8289060: Undefined Behaviour in class VMReg

[theRealAph]

Like class Register, class VMReg exhibits undefined behaviour, in particular null pointer dereferences.

The right way to fix this is simple: make instances of VMReg point to reified instances of VMRegImpl. We do this by creating a static array of VMRegImpl, and making all VMReg instances point into it, making the code well defined.

However, while VMReg instances are no longer null, and so do not generate compile warnings or errors, there is still a problem in that higher-numbered VMReg instances point outside the static array of VMRegImpl. This is hard to avoid, given that (as far as I can tell) there is no upper limit on the number of stack slots that can be allocated as VMReg instances. While this is in theory UB, it's not likely to cause problems. We could fix this by creating a much larger static array of VMRegImpl, up to the largest plausible size of stack offsets.

We could instead make VMReg instances objects with a single numeric field rather than pointers, but some C++ compilers pass all such objects by reference, so I don't think we should.

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Issue

• JDK-8289060: Undefined Behaviour in class VMReg

Reviewers

• Jorn Vernee (@JornVernee - Reviewer)
• Vladimir Kozlov (@vnkozlov - Reviewer)

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[JornVernee]

We could instead make VMReg instances objects with a single numeric field rather than pointers, but some C++ compilers pass all such objects by reference, so I don't think we should.

I've been thinking about this some more after you fixed the same issue for Register on AArch64 [1]. I think the issue is out-of-line calls to member functions. Since this is a pointer, the compiler is forced to spill the value on the stack to comply with the ABI. i.e. what we'd really want is some way to say "pass this by value". (On x64 Windows, as long as a struct fits in a register, it is not passed by reference).

To avoid that, I think we could theoretically convert every member function to a static function that takes a VMReg as it's first argument. That's an option, but not a very nice one... (just mentioning it for the record).

[1] : #6280 (comment)

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I think the patch looks good overall, but it looks like there are some failures in some of the SA tests.

[theRealAph]

I've been thinking about this some more after you fixed the same issue for Register on AArch64 [1]. I think the issue is out-of-line calls to member functions. Since this is a pointer, the compiler is forced to spill the value on the stack to comply with the ABI. i.e. what we'd really want is some way to say "pass this by value". (On x64 Windows, as long as a struct fits in a register, it is not passed by reference).

Ah, I see. That makes sense.

I think the patch looks good overall, but it looks like there are some failures in some of the SA tests.

Right. I'll start digging.

[theRealAph]

I think the patch looks good overall, but it looks like there are some failures in some of the SA tests.

Right. I'll start digging.

Fixed now.

[JornVernee]

LGTM

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8289060: Undefined Behaviour in class VMReg

Reviewed-by: jvernee, kvn

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At the time when this comment was updated there had been 99 new commits pushed to the master branch:

• feb223a: 8288707: javax/swing/JToolBar/4529206/bug4529206.java: setFloating does not work correctly
• c3addbb: 8288895: LdapContext doesn't honor set referrals limit
• 1305fb5: 8287984: AArch64: [vector] Make all bits set vector sharable for match rules
• 7b5bd25: 8286397: Address possibly lossy conversions in jdk.hotspot.agent
• 28c5e48: 8287094: IGV: show node input numbers in edge tooltips
• da6d1fc: 8289477: Memory corruption with CPU_ALLOC, CPU_FREE on muslc
• 31e50f2: 8286104: use aggressive liveness for unstable_if traps
• dddd4e7: 8289291: HttpServer sets incorrect value for "max" parameter in Keep-Alive header value
• 048bffa: Merge
• cf71544: 8289252: Recommend Locale.of() method instead of the constructor
• ... and 89 more: https://git.openjdk.org/jdk/compare/a802b9816ac5c0cb0fd236cc7f25ed4fdb1349ef...master

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[vnkozlov]

Looks reasonable. Let me test it.

[vnkozlov]

My testing passed.

[theRealAph]

/integrate

[openjdk]

Going to push as commit dfb24ae.
Since your change was applied there have been 153 commits pushed to the master branch:

• 82a8bd7: 8287596: Reorg jdk.test.lib.util.ForceGC
• cc2b792: 8288992: AArch64: CMN should be handled the same way as CMP
• 75c0a5b: 8288824: [arm32] Display isetstate in register output
• 83a5d59: 8278479: RunThese test failure with +UseHeavyMonitors and +VerifyHeavyMonitors
• cbaf6e8: 8288022: c2: Transform (CastLL (AddL into (AddL (CastLL when possible
• 8341895: 8289739: Add G1 specific GC breakpoints for testing
• ac6be16: 8289695: [TESTBUG] TestMemoryAwareness.java fails on cgroups v2 and crun
• 4ad18cf: 8289730: Deprecated code sample in java.lang.ClassCastException
• d8f4e97: 8289146: containers/docker/TestMemoryWithCgroupV1.java fails on linux ppc64le machine with missing Memory and Swap Limit output
• f783244: 8289706: (cs) Avoid redundant TreeMap.containsKey call in AbstractCharsetProvider
• ... and 143 more: https://git.openjdk.org/jdk/compare/a802b9816ac5c0cb0fd236cc7f25ed4fdb1349ef...master

Your commit was automatically rebased without conflicts.

[openjdk]

@theRealAph Pushed as commit dfb24ae.

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