8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops #21172

kimbarrett · 2024-09-24T23:26:08Z

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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Commit message must refer to an issue

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

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/21172/head:pull/21172
$ git checkout pull/21172

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

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 21172

View PR using the GUI difftool:
$ git pr show -t 21172

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/21172.diff

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bridgekeeper · 2024-09-24T23:26:44Z

👋 Welcome back kbarrett! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

openjdk · 2024-09-24T23:27:54Z

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

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

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.

➡️ To integrate this PR with the above commit message to the master branch, type /integrate in a new comment.

openjdk · 2024-09-24T23:28:28Z

@kimbarrett The following label will be automatically applied to this pull request:

hotspot

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

mlbridge · 2024-09-24T23:31:44Z

Webrevs

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

stefank

Looks good. One inquiry below.

stefank · 2024-09-25T07:47:21Z

src/hotspot/share/oops/compressedOops.inline.hpp

@@ -48,6 +48,9 @@ inline oop CompressedOops::decode_raw_not_null(narrowOop v) {
 }

 inline oop CompressedOops::decode_raw(narrowOop v) {
+  // Assume a null base casts to zero.  Otherwise we need more complexity that
+  // we can't test, since this is true for all currently supported platforms.
+  assert(0 == reinterpret_cast<uintptr_t>(nullptr), "null pointer value not zero?");


Could this be a static_assert?

+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.

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.

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

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 · 2024-09-25T08:36:37Z

src/hotspot/share/oops/compressedOops.inline.hpp

@@ -48,6 +48,9 @@ inline oop CompressedOops::decode_raw_not_null(narrowOop v) {
 }

 inline oop CompressedOops::decode_raw(narrowOop v) {
+  // Assume a null base casts to zero.  Otherwise we need more complexity that
+  // we can't test, since this is true for all currently supported platforms.
+  assert(0 == reinterpret_cast<uintptr_t>(nullptr), "null pointer value not zero?");


+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 · 2024-09-26T09:21:39Z

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 · 2024-09-26T11:45:17Z

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 · 2024-09-27T09:03:33Z

@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 · 2024-09-27T10:56:44Z

Thanks for all the reviews.

kimbarrett · 2024-09-27T10:56:55Z

/integrate

openjdk · 2024-09-27T10:58:10Z

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 · 2024-09-27T10:58:15Z

@kimbarrett Pushed as commit 25e8929.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

kimbarrett added 4 commits September 23, 2024 07:52

fix base comparisons to 0 in ppc.ad

d5797d5

fix base comparisons to 0 in s390.ad

6f63d46

fix base comparisons to 0 in shared

f870d04

fix setting base to null

ef0dc56

openjdk bot added the rfr Pull request is ready for review label Sep 24, 2024

openjdk bot added the hotspot hotspot-dev@openjdk.org label Sep 24, 2024

stefank approved these changes Sep 25, 2024

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shipilev approved these changes Sep 25, 2024

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Hamlin-Li approved these changes Sep 26, 2024

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offamitkumar approved these changes Sep 27, 2024

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remove nullptr representation asserts

a99a6cd

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shipilev approved these changes Sep 27, 2024

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kimbarrett deleted the fix-CompressedOops branch September 27, 2024 11:04

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8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops #21172

8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops #21172

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8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops #21172

8340620: Fix -Wzero-as-null-pointer-constant warnings for CompressedOops #21172

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