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Re: [PATCH] Add zero-overhead looping for xtensa backend
- From: "Yangfei (Felix)" <felix dot yang at huawei dot com>
- To: "augustine dot sterling at gmail dot com" <augustine dot sterling at gmail dot com>
- Cc: "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, Felix Yang <fei dot yang0953 at gmail dot com>
- Date: Fri, 24 Oct 2014 06:30:14 +0000
- Subject: Re: [PATCH] Add zero-overhead looping for xtensa backend
- Authentication-results: sourceware.org; auth=none
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>
> 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;
+})