8255046: [lworld] JIT should make use of array layout encoding in markWord

[TobiHartmann]

This patch re-implements the flat array check in C1 and C2 by using the mark word bits instead of the layout helper from the array Klass (see JDK-8247299). Unfortunately, this turned out to be far from trivial to implement in C2.

I've introduced a new FlatArrayCheck macro node to wrap the logic of the new check and to make it easier for the loop unswitching optimization to detect and hoist the check. One major problem is that we can't use immutable memory anymore because we are loading the mark word which is mutable (AliasType::_is_rewritable is true). Although the bits we are interested in are in fact immutable (we check for markWord::unlocked_value), we need to use raw memory to not break anti dependency analysis. As a result, flat array checks are not hoisted out of loops anymore and loop unswitching fails. PhaseIdealLoop::move_flat_array_check_out_of_loop will attempt to still move flat array checks out of loops by walking up the memory edge to before the loop and re-wiring the check accordingly.

This patch also fixes an existing issue in Escape Analysis that was only triggered now that we are able to fold the flat array check in more cases due to the ::Ideal/::Value transformations.

@kuksenko, could you please evaluate performance of this change? Disabling the UseArrayMarkWordCheck flag allows to switch back to the old check using the layout helper.

Thanks,
Tobias

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Progress

• Change must not contain extraneous whitespace

Testing

	Linux x64	Windows x64	macOS x64
Build	✔️ (4/4 passed)	✔️ (2/2 passed)	✔️ (2/2 passed)
Test (tier1)	⏳ (1/9 running)	⏳ (9/9 running)	⏳ (1/9 running)

Issue

• JDK-8255046: [lworld] JIT should make use of array layout encoding in markWord

Reviewers

• Roland Westrelin (@rwestrel - Committer) ⚠️ Review applies to 565d64d

Download

$ git fetch https://git.openjdk.java.net/valhalla pull/256/head:pull/256
$ git checkout pull/256

[bridgekeeper]

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[openjdk]

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

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

8255046: [lworld] JIT should make use of array layout encoding in markWord

Reviewed-by: roland

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[mlbridge]

Webrevs

• 01: Full - Incremental (8320b34)
• 00: Full (565d64d)

[rwestrel]

Looks good to me.

[kuksenko]

Cool.
Performance behavior is as expected.
aaload operation got speedup up +15% on some scenarios.
In general, cost of aaload operation now is 103%-106% from legacy (non valhalla) cost.
and it looks like the minimally possible overhead of aaload operation.
Unfortunately, if array is locked aaload got 20% slowdown (also expected). locking on an array object is quite rare situation.

As for aastore operation - performance unchanged. aastore operation has the same cost in valhalla and in legacy world long before that. The reason of that is either hotspot has precise type information (and doesn't generate checks) or ArrayStoreException check is generated (ArrayStoreException check dominates aastore performance).

Note: I considered benchmarks when ArrayFlattened check can't be hoisted out loop (ref to array is loaded on every loop iteration).

[TobiHartmann]

Thanks @rwestrel for the review and thanks @kuksenko for the evaluation! I've added as small follow-up fix for EA.

[TobiHartmann]

/integrate

[openjdk]

@TobiHartmann Pushed as commit 1b1aefa.

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