Bug in simplify_subreg?

[Richard Earnshaw]

> Richard Henderson <rth@redhat.com> writes:
> > On Fri, Aug 02, 2002 at 01:02:26PM +0200, Lars Brinkhoff wrote:
> > > If I insert the same checks against final_regno as are made to
> > > REGNO (op), everything seems to be fine.  Is this the right thing
> > > to do?
> > No.  If the frame pointer isn't being used, then gen_reg_RTX isn't
> > supposed to return frame_pointer_rtx.
> 
> But it does.  Since outermode==SImode==Pmode and reload_in_progress==0,
> gen_reg_RTX returns frame_pointer_rtx to simplify_subreg.  And so
> ORIGINAL_REGNO of frame_pointer_rtx is clobbered.
> 
> Would the right solution be to check frame_pointer_needed in gen_reg_RTX?

We cannot know for certain whether a frame pointer is needed before reload 
starts.  Even during reload the decision might change -- on some machines 
the decision may depend on which registers are being used by the function 
-- and that can change as we push more reloads.

It's probable that on many machines we can accurately guess whether a 
frame pointer is needed or not before reload starts to run, but there's no 
way to be sure.

R.