First some general remarks:
In general, what you are doing is mostly an exercise in futility and is the reverse of how most people would probably go about performance analysis.
The first point to make is that the peak value you are quoting is for strictly for floating point multiply-add instructions (FMAD), which count as two FLOPS, and can be retired at a maximum rate of one per cycle. Other floating point operations which retire at a maximum rate of one per cycle would formally only be classified as a single FLOP, while others might require many cycles to be retired. So if you decided to quote kernel performance against that peak, you are really comparing your codes performance against a stream of pure FMAD instructions, and nothing more than that.
The second point is that when researchers quote FLOP/s values from a piece of code, they are usually using a model FLOP count for the operation, not trying to count instructions. Matrix multiplication and the Linpack LU factorization benchmarks are classic examples of this approach to performance benchmarking. The lower bound of the operation count of those calculations is exactly known, so the calculated throughput is simply that lower bound divided by the time. The actual instruction count is irrelevent. Programmers often use all sorts of techniques, including rundundant calculations, speculative or predictive calculations, and a host of other ideas to make code run faster. The actual FLOP count of such code is irrelevent, the reference is always the model FLOP count.
Finally, when looking at quantifying performance, there are usually only two points of comparison of any real interest
- Does version A of the code run faster than version B on the same hardware?
- Does hardware A perform better than hardware B doing the task of interest?
In the first case you really only need to measure execution time. In the second, a suitable measure usually isn't FLOP/s, it is useful operations per unit time (records per second in sorting, cells per second in a fluid mechanical simulation, etc). Sometimes, as mentioned above, the useful operations can be the model FLOP count of an operation of known theoretical complexity. But the actual floating point instruction count rarely, if ever, enters into the analysis.
If your interest is really about optimization and understanding the performance of your code, then maybe this presentation by Paulius Micikevicius from NVIDIA might be of interest.
Addressing the bullet point questions:
Is this approach correct?
Strictly speaking, no. If you are counting floating point operations, you would need to know the exact FLOP count from the code the GPU is running. The sqrt operation can consume a lot more than a single FLOP, depending on its implementation and the characteristics of the number it is operating on, for example. The compiler can also perform a lot of optimizations which might change the actual operation/instruction count. The only way to get a truly accurate count would be to disassemble compiled code and count the individual floating point operands, perhaps even requiring assumptions about the characteristics of values the code will compute.
What about comparisons (if(a>b) then....)? Do I have to consider them as well?
They are not floating point multiply-add operations, so no.
Can I use the CUDA profiler for easier and more accurate results? I tried the instructions counter, but I could not figure out, what the figure means.
Not really. The profiler can't differentiate between a floating point intruction and any other type of instruction, so (as of 2011) FLOP count from a piece of code via the profiler is not possible. [EDIT: see Greg's execellent answer below for a discussion of the FLOP counting facilities available in versions of the profiling tools released since this answer was written]
