为什么这个自定义Vector2结构的性能比这个自定义Vector4差这么多？

html5 • 2022年9月21日 pm3:32 • 问答

虽然标杆一些自定义矢量类型，我发现，我的意料，我Vector2类型是许多基本操作慢得多从阵列中读取时，比我的Vector4型（和的Vector3）尽管有代码本身更少的操作，字段和变量。这是一个大大简化的示例，演示了这一点：

using System.Runtime.CompilerServices;
using System.Runtime.InteropServices;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

namespace VectorTest
{
    [StructLayout(LayoutKind.Sequential, Pack = 4)]
    public struct TestStruct4
    {
        public float X, Y, Z, W;

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public TestStruct4(float x, float y, float z, float w)
        {
            X = x;
            Y = y;
            Z = z;
            W = w;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static TestStruct4 operator +(in TestStruct4 a, in TestStruct4 b)
        {
            return new TestStruct4(
                a.X + b.X,
                a.Y + b.Y,
                a.Z + b.Z,
                a.W + b.W);
        }
    }

    [StructLayout(LayoutKind.Sequential, Pack = 4)]
    public struct TestStruct2
    {
        public float X, Y;

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public TestStruct2(float x, float y)
        {
            X = x;
            Y = y;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        public static TestStruct2 operator +(in TestStruct2 a, in TestStruct2 b)
        {
            return new TestStruct2(
                a.X + b.X,
                a.Y + b.Y);
        }
    }

    public class Program
    {
        private const int COUNT = 10000;
        private static readonly TestStruct4[] s_arr4 = new TestStruct4[COUNT];
        private static readonly TestStruct2[] s_arr2 = new TestStruct2[COUNT];

        static unsafe void Main()
        {
            for(int i = 0; i < s_arr4.Length; i++)
                s_arr4[i] = new TestStruct4(i, i * 2, i * 3, i * 4);
            for(int i = 0; i < s_arr2.Length; i++)
                s_arr2[i] = new TestStruct2(i, i * 2);

            BenchmarkRunner.Run<Program>();
        }

        [Benchmark]
        public TestStruct4 BenchmarkTestStruct4()
        {
            TestStruct4 ret = default;
            for (int i = 0; i < COUNT; i++)
                ret += s_arr4[i];
            return ret;
        }

        [Benchmark]
        public TestStruct2 BenchmarkTestStruct2()
        {
            TestStruct2 ret = default;
            for (int i = 0; i < COUNT; i++)
                ret += s_arr2[i];
            return ret;
        }
    }
}

运行这个基准测试结果：