cpython/Modules/_math.h

/* log1p(x) = log(1+x).  The log1p function is designed to avoid the
   significant loss of precision that arises from direct evaluation when x is
   small. Use the substitute from _math.h on all platforms: it includes
   workarounds for buggy handling of zeros.
 */

static double
_Py_log1p(double x)
{ … }

#define m_log1p …