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Re: How do I stop gcc from loading data into registers when that's not needed?
On Tue, May 22, 2018 at 10:49:35AM +0200, Richard Biener wrote:
> On Tue, May 22, 2018 at 2:19 AM Paul Koning <paulkoning@comcast.net> wrote:
> > > On May 18, 2018, at 2:07 PM, Richard Biener <richard.guenther@gmail.com>
> wrote:
> > > On May 18, 2018 8:03:05 PM GMT+02:00, Paul Koning <
> paulkoning@comcast.net> wrote:
> > >> In some targets, like pdp11 and vax, most instructions can reference
> > >> data in memory directly.
> > >>
> > >> So when I have "if (x < y) ..." I would expect something like this:
> > >>
> > >> cmpw x, y
> > >> bgeq 1f
> > >> ...
> > >>
> > >> What I actually see, with -O2 and/or -Os, is:
> > >>
> > >> movw x, r0
> > >> movw y, r1
> > >> cmpw r0, r1
> > >> bgeq 1f
> > >> ...
> > >>
> > >> which is both longer and slower. I can't tell why this happens, or how
> > >> to stop it. The machine description has "general_operand" so it
> > >> doesn't seem to be the place that forces things into registers.
> > >
> > > I would expect combine to merge the load and arithmetic and thus it is
> eventually the target costing that makes that not succeed.
>
> > Thanks Richard. I am not having a whole lot of luck figuring out where
> > precisely I need to adjust or how to make the adjustment. I'm doing the
> > adjusting on the pdp11 port right now. That has a TARGET_RTX_COSTS hook
> > which looks fairly plausible. It doesn't currently have a
> > TARGET_MEMORY_MOVE_COST, or TARGET_ADDRESS_COST, or TARGET_INSN_COST. It
> > is likely that I need some or all of those to get this working better? If
> > yes, any hints you can offer where to start?
-fdump-rtl-combine-all (or just -da or -dap), and then look at the dump
file. Does combine try this combination? If so, it will tell you what
the resulting costs are. If not, why does it not try it?
> Sorry, I'm not very familiar with this area of GCC either. Did you confirm
> that combine at least tries to merge the memory ops into the instruction?
It should, it's a simple reg dependency. In many cases it will even do
it if it is not single-use (via a 3->2 combination).
Segher