[PATCH] Simplify (view_convert ~a) < 0 to (view_convert a) >= 0 [PR middle-end/100738]

[Marc Glisse]

On Fri, 4 Jun 2021, Hongtao Liu via Gcc-patches wrote:

> On Tue, Jun 1, 2021 at 6:17 PM Marc Glisse <marc.glisse@inria.fr> wrote:
>>
>> On Tue, 1 Jun 2021, Hongtao Liu via Gcc-patches wrote:
>>
>>> Hi:
>>>  This patch is about to simplify (view_convert:type ~a) < 0 to
>>> (view_convert:type a) >= 0 when type is signed integer. Similar for
>>> (view_convert:type ~a) >= 0.
>>>  Bootstrapped and regtested on x86_64-linux-gnu{-m32,}.
>>>  Ok for the trunk?
>>>
>>> gcc/ChangeLog:
>>>
>>>        PR middle-end/100738
>>>        * match.pd ((view_convert ~a) < 0 --> (view_convert a) >= 0,
>>>        (view_convert ~a) >= 0 --> (view_convert a) < 0): New GIMPLE
>>>        simplification.
>>
>> We already have
>>
>> /* Fold ~X op C as X op' ~C, where op' is the swapped comparison.  */
>> (for cmp (simple_comparison)
>>       scmp (swapped_simple_comparison)
>>   (simplify
>>    (cmp (bit_not@2 @0) CONSTANT_CLASS_P@1)
>>    (if (single_use (@2)
>>         && (TREE_CODE (@1) == INTEGER_CST || TREE_CODE (@1) == VECTOR_CST))
>>     (scmp @0 (bit_not @1)))))
>>
>> Would it make sense to try and generalize it a bit, say with
>>
>> (cmp (nop_convert1? (bit_not @0)) CONSTANT_CLASS_P)
>>
>> (scmp (view_convert:XXX @0) (bit_not @1))
>>
> Thanks for your advice, it looks great.
> And can I use *view_convert1?* instead of *nop_convert1?* here,
> because the original case is view_convert, and nop_convert would fail
> to simplify the case.

Near the top of match.pd, you can see

/* With nop_convert? combine convert? and view_convert? in one pattern
    plus conditionalize on tree_nop_conversion_p conversions.  */
(match (nop_convert @0)
  (convert @0)
  (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))))
(match (nop_convert @0)
  (view_convert @0)
  (if (VECTOR_TYPE_P (type) && VECTOR_TYPE_P (TREE_TYPE (@0))
       && known_eq (TYPE_VECTOR_SUBPARTS (type),
                    TYPE_VECTOR_SUBPARTS (TREE_TYPE (@0)))
       && tree_nop_conversion_p (TREE_TYPE (type), TREE_TYPE (TREE_TYPE (@0))))))

So at least the intention is that it can handle both NOP_EXPR for scalars 
and VIEW_CONVERT_EXPR for vectors, and I think we alread use it that way 
in some places in match.pd, like

(simplify
  (negate (nop_convert? (bit_not @0)))
  (plus (view_convert @0) { build_each_one_cst (type); }))

(simplify
  (bit_xor:c (nop_convert?:s (bit_not:s @0)) @1)
  (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
   (bit_not (bit_xor (view_convert @0) @1))))

(the 'if' seems redundant for this one)

  (simplify
   (negate (nop_convert? (negate @1)))
   (if (!TYPE_OVERFLOW_SANITIZED (type)
        && !TYPE_OVERFLOW_SANITIZED (TREE_TYPE (@1)))
    (view_convert @1)))

etc.


At some point this got some genmatch help, to handle '?' and numbers, so I 
don't remember all the details, but following these examples should work.

-- 
Marc Glisse