What is the maximum theoretical speed-up due to SSE for a simple binary subtraction?
c, optimization, sse, vectorization
Solution
It depends on the CPU. But the theoretical max won't get above 4x. I don't know of a CPU which can execute more than one SSE instruction per clock cycle, which means that it can at most compute 4 values per cycle.
Most CPU's can do at least one floating point scalar instruction per cycle, so in this case you'd see a theoretical max of a 4x speedup.
But you'll have to look up the specific instruction throughput for the CPU you're running on.
A practical speedup of 3x is pretty good though.
Problem
In trying to figure out whether or not my code's inner loop is hitting a hardware design barrier or a lack of understanding on my part barrier. There's a bit more to it, but the simplest question I can come up with to answer is as follows: If I have the following code: ``` float px[32768],py[32768],pz[32768]; float xref, yref, zref, deltax, deltay, deltaz; initialize_with_random(px); initialize_with_random(py); initialize_with_random(pz); for(i=0;i<32768-1;i++) { xref=px[i]; yref=py[i]; zref=pz[i]; for(j=0;j<32768-1;j++ { deltx=xref-px[j]; delty=yref-py[j]; deltz=zref-pz[j]; } } ``` What type of maximum theoretical speed up would I be able to see by going to SSE instructions in a situation where I have complete control over code (assembly, intrinsics, whatever) but no control over runtime environment other than architecture (i.e. it's a multi-user environment so I can't do anything about how the OS kernel assigns time to my particular process). Right now I'm seeing a speed up of 3x with my code, when I would have thought using SSE would give me much more vector depth than the 3x speed up is indicating (presumably the 3x speed up tells me I have a 4x maximum theoretical throughput). (I've tried things such as letting deltx/delty/deltz be arrays in case the compiler wasn't smart enough to auto-promote them, but I still see only 3x speed up.) I'm using the intel C compiler with the appropriate compiler flags for vectorization, but no intrinsics obviously.