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Intrinsics functions are used in compiled languages to use specific CPU instructions outside the scope of the language.

Intrinsic functions are pseudo-functions used by compilers to represent functionality that is outside the current scope of the language; often times, they may later be incorporated into a language. Some examples are simd and atomic instructions. The compiler has knowledge of the operations of the intrinsics and is able to optimize register use to take advantage of them.

A compiler library usually has actual implementations of the functions, which are used if a lower class CPU (or completely different) is detected at run-time or compile time.

Compiler intrinsics are very similar to inline-assembly. Inline assembler has notations to denote permissible input and output registers as well as clobber values; unless the compiler implicitly parses the inline assembly. With a compiler intrinsic, the register use is already built into the compiler and a developer doesn't need to know as many low level details; although it is often helpful to have some low level assembler knowledge to guide profiling and optimization.