I ran some tests and here is the code I tested:
delegate(float[] inout)
{ // My Original Code
float[][] tempbuf = new float[2][];
int length = inout.Length / 2;
for (int c = 0; c < 2; c++)
{
tempbuf[c] = new float[length];
for (int i = 0, offset = c; i < tempbuf[c].Length; i++, offset += 2)
tempbuf[c][i] = inout[offset];
}
}
delegate(float[] inout)
{ // jerryjvl's recommendation: loop unrolling
float[][] tempbuf = new float[2][];
int length = inout.Length / 2;
for (int c = 0; c < 2; c++)
tempbuf[c] = new float[length];
for (int ix = 0, i = 0; ix < length; ix++)
{
tempbuf[0][ix] = inout[i++];
tempbuf[1][ix] = inout[i++];
}
}
delegate(float[] inout)
{ // Unsafe Code
unsafe
{
float[][] tempbuf = new float[2][];
int length = inout.Length / 2;
fixed (float* buffer = inout)
for (int c = 0; c < 2; c++)
{
tempbuf[c] = new float[length];
float* offset = buffer + c;
fixed (float* buffer2 = tempbuf[c])
{
float* p = buffer2;
for (int i = 0; i < length; i++, offset += 2)
*p++ = *offset;
}
}
}
}
delegate(float[] inout)
{ // Modifying my original code to see if the compiler is not as smart as i think it is.
float[][] tempbuf = new float[2][];
int length = inout.Length / 2;
for (int c = 0; c < 2; c++)
{
float[] buf = tempbuf[c] = new float[length];
for (int i = 0, offset = c; i < buf.Length; i++, offset += 2)
buf[i] = inout[offset];
}
}
and results: (buffer size = 2^17, number iterations timed per test = 200)
Average for test #1: 0.001286 seconds +/- 0.000026
Average for test #2: 0.001193 seconds +/- 0.000025
Average for test #3: 0.000686 seconds +/- 0.000009
Average for test #4: 0.000847 seconds +/- 0.000008
Average for test #1: 0.001210 seconds +/- 0.000012
Average for test #2: 0.001048 seconds +/- 0.000012
Average for test #3: 0.000690 seconds +/- 0.000009
Average for test #4: 0.000883 seconds +/- 0.000011
Average for test #1: 0.001209 seconds +/- 0.000015
Average for test #2: 0.001060 seconds +/- 0.000013
Average for test #3: 0.000695 seconds +/- 0.000010
Average for test #4: 0.000861 seconds +/- 0.000009
I got similar results every test. Obviously the unsafe code is the fastest, but I was surprised to see that the CLS couldn't figure out that that it can drop the index checks when dealing with jagged array. Maybe someone can think of more ways to optimize my tests.
Edit:
I tried loop unrolling with the unsafe code and it didn't have an effect.
I also tried optimizing the loop unrolling method:
delegate(float[] inout)
{
float[][] tempbuf = new float[2][];
int length = inout.Length / 2;
float[] tempbuf0 = tempbuf[0] = new float[length];
float[] tempbuf1 = tempbuf[1] = new float[length];
for (int ix = 0, i = 0; ix < length; ix++)
{
tempbuf0[ix] = inout[i++];
tempbuf1[ix] = inout[i++];
}
}
The results are also a hit-miss compared test#4 with 1% difference. Test #4 is my best way to go, so far.
As I told jerryjvl, the problem is getting the CLS to not index check the input buffer, since adding a second check (&& offset < inout.Length) will slow it down...
Edit 2:
I ran the tests before in the IDE, so here are the results outside:
2^17 items, repeated 200 times
******************************************
Average for test #1: 0.000533 seconds +/- 0.000017
Average for test #2: 0.000527 seconds +/- 0.000016
Average for test #3: 0.000407 seconds +/- 0.000008
Average for test #4: 0.000374 seconds +/- 0.000008
Average for test #5: 0.000424 seconds +/- 0.000009
2^17 items, repeated 200 times
******************************************
Average for test #1: 0.000547 seconds +/- 0.000016
Average for test #2: 0.000732 seconds +/- 0.000020
Average for test #3: 0.000423 seconds +/- 0.000009
Average for test #4: 0.000360 seconds +/- 0.000008
Average for test #5: 0.000406 seconds +/- 0.000008
2^18 items, repeated 200 times
******************************************
Average for test #1: 0.001295 seconds +/- 0.000036
Average for test #2: 0.001283 seconds +/- 0.000020
Average for test #3: 0.001085 seconds +/- 0.000027
Average for test #4: 0.001035 seconds +/- 0.000025
Average for test #5: 0.001130 seconds +/- 0.000025
2^18 items, repeated 200 times
******************************************
Average for test #1: 0.001234 seconds +/- 0.000026
Average for test #2: 0.001319 seconds +/- 0.000023
Average for test #3: 0.001309 seconds +/- 0.000025
Average for test #4: 0.001191 seconds +/- 0.000026
Average for test #5: 0.001196 seconds +/- 0.000022
Test#1 = My Original Code
Test#2 = Optimized safe loop unrolling
Test#3 = Unsafe code - loop unrolling
Test#4 = Unsafe code
Test#5 = My Optimized Code
Looks like loop unrolling is not favorable. My optimized code is still my best way to go and with only 10% difference compared to the unsafe code. If only I could tell the compiler that (i < buf.Length) implies that (offset < inout.Length), it will drop the check (inout[offset]) and I will basically get the unsafe performance.