Re: [RS6000] Fix PR61098, Poor code setting count register

[Alan Modra <amodra@gmail.com>]

On Mon, May 12, 2014 at 08:23:16AM +0930, Alan Modra wrote:
> On Sat, May 10, 2014 at 10:24:34PM -0400, David Edelsohn wrote:
> > On Thu, May 8, 2014 at 10:40 PM, Alan Modra <amodra@gmail.com> wrote:
> > > rs6000_emit_set_const ... always returns a non-zero result ...
> > 
> > Can you help clarify the removal of the code that tests if the
> > splitter failed?
> 
> See above.

On thinking some more, let me retract the patch for the time being.
While it's true that I was only removing dead code in the splitters,
the question of why this code has become dead is worth looking into.

I suspect a previous patch to rs6000_emit_set_const was wrong, and
that we should in fact be failing before reload (but never after),
when an expansion would take too many instructions.  "Too many" being
a sequence that is slower than loading a 64-bit constant from the
TOC.

We try to make that sort of tradeoff in rs6000_emit_move (see
num_insn_constant call), but that is really too early.  Some
manipulations of code modify constants.  I've seen cases where
rs6000_emit_move decided to inline a constant, but later changes to
the value meant the expansion was five instructions.

-- 
Alan Modra
Australia Development Lab, IBM