Re: RTL alternative selection question

[Andrew Stubbs <andrew_stubbs at mentor dot com>]

On 23/09/2019 15:39, Andrew Stubbs wrote:
> On 23/09/2019 15:15, Segher Boessenkool wrote:
> > On Mon, Sep 23, 2019 at 11:56:27AM +0100, Andrew Stubbs wrote:
> > >    [(set (match_operand:DI 0 "register_operand"  "=Sg,v")
> > >          (ashift:DI
> > >            (match_operand:DI 1 "gcn_alu_operand" " Sg,v")
> > >            (match_operand:SI 2 "gcn_alu_operand" " Sg,v")))
> > >     (clobber (match_scratch:BI 3                 "=cs,X"))]
> >
> > > Unfortunately, the compiler (almost?) exclusively selects the second
> > > alternative, even when this means moving the values from one register
> > > file to the other, and then back again.
> > >
> > > The problem is that the scalar instruction clobbers the CC register,
> > > which results in a "reject++" for that alternative in the LRA dump.
> >
> > What kind of reject?  It prints a reason, too.
>
>       0 Non input pseudo reload: reject++

Apparently I was confused by operand "0" versus alternative "0". That 
message did occur, but it wasn't the only one. Here's all of it:

             0 Non input pseudo reload: reject++
             3 Scratch win: reject+=2
           alt=0,overall=9,losers=1,rld_nregs=2
           alt=1,overall=6,losers=1,rld_nregs=2

I don't understand why the "reject++" occurs, but presumably has to do 
with the "Sg" register availability somehow?

The "Scratch win" part comes from this code:

          /* We simulate the behavior of old reload here.
             Although scratches need hard registers and it
             might result in spilling other pseudos, no reload
             insns are generated for the scratches.  So it
             might cost something but probably less than old
             reload pass believes.  */
          if (scratch_p)
            {
              if (lra_dump_file != NULL)
                fprintf (lra_dump_file,
                         "            %d Scratch win: reject+=2\n",
                         nop);
              reject += 2;
            }
        }

Would it make sense to skip this reject when CLASS_LIKELY_SPILLED_P, as 
Jeff suggested?

Unfortunately, removing the "Scratch win" penalty alone is not enough 
for LRA to select the first alternative -- at least, no in my testcase 
-- so I need to understand the "non input pseudo reload" issue as well. 
I can see why it fires for alt0, but not why it does not fire for alt1.

Andrew