8374122: [lworld] compiler/c2/cmove/TestScalarConditionalMoveCmpObj.java fails with ir mismatches when preview enabled, post jdk-27+3

[marc-chevalier]

The test TestScalarConditionalMoveCmpObj.java tests whether code such as

r[i] = (a[i] != b[i]) ? cc : dd;

where a and b are Object[] gives a CMoveNode. Without Valhalla, we get a simple If -> If(True|False) -> Region diamond

that is simplified as a CMove.

Only this simple shape is simplified:

valhalla/src/hotspot/share/opto/loopopts.cpp

Lines 723 to 730 in a4fb7eb

	// Attempt to replace a Phi with a conditional move. We have some pretty
	// strict profitability requirements. All Phis at the merge point must
	// be converted, so we can remove the control flow. We need to limit the
	// number of c-moves to a small handful. All code that was in the side-arms
	// of the CFG diamond is now speculatively executed. This code has to be
	// "cheap enough". We are pretty much limited to CFG diamonds that merge
	// 1 or 2 items with a total of 1 or 2 ops executed speculatively.
	Node *PhaseIdealLoop::conditional_move( Node *region ) {

But with Valhalla, since operands could be value objects, the logic is a lot more complex: are pointer equal? If not, are one of both operands null? If both aren't null, are the operand the same class? If so... All being eventually concluded with a call to isSubstitutable. This can't be simplified as a CMove (or not easily).

Even warm up isn't enough: while a[i] and b[i] are guessed to be exactly Object, this still requires a more complex logic than a diamond. Schematically: are pointers equal? If not, is lhs null (that is enough to conclude they are different objects)? If so, trap; if not, is lhs of the speculated (non-value) type? If so, they are different; if not, trap. That is too complicated to make a CMove.

A solution is to make sure we can prove, rather than speculate, that operands are not value objects. Instead of making a and b arrays of Object that can be value types at runtime, we make them arrays of a non-abstract non-value type. This cannot be derived into a value class, so it's fine. And the test pass again!

I've also done a bit of side fixup in these tests: fixing package name and call init on the arrays used for the int flavor of the test (method private static void TestScalarConditionalMoveCmpObj::init(int[] a) was suspiciously unused).

Thanks,
Marc

---

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Issue

• JDK-8374122: [lworld] compiler/c2/cmove/TestScalarConditionalMoveCmpObj.java fails with ir mismatches when preview enabled, post jdk-27+3 (Bug - P4)

Reviewers

• Quan Anh Mai (@merykitty - Committer)
• Tobias Hartmann (@TobiHartmann - Committer)

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8374122: [lworld] compiler/c2/cmove/TestScalarConditionalMoveCmpObj.java fails with ir mismatches when preview enabled, post jdk-27+3

Reviewed-by: qamai, thartmann

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At the time when this comment was updated there had been 128 new commits pushed to the lworld branch:

• 4c6f902: 8375602: [lworld] Test verifiers should verify the equivalence of the objects if possible
• a4fb7eb: 8375434: [lworld] Add comment for null-check in InlineKlass::read_payload_from_addr
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[merykitty]

LGTM

[TobiHartmann]

Looks good to me too!