion of a literal 0 to a null pointer value is syntactic sugar. The
bit pattern of the result might be something else. There's some good
discussion of this here:
https://stackoverflow.com/questions/2761360/could-i-ever-want-to-access-the-address-zero

[shipilev]

All right, if this assert folds even on lowest optimization level, this all is just nitpicking then. Ship it.

[shipilev]

Looks fine, modulo assert comment:

[shipilev]

+1. Although I would expect any sane compiler to fold it, maybe it is still not optimized with something like -O0. Or maybe just move these asserts to CompressedOops::initialize, so whatever happens, happens once.

[stefank]

FWIW, I think these asserts adds extra noise to these functions and I don't think we will be much more happy about having to read them over and over again when we read this functions / debug code through these functions. I would have preferred if this was one of those things that we require from our platforms and place a check in globalDefinitions, or some other prominent place that checks HotSpot's assumptions of the compilers / platforms.

[kimbarrett]

FWIW, I think these asserts adds extra noise to these functions and I don't think we will be much more happy about having to read them over and over again when we read this functions / debug code through these functions. I would have preferred if this was one of those things that we require from our platforms and place a check in globalDefinitions, or some other prominent place that checks HotSpot's assumptions of the compilers / platforms.

Implementing option 2 (making base() an integral offset) would remove that
assumption here, and allow removal of the assertions currently proposed here.
And I generally prefer placing asserts with the expecting code. OTOH, I think
it's nearly certain there are other places where we make the same assumption.
(And I'd forgotten we have some assumption checks in globalDefinitions.cpp.)
I don't have a strong opinion in this area.

[kimbarrett]

@stefank wrote:

FWIW, I think these asserts adds extra noise to these functions [...]

@kimbarrett replied

Implementing option 2 (making base() an integral offset) would remove that assumption here, and allow removal of the assertions currently proposed here. And I generally prefer placing asserts with the expecting code. OTOH, I think it's nearly certain there are other places where we make the same assumption. (And I'd forgotten we have some assumption checks in globalDefinitions.cpp.) I don't have a strong opinion in this area.

@stefank and I discussed this offline. I've removed the asserts, as they are just kind of executable comments that
aren't going to help catch any bugs. As a separate task we're going to consider adding a file that checks various
assumptions like this, as a way of documenting assumptions we make.

[shipilev]

Still good, ship it.

[kimbarrett]

Thanks for all the reviews.

[kimbarrett]

/integrate

[openjdk]

Going to push as commit 25e8929.
Since your change was applied there have been 89 commits pushed to the master branch:

• 6587909: 8341015: OopStorage location decoder crashes accessing non-initalized OopStorage
• 9003e2c: 8341027: Crash in java/runtime/Unsafe/InternalErrorTest when running with -XX:-UseCompressedClassPointers
• 2a2ecc9: 8339475: Clean up return code handling for pthread calls in library coding
• 85dba47: 8325090: javadoc fails when -subpackages option is used with non-modular -source
• 1bc13a1: 8340552: Harden TzdbZoneRulesCompiler against missing zone names
• e6373b5: 8340679: Misc tests fail assert(!set || SafepointSynchronize::is_at_safepoint()) failed: set once or at safepoint
• 2349bb7: 8340974: Ambiguous name of jtreg property vm.libgraal.enabled
• 5d062e2: 8340576: Some JVMCI flags are inconsistent
• 1447967: 8339261: Logs truncated in test javax/net/ssl/DTLS/DTLSRehandshakeTest.java
• a02d895: 8333403: Write a test to check various components events are triggered properly
• ... and 79 more: https://git.openjdk.org/jdk/compare/dd498794f20df0ac1a73d84e54591905c8a5a5c7...master

Your commit was automatically rebased without conflicts.

[openjdk]

@kimbarrett Pushed as commit 25e8929.

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