[patch, avr, pr71676 and pr71678] Issues with casesi expand

Georg-Johann Lay avr@gjlay.de
Wed Oct 19 14:22:00 GMT 2016


On 17.10.2016 09:27, Pitchumani Sivanupandi wrote:
> On Thursday 13 October 2016 08:42 PM, Georg-Johann Lay wrote:
>> On 13.10.2016 13:44, Pitchumani Sivanupandi wrote:
>>> On Monday 26 September 2016 08:19 PM, Georg-Johann Lay wrote:
>>>> On 26.09.2016 15:19, Pitchumani Sivanupandi wrote:
>>>>> Attached patch for PR71676 and PR71678.
>>>>>
>>>>> PR71676 is for AVR target that generates wrong code when switch case index is
>>>>> more than 16 bits.
>>>>>
>>>>> Switch case index of larger than SImode are checked for out of range before
>>>>> 'casesi' expand. RTL expand of casesi gets index as SImode, but index is
>>>>> compared in HImode and ignores upper 16bits.
>>>>>
>>>>> Attached patch changes the expansion for casesi to make the index comparison
>>>>> in SImode and code generation accordingly.
>>>>>
>>>>> PR71678 is ICE because below pattern in 'casesi' is not recognized.
>>>>> (set (reg:HI 47)
>>>>>      (minus:HI (subreg:HI (subreg:SI (reg:DI 44) 0) 0)
>>>>>                (reg:HI 45)))
>>>>>
>>>>> Fix of PR71676 avoids the above pattern as it changes the comparison
>>>>> to SImode.
>>>>
>>>> But this means that all comparisons are now performed in SImode which is a
>>>> great performance loss for most programs which will switch on 16-bit values.
>>>>
>>>> IMO we need a less intrusive (w.r.t. performance) approach.
>>>
>>> Yes.
>>>
>>> I tried to split 'casesi' into several based on case values so that compare is
>>> done
>>> in less expensive modes (i.e. QI or HI). In few cases it is not possible
>>> without
>>> SImode subtract/ compare.
>>>
>>> Pattern casesi will have index in SI mode. So, out of range checks will be
>>> expensive
>>> as most common uses (in AVR) of case values will be in QI/HI mode.
>>>
>>> e.g.
>>>   if case values in QI range
>>>     if upper three bytes index is set
>>>       goto out_of_range
>>>
>>>     offset = index - lower_bound (QImode)
>>>     if offset > case_range       (QImode)
>>>       goto out_of_range
>>>     goto jump_table + offset
>>>
>>>   else if case values in HI range
>>>     if index[2,3] is set
>>>       goto out_of_range
>>>
>>>     offset = index - lower_bound (HImode)
>>>     if offset > case_range       (HImode)
>>>       goto out_of_range
>>>     goto jump_table + offset
>>>
>>> This modification will not work for the negative index values. Because code to
>>> check
>>> upper bytes of index will be expensive than the SImode subtract/ compare.
>>>
>>> So, I'm trying to update fix to have SImode subtract/ compare if the case
>>> values include
>>> negative integers. For, others will try to optimize as mentioned above. Is that
>>> approach OK?
>>
>> But the above code will be executed at run time and add even more overhead,
>> or am I missing something?  If you conclude statically at expand time from
>> the case ranges then we might hit a similar problem as with the original
>> subreg computation.
>
> No. Lower bound and case range are const_int_operand, known at compile time.

Yes, but if the range if form 10 to 90, say, then you still don't know whether 
HImode and QImode is appropriate or not which adds to code size and register 
pressure.

As I mentioned earlier, I am working on a different approach which would revert 
your changes:  The casesi is basically unaltered (except for operand clean-ups 
and avoidance of clobbering subregs).

The ups of my approach are:

* The original value is known and whether is is QI or HI.

* The signedness is known which allows to use the maximum range of
   QI resp. HI depending on the sign.

* Also works on negative values.

* All is done at compile time, no need for extra code.

* No intermediate 32-bit values, no unnecessary increase of reg pressure.

* Optimization can be switched off by -fdisable if desired.

* Result can be seen in dumps.

The downsides are:

* Also needs some lines of code (~400).

* Makes assumptions on the anatomy of the code, i.e. extension
   precedes casesi.

First we should decide which route to follow as the changes are conflicting 
each other.  I have not so much time to work on the stuff but the results are 
promising.  If you are interested in the changes, I can supply it but it's all 
still work in progress.

Johann

> Tried to optimize code generated based on case values range.
> Attached the revised patch.
>
> Tested with avrtest, no regression found.
>
> Is it OK?
>
>> Unfortunately, the generated code (setting cc0, a reg and pc) cannot be
>> wrapped into an unspec or parallel and then later be rectified...
>>
>> I am thinking about a new avr target pass to tidy up the code if no 32-bit
>> computation is needed, but this will be some effort.
> Ok.
>
>
> Regards,
> Pitchumani
>
> gcc/ChangeLog
>
> 2016-10-17  Pitchumani Sivanupandi <pitchumani.s@atmel.com>
>
>     PR target/71676
>     PR target/71678
>     * config/avr/avr.md (casesi_index_qi, casesi_index_hi, casesi_index_si):
>     Add new expands, called by casesi based on case values range.
>
> gcc/testsuite/ChangeLog
>
> 2016-10-17  Pitchumani Sivanupandi <pitchumani.s@atmel.com>
>
>     PR target/71676
>     PR target/71678
>     * gcc.target/avr/pr71676-1.c: New test.
>     * gcc.target/avr/pr71676.c: New test.
>     * gcc.target/avr/pr71678.c: New test.
>



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