From: rick.c.hodgin@nospicedham.gmail.com   
      
   On Tuesday, August 29, 2017 at 11:14:38 AM UTC-4, Alex wrote:   
   > On 64 bit Windows, stack alignment on a 16 byte boundary is required   
   > before calling all except a leaf function. In the called function, the   
   > stack is 8 mod 16.   
   >   
   > Now, I'm struggling to come up with a way of doing it beyond this code   
   > (which I didn't invent, but I can't for the life of me remember where I   
   > found it.)   
   >   
   >    push   rsp   
   >    push   [rsp]   
   >    and    spl $F0   
   >    call   funkychicken   
   >    pop    rsp   
   >   
   > It seems to be the only way of doing this without branches, flags or   
   > other expensive nonsense. But, as ever, there may be a better way. Any   
   > suggestions?   
      
   I don't think the above solution will work.  If I read it correctly   
   (as it appears to be mixing ISAs), you'll lose your relative stack   
   pointer position with the AND, and since it's an unknown (it will   
   either change the value of rsp or not), then it won't be reliable.   
      
   The only thing I can think of that doesn't involve branching is an   
   algorithm like this (completely untested, and I'm using r15 as an   
   arbitrarily chosen register, it can be changed to any other reg):   
      
       xor     r15,r15   
       test    rsp,0Fh         ; See if we're already aligned   
       setnz   r15b            ; Set to 0 or 1 based on alignment   
       shl     r15,3           ; Multiply by 8   
       sub     rsp,r15         ; Adjust rsp by 0 or 8   
       call    funkychicken    ; Perform the normal call   
       add     rsp,r15         ; Un-adjust rsp by 0 or 8   
      
   Note also that this has the effect of pushing your parameters up   
   on the stack an additional 8-bytes, so you may need to do this   
   first ... I don't know.  I've always kept the stack 8-byte aligned   
   and everything worked, so I'm not really sure what you're doing   
   here.   
      
   Thank you,   
   Rick C. Hodgin   
      
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