Such functions could be invoked automatically by Intel compiler, either as a substitution for the standard memset() and memcpy() functions, or by recognizing a for() loop which performs equivalent functionality. Compiler option Qopt-report:4 may flag where for loops are replaced by these functions.
If using ifort, temporaries generated by array section operations may give rise to fast_memcpy.
A possible reason why such "optimization" (if the copy can't be avoided) might be counter-productive is the case where the strings aren't big enough for the library functions to come up to speed, but the compiler isn't able to recognize that at compile time. Short strings might be better optimized with for loops plus alignment and length assertions (not to mention arranging the application to reduce the copying of data).
