TARGET_PROMOTE_PROTOTYPES question

[Jeff Law]

On 10/20/2015 02:26 PM, Steve Ellcey  wrote:
> I have a question about the TARGET_PROMOTE_PROTOTYPES macro.  This macro
> says that types like short or char should be promoted to ints when
> passed as arguments, even if there is a prototype for the argument.
>
> Now when I look at the code generated on MIPS or x86 it looks like there
> is conversion code in both the caller and the callee.  For example:
>
> int foo(char a, short b) { return a+b; }
> int bar (int a) { return foo(a,a); }
>
>
> In the rtl expand dump (on MIPS) I see this in bar:
>
> (insn 6 3 7 2 (set (reg:SI 200)
>          (sign_extend:SI (subreg:HI (reg/v:SI 199 [ a ]) 2))) x.c:2 -1
>       (nil))
> (insn 7 6 8 2 (set (reg:SI 201)
>          (sign_extend:SI (subreg:QI (reg/v:SI 199 [ a ]) 3))) x.c:2 -1
>       (nil))
>
> Which insures that we pass the arguments as ints.
> And in foo we have:
>
> (insn 8 9 10 2 (set (reg/v:SI 197 [ a+-3 ])
>          (sign_extend:SI (subreg:QI (reg:SI 198) 3))) x.c:1 -1
>       (nil))
> (insn 10 8 11 2 (set (reg/v:SI 199 [ b+-2 ])
>          (sign_extend:SI (subreg:HI (reg:SI 200) 2))) x.c:1 -1
>       (nil))
>
> Which makes sure we do a truncate/extend before using the values.
>
> Now I know that we can't get rid of these truncation/extensions
> entirely, but do we need both?  It seems like foo could say that
> if the original registers (198 and 200) are argument registers
> that were extended to SImode due to TARGET_PROMOTE_PROTOTYPES
> then we don't need to do the truncation/extension in the callee
> and could just use the SImode values directly.  Am I missing
> something?  Or are we doing both just to have belts and suspenders
> and want to keep it that way?
 From reading the docs for TARGET_PROMOTE_PROTYPES, it would seem like 
it's the caller's responsibility.

However, it's not clear from the docs if it's something the callee can 
rely on, particularly when interoperating with a non-GCC compiler.

Does the ABI say anything about sub-word argument handling on the caller 
or callee side?

jeff