[LLVMdev] Destination register needs to be valid after callee saved register restore when tail calling

[Arnold Schwaighofer]

On 8 Aug 2007, at 21:12, Anton Korobeynikov wrote:

> Hello, Arnold.
>
>> with the sentence i tried to express the question whether there is a
>> way to persuade the code generator to use another register to load  
>> (or
>> move) the function pointer to (right before the callee saved register
>> restore) but thinking a little further that's nonsense.
> Why don't define some special op for callee address and custom  
> lower it?
> I really suggest you to look into eh_return. It's used in some pretty
> tricky situtation inside eh runtime: it it used to return from some eh
> runtime code. We already know, how much we should unwind the stack,  
> and
> what is the handler (sounds similar, right?). Also %eax and %edx are
> used to return eh data and should be preserved in such function.  
> So, in
> general, code for eh_return looks like (intel notation here):
>
>          mov ecx, ebp
>          add ecx, offset
>          mov [ecx], handler
>          .... (just usual epilogue)
>          mov esp, ecx
>          ret

Aaaah, yes now i understand i was missing the movrr in  
X86RegisterInfo.cpp.
And yes that's probably a much cleaner way of doing it.

The TC_RETURN would take a register (containing the calleeaddress)/or  
the callee TargetGlobalAdress/TargetExternalSymbol and the size of  
the stack adjustment (difference between caller/callee args)

TAILCALL would then be lowered to loading the callee address to a  
register (if its dynamic). and increasing esp+4.

in X86RegisterInfo.cpp we would then have

if (RetOpcode== X86::TC_RETURN){
	if (isDynamicCallee(RetOpCode)
	   add esp  {stack adjustment tailcall}
	   mov esp {register from TAILCALL}
         } else
	     // remove the ret
	   jmp _targetfunction
}


resulting code
	
	mov  ecx  _targetfunction  #load callee
	epilogue
	sub esp 4 # TAILCALL stackslot for eip
	add esp 8  #caller has 2 more arg
	mov  esp ecx
	ret

if the targetfunction is known

epilogue
#TAILCALL
add esp 8
jmp _targetfunction

Lowering of TAILCALL would also take caring of adjusting the argument  
stores.