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

["augustine dot sterling at gmail dot com" <augustine dot sterling at gmail dot com>]

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;
> +})