[cfe-users] Clang9 UBSan and GMP

[Hans Åberg via cfe-users]

> On 31 Oct 2019, at 01:53, David Blaikie <dblaikie at gmail.com> wrote:
> 
>> Yes, but assuming that the GMP adheres to the C standard, there should be no difference in the arithmetical values produced.
> 
> Not necessarily - C (well, I don't know the C standard as well as I know the C++ standard, admittedly) does allow various variations (implementation and unspecified behavior, for instance). eg: order of evaluation of function arguments (not that this is likely to vary due to optimizations - and doesn't with clang to the best of my knowledge, but as an example of the /sort/ of thing):
> 
>   int f() {
>     static int i = 0;
>     return i++;
>   }
>   int f2(int i, int j) {
>     return i;
>   }
>   int main() {
>     return f2(f(), f());
>   }
> 
> This program could exit with 0 or 1 - both results are valid interpretations of the program per the standard. (again, I don't know the C spec quite as well, so I'm not sure exactly what it says about this code)

Right, but that is something one would avoid when computing arithmetical results.