There appears to be a typo in the User and Reference Guide for the Intel® C++ Compiler 15.0. The "Intrinsics for Advanced Vector Extensions" section of the lists _mm256_undefined_si128 one of the "Intrinsics for Generating Vectors of Undefined Values":
https://software.intel.com/en-us/node/524189
However, this intrinsic is not defined in immintrin.h. _mm256_undefined_si256 is defined, however. The documentation should be updated to reflect this.
--
Nathan Weeks
Systems Analyst
Iowa State University -- Department of Mathematics
http://weeks.public.iastate.edu/