You can use assembly code in x64 code, but not in-line -- it must be in an external .asm file, compiled with ml64.exe, and called as a function. For small functions, the overhead of the function call can negate performance gains obtained by using assembly code.

If you must use assembly code, then realize that you need to provide the alternate path for x64 code, and you must synchronize changes to both paths. Any performance gains on the Win32 platform will not be carried over to the x64 version. It is worthwhile to look for intrinsic functions that provide equivalent functionality to the in-line assembly code that you want to use.

Note: x64 code does not use the same stack frame as the x86.