An intrinsic function is a function built in to the compiler. The compiler knows how to best implement the functionality in the most optimized way for these functions and replaces with a set of machine instruction for a particular backend. Often, the code for the function is inserted inline thus avoiding the overhead of function call (In many cases, we do call the library function. For example, for the functions listed in http://llvm.org/docs/LangRef.html#standard-c-library-intrinsics we make a call to libc). These are also called built-in functions for other compilers.
In LLVM these intrinsics are introduced during code optimization at IR level (Intrinsics written in program code can be emitted through frontend directly). These function names will start with a prefix "llvm.", which is a reserved word in LLVM. These functions are always external and a user cannot specify the body for these functions in his/her code. In our code, we can only call these intrinsic functions.
In...