Classic/Legacy CodeWarriorのトピックRe: Unpredictable Math -- Can't Divide Long Ints???

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

mel — Mon, 23 Oct 2006 21:50:33 GMT

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?

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

CompilerGuru — Tue, 24 Oct 2006 05:16:31 GMT

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

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

Marreshe — Wed, 25 Oct 2006 12:27:32 GMT

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.