Unpredictable Math -- Can't Divide Long Ints???

[mel]

This is making me insane.  I am trying to divide a long by a constant and I can't get the compiler to do it.

unsigned int do_math(ulong long_speed)
{
 ulong pulsetime = (long_speed/1000L);
  
  
  pulsetime = (1000L/pulsetime);
 pulsetime /= DEF_INT_RATE;  // interupt rate

 
  return  (uint)pulsetime;
 
}

If the value of long_speed = 5000L, the first equation results  in pulsetime = 968.  Since when does 5000/1000 = 968???.  What can I do to get the math to work?  I already tried doubling the stack.

I tried making the 1000 1000.0.  I tried way too many things just to get a very simple division to work.

Does anyone have a clue?  Is there a codewarrior compiler guru out there? 

[Marreshe]

If the value of long_speed = 5000L, the first equation results  in pulsetime = 968.  Since when does 5000/1000 = 968???.  What can I do to get the math to work?  I already tried doubling the stack.

I tried making the 1000 1000.0.  I tried way too many things just to get a very simple division to work.

Does anyone have a clue?  Is there a codewarrior compiler guru out there? 

I agree you should probably take a look at the assembly.  Is DEF_INT_RATE a #define?  This maybe part of the problem.  The reason is you are casting pulsetime as a uint on the return...this could cause some unwanted optimization.  First try a #pragma around the function to turn off optimization and make sure DEF_INT_RATE is a const ulong not a #define.  You might also try returning pulsetime as a ulong rather than a uint to see what is going on. 

If you can you might want to bit shift rather than divide...roundoff errors can be murderous.

[CompilerGuru]

as I don't see the problem, so just some questions and hints.
- The "1000L/pulsetime" could be a division by zero, you should handle this case, I guess.
- well, as long as it does not work, its not so important, but it looks like everything except the first division could be performed with (unsigned) int arithmetic.
- for which target/cpu is this? Which tools do you use?
- can you provide a complete compilable source code, possibly with the disassembly listing and with one usage which fails?
(what is DEF_INT_RATE?)
- are you sure this is a functional problem and not a debug info only problem?
- check the preprocessor output. Does not look like this is your problem, but in the end you never know.

Daniel