RFR: 8365570: C2 fails assert(false) failed: Unexpected node in SuperWord truncation: CastII

[Emanuel Peter]

On Mon, 18 Aug 2025 16:34:52 GMT, Jasmine Karthikeyan <jkarthikeyan at openjdk.org> wrote:

> Hi all,
> This is a quick patch for the assert failure in superword truncation with CastII. I've added a check for all constraint cast nodes, and attached a reduced version of the fuzzer test. Thanks!

src/hotspot/share/opto/superword.cpp line 2576:

> 2574: 
> 2575:   // Vector nodes and casts should not truncate.
> 2576:   if (type->isa_vect() != nullptr || type->isa_vectmask() != nullptr || in->is_Reduction() || in->is_ConstraintCast()) {

Why should we not truncate a CastII? What can go wrong?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26827#discussion_r2288072033