这段代码为什么会出现问题：
extern "C" __declspec(dllexport) void Transform(double x[], double y[], int iterations, bool forward)
{
long n, i, i1, j, k, i2, l, l1, l2;
double c1, c2, tx, ty, t1, t2, u1, u2, z;
/* Calculate the number of points */
n = (long)pow((double)2, (double)iterations);
/* Do the bit reversal */
i2 = n >> 1;
j = 0;
for (i = 0; i < n - 1; ++i)
{
if (i < j)
{
tx = x[i];
ty = y[i];
x[i] = x[j];
y[i] = y[j];
x[j] = tx;
y[j] = ty;
}
k = i2;
while (k <= j)
{
j -= k;
k >>= 1;
}
j += k;
}
/* Compute the FFT */
c1 = -1.0;
c2 = 0.0;
l2 = 1;
for (l = 0; l < iterations; ++l)
{
l1 = l2;
l2 <<= 1;
u1 = 1;
u2 = 0;
for (j = 0; j < l1; j++)
{
for (i = j; i < n; i += l2)
{
i1 = i + l1;
t1 = u1 * x[i1] - u2 * y[i1];
t2 = u1 * y[i1] + u2 * x[i1];
x[i1] = x[i] - t1;
y[i1] = y[i] - t2;
x[i] += t1;
y[i] += t2;
}
z = u1 * c1 - u2 * c2;
u2 = u1 * c2 + u2 * c1;
u1 = z;
}
c2 = sqrt((1.0 - c1) / 2.0);
if (forward)
c2 = -c2;
c1 = sqrt((1.0 + c1) / 2.0);
}
/* Scaling for forward transform */
if (forward)
{
for (i = 0; i < n; ++i)
{
x[i] /= n;
y[i] /= n;
}
}
}
这段代码比这个代码快20%吗？
public static void Transform(DataSet data, Direction direction)
{
double[] x = data.Real;
double[] y = data.Imag;
data.Direction = direction;
data.ExtremeImag = 0.0;
data.ExtremeReal = 0.0;
data.IndexExtremeImag = 0;
data.IndexExtremeReal = 0;
long n, i, i1, j, k, i2, l, l1, l2;
double c1, c2, tx, ty, t1, t2, u1, u2, z;
/* Calculate the number of points */
n = (long)Math.Pow(2, data.Iterations);
/* Do the bit reversal */
i2 = n >> 1;
j = 0;
for (i = 0; i < n - 1; ++i)
{
if (i < j)
{
tx = x[i];
ty = y[i];
x[i] = x[j];
y[i] = y[j];
x[j] = tx;
y[j] = ty;
}
k = i2;
while (k <= j)
{
j -= k;
k >>= 1;
}
j += k;
}
/* Compute the FFT */
c1 = -1.0;
c2 = 0.0;
l2 = 1;
for (l = 0; l < data.Iterations; ++l)
{
l1 = l2;
l2 <<= 1;
u1 = 1;
u2 = 0;
for (j = 0; j < l1; j++)
{
for (i = j; i < n; i += l2)
{
i1 = i + l1;
t1 = u1 * x[i1] - u2 * y[i1];
t2 = u1 * y[i1] + u2 * x[i1];
x[i1] = x[i] - t1;
y[i1] = y[i] - t2;
x[i] += t1;
y[i] += t2;
}
z = u1 * c1 - u2 * c2;
u2 = u1 * c2 + u2 * c1;
u1 = z;
}
c2 = Math.Sqrt((1.0 - c1) / 2.0);
if (direction == Direction.Forward)
c2 = -c2;
c1 = Math.Sqrt((1.0 + c1) / 2.0);
}
/* Scaling for forward transform */
if (direction == Direction.Forward)
{
for (i = 0; i < n; ++i)
{
x[i] /= n;
y[i] /= n;
if (Math.Abs(x[i]) > data.ExtremeReal)
{
data.ExtremeReal = x[i];
data.IndexExtremeReal = (int)i;
}
if (Math.Abs(y[i]) > data.ExtremeImag)
{
data.ExtremeImag = y[i];
data.IndexExtremeImag = (int)i;
}
}
}
}
FFT http://www.rghware.com/fft.png
我通过在我的应用程序中选择“Native DLL FFT”选项，在图表中间部分创建了CPU下降。
http://www.rghware.com/InstrumentTuner.zip（源代码）
我认为这将在大多数PC上运行。 您需要安装DirectX。我使用某些硬件的捕获设置遇到了一些问题。 捕获设置应该是可配置的，但是应用程序的开发已因此有趣的发现而受到干扰。
无论如何，为什么我使用本地代码会看到速度提高20％？ 这似乎与我先前的一些假设相悖。
更新
将函数转换为不安全方法并修复长整型问题后，新的不安全方法实际上比本地方法运行得更快（非常酷）。
Profile http://www.rghware.com/profile.png
很明显，数组边界检查是此FFT方法减速20％的原因。 由于它的性质，此方法中的for循环无法优化。
感谢大家的帮助。