8272446 C1: Raw version of UnsafeGet generates load barriers

[stefank]

Please review this patch to remove load barriers from the "raw" version of UnsafeGet.

After rebasing the ZGC Generational code to latest openjdk/jdk we hit a crash because a load barrier was generated for some oops that are not supposed to have a load barrier. In upstream ZGC this isn't a problem because these load barriers are just redundant and will find healed good oops. However, in the Generational version un-barriered roots (e.g. thread stacks) have no metadata bits, and we should never perform normal load barriers on them (they are handled by stack watermarks and nmethod barriers).

This load barrier was introduced with:
JDK-8266746: C1: Replace UnsafeGetRaw with UnsafeGet when setting up OSR entry block

which makes UnsafeGet also handle "raw" loads. However, the "raw" path introduced a load barrier. Note that "raw" is often used to denote a function that loads/stores values without using a barrier.

I've tested this with tier1-3 and have had some initial set of eyes looking over the patch, but it would be good to scrutinize this patch and the usage of UnsafeGet in GraphBuilder::setup_osr_entry_block.

@TheRealMDoerr could you check if this patch works with PPC?

---

Progress

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Issue

• JDK-8272446: C1: Raw version of UnsafeGet generates load barriers

Reviewers

• Erik Österlund (@fisk - Reviewer) ⚠️ Review applies to 6c9f8fc
• Igor Veresov (@veresov - Reviewer) ⚠️ Review applies to 6c9f8fc

Reviewing

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[bridgekeeper]

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[openjdk]

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[mlbridge]

Webrevs

• 01: Full - Incremental (16917907)
• 00: Full (6c9f8fc9)

[fisk]

Looks good.

[openjdk]

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

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

8272446: C1: Raw version of UnsafeGet generates load barriers

Reviewed-by: eosterlund, iveresov

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[veresov]

Looks good.

[stefank]

@fisk @veresov Thanks for reviewing!

[stefank]

/integrate

[openjdk]

Going to push as commit df1427b.
Since your change was applied there have been 10 commits pushed to the master branch:

• 989f39f: 8272552: mark hotspot runtime/cds tests which ignore external VM flags
• ee8bf10: 8272327: Shenandoah: Avoid enqueuing duplicate string candidates
• 3fb1927: 8271227: Missing {@code } in com.sun.source.*
• a5ad772: 8272342: [TEST_BUG] java/awt/print/PrinterJob/PageDialogMarginTest.java catches all exceptions
• ae45592: 8272374: doclint should report missing "body" comments
• b2c272d: 8272305: several hotspot runtime/modules don't check exit codes
• 8268825: 8272297: FileInputStream should override transferTo() for better performance
• 3677734: 8271471: [IR Framework] Rare occurrence of "" in PrintIdeal/PrintOptoAssembly can let tests fail
• 0a03481: 8272231: G1: Refactor G1CardSet::get_card_set to return G1CardSetHashTableValue*
• 83d0e12: 8267833: Improve G1CardSetInlinePtr::add()

Your commit was automatically rebased without conflicts.

[openjdk]

@stefank Pushed as commit df1427b.

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