[LLVMdev] Troubling promotion of return value to Integer ...

[Alireza.Moshtaghi at microchip.com]

Hi Chris, please read below:


> In many cases that is true, but for indirect calls (calls through a
> function pointer) it isn't.
> 
> > All that is needed is to add some kind of sign attribute to the
> > definition of callee function which will be set by the front-end and
> > eventually saved in Function class.
> 
> Sure.  Adding an attribute is what we need to do.  To handle the
> indirect call case, llvm puts attributes both on the function
> definitions and on the calls to those functions.

That makes a lot of sense

> 
> Ok.  We already have this, with the 'signext' attribute.  This code:
> 
> signed char g();
> signed char foo(){
> 	return g();
> }
> 
> compiles into:
> 
> define i8 @foo() signext nounwind  {
> entry:
> 	%tmp1 = tail call i8 (...)* @g( ) signext nounwind
; <i8>
> [#uses=1]
> 	ret i8 %tmp1
> }
> 
> declare i8 @g(...) signext
> 
> Note that the 'signext' attribute is on all of foo, the call to G, and
> the G declaration itself.
> 

OK, so you are giving a new meaning to signext attribute, where in
addition to parameters, now it would also be applicable to return value
of function. 
(BTW I think it's a typo that you didn't promote the return value of foo
:)

I take it from your reply that, by using signext and implementing the
two helper methods, which I mentioned before, there would be no need to
add any new function attributes; right?


> > In both cases DAG.getReturnValueRegisterSizeInBits(...) returns 32
> > (on a
> > 32 bit architecture)
> >
> > The caveat here is when we don't have a prototype for callee in
which
> > case "int ()" is considered, hence known values are returned and no
> > optimization will take effect.
> 
> The IR is already capturing this much information.  If you want to go
> this route, it seems simple to parameterize (in target data?) the size
> of int, and have the code generator extend to that size, instead of
> forcing extension to i32.

Yes, I meant the same... 

Regards
Ali