为何Avx.Multiply运算效率未远高于*运算符？

我在研究SSE与AVX的工作原理及优势时编写了测试代码，结果意外发现System.Runtime.Intrinsics.X86.Avx.Multiply仅比传统*运算符快不足5%；更极端的是，在批量字节乘法的测试中，使用SSE指令的版本速度反而更慢，恳请解惑。

浮点乘法测试代码及结果 #

(long TicksSse2, long TicksAlu) TestFloat()
{
    Vector256<float> x = Vector256.Create((float)255, (float)128, (float)64, (float)32, (float)16, (float)8, (float)4, (float)2);
    Vector256<float> y = Vector256.Create((float).5);

    Stopwatch timerSse = new Stopwatch();
    Stopwatch timerAlu = new Stopwatch();

    for (int cnt = 0; cnt < 100_000_000; cnt++)
    {
        timerSse.Start();
        var xx = Avx.Multiply(x, y);
        timerSse.Stop();

        timerAlu.Start();
        float a = (float)255 * (float).5;
        float b = (float)128 * (float).5;
        float c = (float)64 * (float).5;
        float d = (float)32 * (float).5;
        float e = (float)16 * (float).5;
        float f = (float)8 * (float).5;
        float g = (float)4 * (float).5;
        float h = (float)2 * (float).5;
        timerAlu.Stop();
    }

    return (timerSse.ElapsedMilliseconds, timerAlu.ElapsedMilliseconds);
    // 测试结果：timerSse = 1688ms; timerAlu = 1748ms. 
}

字节乘法测试代码及结果 #

Vector128<byte> MultiplyBytes(Vector128<byte> x, Vector128<byte> y)
{
    Vector128<ushort> xAsUShort = x.AsUInt16();
    Vector128<ushort> yAsUShort = y.AsUInt16();

    Vector128<ushort> dstEven = Sse2.MultiplyLow(xAsUShort, yAsUShort);
    Vector128<ushort> dstOdd = Sse2.MultiplyLow(Sse2.ShiftRightLogical(xAsUShort, 8), Sse2.ShiftRightLogical(yAsUShort, 8));

    return Sse2.Or(Sse2.ShiftLeftLogical(dstOdd, 8), Sse2.And(dstEven, helper)).AsByte();
}

(long TicksSse2, long TicksAlu) TestBytes()
{
    Vector128<byte> x = Vector128.Create((byte)1, (byte)2, (byte)3, (byte)4, (byte)5, (byte)6, (byte)7, (byte)8, (byte)9, (byte)10, (byte)11, (byte)12, (byte)13, (byte)14, (byte)15, (byte)16);
    Vector128<byte> y = Vector128.Create((byte)2);

    Stopwatch timerSse = new Stopwatch();
    Stopwatch timerAlu = new Stopwatch();

    for (int cnt = 0; cnt < 100_000_000; cnt++)
    {
        timerSse.Start();
        var xx = MultiplyBytes(x, y);
        timerSse.Stop();

        timerAlu.Start();
        byte a = (byte)1 * (byte)2;
        byte b = (byte)2 * (byte)2;
        byte c = (byte)3 * (byte)2;
        byte d = (byte)4 * (byte)2;
        byte e = (byte)5 * (byte)2;
        byte f = (byte)6 * (byte)2;
        byte g = (byte)7 * (byte)2;
        byte h = (byte)8 * (byte)2;
        byte i = (byte)9 * (byte)2;
        byte j = (byte)10 * (byte)2;
        byte k = (byte)11 * (byte)2;
        byte l = (byte)12 * (byte)2;
        byte m = (byte)13 * (byte)2;
        byte n = (byte)14 * (byte)2;
        byte o = (byte)15 * (byte)2;
        byte p = (byte)16 * (byte)2;
        timerAlu.Stop();
    }

    return (timerSse.ElapsedMilliseconds, timerAlu.ElapsedMilliseconds);
    // 测试结果：timerSse = 3439ms; timerAlu = 1800ms
}

内容的提问来源于stack exchange，提问作者André Reichelt