Optimize Math.Round for arm64 and AwayFromZero for x64

src/libraries/System.Private.CoreLib/src/System/Math.cs

@@ -1299,6 +1299,18 @@ public static double Round(double value, int digits)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static double Round(double value, MidpointRounding mode)
         {
+            // Inline single-instruction modes
+            if (RuntimeHelpers.IsKnownConstant((int)mode))
+            {
+                if (mode == MidpointRounding.ToEven)
+                    return Round(value);
+
+                // For ARM/ARM64 we can lower it down to a single instruction FRINTA
+                // For XARCH we have to use the common path
+                if (AdvSimd.Arm64.IsSupported && mode == MidpointRounding.AwayFromZero)
+                    return AdvSimd.Arm64.RoundAwayFromZero(Vector128.CreateScalarUnsafe(value)).ToScalar();
+            }
+
             return Round(value, 0, mode);
         }
 

src/libraries/System.Private.CoreLib/src/System/MathF.cs

@@ -429,6 +429,18 @@ public static float Round(float x, int digits)
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         public static float Round(float x, MidpointRounding mode)
         {
+            // Inline single-instruction modes
+            if (RuntimeHelpers.IsKnownConstant((int)mode))
+            {
+                if (mode == MidpointRounding.ToEven)
+                    return Round(x);
+
+                // For ARM/ARM64 we can lower it down to a single instruction FRINTA
+                // For XARCH we have to use the common path
+                if (AdvSimd.IsSupported && mode == MidpointRounding.AwayFromZero)
+                    return AdvSimd.RoundAwayFromZero(Vector64.CreateScalarUnsafe(x)).ToScalar();
+            }
+
             return Round(x, 0, mode);
         }
 

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/RuntimeHelpers.cs

@@ -127,5 +127,8 @@ internal static bool IsPrimitiveType(this CorElementType et)
 
         [Intrinsic]
         internal static bool IsKnownConstant(char t) => false;
+
+        [Intrinsic]
+        internal static bool IsKnownConstant(int t) => false;
     }
 }