8351515: C2 incorrectly removes double negation for double and float

[mhaessig]

Issue Summary

A fuzzer run discovered that code of the form 0 - (0 - x) yields the wrong result for x = -0.0. According to the Java Language Spec 15.8.2 the sum of floating point zeroes of opposite sign is positive zero. Hence, (0 - (0 - (-0.0)) must result in +0.0, but due to the folding of all double negations in SubNode::Identity this was optimized to -0.0

Changeset overview

To fix this issue, I excluded floating point numbers from the folding of double negations, which also includes Float16 values. This might seem excessive at first glance, but we do not track range information for floating point types. Hence, we could still perform the folding of the double negation if a floating point constant is not -0.0. However, constant folding already takes care of this.

Changes:

• IR-Framework: fix IRNode.SUBnot matching SubHFNode
• Add a regression IR-test
• Exclude floating point SubNodes from folding double negations

Testing

• Github Actions
• tier1 through tier5 plus Oracle internal testing

---

Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue

Issue

• JDK-8351515: C2 incorrectly removes double negation for double and float (Bug - P3)

Reviewers

• Christian Hagedorn (@chhagedorn - Reviewer) Review applies to 7805314f
• Tobias Hartmann (@TobiHartmann - Reviewer)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk.git pull/24150/head:pull/24150
$ git checkout pull/24150

Update a local copy of the PR:
$ git checkout pull/24150
$ git pull https://git.openjdk.org/jdk.git pull/24150/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 24150

View PR using the GUI difftool:
$ git pr show -t 24150

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk/pull/24150.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back mhaessig! 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]

@mhaessig 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:

8351515: C2 incorrectly removes double negation for double and float

Reviewed-by: thartmann, chagedorn

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 22 new commits pushed to the master branch:

• 56a4ffa: 8352597: [IR Framework] test bug: TestNotCompilable.java fails on product build
• e23e0f8: 8352591: Missing UnlockDiagnosticVMOptions in VerifyGraphEdgesWithDeadCodeCheckFromSafepoints test
• adfb120: 8351748: Add class init barrier to AOT-cached Method/Var Handles
• ee1577b: 8352652: [BACKOUT] nsk/jvmti/ tests should fail when nsk_jvmti_setFailStatus() is called
• df9210e: 8347706: jvmciEnv.cpp has jvmci includes out of order
• 5dd0acb: 8352477: RISC-V: Print warnings when unsupported intrinsics are enabled
• 334a1ee: 8351375: nsk/jvmti/ tests should fail when nsk_jvmti_setFailStatus() is called
• 7442039: 8337279: Share StringBuilder to format instant
• 22182f7: 8352112: [ubsan] hotspot/share/code/relocInfo.cpp:130:37: runtime error: applying non-zero offset 18446744073709551614 to null pointer
• c2e14b1: 8343891: Test javax/swing/JTabbedPane/TestJTabbedPaneBackgroundColor.java failed
• ... and 12 more: https://git.openjdk.org/jdk/compare/1c0fa0af7847d80fd3fbe38f28207aab270609b3...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.

As you do not have Committer status in this project an existing Committer must agree to sponsor your change. Possible candidates are the reviewers of this PR (@chhagedorn, @TobiHartmann) but any other Committer may sponsor as well.

➡️ To flag this PR as ready for integration with the above commit message, type /integrate in a new comment. (Afterwards, your sponsor types /sponsor in a new comment to perform the integration).

[openjdk]

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

• hotspot-compiler

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]

Webrevs

• 02: Full - Incremental (9c79f93c)
• 01: Full - Incremental (dfcc756f)
• 00: Full (7805314f)

[chhagedorn]

Looks good to me. Nice to also see a Float16 IR test :-)

[SirYwell]

Negation here can be a bit of a confusing term since the JLS § 15.15.4 says:

For floating-point values, negation is not the same as subtraction from zero, because if x is +0.0, then 0.0-x is +0.0, but -x is -0.0

This is also the reason why NegF/D nodes are used separately (NegI/L nodes exist but seem to be unused). The change itself looks good, but maybe it's worth to clarify the comment? Or somehow reference the comment here:

jdk/src/hotspot/share/opto/subnode.hpp

Lines 473 to 478 in b32be18

	//------------------------------NegFNode---------------------------------------
	// Negate value a float. Negating 0.0 returns -0.0, but subtracting from
	// zero returns +0.0 (per JVM spec on 'fneg' bytecode). As subtraction
	// cannot be used to replace negation we have to implement negation as ideal
	// node; note that negation and addition can replace subtraction.
	class NegFNode : public NegNode {

[mhaessig]

@SirYwell thanks for the pointer to the negation section of the spec. I improved the comment by referencing spec and differentiating subtraction from negation.

[TobiHartmann]

Looks good to me otherwise!

[mhaessig]

Thanks for the review and helpful comments everyone!

/integrate

[openjdk]

@mhaessig
Your change (at version 9c79f93) is now ready to be sponsored by a Committer.

[TobiHartmann]

/sponsor

[openjdk]

Going to push as commit 5591f8a.
Since your change was applied there have been 22 commits pushed to the master branch:

• 56a4ffa: 8352597: [IR Framework] test bug: TestNotCompilable.java fails on product build
• e23e0f8: 8352591: Missing UnlockDiagnosticVMOptions in VerifyGraphEdgesWithDeadCodeCheckFromSafepoints test
• adfb120: 8351748: Add class init barrier to AOT-cached Method/Var Handles
• ee1577b: 8352652: [BACKOUT] nsk/jvmti/ tests should fail when nsk_jvmti_setFailStatus() is called
• df9210e: 8347706: jvmciEnv.cpp has jvmci includes out of order
• 5dd0acb: 8352477: RISC-V: Print warnings when unsupported intrinsics are enabled
• 334a1ee: 8351375: nsk/jvmti/ tests should fail when nsk_jvmti_setFailStatus() is called
• 7442039: 8337279: Share StringBuilder to format instant
• 22182f7: 8352112: [ubsan] hotspot/share/code/relocInfo.cpp:130:37: runtime error: applying non-zero offset 18446744073709551614 to null pointer
• c2e14b1: 8343891: Test javax/swing/JTabbedPane/TestJTabbedPaneBackgroundColor.java failed
• ... and 12 more: https://git.openjdk.org/jdk/compare/1c0fa0af7847d80fd3fbe38f28207aab270609b3...master

Your commit was automatically rebased without conflicts.

[openjdk]

@TobiHartmann @mhaessig Pushed as commit 5591f8a.

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