8324794: C2 SuperWord: do not ignore reductions in SuperWord::unrolling_analysis

[eme64]

Subtask of #16620

Ignoring reductions in unrolling_analysis is unnecessary, and it adds unnecessary dependency of unrolling_analysis on reduction-analysis. That dependency needs to be removed for further refactoring.

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Issue

• JDK-8324794: C2 SuperWord: do not ignore reductions in SuperWord::unrolling_analysis (Sub-task - P4)

Reviewers

• Christian Hagedorn (@chhagedorn - Reviewer)
• Vladimir Kozlov (@vnkozlov - Reviewer)

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[mlbridge]

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[vnkozlov]

Change match the subject but your description is confusing:

Ignoring reductions in unrolling_analysis is unnecessary, and it adds unnecessary dependency of unrolling_analysis on reduction-analysis. That dependency needs to be removed for further refactoring.

The only dependency you are talking about is the 'is_marked_reduction()` call in the condition you are removing. Right?

Do you know why this check was added?

[eme64]

@vnkozlov Yes, exactly, the call to is_marked_reduction. Other than that, unrolling_analysis could be static, and does not need any information from SuperWord. I'd like to splitt off unrolling_analysis, and so I'll have to remove the call to is_marked_reduction.

It seems like this was in from the begginning, when Michael Berg added the unrolling_analysis with 7c7b918

I can see no reason stated in the RFE or the code itself.

I can only speculate: maybe the idea was that reductions are not profitable, unless there are other nodes, like stores and loads. So if we only find reductions, then we would not adjust the unrolling, since we are not expecting vectorization anyway. Again: only speculation. You reviewed the code in 2015, maybe you still remember the reason ;)

FYI: only reductions may in the not too distant future become vectorizable in a profitable way, so I think removing this is good anyway.

[chhagedorn]

That looks reasonable.

[openjdk]

@eme64 This change now passes all automated pre-integration checks.

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After integration, the commit message for the final commit will be:

8324794: C2 SuperWord: do not ignore reductions in SuperWord::unrolling_analysis

Reviewed-by: chagedorn, kvn

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 38 new commits pushed to the master branch:

• f0024f5: 8324734: Relax too-strict assert(VM_Version::supports_evex()) in Assembler::locate_operand()
• fd8adf3: 8324856: Serial: Move Generation::is_in to DefNewGeneration
• a1d65eb: 8324125: Improve class initialization barrier in TemplateTable::_new for RISC-V
• b6d364a: 8324865: windows-x64-slowdebug still does not build after JDK-8324840
• 64c3642: 8242564: javadoc crashes:: class cast exception com.sun.tools.javac.code.Symtab$6
• e999dfc: 8323503: x86: Shorter movptr(reg, imm) for 32-bit unsigned immediates
• 84deeb6: 8324667: fold Parse::seems_stable_comparison()
• fb07bbe: 8324717: Remove HotSpotJVMCICompilerFactory
• d1e6763: 8324733: [macos14] Problem list tests which fail due to macOS bug described in JDK-8322653
• c1281e6: 8324678: Replace NULL with nullptr in HotSpot gtests
• ... and 28 more: https://git.openjdk.org/jdk/compare/781f368d421a94857929e4168974f43e890637d8...master

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[vnkozlov]

I can only speculate: maybe the idea was that reductions are not profitable, unless there are other nodes, like stores and loads. So if we only find reductions, then we would not adjust the unrolling, since we are not expecting vectorization anyway. Again: only speculation. You reviewed the code in 2015, maybe you still remember the reason ;)

I looked and there was no discussion about that during review. Originally it was not SuperWord analysis - it only looked for arithmetic Phi nodes in loop. Last year we changed it: 1be80a44
I think the check simplify unrolling_analysis code since we skip nodes we already know about them.

[eme64]

@vnkozlov this change here is in preparation for this next RFE: #17624

[vnkozlov]

After offline discussion I approve these changes.

[eme64]

Thanks @vnkozlov and @chhagedorn for the reviews!
/integrate

[openjdk]

Going to push as commit 11e28bd.
Since your change was applied there have been 40 commits pushed to the master branch:

• d51aaf6: 8323950: Null CLD while loading shared lambda proxy class with javaagent active
• f57c722: 8324880: Rename get_stack_trace.h
• f0024f5: 8324734: Relax too-strict assert(VM_Version::supports_evex()) in Assembler::locate_operand()
• fd8adf3: 8324856: Serial: Move Generation::is_in to DefNewGeneration
• a1d65eb: 8324125: Improve class initialization barrier in TemplateTable::_new for RISC-V
• b6d364a: 8324865: windows-x64-slowdebug still does not build after JDK-8324840
• 64c3642: 8242564: javadoc crashes:: class cast exception com.sun.tools.javac.code.Symtab$6
• e999dfc: 8323503: x86: Shorter movptr(reg, imm) for 32-bit unsigned immediates
• 84deeb6: 8324667: fold Parse::seems_stable_comparison()
• fb07bbe: 8324717: Remove HotSpotJVMCICompilerFactory
• ... and 30 more: https://git.openjdk.org/jdk/compare/781f368d421a94857929e4168974f43e890637d8...master

Your commit was automatically rebased without conflicts.

[openjdk]

@eme64 Pushed as commit 11e28bd.

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