[PATCH, rs6000]: mark clobber for registers changed by untpyed_call

[Segher Boessenkool]

Hi!

On Thu, Feb 06, 2020 at 10:49:36AM +0800, Jiufu Guo wrote:
> >   emit_call_insn (gen_call (operands[0], const0_rtx, const0_rtx));
> >
> >   for (i = 0; i < XVECLEN (operands[2], 0); i++)
> >     {
> >       rtx set = XVECEXP (operands[2], 0, i);
> >       emit_move_insn (SET_DEST (set), SET_SRC (set));
> >     }
> >
> > ... and nothing in the rtl stream says that those return registers are
> > actually set by that call.  Maybe we should use gen_call_value?  Can we
> > ever be asked to return more than a single thing here?
> I was also thinking about using "gen_call_value" or "emit_clobber (r3)"
> which could generate rtl: "%3:DI=call [foo]" or "call [foo]; clobber
> r3".  This could tell optimizer that %3 is changed.

The problem with "call ; clobber r3" is that some set+use of a pseudo can
be moved between these, and then rnreg can rename that to r3 again.  We
really need to show the call sets r3, in the general case (or that r3 is
live after the call, at least).

> While there are
> potential issues that untyped_call may change other registers.  So, mark
> clobber for all touched registers maybe more safe.

Well, we can derive what registers it sets, perhaps?  What does x86 do
here?  It does something, I know that, haven't looked much deeper yet
though :-)

In general: this is not a problem for us only; some other archs may have
found a good solution already.


Segher