The x64 Application Binary Interface (ABI) is a 4 register fast-call calling convention, with stack-backing for those registers. There is a strict one-to-one correspondence between arguments in a function, and the registers for those arguments. Any argument that doesn’t fit in 8 bytes, or is not 1, 2, 4, or 8 bytes, must be passed by reference. There is no attempt to spread a single argument across multiple registers. The x87 register stack is unused. It may be used, but must be considered volatile across function calls. All floating point operations are done using the 16 XMM registers. The arguments are passed in registers RCX, RDX, R8, and R9. If the arguments are float/double, they are passed in XMM0L, XMM1L, XMM2L, and XMM3L. 16 byte arguments are passed by reference. Parameter passing is described in detail in Parameter Passing. In addition to these registers, RAX, R10, R11, XMM4, and XMM5 are volatile. All other registers are non-volatile. Register usage is documented in detail in Register Usage and Caller/Callee Saved Registers.

proc return input:qword 
        fld     qword [input]  
        ret
endp    

proc return input
        movsd   xmm0,[input] 
        ret
endp    

proc return
        movsd   xmm0,[rcx] 
        ret
endp    

Since the macros handle the input of a double array well in 32-bits I was expecting what worked there to also work in 64-bits, but I guess I was either wrong, did it wrong or there is some bug in the proc macro in handling the parameters. Doing it manually and using the registers is working fine.

It should be noted however that in the calling convention used in 64-bit Windows first four parameters are passed in registers (RCX, RDX, R8 and R9), and therefore, even though there is a space reserved for them at the stack and it is labelled with name provided in the procedure definition, those four parameters will not initially reside there. They should be accessed by directly reading the registers.

proc WindowProc uses rbx rsi rdi, hwnd,wmsg,wparam,lparam

; Note that first four parameters are passed in registers,
; while names given in the declaration of procedure refer to the stack
; space reserved for them - you may store them there to be later accessible
; if the contents of registers gets destroyed. This may look like:
;       mov     [hwnd],rcx
;       mov     [wmsg],edx
;       mov     [wparam],r8
;       mov     [lparam],r9

        cmp     edx,WM_DESTROY
        je      .wmdestroy
  .defwndproc:
        invoke  DefWindowProc,rcx,rdx,r8,r9
        jmp     .finish
  .wmdestroy:
        invoke  PostQuitMessage,0
        xor     eax,eax
  .finish:
        ret

endp