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

[Chris Lattner]

On May 29, 2008, at 10:30 AM, <Alireza.Moshtaghi at microchip.com> <Alireza.Moshtaghi at microchip.com 
 > wrote:

>
>> On further reflection, I actually like the idea of adding 4  
>> attributes
>> better than adding knowledge of "int" into TargetData.  "int" is a C
>> notion that doesn't really belong in TargetData, and it is better for
>> the optimizers to have the front-end expose the implicit promotions.
>
> This is very true and indeed applicable to this problem...
> So I have to agree :)
>
> Let me summarize what we discussed so far:
>
> 1) The return value promotion will be removed from llvm backend and
>   implemented in both front-ends (clang and llvm-gcc)
>
> 2) The promotions are only applied to the return value in the body
>   of the function.
>   Return value of function definition and declaration will not be
> promoted
>
>   Example for a Target with 32-bit int type:
>
>   char foo() {
>     char c = ...
>     return c;
>   }
>
>   Is translated to:
>
>   define i8 @foo() {
>      ...
>      %tmp = sext i8 ... to i32
>      ret i32 %tmp
>   }

Close.  The return value of declarations also has to be promoted.  foo  
should be translated to:

>   define i32 @foo() sign_ext_from_i8 {
>      ...
>      %tmp = sext i8 ... to i32
>      ret i32 %tmp
>   }

> 3) Return value promotion is the default behavior, however,
>   hooks will be provided to allow the Target to disable it
>   and emit diagnostics.

Sure, the front-end can do that.

> 4) There will be 4 new function attributes:
>   sign_ext_from_i8, sign_ext_from_i16
>   zero_ext_from_i8, zero_ext_from_i16
>   These attributes will be placed on the function CALL node by
> front-end
>   to inform the backend about such promotions and enable optimization
> of
>   return value. This should be sufficient for direct and indirect  
> call.
>   (syntax of these attributes to be defined)

They also go on the function definition, as above.

> Am I capturing everything?

Yep!  The place to start with this is introducing the new attributes.   
Given the new attributes, we can mock up .ll code that uses them and  
improve the optimizers to use them.  When everything there is correct,  
we can move each front-end over to start using them.

-Chris