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

Reviewed-by: chagedorn, thartmann, epeter

Author: jaskarth

src/hotspot/share/opto/superword.cpp

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2007, 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2007, 2026, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -2482,6 +2482,11 @@ static bool can_subword_truncate(Node* in, const Type* type) {
     return false;
   }
 
+  // Since casts specifically change the type of a node, stay on the safe side and do not truncate them.
+  if (in->is_ConstraintCast()) {
+    return false;
+  }
+
   // Cannot be truncated:
   switch (opc) {
   case Op_AbsI:

test/hotspot/jtreg/compiler/vectorization/TestSubwordTruncation.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2025, 2026, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -479,6 +479,32 @@ public Object[] testRoundD(byte[] in) {
         return new Object[] { in, res };
     }
 
+    @Test
+    @IR(counts = { IRNode.CAST_II, ">0" })
+    @Warmup(0)
+    public Object[] testCastII() {
+        byte[] bytes = new byte[400];
+        intField = 6;
+        int i = 0;
+        int j = 1;
+
+        do {
+            bytes[j] = (byte) i;
+            int k = 1;
+
+            do {
+                i <<= intField;
+                i += (k ^ i);
+                i -= j;
+
+                for (int u = 1; 1 > u; u++) {
+                }
+            } while (++k < 8);
+        } while (++j < 191);
+
+        return new Object[] { bytes };
+    }
+
     public static void main(String[] args) {
         TestFramework.run();
     }