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

[Alan Modra <amodra@gmail.com>]

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:
> 
> >> Please do not remove all of the comments from the two functions. The
> >> comments should provide some documentation about the different
> >> purposes of the two functions other than setting DEST to a CONST.
> >
> > I believe my updated comment covers the complete purpose of the
> > function nowadays.  The comments I removed are out-dated, and should
> > have been removed a long time ago..  rs6000_emit_set_const does not
> > even look at N, it always returns a non-zero result, and the return is
> > only tested for non-zero.  I removed MODE too, because that is always
> > the same as GET_MODE (dest).
> 
> It is helpful if the comment expresses more than restating the
> information one can glean from the function name. It's useful to note
> that rs6000_emit_set_long_const is a standard decomposition with a
> bounded number of instructions.
> 
> >> I think that the way you rearranged the invocations of copy_rtx() in
> >> rs6000_emit_set_long_const() is okay, but it would be good for someone
> >> else to double check.
> >
> > Yeah, that function is a bit messy.  I took the approach of always use
> > a bare "dest" once in the last instruction emitted, with every other
> > use getting hit with copy_rtx.  The previous approach was similar,
> > but used the bare "dest" on the first instruction emitted.  Obviously
> > you don't need copy_rtx anywhere with the new code when
> > can_create_pseudo_p is true, but I felt it wasn't worth optimising
> > that for the added source complication.
> 
> Can you help clarify the removal of the code that tests if the
> splitter failed?  The splitters in the Alpha port follow mostly the
> same rhythm, with a little bit of further cleanup and consolidation
> relative to the rs6000 port. alpha_split_const_mov() falls back on
> alpha_emit_set_long_const(), but checks that the target is valid and
> allows the splitter to fail. Either the Alpha port is doing
> unnecessary work or this cleanup patch is too aggressive. Either way,
> a comment seems necessary.

OK, I've had a good look at the history of this code.

rs6000_emit_set_const and rs6000_emit_set_long_const were introduced
with revision 44516, a largish patch by Dan Berlin.  As you hint
above, it seems the functions were copied from alpha.  So the
parameters were unnecessary and the comments just plain wrong for the
rs6000 version of code right from the initial commit.  Worse, only
half of necessary infrastructure was copied from alpha..

So let me lay out what I believe should be happening with
(set (reg) (constant))

At expand time, if the above set can't be implemented in a single
instruction, then it should be decomposed to the equivalent set high
part, ori low part, and possibly shift instructions so long as the
resulting sequence is small.  I think we basically do this correctly
in rs6000_emit_move.  See the num_insns_constant call there.
Constants that can't be evaluated inline by two (or three)
instructions will be replaced with a load from the TOC.

The same thing ought to happen in the splitters that use
rs6000_emit_set_const.  rs6000_emit_set_const should refuse to expand
to too many instructions (just like alpha).  We don't do this, but if
we did, this would leave some (set (reg) (constant)) instructions in
the RTL.  Alternatively the splitters could generate loads from the
TOC, but see pr57836, which shows the loads from the TOC we crafted
at expand time being reduced back to (set (reg) (constant)).

Finally, at reload time, any remaining (set (reg) (constant))
(ie. those that result in a long inline sequence) should be forced to
the TOC.  This is the missing part of the infrastructure that wasn't
copied from alpha.  Our legitimate_constant_p needs to reject some
constants..  As it is, reload simply expands to a four or five
inline instruction sequence.

David, I'd like some help with the legitimate_constant_p
implementation.  I have something that seems to work (not yet
regression tested) but there are a number of things that I'm not clear
on (eg. the revision 20229 change) so likely will get it wrong.

-- 
Alan Modra
Australia Development Lab, IBM