zlacker

I would go even further and state that "you should never assume that floating point functions will evaluate the same on two different computers, or even on two different versions of the same application", as the results of floating point evaluations can differ depending on platform, compiler optimizations, compilation-flags, run-time FPU environment (rounding mode, &c.), and even memory alignment of run-time data.

There's a C++26 paper about compile time math optimizations with a good overview and discussion about some of these issues [P1383]. The paper explicitly states:

1. It is acceptable for evaluation of mathematical functions to differ between translation time and runtime.

2. It is acceptable for constant evaluation of mathematical functions to differ between platforms.

So C++ has very much accepted the fact that floating point functions should not be presumed to give identical results in all circumstances.

Now, it is of course possible to ensure that floating point-related functions give identical results on all your target machines, but it's usually not worth the hassle.

[P1383]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p13...

>>DavidV+(OP)
Even the exact same source code compiled with different compilers, or the same compiler with different compiler options.

Intel Compiler for e.g. uses less than IEEE764 precision for floating point ops by default, for example.