Intrinsics and Inline Assembly
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. In general, this will be the same list of intrinsic functions implemented for ALPHA and the Itanium but will include x64 specific functions as well.
The intrinsics supported by the compiler are described in Compiler Intrinsics.