Insn combine trying (ior:HI (clobber:HI (const_int 0)))

[Jeff Law]

On 7/16/24 4:29 AM, Georg-Johann Lay wrote:
> 
> 
> Am 15.07.24 um 19:53 schrieb Richard Sandiford:
>> Georg-Johann Lay <avr@gjlay.de> writes:
>>> In a test case I see insn combine trying to match such
>>> expressions, which do not make any sense to me, like:
>>>
>>> Trying 2 -> 7:
>>>       2: r45:HI=r48:HI
>>>         REG_DEAD r48:HI
>>>       7: {r47:HI=r45:HI|r46:PSI#0;clobber scratch;}
>>>         REG_DEAD r46:PSI
>>>         REG_DEAD r45:HI
>>> Failed to match this instruction:
>>> (parallel [
>>>           (set (reg:HI 47 [ _4 ])
>>>               (ior:HI (clobber:HI (const_int 0 [0]))
>>>                   (reg:HI 48)))
>>>           (clobber (scratch:QI))
>>>       ])
>>>
>>> and many other occasions like that.
>>>
>>> Is this just insn combine doing its business?
>>>
>>> Or should this be some sensible RTL instead?
>>>
>>> Seen on target avr with v14 and trunk,
>>> attached test case and dump compiled with
>>
>> (clobber:M (const_int 0)) is combine's way of representing
>> "something went wrong here".  And yeah, recog acts as an error
>> detection mechanism in these cases.  In other words, the idea
>> is that recog should eventually fail on nonsense rtl like that,
>> so earlier code doesn't need to check explicitly.
>>
>> Richard
> 
> Hi Richard,
> 
> I never saw this before; and the point is that in some functions,
> combine is almost exclusively producing these rtxes with clobber
> const_int, but almost no meaningful combinations.  Like in:
Perhaps, but the convention of using a (clobber (const_int 0)) for cases 
where combine gets "lost" has been around for decades.


Jeff