Re: [PATCH v3, rs6000] Disable TImode from Bool expanders [PR100694, PR93123]

[HAO CHEN GUI <guihaoc@linux.ibm.com>]

Hi Segher,
  Thanks for your comments.

On 13/7/2022 上午 1:26, Segher Boessenkool wrote:
>> --- a/gcc/config/rs6000/rs6000.md
>> +++ b/gcc/config/rs6000/rs6000.md
>> @@ -7078,27 +7078,38 @@ (define_expand "subti3"
>>  })
>>  \f
>>  ;; 128-bit logical operations expanders
>> +;; Fail TImode in all 128-bit logical operations expanders and split it into
>> +;; two DI registers.
>>
>>  (define_expand "and<mode>3"
>>    [(set (match_operand:BOOL_128 0 "vlogical_operand")
>>  	(and:BOOL_128 (match_operand:BOOL_128 1 "vlogical_operand")
>>  		      (match_operand:BOOL_128 2 "vlogical_operand")))]
>>    ""
>> -  "")
>> +{
>> +  if (<MODE>mode == TImode)
>> +    FAIL;
>> +})
> It is better to not FAIL it, but simply not have a pattern for the
> TImode version at all.
> 
> Does nothing depend on the :TI version to exist?
> 
> What about the :PTI version?  Getting rid of that as well will allow
> some nice optimisations.
> 
> Of course we *do* have instructions to do such TImode ops, on newer
> CPUs, but in vector registers only.  It isn't obvious what is faster.
> 

During expand, TI mode is split to two registers when it can't match
any expands. So I failed TI mode in each expand and expect to be
split at expand. TI mode is still in some insn_and_split patterns
(e.g. "*and<mode>3_internal"). If later rtl passes generate TI mode
logical operations, they still can be matched.

Originally, the TI mode is split after reload pass by
rs6000_split_logical. It's too late to catch some rtl optimizations.

For the PTI, it can't be split to two registers during expand. PTI
requires an even/odd register pair. So splitting it after reload can
make sure it gets correct registers, I think.

From my understanding, it's sub-optimal to use vector logical operation
instructions for TI mode if the destination is an integer operand. It
needs three instructions (move to vector register, vector logical
operation and move from vector register). When splitting TImode, it only
needs two logical instructions on two separate registers.

Thanks again
Gui Haochen