8264020: Optimize double negation elimination

[e1iu]

For the double negation '-(-x)', c2 could reduce the redundant 'sub'
through SubNode::Ideal. But every time when 'sub' is removed, there
would generate two useless nodes SubNode(x, 0)[1] and AddNode(x, -0)[2],
which would be removed by later phases but should have optimized better
to 'x' itself without generating those new stuffs.

This patch is a small fix for SubNode's Ideal, by leaving this special
case to 'Identity' which handled double negation already[3].

[1] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/subnode.cpp#L238
[2] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/subnode.cpp#L181
[3] https://github.com/openjdk/jdk/blob/master/src/hotspot/share/opto/subnode.cpp#L55

---

Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change must be properly reviewed

Issue

• JDK-8264020: Optimize double negation elimination

Reviewers

• Tobias Hartmann (@TobiHartmann - Reviewer)
• Christian Hagedorn (@chhagedorn - Reviewer)

Reviewing

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Update a local copy of the PR:
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$ git pull https://git.openjdk.java.net/jdk pull/3231/head

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$ git pr checkout 3231

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https://git.openjdk.java.net/jdk/pull/3231.diff

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[mlbridge]

Webrevs

• 01: Full - Incremental (d553a68)
• 00: Full (934fe11)

[TobiHartmann]

Otherwise, looks good to me.

[openjdk]

@theRealELiu 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:

8264020: Optimize double negation elimination

Reviewed-by: thartmann, chagedorn

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At the time when this comment was updated there had been 125 new commits pushed to the master branch:

• 1a681fa: 8263560: Remove needless wrapping with BufferedInputStream
• 182b11c: 8264016: [JVMCI] add some thread local fields for use by JVMCI
• 8cf1c62: 8263754: HexFormat 'fromHex' methods should be static
• a5d7de2: 8263404: RsaPrivateKeySpec is always recognized as RSAPrivateCrtKeySpec in RSAKeyFactory.engineGetKeySpec
• 128c0c9: 8248418: jpackage fails to extract main class and version from app module linked in external runtime
• fd45694: 8264344: Outdated links in JavaComponentAccessibility.m
• f17ea9e: 8262899: TestRedirectLinks fails
• 963f1fc: 8264309: JFR: Improve .jfc parser
• 364cce1: 8264332: Use the blessed modifier order in jdk.charsets
• fbbd98b: 8264029: Replace uses of StringBuffer with StringBuilder in java.base
• ... and 115 more: https://git.openjdk.java.net/jdk/compare/a5e7a890fe7c7845cd06cfd3770e32f21d8d0573...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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➡️ 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).

[TobiHartmann]

Thanks for updating! Looks good to me.

[chhagedorn]

Looks good!

[e1iu]

/integrate

[openjdk]

@theRealELiu
Your change (at version d553a68) is now ready to be sponsored by a Committer.

[nsjian]

/sponsor

[openjdk]

@nsjian @theRealELiu Since your change was applied there have been 131 commits pushed to the master branch:

• 4ffa41c: 8263615: Cleanup tightly_coupled_allocation
• 4ea6abf: 8264324: Simplify allocation list management in OopStorage::reduce_deferred_updates
• 8735259: 8264333: Use the blessed modifier order in jdk.jshell
• d2a63f2: 8264360: Loop strip mining verification fails with "should be on the backedge"
• 8100a20: 8263971: C2 crashes with SIGFPE with -XX:+StressGCM and -XX:+StressIGVN
• bcdf469: 8264337: VM crashed when -XX:+VerifySharedSpaces
• 1a681fa: 8263560: Remove needless wrapping with BufferedInputStream
• 182b11c: 8264016: [JVMCI] add some thread local fields for use by JVMCI
• 8cf1c62: 8263754: HexFormat 'fromHex' methods should be static
• a5d7de2: 8263404: RsaPrivateKeySpec is always recognized as RSAPrivateCrtKeySpec in RSAKeyFactory.engineGetKeySpec
• ... and 121 more: https://git.openjdk.java.net/jdk/compare/a5e7a890fe7c7845cd06cfd3770e32f21d8d0573...master

Your commit was automatically rebased without conflicts.

Pushed as commit f3726a8.

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