sh4 abi doc

[Andrew Cagney]

> Clarke, Stephen writes:
>  > > From: Elena Zannoni [mailto:ezannoni@redhat.com] 
>  > > Sent: Thursday, September 26, 2002 10:43 AM
>  > > 
>  > > The cutoff is 8 bytes.
>  > 
>  > Yes: anything bigger than 8 bytes is definitely
>  > returned in memory.
>  > 
>  > But the cutoff is not "clean", because structs smaller
>  > than this could also be returned in memory, if they
>  > don't pass the "same size and alignment as an integer
>  > type" rule.
>  > 
>  > For example, in
>  > 
>  >   struct s { char c[3]; } wibble;
>  >   struct s foo(void) {  return wibble; }
>  > 
>  > the return value from foo() will be in memory, not
>  > in R0, because there is no 3-byte integer type.
>  > 
>  > Sorry if that was obvious already, I just wanted to
>  > be sure.
>  > 
>  > Steve.
> 
> 
> Very helpful. I hadn't noticed this variant, indeed.

Ah, the ``gcc struct return'' rule.  Perhaphs we need a generic function 
that specifies this.

enjoy,
Andrew