Intrinsics and Inline Assembly
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One of the constraints for the x64 compiler is to have no inline assembler support. This means that functions that cannot be written in C or C++ will either have to be written as subroutines or as intrinsic functions supported by the compiler. Certain functions are performance sensitive while others are not. Performance-sensitive functions should be implemented as intrinsic functions.
The intrinsics supported by the compiler are described in Compiler Intrinsics.