Re: match.pd: Optimize (x & y) ^ (x | y)

[Marc Glisse <marc.glisse@inria.fr>]

On Fri, 12 Jun 2015, Richard Biener wrote:

>> Not judging at all whether it is desirable or not, but you might have 
>> hit the issue that when you want several convert?, you need to use the 
>> spelling convert1?, convert2?, and it stops there, while here you would 
>> probably want at least 4 (maybe 6?) for this case. You might be able to 
>> work around it with a user-defined predicate, but I keep getting errors 
>> like
>> generic-match.c:6655:16: error: redeclaration of ‘tree_node* o20_pops [2]’
>>
>> If you want to reproduce the error (this is probably not good as is, it 
>> is only provided as a reproducer)
>>
>> (match (nopand @0 @1)
>>  (bit_and (convert1? @0) (convert2? @1))
>>  (if (tree_nop_conversion_p (type, TREE_TYPE (@0))
>>       && tree_nop_conversion_p (type, TREE_TYPE (@1)))))
>> (match (nopior @0 @1)
>>  (bit_ior (convert1? @0) (convert2? @1))
>>  (if (tree_nop_conversion_p (type, TREE_TYPE (@0))
>>       && tree_nop_conversion_p (type, TREE_TYPE (@1)))))
>> (simplify
>>  (bit_xor:c (convert1? (nopand@2 @0 @1))
>>             (convert2? (nopior@3 @0 @1)))
>>  (if (tree_nop_conversion_p (type, TREE_TYPE (@2))
>>       && tree_nop_conversion_p (type, TREE_TYPE (@3)))
>>   (bit_xor (convert @0) (convert @1))))
>>
>>
>> fold-const.c traditionally avoided the combinatorial explosion by using
>> strip_nops.
>
> Yeah.  We can probably special case conditional conversions in code 
> generation instead of lowering it.  And then go the full way and special 
> case nop conversions so you can avoid writing the predicate as well.

Without special casing, I currently have:

(match (nopcvt @0)
  (convert? @0)
  (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))))
(simplify
  (bit_xor:c (convert1? (bit_and@2 (nopcvt @0) (nopcvt @1)))
             (convert2? (bit_ior:c (nopcvt @0) (nopcvt @1))))
  (if (tree_nop_conversion_p (type, TREE_TYPE (@2)))
   (bit_xor (convert @0) (convert @1))))

which simplifies Jakub's testcase without exploding the size of *-match.c, 
but it is still not very satisfying.

-- 
Marc Glisse