Local variables optimized for reuse and lowered registry pressure.

EvenPerfectBitScanner.Benchmarks/MutableUInt128Mul64Benchmarks.cs

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+using System.Runtime.CompilerServices;
+using BenchmarkDotNet.Attributes;
+using BenchmarkDotNet.Jobs;
+using PerfectNumbers.Core;
+
+namespace EvenPerfectBitScanner.Benchmarks;
+
+[ShortRunJob(RuntimeMoniker.Net80)]
+[MemoryDiagnoser]
+public class MutableUInt128Mul64Benchmarks
+{
+	private static readonly BenchmarkInput[] Inputs =
+	[
+		new(Create(0UL, 0x0000_0000_0000_0007UL), 0x3UL, "TinyOperands"),
+		new(Create(0x1234_5678_9ABC_DEF0UL, 0x0FED_CBA9_8765_4321UL), 0xFFFF_FFFF_0000_0001UL, "MixedOperands"),
+		new(Create(0xFFFF_FFFF_FFFF_FFFFUL, 0xFFFF_FFFF_FFFF_FFFFUL), 0xFFFF_FFFF_FFFF_FFFBUL, "NearMaxOperands"),
+    ];
+
+    [ParamsSource(nameof(GetInputs))]
+    public BenchmarkInput Input { get; set; }
+
+    public static IEnumerable<BenchmarkInput> GetInputs() => Inputs;
+
+	private static MutableUInt128 Create(ulong high, ulong low) => new(high, low);
+
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    private static ulong Mul64(ulong left, ulong right, out ulong low)
+    {
+        const ulong Mask32 = 0xFFFF_FFFFUL;
+
+        ulong leftLow = left & Mask32;
+        ulong leftHigh = left >> 32;
+        ulong rightLow = right & Mask32;
+        ulong rightHigh = right >> 32;
+
+        ulong lowLow = leftLow * rightLow;
+        ulong lowHigh = leftLow * rightHigh;
+        ulong highLow = leftHigh * rightLow;
+        ulong highHigh = leftHigh * rightHigh;
+
+        ulong carry = (lowLow >> 32) + (lowHigh & Mask32) + (highLow & Mask32);
+        ulong high = highHigh + (lowHigh >> 32) + (highLow >> 32) + (carry >> 32);
+
+        low = ((carry & Mask32) << 32) | (lowLow & Mask32);
+        return high;
+    }
+
+	[MethodImpl(MethodImplOptions.AggressiveInlining)]
+	private static void StandardInternal(ref MutableUInt128 source, ulong value)
+	{
+        ulong currentHigh = source.High;
+        ulong currentLow = source.Low;
+
+        ulong carry = Mul64(currentLow, value, out ulong newLow);
+        Mul64(currentHigh, value, out ulong shiftedHigh);
+        ulong newHigh = shiftedHigh + carry;
+
+        source.Low = newLow;
+        source.High = newHigh;
+	}
+
+	/// <summary>
+	/// Baseline MultiplyAdd implementation that chains Multiply and Add; measured 3.113 ns with mixed operands,
+	/// 3.128 ns when both limbs and inputs are near their maxima, and 3.131 ns with tiny operands.
+	/// </summary>
+	/// <remarks>
+	/// Observed means: MixedOperands 3.113 ns, NearMaxOperands 3.128 ns, TinyOperands 3.131 ns.
+	/// </remarks>
+	[Benchmark(Baseline = true)]
+	public MutableUInt128 Standard()
+	{
+		MutableUInt128 value = Input.InitialValue;
+		StandardInternal(ref value, Input.Addend);
+		return value;
+	}
+
+    /// <summary>
+    /// Fully inlined MultiplyAdd implementation that kept all arithmetic inside the method; landed at 2.969 ns on mixed operands,
+    /// 2.912 ns on near-max inputs, and 2.935 ns on tiny operands.
+    /// </summary>
+    /// <remarks>
+    /// Observed means: MixedOperands 2.969 ns, NearMaxOperands 2.912 ns, TinyOperands 2.935 ns.
+	/// Status: Implemented
+    /// </remarks>
+    [Benchmark]
+    public MutableUInt128 Optimized()
+    {
+        MutableUInt128 value = Input.InitialValue;
+        value.Mul(Input.Addend);
+        return value;
+    }
+
+    public readonly record struct BenchmarkInput(MutableUInt128 InitialValue, ulong Addend, string Name)
+    {
+        public override string ToString()
+        {
+            return Name;
+        }
+    }
+}