8369898: C2 SuperWord: assert(has_ctrl(i)) failed: should be control, not loop

[eme64]

In PhaseIdealLoop::create_new_if_for_multiversion, we replace multiversion_slow_proj with the new region that merges the new_multiversion_slow_proj and new_if_false. Using igvn.replace_node moves all the control inputs of the outputs of the old multiversion_slow_proj to the new region. This is sufficient during IGVN, but not during loop-opts: the "controllees" of multiversion_slow_proj (the nodes that used to answer to get_ctrl with multiversion_slow_proj) should now be "controllees" of region (answer region for get_ctrl).

This is what lazy_replace is for:

• It puts a "forwarding" in the _loop_and_ctrl table: instead of mapping multiversion_slow_proj to its loop, it now points to the new ctrl node region.
• When we call get_ctrl on a "controllee" of the old multiversion_slow_proj, we then skip over multiversion_slow_proj via the "forwarding" to the new region.

I'm proposing a PR to improve the documentation and some renamings around get_ctrl and lazy_replace:
#27892

A previous PR that used lazy_replace, in case you want to understand more:
#15720

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Issue

• JDK-8369898: C2 SuperWord: assert(has_ctrl(i)) failed: should be control, not loop (Bug - P3)

Reviewers

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

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[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:

8369898: C2 SuperWord: assert(has_ctrl(i)) failed: should be control, not loop

Reviewed-by: chagedorn, kvn

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At the time when this comment was updated there had been 118 new commits pushed to the master branch:

• a1be297: 8369854: (ch) Refine specification of behavior of {Gathering,Writable}ByteChannel.write
• 257bb2b: 8370079: Re-enable vmTestbase/gc/vector/CircularListLow and LinearListLow with SerialGC
• dc6858f: 8370230: Bad copyright in NMTPrintMallocSiteOfCorruptedMemory.java after JDK-8369527
• ... and 115 more: https://git.openjdk.org/jdk/compare/9feb8f21b5d000f8901938f1dde89638c79ca805...master

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Webrevs

• 01: Full - Incremental (6e23865a)
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[chhagedorn]

The fix looks reasonable to me and I agree that we should improve the documentation for lazy_replace() and also think about it's naming - by just looking at the name, one could first guess that the actual node replacement is delayed which is not the case!

Maybe @rwestrel also wants to have a look who worked with lazy_replace() in the mentioned PR #15720.

[eme64]

@chhagedorn Thanks for the suggestions, they are all applied :)

[chhagedorn]

Update looks good, thanks!

[vnkozlov]

Good.

[eme64]

@chhagedorn @vnkozlov Thanks for the reviews!

/integrate

[openjdk]

Going to push as commit 634746a.
Since your change was applied there have been 124 commits pushed to the master branch:

• 207fe55: 8369902: C2 SuperWord: wrong result because filterin NaN instead of zero in MemPointerParser::canonicalize_raw_summands
• eee2908: 8370257: Remove ProblemListed tests from ProblemList.txt
• 2de8d58: 8366333: AArch64: Enhance SVE subword type implementation of vector compress
• ... and 121 more: https://git.openjdk.org/jdk/compare/9feb8f21b5d000f8901938f1dde89638c79ca805...master

Your commit was automatically rebased without conflicts.

[openjdk]

@eme64 Pushed as commit 634746a.

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