undefined behavior in fast_expf

[Dr15Jones]

While using VDT while using the gcc undefined behavior compiles flag, we get the report

vdt/exp.h:144:38: runtime error: left shift of negative value -109

Which is
https://github.com/dpiparo/vdt/blob/master/include/exp.h#L144

The happens when calling vdt::fast_expf with the value

vdt::fast_expf (initial_x=-163.483994)

The C++ standard does say that left shifting a negative number is undefined.

[Dr15Jones]

It looks like the simplest solution would be to check the bounds of initial_x at the beginning of the function, instead of at the end:
https://github.com/dpiparo/vdt/blob/master/include/exp.h#L147
and return immediately.

[VinInn]

On 16 Jul, 2019, at 4:35 PM, Chris Jones ***@***.***> wrote: It looks like the simplest solution would be to check the bounds of initial_x at the beginning of the function, instead of at the end: https://github.com/dpiparo/vdt/blob/master/include/exp.h#L147 and return immediately.

will not vectorize at best x = (initial_x < details::MINLOGF) ? details::MINLOGF : initial_x; or std::max(initial_x, details::MINLOGF); same for the positive overflow

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