Re: RE: [Algorithms] C style math libraries versus "proper" C++ maths

I remember some passed days in game programming.

3D matrix operations and its implementation on PIII 900MHz.
a class containing an array of float, something very simple.
And for basic operation, you can pass on reference (using & keyword) or pointer (* keyword) or pass directly the array (so all floats are placed into stack.
Case 1 & Case 2 : only 4 bytes on the stack but all computing is using a deference (-> operator)
Case 3 : for 4x4 array (x,y,z,w), 16 floats so 16 x 4 = 64 bytes per matrix.

At this time the fastest method was the 3.

So better code isn't always the faster one ;)

  ----- Original Message ----- 
  From: Stephane Etienne 
  To: [email protected] 
  Sent: Saturday, March 25, 2006 11:00 PM
  Subject: [GD-General] RE: [Algorithms] C style math libraries versus "proper" C++ maths


  To do a fair comparison, we would need to see your C implementation. You implemented operator+, operator*, etc but these return a temporary which may or may not be optimized out. The best way to find out is to look at the disassembly. Also, different compiler may do different things. If your C version usage does not need any temporaries, then it will most likely be faster indeed. However if u use operator+= or operator *=, you should definitely get similar performances. Also never profile debug builds. Even functions marked inline will not be inlined in debug builds for example.



  Since C99 support the inline keyword, you should not see any difference between a C and C++ implementation. As Jarkko said, beware of exceptions. They can easily make your code a lot slower.



  I have a few suggestions regarding your code. They will not improve execution speed but you wrote several things that are unnecessary:



  -          the inline keyword is not necessary for inlined method inside the class body (less to read means it is also easier to read)

  -          Initializer list are only useful when the default constructor of constructed objects actually does something. Your use of initializer list here does not buy you anything.

  -          Prefer C style functions over C++ member methods. See Scott Meyer, "How non member functions improve encapsulation" for more info.

  -          No need to have a default destructor that does not do anything for non virtual classes.

  -          No need to implement the default copy operator unless you need to do something special. In your case, the default copy constructors and operators provided by the compiler will do just fine.

  -          Your use of statics to implement zero vector, etc could mess with the cache on platforms such the Xbox 360 and the PS3.



  Understand your code. Look at the disassembly and profile over an over again to understand why something is slow.



  Stephan.



  -----Original Message-----
  From: [email protected] [mailto:[email protected]] On Behalf Of Richard Fabian
  Sent: Saturday, March 25, 2006 5:40 AM
  To: [email protected]
  Subject: [Algorithms] C style math libraries versus "proper" C++ maths



  We've always used a math library that has a function per operation you want to apply to each math construct. We do this because "back in the day" we found that C++ style math libraries lead to lots of unecessary destructors being made... at least in debug, but we suspected in release also. 

  its been a few years, two major microsoft compilers later, a new SN compiler for PS2, and GCC still getting better and better...

  so, after hearing a workmate claim that he was using a C++ math library "because it was faster" i decided to update my old C++ math library and make myself a profiling testbed. 
  The sad news is: my old and horrible to read and work with function based math library still outstrips the C++ math library by about a 5% speed margin, even on trivial stuff.

  I'm not a C++ newb, i do know about constructor return optimisation, operator overloads copy constructors and the like, but as i tend to write code that is more architectural than the low level stuff like math libraries, i wondered what kind of performance difference (and which way) you were seeing. 

  neither math library uses any platform specific optimisations, such as SSE, SIMD, VU. This is pure C++ only.

  I have pasted my C++ math "header" that i was using for profiling if you want to take a look at my code and maybe point me in a direction of betterly optimised code..

  http://rafb.net/paste/results/P2kY2O59.html