On Tue, 20 Jan 2026 08:49:02 GMT, Marc Chevalier <[email protected]> wrote:
> 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 > > <img width="2047" height="1382" alt="diamond" > src="https://github.com/user-attachments/assets/811d4a51-6c0e-4789-bf37-3b1fa19ba0f5"; > /> > > that is simplified as a CMove. > > <img width="2156" height="1061" alt="cmove" > src="https://github.com/user-attachments/assets/f1c3277d-57e6-4af4-be3d-2ec52da7e5bc"; > /> > > Only this simple shape is simplified: > > https://github.com/openjdk/valhalla/blob/a4fb7ebd5af316d3e99d10bffb44d5be3aab5548/src/hotspot/share/opto/loopopts.cpp#L723-L730 > > 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 Thanks @merykitty and @TobiHartmann for reviews! ------------- PR Comment: https://git.openjdk.org/valhalla/pull/1934#issuecomment-3784264876
