naked popcnt function

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Hello, I would like to write a "popcnt" function. This works fine

ulong popcnt(ulong x)
{
asm { mov RAX, x ; popcnt RAX, RAX ; }
}

However, if I add the "naked" keyword ( which should improve 
performance? ) it doesn't work anymore and I can't figure out 
what change I am supposed to make ( aside from x[RBP] instead of 
x )
This function is going to be *heavily* used.

Thanks for any help.