Re: [PATCH] Add zero-overhead looping for xtensa backend

["Yangfei (Felix)" <felix dot yang at huawei dot com>]

1. The original xtensa port never generates "loop" instruction at all.
2. A port doesn't need to implement hwloop_pattern_reg hook if it has no zero-cost loop instruction.

Is that clear?

> 
> I mean without your patch at all.
> 
> On Thu, Oct 23, 2014 at 11:30 PM, Yangfei (Felix) <felix.yang@huawei.com>
> wrote:
> >>
> >> On Thu, Oct 23, 2014 at 9:12 PM, Yangfei (Felix) <felix.yang@huawei.com>
> wrote:
> >> >> Here the key point is we need a general purpose register for the "loop"
> >> >> instruction.
> >>
> >> So the question to ask here is, "How does this work today, without
> >> loop instructions?" Somehow--even when it has been spilled--a branch
> >> instruction can test the trip count. There should be no difference.
> >>
> >> >> And we cannot use zero-cost looping in this situation.
> >> >> And that's why I spilt the zero_cost_loop_end into a normal test and
> branch.
> >>
> >> > Also note that the hwloop_pattern_reg interface also expects a
> >> > general
> >> purpose register in the doloop_end pattern.
> >>
> >> If there were no loop instruction, how would this work?
> >
> >
> > Just take a look at my patch. I handle this in the new define_split:
> >
> > +(define_split
> > +  [(set (pc)
> > +        (if_then_else (ne (match_operand:SI 0 "nonimmediate_operand"
> "")
> > +                          (const_int 1))
> > +                      (label_ref (match_operand 1 "" ""))
> > +                      (pc)))
> > +   (set (match_operand:SI 2 "nonimmediate_operand" "")
> > +        (plus:SI (match_dup 0)
> > +                 (const_int -1)))
> > +   (unspec [(const_int 0)] UNSPEC_LSETUP_END)
> > +   (clobber (match_scratch 3))]
> > +  "TARGET_LOOPS && optimize && reload_completed"
> > +  [(const_int 0)]
> > +{
> > +  if (!REG_P (operands[0]))
> > +    {
> > +      rtx test;
> > +
> > +      /* Fallback into a normal conditional branch insn.  */
> > +      emit_move_insn (operands[3], operands[0]);
> > +      emit_insn (gen_addsi3 (operands[3], operands[3], constm1_rtx));
> > +      emit_move_insn (operands[0], operands[3]);
> > +      test = gen_rtx_NE (VOIDmode, operands[3], const0_rtx);
> > +      emit_jump_insn (gen_cbranchsi4 (test, operands[3],
> > +                                      const0_rtx, operands[1]));
> > +    }
> > +  else
> > +    {
> > +      emit_jump_insn (gen_loop_end (operands[0], operands[1],
> operands[2]));
> > +    }
> > +
> > +  DONE;
> > +})