From: alex@nospicedham.rivadpm.com   
      
   On 29-Aug-17 18:56, Rick C. Hodgin wrote:   
   > On Tuesday, August 29, 2017 at 1:14:49 PM UTC-4, Alex wrote:   
   >> On 29-Aug-17 17:07, Rick C. Hodgin wrote:   
   >>> 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.   
   >>   
   >> It does. Either the AND doesn't change RSP and we pop the second copy,   
   >> or it does and we pop the first.   
   >>   
   >> If I'm passing parameters beyond 4, then that needs parameters pushed on   
   >> the stack. An even number can use AND SPL $F0 and an odd number OR SPL   
   >> $08 as in   
   >>   
   >>        push   rsp   
   >>        push   [rsp]   
   >>        and    spl $F0    ; align to 16   
   >>        push   r14        ; two extra parameters = 16   
   >>        push   r15   
   >>        call   funkychicken   
   >>        add    rsp $8   
   >>        pop    rsp   
   >>   
   >> or   
   >>   
   >>        push   rsp   
   >>        push   [rsp]   
   >>        or     spl $08   ; align to 8 byte   
   >>        push   r15       ; one extra parameter   
   >>        call   funkychicken   
   >>        add    rsp $4   
   >>        pop    rsp   
   >   
   > #1 -- That will change the value of the stack pointer, but you   
   >        don't know if it changed or not.  If it was already aligned,   
   >        it would've done nothing, but if it wasn't aligned, it   
   >        would've adjusted it.  And when you return, you'll be at a   
   >        point of not knowing whether or not it should be adjusted.   
   >        Your subsequent pop rsp will be off by 8 bytes potentially.   
      
   That's why there are two identical values on the stack provided by the   
   pushes of RSP and [RSP] (which is RSP at the point of the first push).   
      
   >   
   > #2 -- What is "spl"?   
      
   The low order byte of RSP. It's key to understanding why it works.   
   Anyhow, I'm looking for an alternative, and might use the register version.   
      
      
   --   
   Alex   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)