[release/8.0-staging] Fix constant folding for arm64 MultiplyByScalar operations involving Vector2.One

src/coreclr/jit/valuenum.cpp

@@ -7734,10 +7734,38 @@ ValueNum ValueNumStore::EvalHWIntrinsicFunBinary(var_types      type,
             }
 
 #ifdef TARGET_ARM64
-            case NI_AdvSimd_Multiply:
             case NI_AdvSimd_MultiplyByScalar:
-            case NI_AdvSimd_Arm64_Multiply:
             case NI_AdvSimd_Arm64_MultiplyByScalar:
+            {
+                if (!varTypeIsFloating(baseType))
+                {
+                    // Handle `x * 0 == 0` and `0 * x == 0`
+                    // Not safe for floating-point when x == -0.0, NaN, +Inf, -Inf
+                    ValueNum zeroVN = VNZeroForType(TypeOfVN(cnsVN));
+
+                    if (cnsVN == zeroVN)
+                    {
+                        return VNZeroForType(type);
+                    }
+                }
+
+                assert((TypeOfVN(arg0VN) == type) && (TypeOfVN(arg1VN) == TYP_SIMD8));
+
+                // Handle x * 1 => x, but only if the scalar RHS is <1, ...>.
+                if (IsVNConstant(arg1VN))
+                {
+                    if (EvaluateSimdGetElement(this, TYP_SIMD8, baseType, arg1VN, 0) == VNOneForType(baseType))
+                    {
+                        return arg0VN;
+                    }
+                }
+                break;
+            }
+#endif
+
+#ifdef TARGET_ARM64
+            case NI_AdvSimd_Multiply:
+            case NI_AdvSimd_Arm64_Multiply:
 #else
             case NI_SSE_Multiply:
             case NI_SSE2_Multiply:

src/tests/JIT/Regression/JitBlue/Runtime_93876/Runtime_93876.cs

@@ -0,0 +1,25 @@
+// 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 System.Numerics;
+using Xunit;
+
+public static class Runtime_93876
+{
+    [Fact]
+    public static void Problem()
+    {
+        Vector4 v = Mul(0, 1);
+        Assert.Equal(Vector4.One, v);
+        Vector64<float> v64 = Mul64(0, 1);
+        Assert.Equal(Vector64<float>.One, v64);
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static Vector4 Mul(float a, float b) => Vector4.Multiply(a + b, Vector4.One);
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static Vector64<float> Mul64(float a, float b) => Vector64.Multiply(a + b, Vector64<float>.One);
+}

src/tests/JIT/Regression/JitBlue/Runtime_93876/Runtime_93876.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <Optimize>True</Optimize>
+  </PropertyGroup>
+  <ItemGroup>
+    <Compile Include="$(MSBuildProjectName).cs" />
+  </ItemGroup>
+</Project>