[Bug target/45000] RX signed extened unsigned char or short return value.

["nickc at redhat dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #4 from nickc at redhat dot com  2010-07-28 14:05 -------
Hi Kazuhiro-san,

> If the func() is external function, output code is the following.
>  bsr    _func
>  mouv.B r1, r1
> If the return value is zero exteneded,
> "movu.B r1, r1" code can be removed.

Not really.  The problem is that the compiler cannot guarantee the behaviour of
the func function, since it is external to the compilation unit.  It might not
actually return an unsigned byte value.


> Is the explanation san integer conversion rank?

I am not sure what you mean here.  The simplest thing that I can say is that
the code produced by gcc is what is required by the ISO C99 standard.  If the
RX ABI requires a different behaviour then it is not compatible with the ISO
C99 standard.

That said I have uploaded a patch which offers a compromise.  It makes
functions which return small unsigned values insert a zero-extend instruction
into their epilogues.  This is less efficient than the change that you were
requesting (using MOVU.B to load the value from memory in the first place), but
it does mean that the code produced by gcc will work both with other code
produced by gcc and with code produced by Renesas's own compiler.

May I also suggest that you contact Matt Newsome <Matt.Newsome@renesas.com> at
Renesas who is also concerned with this particular problem.

Cheers
  Nick


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45000