Preserve element types when folding SIMD Dot broadcasts

src/coreclr/jit/morph.cpp

@@ -9483,8 +9483,10 @@ GenTree* Compiler::fgOptimizeHWIntrinsic(GenTreeHWIntrinsic* node)
                 break;
             }
 
-            // Must be working with the same types of vectors.
-            if (hwop1->TypeGet() != retType)
+            // Must have matching vector sizes and compatible element types.
+            // Signedness-only differences preserve the broadcast bits.
+            if ((hwop1->TypeGet() != retType) ||
+                (varTypeToSigned(hwop1->GetSimdBaseType()) != varTypeToSigned(simdBaseType)))
             {
                 break;
             }

src/tests/JIT/Regression_ro_2/Runtime_134479.cs

@@ -0,0 +1,46 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.CompilerServices;
+using System.Runtime.Intrinsics;
+using Xunit;
+
+public class Runtime_134479
+{
+    [Theory]
+    [InlineData(1, 8, 8u)]
+    [InlineData(-1, -8, 65528u)]
+    [InlineData(32767, -8, 65528u)]
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    public static void Widening128(int value, int expectedSigned, uint expectedUnsigned)
+    {
+        Vector128<short> signed = Vector128.Create((short)value);
+        Vector128<ushort> unsigned = signed.AsUInt16();
+        Assert.Equal(Vector128.Create(expectedSigned), Vector128.Create((int)Vector128.Dot(signed, Vector128<short>.One)));
+        Assert.Equal(Vector128.Create(expectedUnsigned), Vector128.Create((uint)Vector128.Dot(unsigned, Vector128<ushort>.One)));
+    }
+
+    [Theory]
+    [InlineData(1, 8, 8u)]
+    [InlineData(-1, -8, 248u)]
+    [InlineData(127, -8, 248u)]
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    public static void Widening64(int value, int expectedSigned, uint expectedUnsigned)
+    {
+        Vector64<sbyte> signed = Vector64.Create((sbyte)value);
+        Vector64<byte> unsigned = signed.AsByte();
+        Assert.Equal(Vector64.Create(expectedSigned), Vector64.Create((int)Vector64.Dot(signed, Vector64<sbyte>.One)));
+        Assert.Equal(Vector64.Create(expectedUnsigned), Vector64.Create((uint)Vector64.Dot(unsigned, Vector64<byte>.One)));
+    }
+
+    [Theory]
+    [InlineData(1, 4, 4u)]
+    [InlineData(-1, -4, 4294967292u)]
+    [MethodImpl(MethodImplOptions.NoInlining | MethodImplOptions.AggressiveOptimization)]
+    public static void CompatibleBroadcasts(int value, int expectedSigned, uint expectedUnsigned)
+    {
+        Vector128<int> vector = Vector128.Create(value);
+        Assert.Equal(Vector128.Create(expectedSigned), Vector128.Create(Vector128.Dot(vector, Vector128<int>.One)));
+        Assert.Equal(Vector128.Create(expectedUnsigned), Vector128.Create(unchecked((uint)Vector128.Dot(vector, Vector128<int>.One))));
+    }
+}