kernels(cpu)(matmul): F32 outer-product tiled GEMM kernel for prefill

benchmarks/DotLLM.Benchmarks/OuterProductGemmF32Benchmark.cs

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+using System.Runtime.InteropServices;
+using BenchmarkDotNet.Attributes;
+using DotLLM.Cpu.Kernels;
+
+namespace DotLLM.Benchmarks;
+
+/// <summary>
+/// Compares the new <see cref="OuterProductGemm.OuterProductGemmF32"/> against the
+/// production <see cref="MatMul.GemmF32(float*, float*, float*, int, int, int)"/>
+/// at prefill-shaped workloads (multi-token, contraction along K).
+///
+/// Convention: <c>C[N,M] = B[N,K] × A[M,K]^T</c> — N is the batch (token count),
+/// M is the output dim (e.g. hidden size), K is the contraction dim (e.g. K-proj
+/// from the attention block input).
+///
+/// Run with:
+///   dotnet run -c Release -- --filter '*OuterProductGemmF32Benchmark*'
+/// </summary>
+[MemoryDiagnoser]
+[SimpleJob(warmupCount: 5, iterationCount: 15)]
+public unsafe class OuterProductGemmF32Benchmark : IDisposable
+{
+    // Three prefill profiles spanning typical attention-projection shapes.
+    // K=4096 mirrors Llama-3-8B's hidden_size = 4096 and 32-head q_proj output.
+    [Params(128, 512, 2048)]
+    public int M { get; set; }
+
+    public int K { get; set; } = 4096;
+
+    public int N { get; set; } = 32;
+
+    private float* _a;
+    private float* _b;
+    private float* _c;
+
+    [GlobalSetup]
+    public void Setup()
+    {
+        var rng = new Random(42);
+        long aLen = (long)M * K;
+        long bLen = (long)N * K;
+        long cLen = (long)N * M;
+
+        _a = (float*)NativeMemory.AlignedAlloc((nuint)(aLen * sizeof(float)), 64);
+        _b = (float*)NativeMemory.AlignedAlloc((nuint)(bLen * sizeof(float)), 64);
+        _c = (float*)NativeMemory.AlignedAlloc((nuint)(cLen * sizeof(float)), 64);
+
+        for (long i = 0; i < aLen; i++) _a[i] = rng.NextSingle() * 2f - 1f;
+        for (long i = 0; i < bLen; i++) _b[i] = rng.NextSingle() * 2f - 1f;
+    }
+
+    public void Dispose()
+    {
+        if (_a != null) { NativeMemory.AlignedFree(_a); _a = null; }
+        if (_b != null) { NativeMemory.AlignedFree(_b); _b = null; }
+        if (_c != null) { NativeMemory.AlignedFree(_c); _c = null; }
+        GC.SuppressFinalize(this);
+    }
+
+    /// <summary>Baseline: production tiled GEMM path used by the engine today.</summary>
+    [Benchmark(Baseline = true)]
+    public void GemmF32_Baseline()
+    {
+        MatMul.GemmF32(_a, _b, _c, M, K, N);
+    }
+
+    /// <summary>Candidate: new outer-product 4×3 AVX2 microkernel.</summary>
+    [Benchmark]
+    public void OuterProductGemmF32_Avx2()
+    {
+        OuterProductGemm.OuterProductGemmF32(_a, _b, _c, M, K, N);
+    }
+}