I'm using Delphi XE6 to perform a complicated floating point calculation. I realize the limitations of floating point numbers so understand the inaccuracies inherent in FP numbers. However this particular case, I always get 1 of 2 different values at the end of the calculation.
The first value and after a while (I haven't figured out why and when), it flips to the second value, and then I can't get the first value again unless I restart my application. I can't really be more specific as the calculation is very complicated. I could almost understand if the value was somewhat random, but just 2 different states is a little confusing. This only happens in the 32-bit compiler, the 64 bit compiler gives one single answer no matter how many times I try it. This number is different from the 2 from the 32-bit calculation, but I understand why that is happening and I'm fine with it. I need consistency, not total accuracy.
My one suspicion is that perhaps the FPU is being left in a state after some calculation that affects subsequent calculations, hence my question about clearing all registers and FPU stack to level out the playing field. I'd call this CLEARFPU before I start of the calculation.
After some more investigation I realized I was looking in the wrong place. What you see is not what you get with floating point numbers. I was looking at the string representation of the numbers and thinking here are 4 numbers going into a calculation ALL EQUAL and the result is different. Turns out the numbers only seemed to be the same. I started logging the hex equivalent of the numbers, worked my way back and found an external dll used for matrix multiplication the cause of the error. I replaced the matrix multiplication with a routine written in Delphi and all is well.
Floating point calculations are deterministic. The inputs are the input data and the floating point control word. With the same input, the same calculation will yield repeatable output.
If you have unpredictable results, then there will be a reason for it. Either the input data or the floating point control word is varying. You have to diagnose what that reason for that is. Until you understand the problem fully, you should not be looking for a problem. Do not attempt to apply a sticking plaster without understanding the disease.
So the next step is to isolate and reproduce the problem in a simple piece of code. Once you can reproduce the issue you can solve the problem.
Possible explanations include using uninitialized data, or external code modifying the floating point control word. But there could be other reasons.
Uninitialized data is plausible. Perhaps more likely is that some external code is modifying the floating point control word. Instrument your code to log the floating point control word at various stages of execution, to see if it ever changes unexpectedly.