Built-in functions in V8 come in different flavors w.r.t. implementation, depending on their functionality, performance requirements, and sometimes plain historical development.
--allow-natives-syntax. Some runtime functions are directly embedded by the compiler into generated code. For a list, see
Other functions are implemented as built-ins, which themselves can be implemented in a number of different ways. Some are implemented directly in platform-dependent assembly. Some are implemented in CodeStubAssembler, a platform-independent abstraction. Yet others are directly implemented in C++. Built-ins are sometimes also used to implement pieces of glue code, not necessarily entire functions. For a list, see