Re: [PATCH] S/390: Allow to use r1 to r4 as literal pool base.

["Ulrich Weigand" <uweigand@de.ibm.com>]

Dominik Vogt wrote:
> On Mon, Dec 14, 2015 at 04:08:32PM +0100, Ulrich Weigand wrote:
> > I don't think that r1 is actually safe here.  Note that it may be used
> > (unconditionally) as temp register in s390_emit_prologue in certain cases;
> > the upcoming split-stack code will also need to use r1 in some cases.
> 
> How about the attached patch?  It also allows to use r0 as the
> temp register if possible (needs more testing).

This doesn't look safe either.  In particular:

- you use cfun_save_high_fprs_p at a place where its value might not yet
  have been determined (when calling s390_get_prologue_temp_regno from
  s390_init_frame_layout *before* the s390_register_info/s390_frame_info
  calls)

- r0 might hold the incoming static chain value, in which case it cannot
  be used as temp register

> If that's too
> much effort, I'm fine with limiting the original patch to r4 to
> r2.

That seems preferable to me.

> > r2 through r4 should be fine.  [ Not sure if there will be many (any?) cases
> > where one of those is unused but r5 isn't, however. ]
> 
> This can happen if the function only uses register pairs
> (__int128).  Actually I'm not sure whether r2 and r4 are valid
> candidates.

Huh?  Why not?

Bye,
Ulrich

-- 
  Dr. Ulrich Weigand
  GNU/Linux compilers and toolchain
  Ulrich.Weigand@de.ibm.com