Operations speed

I'm coding a game and it's very important to make speed calculations in render-code.

How can I get the speed of some operations?

For example, how to know whether multiplying is faster then sqrt, etc? Or I have to make tests and calcul开发者_JAVA百科ate the time.

Programming language is c++, thanks.

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This kind of micro-optimisation is just the thing to waste your time for minimal gain.

Use a profiler and start by improving your own algorithms and code wherever the profiler tells you that the game is spending most of its time.

Note that in some cases you may have to overhaul the whole software - or a major part of it - in order to implement a more efficient design. In that case the profiler results can be misleading to the inexperienced. E.g. optimising a complex computation may procure minimal gain, when compared to caching its result once and for all.

See also this somewhat related thread.

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Determining the speed of a particular operation is often known as profiling.The best solution for profiling an operation is to use a profiler. Visual Studio has a good profiler. Linux has gprof . If your compiler doesn't have a profiler, it might be worthwhile purchasing a compiler that does if you will often be profiling your code.

If you have to get by without using a professional profiler, then you can usually get by embedding your own into your program

check this out for codes of some profilers.

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Your best bet is to use a tool like AQTime and do a profiling run. Then you will know where to spend your time optimizing. But doing it prematurely or based on guess work likely wont get you much, and just complicate your code or break something. The best thing is to take any floating point calculations, especially sin, cos and the like, and sqrt out of any loops if you can.

I once had something like this:

for i = 0 to nc
  for j = 0 to nc
      aij = sqrt(a[i]*b[j])

which calculates nc*nc square roots. But since sqrt(a*b) is equal to sqrt(a)*sqrt(b), you can precompute the square roots for all the a's and b's beforehand so the loop then just becomes what is shown below. So instead of nc*nc square roots, you have 2*nc square roots.

for i = 0 to nc
  for j = 0 to nc
      aij = asqrt[i]*bsqrt[j]

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The question you are asking is highly dependent on the platform you are developing for at the hardware level. Not only will there be variation between different chipsets (Intel / AMD) but there will also be variations on the platform (I suspect that the iPhone doesn't have as many instructions for doing certain things quicker).

You state in your question that you are talking about 'render code'. The rules change massively if you're talking about code that will actually run on the GPU (shader code) instead of the CPU.

As @thkala states, I really wouldn't worry about this before you start. I've found it not only easier, but quicker to code it in a way that works first, and then (only if it needs improving) rewriting the bits that are slow when you profile your code. Better algorithms will usually provide better performance than trying to make use of only specific functions.

In the game(s) that we're developing for the iPhone, the only thing that I've kept in mind are that big math operations (sqrt) are slow (not basic maths) and that for loops that run every frame can quickly eat up CPU. Keeping that in mind, we've not had to optimise hardly any code - as it's all running at 60fps anyway - so I'm glad I didn't worry about it at the start.