Its one of the caveats of the C-family that developers are supposed to be aware of, but often aren't. It doesn't support IEEE 754 fully. There is a standard to do so, but no one has actually implemented it.
Of course in my case what I'm actually concerned with is the behavior surrounding inf and NaN. Thankfully I've never been forced to write code that relied on subtle precision or rounding differences. If it ever comes up I'd hope to keep it to a platform independent fixed point library.
But, for example, LLVM does not fully support IEEE 754 [0].
And nor does GCC - who list it as unsupported, despite defining the macro and having partial support. [1]
The biggest caveat is in Annex F of the C standard:
> The C functions in the following table correspond to mathematical operations recommended by IEC 60559. However, correct rounding, which IEC 60559 specifies for its operations, is not required for the C functions in the table.
The C++ standard [2] barely covers support, but if a type supports any of the properties of ISO 60559, then it gets is_iec559 - even if that support is _incomplete_.
This paper [3] is a much deeper dive - but the current state for C++ is worse than C. Its underspecified.
> When built with version 18.1.0 of the clang C++ compiler, without specifying any compiler options, the output is:
> distance: 0.0999999
> proj_vector_y: -0.0799999
> Worse, if -march=skylake is passed to the clang C++ compiler, the output is:
> distance: 0.1
> proj_vector_y: -0.08
[0] https://github.com/llvm/llvm-project/issues/17379
[1] https://www.gnu.org/software/gcc/projects/c-status.html
[2] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/n49...
Huh. https://en.cppreference.com/w/c/23.html says the "Old feature-test macro" __STDC_IEC_559__ was deprecated in C23, in favor of __STDC_IEC_60559_BFP__ .