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Re: ARM peephole2 from 2003 never merged, still valid


On 06/06/2019 00:46, Segher Boessenkool wrote:
> On Wed, Jun 05, 2019 at 05:02:53PM -0600, Jeff Law wrote:
>> On 6/2/19 6:28 AM, Segher Boessenkool wrote:
>>> On Sat, Jun 01, 2019 at 11:41:30PM +0000, Fredrik Hederstierna wrote:
>>>> +(define_peephole2
>>>> +  [(set (match_operand:SI 0 "arm_general_register_operand" "")
>>>> +       (match_operand:SI 1 "arm_general_register_operand" ""))
>>>> +   (set (reg:CC CC_REGNUM)
>>>> +       (compare:CC (match_dup 0) (const_int 0)))]
>>>> +  "TARGET_ARM"
>>>> +  [(parallel [(set (reg:CC CC_REGNUM) (compare:CC (match_dup 1) (const_int 0)))
>>>> +             (set (match_dup 0) (match_dup 1))])]
>>>> +  ""
>>>> +)
>>>
>>> Do you have a testcase for this?  I wonder if it would be better handled
>>> during combine, and what that then tried; or perhaps these opportunities
>>> are created later, making a peephole a more attractive solution.
>> We have two independent insns with no output/true dependency between
>> them.  So there's really not anything for combine to do here.
> 
> op0 := op1;
> CC := op0 cmp 0;
> 
> That's a perfectly fine dependency I think?

But if op0 isn't dead after the second insn, combine won't consider it,
right?  Even though in this case the combination would be safe as we
still produce op0.

R.

> 
>> What is more interesting to me is whether or not this could be handled
>> by compare-elim and a define_insn rather than a define_peephole2.  THe
>> existing pattern and the new one both seem well suited for compare-elim.
>>
>> I do think a testcase is warranted here.  Fredrik, if you could reduce
>> one from CSiBE that might be appropriate.
> 
> Yeah, let's see what is really going on :-)
> 
> 
> Segher
> 


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