https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78696
--- Comment #9 from joseph at codesourcery dot com <joseph at codesourcery dot com> --- Implementation-specific can in practice include cases where the implementation deviates from the standard (e.g. Windows 3-digit exponents, though disabling the optimization for floating-point formats should hide that). It's clearly not a good idea to convert a minor conformance bug in the library into a buffer overrun by causing a program to allocate enough memory for the standard output from a format but not enough for what the library implementation actually outputs. (It wouldn't surprise me if there are printf implementations whose results for some floating-point values are not one of the nearest decimals in each direction and so could take more characters than either of those take in some cases.)
