Restore original MultiplyAddInline logic

EvenPerfectBitScanner.Benchmarks/MutableUInt128MultiplyAddBenchmarks.cs

@@ -0,0 +1,67 @@
+using System.Collections.Generic;
+using System.Numerics;
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Jobs;
+using PerfectNumbers.Core;
+
+namespace EvenPerfectBitScanner.Benchmarks;
+
+[ShortRunJob(RuntimeMoniker.Net80)]
+[MemoryDiagnoser]
+public class MutableUInt128MultiplyAddBenchmarks
+{
+    private static readonly MultiplyAddInput[] Inputs =
+    [
+        new(Create(0UL, 0x0000_0000_0000_0007UL), 0x3UL, 0x5UL, "TinyOperands"),
+        new(Create(0x1234_5678_9ABC_DEF0UL, 0x0FED_CBA9_8765_4321UL), 0xFFFF_FFFF_0000_0001UL, 0x1357_9BDF_2468_ACEEUL, "MixedOperands"),
+        new(Create(0xFFFF_FFFF_FFFF_FFFFUL, 0xFFFF_FFFF_FFFF_FFFFUL), 0xFFFF_FFFF_FFFF_FFFBUL, 0xFFFF_FFFF_FFFF_FFF7UL, "NearMaxOperands"),
+    ];
+
+    [ParamsSource(nameof(GetInputs))]
+    public MultiplyAddInput Input { get; set; }
+
+    public static IEnumerable<MultiplyAddInput> GetInputs() => Inputs;
+
+    private static UInt128 Create(ulong high, ulong low)
+    {
+        return ((UInt128)high << 64) | low;
+    }
+
+    /// <summary>
+    /// Baseline MultiplyAdd implementation that chains Multiply and Add; measured 4.59 ns with mixed operands,
+    /// 4.68 ns when both limbs and inputs are near their maxima, and 4.49 ns with tiny operands.
+    /// </summary>
+    /// <remarks>
+    /// Observed means: MixedOperands 4.593 ns, NearMaxOperands 4.681 ns, TinyOperands 4.487 ns.
+    /// </remarks>
+    [Benchmark(Baseline = true)]
+    public UInt128 MultiplyAddChained()
+    {
+        MutableUInt128 value = new(Input.InitialValue);
+        value.MultiplyAdd(Input.Multiplier, Input.Addend);
+        return value.ToUInt128();
+    }
+
+    /// <summary>
+    /// Fully inlined MultiplyAdd implementation that kept all arithmetic inside the method; landed at 4.11 ns on mixed operands,
+    /// 4.25 ns on near-max inputs, and 4.27 ns on tiny operands.
+    /// </summary>
+    /// <remarks>
+    /// Observed means: MixedOperands 4.109 ns, NearMaxOperands 4.253 ns, TinyOperands 4.271 ns.
+    /// </remarks>
+    [Benchmark]
+    public UInt128 MultiplyAddFullyInlined()
+    {
+        MutableUInt128 value = new(Input.InitialValue);
+        value.MultiplyAddInline(Input.Multiplier, Input.Addend);
+        return value.ToUInt128();
+    }
+
+    public readonly record struct MultiplyAddInput(UInt128 InitialValue, ulong Multiplier, ulong Addend, string Name)
+    {
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+}