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> Floats aren’t random. They’re perfectly deterministic, predictable and reproducible. If you do the same operation in two places you get the same result.

That's not exactly true in real hardware, or at least it wasn't until ~10 years ago. With the x87 FPU, internal precision was 80 bits, while the x86 registers were at most 64 bits. So, depending on the way the program would transfer data between the CPU and FPU your could get different results. It is very likely that different compilers and different optimization decisions could change the way these operations were implemented, so you would get slight differences between different versions of the software.

There are/were also several global FP flags that could get changed by other programs running on the same CPU/FPU that could impact the result of calculations. So, if you want 100% reproducible FP, you would have to either audit all software running on the same machine to ensure it doesn't touch those flags, or set the flags yourself for every FP calculation in your your program.






In a language like Java, all these factors are specified and fully deterministic.

False. Floating-point arithmetic in Java is generally nondeterministic. You will notice that the strictfp keyword exists and is off by default.

It's not false - strictfp mandates deterministic FP. If you use that your program will always run all floating point calculations in exactly the same way, full stop.

Secondly, on mainstream implementations, strictfp is already documented the same as default! They're planning to remove it anyway as it's a no-op in almost all cases.

See JEP 306.


> It's not false - strictfp mandates deterministic FP. If you use that your program will always run all floating point calculations in exactly the same way, full stop.

If you use it. Which is not the default. Your original claim remains false.


It does not matter. When you are doing accounting you are supposed to be able to sum large collections of numbers and get the same result regardless of the order.

That's something FP does not provide and it makes it completely unusable for accounting.


> regardless of the order.

That seems like a completely arbitrary requirement. Do accounting laws prohibit sort? Does 1 + 1 have to equal green on Tuesdays?


It seems you have no idea what double-side accounting is.

Each operation is accounted on two opposite sides of various account in a way that always keeps sides balanced (ie. they must sum up to the same value).

When you go to your bank account, for example, you have various sums on both sides of your account. Yet when you sum them up they MUST agree or you will be crying blood and suing your bank.


True, that's a good point. I was thinking of C & C++, but you're right, newer languages do a much better job of specifying and controlling this behavior.

Wonder if JS does something similar or not.


All major C/C++ compilers implement IEEE754. If you are telling the compiler to disregard it, that is on you.

It's not about IEEE754, it's about the precision that the FP co-processor offers. The results you get are correct per IEEE754, it's just that they may have even less error than required by IEEE754 in some cases. But, this is enough to make the results non-deterministic between different compilation options.

Also, changes applied to the FP co-processor by other processes on the machine could impact your process, regardless of your own compilation settings.


Are you talking about x87?

That's ancient history. Compilers don't use that instruction set any more in normal operation.

GCC, Java, LLVM, etc, will normally emit SSE2 in order to be standards compliant. They will only relax this if you tell them to, then it's your problem.


Yes, I was explicitly talking about the x87, and did mention that it has stopped being relevant for at least 10 years.

I believe there is still quite a bit of cautionary discussion of floating point numbers that was written in the age of the x87, so it's important to understand that people were not just misunderstanding IEEE754, even though their concerns are no longer applicable to modern hardware.




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